SRFI 13: String Libraries

# Title

SRFI 13: String Libraries

# Author

Olin Shivers

# Status

This SRFI is currently in ``final'' status. To see an explanation of each status that a SRFI can hold, see here. You can access the discussion via the archive of the mailing list.

- Received: 1999/10/17
- Draft: 1999/10/18-1999/12/16
- Revised: 1999/10/31
- Revised: 1999/11/13
- Revised: 1999/11/22
- Revised: 2000/04/30
- Revised: 2000/06/09
- Revised: 2000/12/23

# Table of contents

# Abstract

R5RS Scheme has an impoverished set of string-processing utilities, which is a problem for authors of portable code. This SRFI proposes a coherent and comprehensive set of string-processing procedures; it is accompanied by a reference implementation of the spec. The reference implementation is

- portable
- efficient
- open source

The routines in this SRFI are backwards-compatible with the string-processing routines of R5RS.

# Procedure Index

Here is a list of the procedures provided by the string-lib and string-lib-internals packages. R5RS procedures are shown in bold; extended R5RS procedures, in bold italic.

Predicates

      string? string-null?
      string-every string-any

Constructors

      make-string string string-tabulate

List & string conversion

      string->list list->string
      reverse-list->string string-join

Selection

      string-length
      string-ref
      string-copy
      substring/shared
      string-copy!
      string-take string-take-right
      string-drop string-drop-right
      string-pad  string-pad-right
      string-trim string-trim-right string-trim-both

Modification

      string-set! string-fill!

Comparison

      string-compare string-compare-ci
      string<>     string=    string<    string>    string<=    string>=
      string-ci<>  string-ci= string-ci< string-ci> string-ci<= string-ci>=
      string-hash  string-hash-ci

Prefixes & suffixes

      string-prefix-length    string-suffix-length
      string-prefix-length-ci string-suffix-length-ci

      string-prefix?    string-suffix?
      string-prefix-ci? string-suffix-ci?

Searching

      string-index string-index-right
      string-skip  string-skip-right
      string-count
      string-contains string-contains-ci

Alphabetic case mapping

      string-titlecase  string-upcase  string-downcase
      string-titlecase! string-upcase! string-downcase!

Reverse & append

      string-reverse string-reverse!
      string-append
      string-concatenate
      string-concatenate/shared string-append/shared
      string-concatenate-reverse string-concatenate-reverse/shared

Fold, unfold & map

      string-map      string-map!
      string-fold     string-fold-right
      string-unfold   string-unfold-right
      string-for-each string-for-each-index

Replicate & rotate

      xsubstring string-xcopy!

Miscellaneous: insertion, parsing

      string-replace string-tokenize

Filtering & deleting

      string-filter string-delete

Low-level procedures

      string-parse-start+end
      string-parse-final-start+end
      let-string-start+end

      check-substring-spec
      substring-spec-ok?

      make-kmp-restart-vector kmp-step string-kmp-partial-search

# Rationale

This SRFI defines two libraries that provide a rich set of operations for manipulating strings. These are frequently useful for scripting and other text-manipulation applications. The library's design was influenced by the string libraries found in MIT Scheme, Gambit, RScheme, MzScheme, slib, Common Lisp, Bigloo, guile, Chez, APL, Java, and the SML standard basis.

All procedures involving character comparison are available in both case-sensitive and case-insensitive forms.

All functionality is available in substring and full-string forms.

## Strings are code-point sequences

This SRFI considers strings simply to be a sequence of "code points" or character encodings. Operations such as comparison or reversal are always done code point by code point. See the comments below on super-ASCII character types for implications that follow.

It's entirely possible that a legal string might not be a sensible "text" sequence. For example, consider a string comprised entirely of zero-width Unicode accent characters with no preceding base character to modify -- this is a legal string, albeit one that does not make a great deal of sense when interpreted as a sequence of natural-language text. The routines in this SRFI do not handle these "text" concerns; they restrict themselves to the underlying view of strings as merely a sequence of "code points."

## String operations are locale- and context-independent

This SRFI defines string operations that are locale- and context-independent. While it is certainly important to have a locale-sensitive comparison or collation procedure when processing text, it is also important to have a suite of operations that are reliably invariant for basic string processing --- otherwise, a change of locale could cause data structures such as hash tables, b-trees, symbol tables, directories of filenames, etc. to become corrupted.

Locale- and context-sensitive text operations, such as collation, are explicitly deferred to a subsequent, companion "text" SRFI.

## Internationalisation & super-ASCII character types

The major issue confronting this SRFI is the existence of super-ASCII character encodings, such as eight-bit Latin-1 or 16- and 32-bit Unicode. It is a design goal of this SRFI for the API to be portable across string implementations based on at least these three standard encodings. Unfortunately, this places strong limitations on the API design. Here are some relevant issues. Be warned that life in a super-ASCII world is significantly more complex; there are no easy answers for many of these issues.

### Case mapping and case-folding

Upper- and lower-casing characters is complex in super-ASCII encodings.

- Some characters case-map to more than one character. For example, the Latin-1 German eszet character upper-cases to "SS."
  - This means that the R5RS function `char-upcase` is not well-defined, since it is defined to produce a (single) character result.
  - It means that an in-place `string-upcase!` procedure cannot be reliably defined, since the original string may not be long enough to contain the result -- an N-character string might upcase to a 2N-character result.
  - It means that case-insensitive string-matching or searching is quite tricky. For example, an n-character string s might match a 2N-character string s'.
- Some characters case-map in different ways depending upon their surrounding context. For example, the Unicode Greek capital sigma character downcases differently depending upon whether or not it is the final character in a word. Again, this spells trouble for the simple R5RS `char-downcase` function.
- Unicode defines three cases: lowercase, uppercase and titlecase. The distinction between uppercase and titlecase arises in the presence of Unicode's compound characters. For example, Unicode has a single character representing the compound pair "dz." Uppercasing the "dz" character produces the compound character "DZ", while titlecasing (or, as Americans say, capitalizing) it produces compound character "Dz".
- Turkish actually has different case-mappings from other languages.

The Unicode Consortium's web site

http://www.unicode.org/

has detailed discussions of the issues. See in particular technical report 21 on case mappings

http://www.unicode.org/unicode/reports/tr21/

SRFI 13 makes no attempt to deal with these issues; it uses a simple 1-1 locale- and context-independent case-mapping, specifically Unicode's 1-1 case-mappings given in

ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

The format of this file is explained in

ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.html

Note that this means that German eszet upper-cases to itself, not "SS".

Case-mapping and case-folding operations in SRFI 13 are locale-independent so that shifting locales won't wreck hash tables, b-trees, symbol tables, etc.

### String equality & string normalisation

Comparing strings for equality is complicated because in some cases Unicode actually provides multiple encodings for the "same" character, and because what we usually think of as a "character" can be represented in Unicode as a sequence of several code-points. For example, consider the letter "e" with an acute accent. There is a single Unicode character for this. However, Unicode also allows one to represent this with a two-character sequence: the "e" character followed by a zero-width acute-accent character. As another example, Unicode provides some Asian characters in "narrow" and "full" widths.

There are multiple ways we might want to compare strings for equality. In (roughly) decreasing order of precision,

- we might want a precise comparison of the actual encoding, so that <e-acute> would not compare equal to <e, acute>.
- We might want a "normalised" comparison, where these two sequences would compare equal.
- We might want an even more-permissive normalisation, where visually-distinct properties of "the same" character would be ignored. For example, we might want narrow/full-width versions of the same Asian character to compare equal.
- We might want comparisons that are insensitive to accents and diacritical marks.
- We might want comparisons that are case-insensitive.
- We might want comparisons that are insensitive to several of the above properties.
- We might want ways to "normalise" strings into various canonical forms.

This library does not address these complexities. SRFI 13 string equality is simply based upon comparing the encoding values used for the characters. Accent-insensitive and other types of comparison are not provided; only a simple form of case-insensitive comparison is provided, which uses the 1-1 case mappings specified by Unicode in

ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

These are adequate for "program" or "systems" use of strings (e.g., to manipulate program identifiers and operating-system filenames).

### String inequality

Above and beyond the issues arising in string-equality, when we attempt to order strings there are even further considerations.

- French orders accents with right-to-left significance -- the reverse of the significance of the characters.
- Case-insensitive ordering is not well defined by simple "code-point" considerations, even for simple ASCII: there are punctuation characters between the ASCII's upper-case range of letters and its lower-case range (left-bracket, backslash, right-bracket, caret, underbar and backquote). Does left-bracket compare less-than or greater-than "a" in a case-insensitive comparison?
- The German eszet character should sort as if it were the pair of letters "ss".

Unicode defines a complex set of machinery for ordering or "collating" strings, which involves mapping each string to a multi-byte sort key, and then doing simple lexicographic sorting with these keys. These rules can be overlaid by additional domain- or language-specific rules. Again, this SRFI does not address these issues. SRFI 13 string ordering is strictly based upon a character-by-character comparison of the values used for representing the string.

## Naming conventions

This library contains a large number of procedures, but they follow a consistent naming scheme, and are consistent with the conventions developed in SRFI 1. The names are composed of smaller lexemes in a regular way that exposes the structure and relationships between the procedures. This should help the programmer to recall or reconstitute the name of the particular procedure that he needs when writing his own code. In particular

- Procedures whose names end in "-ci" are case-insensitive variants.
- Procedures whose names end in "!" are side-effecting variants. What values these procedures return is usually not specified.
- The order of common parameters is consistent across the different procedures.
- Left/right/both directionality: Procedures that have left/right directional variants use the following convention:

  Direction |  | Suffix
  --- | --- | ---
  left-to-right |  | none
  right-to-left |  | `-right`
  both |  | `-both`

  This is a general convention that was established in SRFI 1. The value of a convention is proportional to the extent of its use.

## Shared storage

Some Scheme implementations, e.g. guile and T, provide ways to construct substrings that share storage with other strings. This facility is called "shared-text substrings." Shared-text substrings can be used to eliminate the allocation and copying time and space required to produce substrings, which can be a tremendous savings for some applications, reducing a linear-time operation to constant time. Additionally, some algorithms rely on the sharing property of these substrings -- the application assumes that if the underlying storage is mutated, then all strings sharing that storage will show the change. However, shared-text substrings are not a common feature; most Scheme implementations do not provide them.

SRFI 13 takes a middle ground with respect to shared-text substrings. In particular, a Scheme implementation does not need to have shared-text substrings in order to implement this SRFI.

There is an additional form of storage sharing enabled by some SRFI 13 procedures, even without the benefit of shared-text substrings. In some cases, some SRFI 13 routines are allowed to return as a result one of the strings that was passed in as a parameter. For example, when constructing a substring with the `substring/shared` procedure, if the requested substring is the entire string, the procedure is permitted simply to return the original value. That is,

    (eq? s (substring/shared s 0 (string-length s))) => true or false

whereas the R5RS `substring` function is required to allocate a fresh copy

    (eq? s (substring s 0 (string-length s))) => false.

In keeping with SRFI 13's general approach to sharing, compliant implementations are allowed, but not required, to provide this kind of sharing. Hence, procedures may not rely upon sharing in these cases.

Most procedures that permit results to share storage with inputs have equivalent procedures that require allocating fresh storage for results. If an application wishes to be sure a new, fresh string is allocated, then these "pure" procedures should be used.

Fresh copy guaranteed | Sharing permitted
--- | ---
`string-copy` | `substring/shared`
`string-copy` | `string-take string-take-right`
`string-copy` | `string-drop string-drop-right`
`string-concatenate` | `string-concatenate/shared`
`string-append` | `string-append/shared`
`string-concatenate-reverse` | `string-concatenate-reverse/shared`
| `string-pad string-pad-right`
| `string-trim string-trim-right`
| `string-trim-both`
| `string-filter string-delete`

On the other hand, the functionality is present to allow one to write efficient code without shared-text substrings. You can write efficient code that works by passing around start/end ranges indexing into a string instead of simply building a shared-text substring. The API would be much simpler without this consideration -- if we had cheap shared-text substrings, all the start/end index parameters would vanish. However, since SRFI 13 does not require implementations to provide shared-text substrings, the extended API is provided.

## R4RS/R5RS procedures

The R4RS and R5RS reports define 22 string procedures. The string-lib package includes 8 of these exactly as defined, 3 in an extended, backwards-compatible way, and drops the remaining 11 (whose functionality is available via other bindings).

The 8 procedures provided exactly as documented in the reports are `string?`, `make-string`, `string`, `string-length`, `string-ref`, `string-set!`, `string-append`, and `list->string`.

The eleven functions not included are `string=?`, `string-ci=?`, `string<?`, `string-ci<?`, `string>?`, `string-ci>?`, `string<=?`, `string-ci<=?`, `string>=?`, `string-ci>=?`, and `substring`. The string-lib package provides alternate bindings and extended functionality.

Additionally, the three extended procedures are

    string-fill! s char [start end] -> unspecified
    string->list s [start end] -> char-list
    string-copy  s [start end] -> string

They are uniformly extended to take optional start/end parameters specifying substring ranges.

## Extra-SRFI recommendations

This SRFI recommends the following

- A SRFI be defined for shared-text substrings, allowing programs to be written that actually rely on the shared-storage properties of these data structures.
- A SRFI be defined for manipulating Unicode text -- various normalisation operations, collation, searching, etc. Collation operations might be parameterised by a "collation" structure representing collation rules for a particular locale or language. Alternatively, a data structure specifying collation rules could be activated with dynamic scope by special procedures, possibly overridden by allowing collation rules to be optional arguments to procedures that need to order strings, e.g.

      (with-locale* denmark-locale
        (lambda ()
          (f x)
          (g 42)))

      (with-locale taiwan-locale
        (f x)
        (h denmark-locale)
        (g 42))

      (set-locale! denmark-locale)

- A SRFI be defined for manipulating characters that is portable across at least ASCII, Latin-1 and Unicode.
  - For backwards-compatibility, `char-upcase` and `char-downcase` should be defined to use the 1-1 locale- and context-insensitive case mappings given by Unicode's UnicodeData.txt table.
  - numeric codes for standard functions that map between characters and integers should be required to use the Unicode/Latin-1/ASCII mapping. This allows programmers to write portable code.
  - `char-titlecase` be added to `char-upcase` and `char-downcase`.
  - `char-titlecase?` be added to `char-upcase?` and `char-downcase?`.
  - Title/up/down-case functions be added to the character-processing suite which allow 1->n case maps by returning immutable, possibly-multi-character strings instead of single characters. These case mappings need not be locale- or context-sensitive.

These recommendations are not a part of the SRFI 13 spec. Note also that requiring a Unicode/Latin-1/ASCII interface to integer/char mapping functions does not imply anything about the actual underlying encodings of characters.

# Procedure Specification

In the following procedure specifications:

- An s parameter is a string.
- A char parameter is a character.
- Start and end parameters are half-open string indices specifying a substring within a string parameter; when optional, they default to 0 and the length of the string, respectively. When specified, it must be the case that 0 <= start <= end <= `(string-length s)`, for the corresponding parameter s. They typically restrict a procedure's action to the indicated substring.
- A pred parameter is a unary character predicate procedure, returning a true/false value when applied to a character.
- A char/char-set/pred parameter is a value used to select/search for a character in a string. If it is a character, it is used in an equality test; if it is a character set, it is used as a membership test; if it is a procedure, it is applied to the characters as a test predicate.
- An i parameter is an exact non-negative integer specifying an index into a string.
- Len and nchars parameters are exact non-negative integers specifying a length of a string or some number of characters.
- An obj parameter may be any value at all.

Passing values to procedures with these parameters that do not satisfy these types is an error.

Parameters given in square brackets are optional. Unless otherwise noted in the text describing the procedure, any prefix of these optional parameters may be supplied, from zero arguments to the full list. When a procedure returns multiple values, this is shown by listing the return values in square brackets, as well. So, for example, the procedure with signature

    halts? f [x init-store] -> [boolean integer]

would take one (f), two (f, x) or three (f, x, init-store) input parameters, and return two values, a boolean and an integer.

A parameter followed by "`...`" means zero-or-more elements. So the procedure with the signature

    sum-squares x ...  -> number

takes zero or more arguments (x ...), while the procedure with signature

    spell-check doc dict1 dict2 ... -> string-list

takes two required parameters (doc and dict1) and zero or more optional parameters (dict2 ...).

If a procedure is said to return "unspecified," this means that nothing at all is said about what the procedure returns. Such a procedure is not even required to be consistent from call to call. It is simply required to return a value (or values) that may be passed to a command continuation, e.g. as the value of an expression appearing as a non-terminal subform of a `begin` expression. Note that in R5RS, this restricts such a procedure to returning a single value; non-R5RS systems may not even provide this restriction.

## Main procedures

In a Scheme system that has a module or package system, these procedures should be contained in a module named "string-lib".

### Predicates

`string?` obj -> boolean
  [R5RS] Returns `#t` if obj is a string, otherwise returns `#f`.
`string-null?` s -> boolean
  Is s the empty string?
`string-every` char/char-set/pred s [start end] -> value
`string-any` char/char-set/pred s [start end] -> value
  Checks to see if the given criteria is true of every / any character in s, proceeding from left (index start) to right (index end).

  If char/char-set/pred is a character, it is tested for equality with the elements of s.

  If char/char-set/pred is a character set, the elements of s are tested for membership in the set.

  If char/char-set/pred is a predicate procedure, it is applied to the elements of s. The predicate is "witness-generating:"

  - If `string-any` returns true, the returned true value is the one produced by the application of the predicate.
  - If `string-every` returns true, the returned true value is the one produced by the final application of the predicate to s[end]. If `string-every` is applied to an empty sequence of characters, it simply returns `#t`.
  If `string-every` or `string-any` apply the predicate to the final element of the selected sequence (i.e., s[end-1]), that final application is a tail call.

  The names of these procedures do not end with a question mark -- this is to indicate that, in the predicate case, they do not return a simple boolean (`#t` or `#f`), but a general value.

### Constructors

`make-string` len [char] -> string
  [R5RS] `make-string` returns a newly allocated string of length len. If char is given, then all elements of the string are initialized to char, otherwise the contents of the string are unspecified.
`string` char1 ... -> string
  [R5RS] Returns a newly allocated string composed of the argument characters.
`string-tabulate` proc len -> string
  Proc is an integer->char procedure. Construct a string of size len by applying proc to each index to produce the corresponding string element. The order in which proc is applied to the indices is not specified.

### List & string conversion

`string->list` s [start end] -> char-list
`list->string` char-list -> string
  [R5RS+] `string->list` returns a newly allocated list of the characters that make up the given string. `list->string` returns a newly allocated string formed from the characters in the list char-list, which must be a list of characters. `string->list` and `list->string` are inverses so far as `equal?` is concerned.

  `string->list` is extended from the R5RS definition to take optional start/end arguments.

`reverse-list->string` char-list -> string
  An efficient implementation of `(compose list->string reverse)`:

      (reverse-list->string '(#\a #\B #\c)) -> "cBa"

  This is a common idiom in the epilog of string-processing loops that accumulate an answer in a reverse-order list. (See also `string-concatenate-reverse` for the "chunked" variant.)
`string-join` string-list [delimiter grammar] -> string
  This procedure is a simple unparser --- it pastes strings together using the delimiter string.

  The grammar argument is a symbol that determines how the delimiter is used, and defaults to `'infix`.

  - `'infix` means an infix or separator grammar: insert the delimiter between list elements. An empty list will produce an empty string -- note, however, that parsing an empty string with an infix or separator grammar is ambiguous. Is it an empty list, or a list of one element, the empty string?
  - `'strict-infix` means the same as `'infix`, but will raise an error if given an empty list.
  - `'suffix` means a suffix or terminator grammar: insert the delimiter after every list element. This grammar has no ambiguities.
  - `'prefix` means a prefix grammar: insert the delimiter before every list element. This grammar has no ambiguities.
  The delimiter is the string used to delimit elements; it defaults to a single space " ".

      (string-join '("foo" "bar" "baz") ":")         => "foo:bar:baz"
      (string-join '("foo" "bar" "baz") ":" 'suffix) => "foo:bar:baz:"

      ;; Infix grammar is ambiguous wrt empty list vs. empty string,
      (string-join '()   ":") => ""
      (string-join '("") ":") => ""

      ;; but suffix & prefix grammars are not.
      (string-join '()   ":" 'suffix) => ""
      (string-join '("") ":" 'suffix) => ":"

### Selection

`string-length` s -> integer
  [R5RS] Returns the number of characters in the string s.
`string-ref` s i -> char
  [R5RS] Returns character s[i] using zero-origin indexing. I must be a valid index of s.
`string-copy` s [start end] -> string
`substring/shared` s start [end] -> string
  [R5RS+] `substring/shared` returns a string whose contents are the characters of s beginning with index start (inclusive) and ending with index end (exclusive). It differs from the R5RS `substring` in two ways:
  - The end parameter is optional, not required.
  - `substring/shared` may return a value that shares memory with s or is `eq?` to s.

  `string-copy` is extended from its R5RS definition by the addition of its optional start/end parameters. In contrast to `substring/shared`, it is guaranteed to produce a freshly-allocated string.

  Use `string-copy` when you want to indicate explicitly in your code that you wish to allocate new storage; use `substring/shared` when you don't care if you get a fresh copy or share storage with the original string.

      (string-copy "Beta substitution") => "Beta substitution"
      (string-copy "Beta substitution" 1 10)
          => "eta subst"
      (string-copy "Beta substitution" 5) => "substitution"

`string-copy!` target tstart s [start end] -> unspecified
  Copy the sequence of characters from index range [start,end) in string s to string target, beginning at index tstart. The characters are copied left-to-right or right-to-left as needed -- the copy is guaranteed to work, even if target and s are the same string.

  It is an error if the copy operation runs off the end of the target string, e.g.

      (string-copy! (string-copy "Microsoft") 0
                    "Regional Microsoft Operating Companies") => error

`string-take` s nchars -> string
`string-drop` s nchars -> string
`string-take-right` s nchars -> string
`string-drop-right` s nchars -> string
  `string-take` returns the first nchars of s; `string-drop` returns all but the first nchars of s. `string-take-right` returns the last nchars of s; `string-drop-right` returns all but the last nchars of s. If these procedures produce the entire string, they may return either s or a copy of s; in some implementations, proper substrings may share memory with s.

      (string-take "Pete Szilagyi" 6) => "Pete S"
      (string-drop "Pete Szilagyi" 6) => "zilagyi"

      (string-take-right "Beta rules" 5) => "rules"
      (string-drop-right "Beta rules" 5) => "Beta "

  It is an error to take or drop more characters than are in the string:

      (string-take "foo" 37) => error

`string-pad` s len [char start end] -> string
`string-pad-right` s len [char start end] -> string
  Build a string of length len comprised of s padded on the left (right) by as many occurrences of the character char as needed. If s has more than len chars, it is truncated on the left (right) to length len. Char defaults to #\space.

  If len <= end-start, the returned value is allowed to share storage with s, or be exactly s (if len = end-start).

      (string-pad     "325" 5) => "  325"
      (string-pad   "71325" 5) => "71325"
      (string-pad "8871325" 5) => "71325"

`string-trim` s [char/char-set/pred start end] -> string
`string-trim-right` s [char/char-set/pred start end] -> string
`string-trim-both` s [char/char-set/pred start end] -> string
  Trim s by skipping over all characters on the left / on the right / on both sides that satisfy the second parameter char/char-set/pred:
  - if it is a character char, characters equal to char are trimmed;
  - if it is a char set cs, characters contained in cs are trimmed;
  - if it is a predicate pred, it is a test predicate that is applied to the characters in s; a character causing it to return true is skipped.
  Char/char-set/pred defaults to the character set `char-set:whitespace` defined in SRFI 14.

  If no trimming occurs, these functions may return either s or a copy of s; in some implementations, proper substrings may share memory with s.

      (string-trim-both "  The outlook wasn't brilliant,  \n\r")
          => "The outlook wasn't brilliant,"

### Modification

`string-set!` s i char -> unspecified
  [R5RS] I must be a valid index of s. `string-set!` stores char in element i of s. Constant string literals appearing in code are immutable; it is an error to use them in a `string-set!.`

      (define (f) (make-string 3 #\*))
      (define (g) "***")
      (string-set! (f) 0 #\?)                ==>  unspecified
      (string-set! (g) 0 #\?)                ==>  error
      (string-set! (symbol->string 'immutable)
                   3
                   #\?)                      ==>  error

`string-fill!` s char [start end] -> unspecified
  [R5RS+] Stores char in every element of s.

  `string-fill` is extended from the R5RS definition to take optional start/end arguments.

### Comparison

`string-compare` s1 s2 proc< proc= proc> [start1 end1 start2 end2] -> values
`string-compare-ci` s1 s2 proc< proc= proc> [start1 end1 start2 end2] -> values
  Apply proc<, proc=, or proc> to the mismatch index, depending upon whether s1 is less than, equal to, or greater than s2. The "mismatch index" is the largest index i such that for every 0 <= j < i, s1[j] = s2[j] -- that is, i is the first position that doesn't match.

  `string-compare-ci` is the case-insensitive variant. Case-insensitive comparison is done by case-folding characters with the operation

      (char-downcase (char-upcase c))

  where the two case-mapping operations are assumed to be 1-1, locale- and context-insensitive, and compatible with the 1-1 case mappings specified by Unicode's UnicodeData.txt table:
  ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

  The optional start/end indices restrict the comparison to the indicated substrings of s1 and s2. The mismatch index is always an index into s1; in the case of proc=, it is always end1; we observe the protocol in this redundant case for uniformity.

      (string-compare "The cat in the hat" "abcdefgh"
                      values values values
                      4 6         ; Select "ca"
                      2 4)        ; & "cd"
          => 5    ; Index of S1's "a"

  Comparison is simply done on individual code-points of the string. True text collation is not handled by this SRFI.
`string=` s1 s2 [start1 end1 start2 end2] -> boolean
`string<>` s1 s2 [start1 end1 start2 end2] -> boolean
`string<` s1 s2 [start1 end1 start2 end2] -> boolean
`string>` s1 s2 [start1 end1 start2 end2] -> boolean
`string<=` s1 s2 [start1 end1 start2 end2] -> boolean
`string>=` s1 s2 [start1 end1 start2 end2] -> boolean
  These procedures are the lexicographic extensions to strings of the corresponding orderings on characters. For example, `string<` is the lexicographic ordering on strings induced by the ordering `char<?` on characters. If two strings differ in length but are the same up to the length of the shorter string, the shorter string is considered to be lexicographically less than the longer string.

  The optional start/end indices restrict the comparison to the indicated substrings of s1 and s2.

  Comparison is simply done on individual code-points of the string. True text collation is not handled by this SRFI.

`string-ci=` s1 s2 [start1 end1 start2 end2] -> boolean
`string-ci<>` s1 s2 [start1 end1 start2 end2] -> boolean
`string-ci<` s1 s2 [start1 end1 start2 end2] -> boolean
`string-ci>` s1 s2 [start1 end1 start2 end2] -> boolean
`string-ci<=` s1 s2 [start1 end1 start2 end2] -> boolean
`string-ci>=` s1 s2 [start1 end1 start2 end2] -> boolean
  Case-insensitive variants.

  Case-insensitive comparison is done by case-folding characters with the operation

      (char-downcase (char-upcase c))

  where the two case-mapping operations are assumed to be 1-1, locale- and context-insensitive, and compatible with the 1-1 case mappings specified by Unicode's UnicodeData.txt table:
  ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt
`string-hash` s [bound start end] -> integer
`string-hash-ci` s [bound start end] -> integer
  Compute a hash value for the string s. Bound is a non-negative exact integer specifying the range of the hash function. A positive value restricts the return value to the range [0,bound).

  If bound is either zero or not given, the implementation may use an implementation-specific default value, chosen to be as large as is efficiently practical. For instance, the default range might be chosen for a given implementation to map all strings into the range of integers that can be represented with a single machine word.

  The optional start/end indices restrict the hash operation to the indicated substring of s.

  `string-hash-ci` is the case-insensitive variant. Case-insensitive comparison is done by case-folding characters with the operation

      (char-downcase (char-upcase c))

  where the two case-mapping operations are assumed to be 1-1, locale- and context-insensitive, and compatible with the 1-1 case mappings specified by Unicode's UnicodeData.txt table:
  ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

  Invariants:

      (<= 0 (string-hash s b) (- b 1)) ; When B > 0.
      (string=    s1 s2)  =>  (= (string-hash s1 b)    (string-hash s2 b))
      (string-ci= s1 s2)  =>  (= (string-hash-ci s1 b) (string-hash-ci s2 b))

  A legal but nonetheless discouraged implementation:

      (define (string-hash    s . other-args) 1)
      (define (string-hash-ci s . other-args) 1)

  Rationale: allowing the user to specify an explicit bound simplifies user code by removing the mod operation that typically accompanies every hash computation, and also may allow the implementation of the hash function to exploit a reduced range to efficiently compute the hash value. E.g., for small bounds, the hash function may be computed in a fashion such that intermediate values never overflow into bignum integers, allowing the implementor to provide a fixnum-specific "fast path" for computing the common cases very rapidly.

### Prefixes & suffixes

`string-prefix-length` s1 s2 [start1 end1 start2 end2] -> integer
`string-suffix-length` s1 s2 [start1 end1 start2 end2] -> integer
`string-prefix-length-ci` s1 s2 [start1 end1 start2 end2] -> integer
`string-suffix-length-ci` s1 s2 [start1 end1 start2 end2] -> integer
  Return the length of the longest common prefix/suffix of the two strings. For prefixes, this is equivalent to the "mismatch index" for the strings (modulo the starti index offsets).

  The optional start/end indices restrict the comparison to the indicated substrings of s1 and s2.

  `string-prefix-length-ci` and `string-suffix-length-ci` are the case-insensitive variants. Case-insensitive comparison is done by case-folding characters with the operation

      (char-downcase (char-upcase c))

  where the two case-mapping operations are assumed to be 1-1, locale- and context-insensitive, and compatible with the 1-1 case mappings specified by Unicode's UnicodeData.txt table:
  ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt
  Comparison is simply done on individual code-points of the string.
`string-prefix?` s1 s2 [start1 end1 start2 end2] -> boolean
`string-suffix?` s1 s2 [start1 end1 start2 end2] -> boolean
`string-prefix-ci?` s1 s2 [start1 end1 start2 end2] -> boolean
`string-suffix-ci?` s1 s2 [start1 end1 start2 end2] -> boolean
  Is s1 a prefix/suffix of s2?

  The optional start/end indices restrict the comparison to the indicated substrings of s1 and s2.

  `string-prefix-ci?` and `string-suffix-ci?` are the case-insensitive variants. Case-insensitive comparison is done by case-folding characters with the operation

      (char-downcase (char-upcase c))

  where the two case-mapping operations are assumed to be 1-1, locale- and context-insensitive, and compatible with the 1-1 case mappings specified by Unicode's UnicodeData.txt table:
  ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

  Comparison is simply done on individual code-points of the string.

### Searching

`string-index` s char/char-set/pred [start end] -> integer or #f
`string-index-right` s char/char-set/pred [start end] -> integer or #f
`string-skip` s char/char-set/pred [start end] -> integer or #f
`string-skip-right` s char/char-set/pred [start end] -> integer or #f
  `string-index` (`string-index-right`) searches through the string from the left (right), returning the index of the first occurrence of a character which
  - equals char/char-set/pred (if it is a character);
  - is in char/char-set/pred (if it is a character set);
  - satisfies the predicate char/char-set/pred (if it is a procedure).
  If no match is found, the functions return false.

  The start and end parameters specify the beginning and end indices of the search; the search includes the start index, but not the end index. Be careful of "fencepost" considerations: when searching right-to-left, the first index considered is

  end-1
  whereas when searching left-to-right, the first index considered is
  start
  That is, the start/end indices describe a same half-open interval [start,end) in these procedures that they do in all the other SRFI 13 procedures.

  The skip functions are similar, but use the complement of the criteria: they search for the first char that doesn't satisfy the test. E.g., to skip over initial whitespace, say

      (cond ((string-skip s char-set:whitespace) =>
             (lambda (i) ...)) ; s[i] is not whitespace.
            ...)

`string-count` s char/char-set/pred [start end] -> integer
  Return a count of the number of characters in s that satisfy the char/char-set/pred argument. If this argument is a procedure, it is applied to the character as a predicate; if it is a character set, the character is tested for membership; if it is a character, it is used in an equality test.
`string-contains` s1 s2 [start1 end1 start2 end2] -> integer or false
`string-contains-ci` s1 s2 [start1 end1 start2 end2] -> integer or false
  Does string s1 contain string s2?

  Return the index in s1 where s2 occurs as a substring, or false. The optional start/end indices restrict the operation to the indicated substrings.

  The returned index is in the range [start1,end1). A successful match must lie entirely in the [start1,end1) range of s1.

      (string-contains "eek -- what a geek." "ee"
                       12 18) ; Searches "a geek"
          => 15

  `string-contains-ci` is the case-insensitive variant. Case-insensitive comparison is done by case-folding characters with the operation

      (char-downcase (char-upcase c))

  where the two case-mapping operations are assumed to be 1-1, locale- and context-insensitive, and compatible with the 1-1 case mappings specified by Unicode's UnicodeData.txt table:
  ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

  Comparison is simply done on individual code-points of the string.

  The names of these procedures do not end with a question mark -- this is to indicate that they do not return a simple boolean (`#t` or `#f`). Rather, they return either false (`#f`) or an exact non-negative integer.

### Alphabetic case mapping

`string-titlecase` s [start end] -> string
`string-titlecase!` s [start end] -> unspecified
  For every character c in the selected range of s, if c is preceded by a cased character, it is downcased; otherwise it is titlecased.

  `string-titlecase` returns the result string and does not alter its s parameter. `string-titlecase!` is the in-place side-effecting variant.

      (string-titlecase "--capitalize tHIS sentence.") =>
        "--Capitalize This Sentence."

      (string-titlecase "see Spot run. see Nix run.") =>
        "See Spot Run. See Nix Run."

      (string-titlecase "3com makes routers.") =>
        "3Com Makes Routers."

  Note that if a start index is specified, then the character preceding s[start] has no effect on the titlecase decision for character s[start]:

      (string-titlecase "greasy fried chicken" 2) => "Easy Fried Chicken"

  Titlecase and cased information must be compatible with the Unicode specification.

`string-upcase` s [start end] -> string
`string-upcase!` s [start end] -> unspecified
`string-downcase` s [start end] -> string
`string-downcase!` s [start end] -> unspecified
  Raise or lower the case of the alphabetic characters in the string.

  `string-upcase` and `string-downcase` return the result string and do not alter their s parameter. `string-upcase!` and `string-downcase!` are the in-place side-effecting variants.

  These procedures use the locale- and context-insensitive 1-1 case mappings defined by Unicode's UnicodeData.txt table:

  ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt

### Reverse & append

`string-reverse` s [start end] -> string
`string-reverse!` s [start end] -> unspecified
  Reverse the string.

  `string-reverse` returns the result string and does not alter its s parameter. `string-reverse!` is the in-place side-effecting variant.

      (string-reverse "Able was I ere I saw elba.")
          => ".able was I ere I saw elbA"

      ;;; In-place rotate-left, the Bell Labs way:
      (lambda (s i)
        (let ((i (modulo i (string-length s))))
          (string-reverse! s 0 i)
          (string-reverse! s i)
          (string-reverse! s)))

  Unicode note: Reversing a string simply reverses the sequence of code-points it contains. So a zero-width accent character a coming after a base character b in string s would come out before b in the reversed result.

`string-append` s1 ... -> string
  [R5RS] Returns a newly allocated string whose characters form the concatenation of the given strings.
`string-concatenate` string-list -> string
  Append the elements of `string-list` together into a single string. Guaranteed to return a freshly allocated string.

  Note that the `(apply string-append string-list)` idiom is not robust for long lists of strings, as some Scheme implementations limit the number of arguments that may be passed to an n-ary procedure.

`string-concatenate/shared` string-list -> string
`string-append/shared` s1 ... -> string
  These two procedures are variants of `string-concatenate` and `string-append` that are permitted to return results that share storage with their parameters. In particular, if `string-append/shared` is applied to just one argument, it may return exactly that argument, whereas `string-append` is required to allocate a fresh string.
`string-concatenate-reverse` string-list [final-string end] -> string
`string-concatenate-reverse/shared` string-list [final-string end] -> string
  With no optional arguments, these functions are equivalent to

      (string-concatenate (reverse string-list))

  and

      (string-concatenate/shared (reverse string-list))

  respectively.

  If the optional argument final-string is specified, it is consed onto the beginning of string-list before performing the list-reverse and string-concatenate operations.

  If the optional argument end is given, only the first end characters of final-string are added to the string list, thus producing

      (string-concatenate
        (reverse (cons (substring/shared final-string 0 end)
                       string-list)))

  E.g.

      (string-concatenate-reverse '(" must be" "Hello, I") " going.XXXX" 7)
        => "Hello, I must be going."

  This procedure is useful in the construction of procedures that accumulate character data into lists of string buffers, and wish to convert the accumulated data into a single string when done.

  Unicode note: Reversing a string simply reverses the sequence of code-points it contains. So a zero-width accent character ac coming after a base character bc in string s would come out before bc in the reversed result.

### Fold, unfold & map

`string-map` proc s [start end] -> string
`string-map!` proc s [start end] -> unspecified
  Proc is a char->char procedure; it is mapped over s.

  `string-map` returns the result string and does not alter its s parameter. `string-map!` is the in-place side-effecting variant.

  Note: The order in which proc is applied to the elements of s is not specified.

`string-fold` kons knil s [start end] -> value
`string-fold-right` kons knil s [start end] -> value
  These are the fundamental iterators for strings.

  The left-fold operator maps the kons procedure across the string from left to right

      (... (kons s[2] (kons s[1] (kons s[0] knil))))

  In other words, `string-fold` obeys the (tail) recursion

      (string-fold kons knil s start end) =
          (string-fold kons (kons s[start] knil) start+1 end)

  The right-fold operator maps the kons procedure across the string from right to left

      (kons s[0] (... (kons s[end-3] (kons s[end-2] (kons s[end-1] knil)))))

  obeying the (tail) recursion

      (string-fold-right kons knil s start end) =
          (string-fold-right kons (kons s[end-1] knil) start end-1)

  Examples:

      ;;; Convert a string to a list of chars.
      (string-fold-right cons '() s)

      ;;; Count the number of lower-case characters in a string.
      (string-fold (lambda (c count)
                     (if (char-lower-case? c)
                         (+ count 1)
                         count))
                   0
                   s)

      ;;; Double every backslash character in S.
      (let* ((ans-len (string-fold (lambda (c sum)
                                     (+ sum (if (char=? c #\\) 2 1)))
                                   0 s))
             (ans (make-string ans-len)))
        (string-fold (lambda (c i)
                       (let ((i (if (char=? c #\\)
                                    (begin (string-set! ans i #\\) (+ i 1))
                                    i)))
                         (string-set! ans i c)
                         (+ i 1)))
                     0 s)
        ans)

  The right-fold combinator is sometimes called a "catamorphism."

`string-unfold` p f g seed [base make-final] -> string
  This is a fundamental constructor for strings.
  - G is used to generate a series of "seed" values from the initial seed:
    seed, (g seed), (g2 seed), (g3 seed), ...
  - P tells us when to stop -- when it returns true when applied to one of these seed values.
  - F maps each seed value to the corresponding character in the result string. These chars are assembled into the string in a left-to-right order.
  - Base is the optional initial/leftmost portion of the constructed string; it defaults to the empty string "".
  - Make-final is applied to the terminal seed value (on which p returns true) to produce the final/rightmost portion of the constructed string. It defaults to `(lambda (x) "")`.

  More precisely, the following (simple, inefficient) definitions hold:

      ;;; Iterative
      (define (string-unfold p f g seed base make-final)
        (let lp ((seed seed) (ans base))
          (if (p seed)
              (string-append ans (make-final seed))
              (lp (g seed) (string-append ans (string (f seed)))))))

      ;;; Recursive
      (define (string-unfold p f g seed base make-final)
        (string-append base
                       (let recur ((seed seed))
                         (if (p seed) (make-final seed)
                             (string-append (string (f seed))
                                            (recur (g seed)))))))

  `string-unfold` is a fairly powerful string constructor -- you can use it to convert a list to a string, read a port into a string, reverse a string, copy a string, and so forth. Examples:

      (port->string p) = (string-unfold eof-object? values
                                        (lambda (x) (read-char p))
                                        (read-char p))

      (list->string lis) = (string-unfold null? car cdr lis)

      (string-tabulate f size) = (string-unfold (lambda (i) (= i size)) f add1 0)

  To map f over a list lis, producing a string:

      (string-unfold null? (compose f car) cdr lis)

  Interested functional programmers may enjoy noting that `string-fold-right` and `string-unfold` are in some sense inverses. That is, given operations knull?, kar, kdr, kons, and knil satisfying

      (kons (kar x) (kdr x)) = x  and (knull? knil) = #t

  then

      (string-fold-right kons knil (string-unfold knull? kar kdr x)) = x

  and

      (string-unfold knull? kar kdr (string-fold-right kons knil s)) = s.

  The final string constructed does not share storage with either base or the value produced by make-final.

  This combinator sometimes is called an "anamorphism."

  Note: implementations should take care that runtime stack limits do not cause overflow when constructing large (e.g., megabyte) strings with `string-unfold`.

`string-unfold-right` p f g seed [base make-final] -> string
  This is a fundamental constructor for strings.
  - G is used to generate a series of "seed" values from the initial seed: seed, (g seed), (g2 seed), (g3 seed), ...
  - P tells us when to stop -- when it returns true when applied to one of these seed values.
  - F maps each seed value to the corresponding character in the result string. These chars are assembled into the string in a right-to-left order.
  - Base is the optional initial/rightmost portion of the constructed string; it defaults to the empty string "".
  - Make-final is applied to the terminal seed value (on which P returns true) to produce the final/leftmost portion of the constructed string. It defaults to `(lambda (x) "")`.

  More precisely, the following (simple, inefficient) definitions hold:

      ;;; Iterative
      (define (string-unfold-right p f g seed base make-final)
        (let lp ((seed seed) (ans base))
          (if (p seed)
              (string-append (make-final seed) ans)
              (lp (g seed) (string-append (string (f seed)) ans)))))

      ;;; Recursive
      (define (string-unfold-right p f g seed base make-final)
        (string-append (let recur ((seed seed))
                         (if (p seed) (make-final seed)
                             (string-append (recur (g seed))
                                            (string (f seed)))))
                       base))

  Interested functional programmers may enjoy noting that `string-fold` and `string-unfold-right` are in some sense inverses. That is, given operations knull?, kar, kdr, kons, and knil satisfying
  `(kons (kar x) (kdr x))` = x and `(knull? knil)` = #t
  then

      (string-fold kons knil (string-unfold-right knull? kar kdr x)) = x

  and

      (string-unfold-right knull? kar kdr (string-fold kons knil s)) = s.

  The final string constructed does not share storage with either base or the value produced by make-final.

  Note: implementations should take care that runtime stack limits do not cause overflow when constructing large (e.g., megabyte) strings with `string-unfold-right.`

`string-for-each` proc s [start end] -> unspecified
  Apply proc to each character in s. `string-for-each` is required to iterate from start to end in increasing order.
`string-for-each-index` proc s [start end] -> unspecified
  Apply proc to each index of s, in order. The optional start/end pairs restrict the endpoints of the loop. This is simply a method of looping over a string that is guaranteed to be safe and correct. Example:

      (let* ((len (string-length s))
             (ans (make-string len)))
        (string-for-each-index
            (lambda (i) (string-set! ans (- len i) (string-ref s i)))
            s)
        ans)

### Replicate & rotate

`xsubstring` s from [to start end] -> string
  This is the "extended substring" procedure that implements replicated copying of a substring of some string.

  S is a string; start and end are optional arguments that demarcate a substring of s, defaulting to 0 and the length of s (i.e., the whole string). Replicate this substring up and down index space, in both the positive and negative directions. For example, if s = "abcdefg", start=3, and end=6, then we have the conceptual bidirectionally-infinite string

  ... | d | e | f | d | e | f | d | e | f | d | e | f | d | e | f | d | e | f | d | ...
  ... | -9 | -8 | -7 | -6 | -5 | -4 | -3 | -2 | -1 | 0 | +1 | +2 | +3 | +4 | +5 | +6 | +7 | +8 | +9 | ...

  `xsubstring` returns the substring of this string beginning at index from, and ending at to (which defaults to from+(end-start)).

  You can use `xsubstring` to perform a variety of tasks:

  - To rotate a string left: `(xsubstring "abcdef" 2)` => `"cdefab"`
  - To rotate a string right: `(xsubstring "abcdef" -2)` => `"efabcd"`
  - To replicate a string: `(xsubstring "abc" 0 7)` => `"abcabca"`

  Note that

  - The from/to indices give a half-open range -- the characters from index from up to, but not including, index to.
  - The from/to indices are not in terms of the index space for string s. They are in terms of the replicated index space of the substring defined by s, start, and end.

  It is an error if start=end -- although this is allowed by special dispensation when from=to.

`string-xcopy!` target tstart s sfrom [sto start end] -> unspecified
  Exactly the same as `xsubstring,` but the extracted text is written into the string target starting at index tstart. This operation is not defined if `(eq? target s)` or these two arguments share storage -- you cannot copy a string on top of itself.

### Miscellaneous: insertion, parsing

`string-replace` s1 s2 start1 end1 [start2 end2] -> string
  Returns

      (string-append (substring/shared s1 0 start1)
                     (substring/shared s2 start2 end2)
                     (substring/shared s1 end1 (string-length s1)))

  That is, the segment of characters in s1 from start1 to end1 is replaced by the segment of characters in s2 from start2 to end2. If start1=end1, this simply splices the s2 characters into s1 at the specified index.

  Examples:

      (string-replace "The TCL programmer endured daily ridicule."
                      "another miserable perl drone" 4 7 8 22 ) =>
          "The miserable perl programmer endured daily ridicule."

      (string-replace "It's easy to code it up in Scheme." "lots of fun" 5 9) =>
          "It's lots of fun to code it up in Scheme."

      (define (string-insert s i t) (string-replace s t i i))

      (string-insert "It's easy to code it up in Scheme." 5 "really ") =>
          "It's really easy to code it up in Scheme."

`string-tokenize` s [token-set start end] -> list
  Split the string s into a list of substrings, where each substring is a maximal non-empty contiguous sequence of characters from the character set token-set.
  - token-set defaults to `char-set:graphic` (see SRFI 14 for more on character sets and `char-set:graphic`).
  - If start or end indices are provided, they restrict `string-tokenize` to operating on the indicated substring of s.

  This function provides a minimal parsing facility for simple applications. More sophisticated parsers that handle quoting and backslash effects can easily be constructed using regular-expression systems; be careful not to use `string-tokenize` in contexts where more serious parsing is needed.

      (string-tokenize "Help make programs run, run, RUN!") =>
        ("Help" "make" "programs" "run," "run," "RUN!")

### Filtering & deleting

`string-filter` char/char-set/pred s [start end] -> string
`string-delete` char/char-set/pred s [start end] -> string
  Filter the string s, retaining only those characters that satisfy / do not satisfy the char/char-set/pred argument. If this argument is a procedure, it is applied to the character as a predicate; if it is a char-set, the character is tested for membership; if it is a character, it is used in an equality test.

  If the string is unaltered by the filtering operation, these functions may return either s or a copy of s.

## Low-level procedures

The following procedures are useful for writing other string-processing functions. In a Scheme system that has a module or package system, these procedures should be contained in a module named "string-lib-internals".

### Start/end optional-argument parsing & checking utilities

`string-parse-start+end` proc s args -> [rest start end]
`string-parse-final-start+end` proc s args -> [start end]
  `string-parse-start+end` may be used to parse a pair of optional start/end arguments from an argument list, defaulting them to 0 and the length of some string s, respectively. Let the length of string s be slen.
  - If args = (), the function returns `(values '() 0 slen)`
  - If args = (i), i is checked to ensure it is an exact integer, and that 0 <= i <= slen. Returns `(values (cdr args) i slen)`.
  - If args = `(i j ...)`, i and j are checked to ensure they are exact integers, and that 0 <= i <= j <= slen. Returns `(values (cddr args) i j)`.

  If any of the checks fail, an error condition is raised, and proc is used as part of the error condition -- it should be the client procedure whose argument list `string-parse-start+end` is parsing.

  `string-parse-final-start+end` is exactly the same, except that the args list passed to it is required to be of length two or less; if it is longer, an error condition is raised. It may be used when the optional start/end parameters are final arguments to the procedure.

  Note that in all cases, these functions ensure that s is a string (by necessity, since all cases apply `string-length` to s either to default end or to bounds-check it).

`let-string-start+end` (start end [rest]) proc-exp s-exp args-exp body ... -> value(s)
  [Syntax] Syntactic sugar for an application of `string-parse-start+end` or `string-parse-final-start+end.`

  If a rest variable is given, the form is equivalent to

      (call-with-values
          (lambda () (string-parse-start+end proc-exp s-exp args-exp))
        (lambda (rest start end) body ...))

  If no rest variable is given, the form is equivalent to

      (call-with-values
          (lambda () (string-parse-final-start+end proc-exp s-exp args-exp))
        (lambda (start end) body ...))

`check-substring-spec` proc s start end -> unspecified
`substring-spec-ok?` s start end -> boolean
  Check values s, start and end to ensure they specify a valid substring. This means that s is a string, start and end are exact integers, and 0 <= start <= end <= `(string-length s)`

  If the values are not proper

  - `check-substring-spec` raises an error condition. proc is used as part of the error condition, and should be the procedure whose parameters we are checking.
  - `substring-spec-ok?` returns false.
  Otherwise, `substring-spec-ok?` returns true, and `check-substring-spec` simply returns (what it returns is not specified).

### Knuth-Morris-Pratt searching

The Knuth-Morris-Pratt string-search algorithm is a method of rapidly scanning a sequence of text for the occurrence of some fixed string. It has the advantage of never requiring backtracking -- hence, it is useful for searching not just strings, but also other sequences of text that do not support backtracking or random-access, such as input ports. These routines package up the initialisation and searching phases of the algorithm for general use. They also support searching through sequences of text that arrive in buffered chunks, in that intermediate search state can be carried across applications of the search loop from the end of one buffer application to the next.

A second critical property of KMP search is that it requires the allocation of auxiliary memory proportional to the length of the pattern, but constant in the size of the character type. Alternate searching algorithms frequently require the construction of a table with an entry for every possible character -- which can be prohibitively expensive in a 16- or 32-bit character representation.

`make-kmp-restart-vector` s [c= start end] -> integer-vector
  Build a Knuth-Morris-Pratt "restart vector," which is useful for quickly searching character sequences for the occurrence of string s (or the substring of s demarcated by the optional start/end parameters, if provided). C= is a character-equality function used to construct the restart vector. It defaults to `char=?`; use `char-ci=?` instead for case-folded string search.

  The definition of the restart vector rv for string s is: If we have matched chars 0..i-1 of s against some search string ss, and s[i] doesn't match ss[k], then reset i := rv[i], and try again to match ss[k]. If rv[i] = -1, then punt ss[k] completely, and move on to ss[k+1] and s[0].

  In other words, if you have matched the first i chars of s, but the i+1'th char doesn't match, rv[i] tells you what the next-longest prefix of s is that you have matched.

  The following string-search function shows how a restart vector is used to search. Note the attractive feature of the search process: it is "on line," that is, it never needs to back up and reconsider previously seen data. It simply consumes characters one-at-a-time until declaring a complete match or reaching the end of the sequence. Thus, it can be easily adapted to search other character sequences (such as ports) that do not provide random access to their contents.

      (define (find-substring pattern source start end)
        (let ((plen (string-length pattern))
              (rv (make-kmp-restart-vector pattern)))

          ;; The search loop. SJ & PJ are redundant state.
          (let lp ((si start) (pi 0)
                   (sj (- end start))     ; (- end si)  -- how many chars left.
                   (pj plen))             ; (- plen pi) -- how many chars left.

            (if (= pi plen) (- si plen)                   ; Win.

                (and (<= pj sj)                           ; Lose.

                     (if (char=? (string-ref source si)           ; Test.
                                 (string-ref pattern pi))
                         (lp (+ 1 si) (+ 1 pi) (- sj 1) (- pj 1)) ; Advance.

                         (let ((pi (vector-ref rv pi)))           ; Retreat.
                           (if (= pi -1)
                               (lp (+ si 1)  0   (- sj 1)  plen)  ; Punt.
                               (lp si        pi  sj        (- plen pi))))))))))

  The optional start/end parameters restrict the restart vector to the indicated substring of pat; rv is end - start elements long. If start > 0, then rv is offset by start elements from pat. That is, rv[i] describes pattern element pat[i + start]. Elements of rv are themselves indices that range just over [0, end-start), not [start, end).

  Rationale: the actual value of rv is "position independent" -- it does not depend on where in the pat string the pattern occurs, but only on the actual characters comprising the pattern.

`kmp-step` pat rv c i c= p-start -> integer
  This function encapsulates the work performed by one step of the KMP string search; it can be used to scan strings, input ports, or other on-line character sources for fixed strings.

  Pat is the non-empty string specifying the text for which we are searching. Rv is the Knuth-Morris-Pratt restart vector for the pattern, as constructed by `make-kmp-restart-vector.` The pattern begins at pat[p-start], and is `(string-length rv)` characters long. C= is the character-equality function used to construct the restart vector, typically `char=?` or `char-ci=?`.

  Suppose the pattern is N characters in length: pat[p-start, p-start + n). We have already matched i characters: pat[p-start, p-start + i). (P-start is typically zero.) C is the next character in the input stream. `kmp-step` returns the new i value -- that is, how much of the pattern we have matched, including character c. When i reaches n, the entire pattern has been matched.

  Thus a typical search loop looks like this:

      (let lp ((i 0))
        (or (= i n)                           ; Win -- #t
            (and (not (end-of-stream))        ; Lose -- #f
                 (lp (kmp-step pat rv (get-next-character) i char=? 0)))))

  Example:

      ;; Read chars from IPORT until we find string PAT or hit EOF.
      (define (port-skip pat iport)
        (let* ((rv (make-kmp-restart-vector pat))
               (patlen (string-length pat)))
          (let lp ((i 0) (nchars 0))
            (if (= i patlen) nchars                    ; Win -- nchars skipped
                (let ((c (read-char iport)))
                  (if (eof-object? c) c                ; Fail -- EOF
                      (lp (kmp-step pat rv c i char=? 0) ; Continue
                          (+ nchars 1))))))))

  This procedure could be defined as follows:

      (define (kmp-step pat rv c i c= p-start)
        (let lp ((i i))
          (if (c= c (string-ref pat (+ i p-start)))     ; Match =>
              (+ i 1)                                   ;   Done.
              (let ((i (vector-ref rv i)))              ; Back up in PAT.
                (if (= i -1) 0                          ; Can't back up more.
                    (lp i)))))))                        ; Keep going.

  Rationale: this procedure takes no optional arguments because it is intended as an inner-loop primitive and we do not want any run-time penalty for optional-argument parsing and defaulting, nor do we wish barriers to procedure integration/inlining.

`string-kmp-partial-search` pat rv s i [c= p-start s-start s-end] -> integer
  Applies `kmp-step` across s; optional s-start/s-end bounds parameters restrict search to a substring of s. The pattern is `(vector-length rv)` characters long; optional p-start index indicates non-zero start of pattern in pat.

  Suppose plen = `(vector-length rv)` is the length of the pattern. I is an integer index into the pattern (that is, 0 <= i < plen) indicating how much of the pattern has already been matched. (This means the pattern must be non-empty -- plen > 0.)

  - On success, returns -j, where j is the index in s bounding the end of the pattern -- e.g., a value that could be used as the end parameter in a call to `substring/shared`.
  - On continue, returns the current search state i' (an index into rv) when the search reached the end of the string. This is a non-negative integer.
  Hence:
  - A negative return value indicates success, and says where in the string the match occured.
  - A non-negative return value provides the i to use for continued search in a following string.

  This utility is designed to allow searching for occurrences of a fixed string that might extend across multiple buffers of text. This is why, for example, we do not provide the index of the start of the match on success -- it may have occurred in a previous buffer.

  To search a character sequence that arrives in "chunks," write a loop of this form:

      (let lp ((i 0))
        (and (not (end-of-data?))             ; Lose -- return #f.
             (let* ((buf (get-next-chunk))    ; Get or fill up the buffer.
                    (i (string-kmp-partial-search pat rv buf i)))
               (if (< i 0) (- i)              ; Win -- return end index.
                   (lp i)))))                 ; Keep looking.

  Modulo start/end optional-argument parsing, this procedure could be defined as follows:

      (define (string-kmp-partial-search pat rv s i c= p-start s-start s-end)
        (let ((patlen (vector-length rv)))
          (let lp ((si s-start)       ; An index into S.
                   (vi i))            ; An index into RV.
            (cond ((= vi patlen) (- si))      ; Win.
                  ((= si end) vi)             ; Ran off the end.
                  (else (lp (+ si 1)          ; Match s[si] & loop.
                            (kmp-step pat rv (string-ref s si)
                                      vi c= p-start)))))))

# Reference implementation

This SRFI comes with a reference implementation. It can be found at:

http://srfi.schemers.org/srfi-13/srfi-13.scm

I have placed this source on the Net with an unencumbered, "open" copyright. The prefix/suffix and comparison routines in this code had (extremely distant) origins in MIT Scheme's string lib, and were substantially reworked by myself. Being derived from that code, they are covered by the MIT Scheme copyright, which is a generic BSD-style open-source copyright. See the source file for details.

The KMP string-search code was influenced by implementations written by Stephen Bevan, Brian Denheyer and Will Fitzgerald. However, this version was written from scratch by myself.

The remainder of the code was written by myself for scsh or for this SRFI; I have placed this code under the scsh copyright, which is also a generic BSD-style open-source copyright.

The code is written for portability and should be straightforward to port to any Scheme. The source comments contains detailed notes describing the non-R5RS dependencies.

The library is written for clarity and well-commented; the current source is approximately 1000 lines of source code and 1000 lines of comments and white space. It is also written for efficiency. Fast paths are provided for common cases. This is not to say that the implementation can't be tuned up for a specific Scheme implementation. There are notes in the comments addressing ways implementors can tune the reference implementation for performance.

In short, I've written the reference implementation to make it as painless as possible for an implementor -- or a regular programmer -- to adopt this library and get good results with it.

# Acknowledgements

The design of this library benefited greatly from the feedback provided during the SRFI discussion phase. Among those contributing thoughtful commentary and suggestions, both on the mailing list and by private discussion, were Paolo Amoroso, Lars Arvestad, Alan Bawden, Jim Bender, Dan Bornstein, Per Bothner, Will Clinger, Brian Denheyer, Mikael Djurfeldt, Kent Dybvig, Sergei Egorov, Marc Feeley, Matthias Felleisen, Will Fitzgerald, Matthew Flatt, Arthur A. Gleckler, Ben Goetter, Sven Hartrumpf, Erik Hilsdale, Richard Kelsey, Oleg Kiselyov, Bengt Kleberg, Donovan Kolbly, Bruce Korb, Shriram Krishnamurthi, Bruce Lewis, Tom Lord, Brad Lucier, Dave Mason, David Rush, Klaus Schilling, Jonathan Sobel, Mike Sperber, Mikael Staldal, Vladimir Tsyshevsky, Donald Welsh, and Mike Wilson. I am grateful to them for their assistance.

I am also grateful the authors, implementors and documentors of all the systems mentioned in the introduction. Aubrey Jaffer and Kent Pitman should be noted for their work in producing Web-accessible versions of the R5RS and Common Lisp spec, which was a tremendous aid.

This is not to imply that these individuals necessarily endorse the final results, of course.

During this document's long development period, great patience was exhibited by Mike Sperber, who is the editor for the SRFI, and by Hillary Sullivan, who is not.

# References & links

[Case-map]
  Case mappings.
  Unicode Technical Report 21.
  http://www.unicode.org/unicode/reports/tr21/
[CommonLisp]
  Common Lisp: the Language.
  Guy L. Steele Jr. (editor).
  Digital Press, Maynard, Mass., second edition 1990.
  Available at http://www.elwood.com/alu/table/references.htm#cltl2.

  The Common Lisp "HyperSpec," produced by Kent Pitman, is essentially the ANSI spec for Common Lisp: http://www.harlequin.com/education/books/HyperSpec/.

[Java]
  The following URLs provide documentation on relevant Java classes.
  http://java.sun.com/products/jdk/1.2/docs/api/java/lang/Character.html
  http://java.sun.com/products/jdk/1.2/docs/api/java/lang/String.html
  http://java.sun.com/products/jdk/1.2/docs/api/java/lang/StringBuffer.html
  http://java.sun.com/products/jdk/1.2/docs/api/java/text/Collator.html
  http://java.sun.com/products/jdk/1.2/docs/api/java/text/package-summary.html
[MIT-Scheme]
  http://www.swiss.ai.mit.edu/projects/scheme/
[R5RS]
  Revised5 report on the algorithmic language Scheme.
  R. Kelsey, W. Clinger, J. Rees (editors).
  Higher-Order and Symbolic Computation, Vol. 11, No. 1, September, 1998.
  and ACM SIGPLAN Notices, Vol. 33, No. 9, October, 1998.
  Available at http://www.schemers.org/Documents/Standards/.
[SRFI]
  The SRFI web site.
  http://srfi.schemers.org/
[SRFI-13]
  SRFI-13: String libraries.
  http://srfi.schemers.org/srfi-13/
  This document, in HTML:
    http://srfi.schemers.org/srfi-13/srfi-13.html
  This document, in plain text format:
    http://srfi.schemers.org/srfi-13/srfi-13.txt
  Source code for the reference implementation:
    http://srfi.schemers.org/srfi-13/srfi-13.scm
  Scheme 48 module specification, with typings:
    http://srfi.schemers.org/srfi-13/srfi-13-s48-module.scm
[SRFI-14]
  SRFI-14: Character-set library.
  http://srfi.schemers.org/srfi-14/
  The SRFI 14 char-set library defines a character-set data type, which is used by some procedures in this library.
[Unicode]
  http://www.unicode.org/
[UnicodeData]
  The Unicode character database.
  ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.txt
  ftp://ftp.unicode.org/Public/UNIDATA/UnicodeData.html

# Copyright

Certain portions of this document -- the specific, marked segments of text describing the R5RS procedures -- were adapted with permission from the R5RS report.

All other text is copyright (C) Olin Shivers (1998, 1999, 2000). All Rights Reserved.

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.