(**************************************************************************)
(*                                                                        *)
(*                                 OCaml                                  *)
(*                                                                        *)
                , projet Cristal , INRIA Rocquencourt           
(*                                                                        *)
    Copyright 1996 Institut National de Recherche en Informatique et     
(*     en Automatique.                                                    *)
(*                                                                        *)
(*   All rights reserved.  This file is distributed under the terms of    *)
    the GNU Lesser General Public License version 2.1 , with the          
(*   special exception on linking described in the file LICENSE.          *)
(*                                                                        *)
(**************************************************************************)

* Byte sequence operations . 

    A byte sequence is a mutable data structure that contains a 
    fixed - length sequence of bytes . Each byte can be indexed in 
    constant time for reading or writing . 

    Given a byte sequence [ s ] of length [ l ] , we can access each of the 
    [ l ] bytes of [ s ] via its index in the sequence . Indexes start at 
    [ 0 ] , and we will call an index valid in [ s ] if it falls within the 
    range [ [ 0 ... l-1 ] ] ( inclusive ) . A position is the point between two 
    bytes or at the beginning or end of the sequence .   We call a 
    position valid in [ s ] if it falls within the range [ [ 0 ... l ] ] 
    ( inclusive ) . Note that the byte at index [ n ] is between positions 
    [ n ] and [ n+1 ] . 

    Two parameters [ start ] and [ len ] are said to designate a valid 
    range of [ s ] if [ len > = 0 ] and [ start ] and [ start+len ] are valid 
    positions in [ s ] . 

    Byte sequences can be modified in place , for instance via the [ set ] 
    and [ blit ] functions described below .   See also strings ( module 
    { ! String } ) , which are almost the same data structure , but can not be 
    modified in place . 

    Bytes are represented by the OCaml type [ char ] . 

    @since 4.02.0 
 

   A byte sequence is a mutable data structure that contains a
   fixed-length sequence of bytes. Each byte can be indexed in
   constant time for reading or writing.

   Given a byte sequence [s] of length [l], we can access each of the
   [l] bytes of [s] via its index in the sequence. Indexes start at
   [0], and we will call an index valid in [s] if it falls within the
   range [[0...l-1]] (inclusive). A position is the point between two
   bytes or at the beginning or end of the sequence.  We call a
   position valid in [s] if it falls within the range [[0...l]]
   (inclusive). Note that the byte at index [n] is between positions
   [n] and [n+1].

   Two parameters [start] and [len] are said to designate a valid
   range of [s] if [len >= 0] and [start] and [start+len] are valid
   positions in [s].

   Byte sequences can be modified in place, for instance via the [set]
   and [blit] functions described below.  See also strings (module
   {!String}), which are almost the same data structure, but cannot be
   modified in place.

   Bytes are represented by the OCaml type [char].

   @since 4.02.0
 *)

external length : bytes -> int = "%bytes_length"
(** Return the length (number of bytes) of the argument. *)

external get : bytes -> int -> char = "%bytes_safe_get"
(** [get s n] returns the byte at index [n] in argument [s].

    Raise [Invalid_argument] if [n] is not a valid index in [s]. *)

external set : bytes -> int -> char -> unit = "%bytes_safe_set"
(** [set s n c] modifies [s] in place, replacing the byte at index [n]
    with [c].

    Raise [Invalid_argument] if [n] is not a valid index in [s]. *)

external create : int -> bytes = "caml_create_bytes"
* [ create n ] returns a new byte sequence of length [ n ] . The 
     sequence is uninitialized and contains arbitrary bytes . 

     Raise [ Invalid_argument ] if [ n < 0 ] or [ n > ] { !  } .
    sequence is uninitialized and contains arbitrary bytes.

    Raise [Invalid_argument] if [n < 0] or [n > ]{!Sys.max_string_length}. *)

val make : int -> char -> bytes
* [ make n c ] returns a new byte sequence of length [ n ] , filled with 
     the byte [ c ] . 

     Raise [ Invalid_argument ] if [ n < 0 ] or [ n > ] { !  } .
    the byte [c].

    Raise [Invalid_argument] if [n < 0] or [n > ]{!Sys.max_string_length}. *)

val init : int -> (int -> char) -> bytes
* [ Bytes.init n f ] returns a fresh byte sequence of length [ n ] , with 
     character [ i ] initialized to the result of [ f i ] ( in increasing 
     index order ) . 

     Raise [ Invalid_argument ] if [ n < 0 ] or [ n > ] { !  } .
    character [i] initialized to the result of [f i] (in increasing
    index order).

    Raise [Invalid_argument] if [n < 0] or [n > ]{!Sys.max_string_length}. *)

val empty : bytes
* A byte sequence of size 0 .

val copy : bytes -> bytes
(** Return a new byte sequence that contains the same bytes as the
    argument. *)

val of_string : string -> bytes
(** Return a new byte sequence that contains the same bytes as the
    given string. *)

val to_string : bytes -> string
(** Return a new string that contains the same bytes as the given byte
    sequence. *)

val sub : bytes -> int -> int -> bytes
(** [sub s start len] returns a new byte sequence of length [len],
    containing the subsequence of [s] that starts at position [start]
    and has length [len].

    Raise [Invalid_argument] if [start] and [len] do not designate a
    valid range of [s]. *)

val sub_string : bytes -> int -> int -> string
(** Same as [sub] but return a string instead of a byte sequence. *)

val extend : bytes -> int -> int -> bytes
* [ extend s left right ] returns a new byte sequence that contains 
     the bytes of [ s ] , with [ left ] uninitialized bytes prepended and 
     [ right ] uninitialized bytes appended to it . If [ left ] or [ right ] 
     is negative , then bytes are removed ( instead of appended ) from 
     the corresponding side of [ s ] . 

     Raise [ Invalid_argument ] if the result length is negative or 
     longer than { !  } bytes .
    the bytes of [s], with [left] uninitialized bytes prepended and
    [right] uninitialized bytes appended to it. If [left] or [right]
    is negative, then bytes are removed (instead of appended) from
    the corresponding side of [s].

    Raise [Invalid_argument] if the result length is negative or
    longer than {!Sys.max_string_length} bytes. *)

val fill : bytes -> int -> int -> char -> unit
(** [fill s start len c] modifies [s] in place, replacing [len]
    characters with [c], starting at [start].

    Raise [Invalid_argument] if [start] and [len] do not designate a
    valid range of [s]. *)

val blit : bytes -> int -> bytes -> int -> int -> unit
(** [blit src srcoff dst dstoff len] copies [len] bytes from sequence
    [src], starting at index [srcoff], to sequence [dst], starting at
    index [dstoff]. It works correctly even if [src] and [dst] are the
    same byte sequence, and the source and destination intervals
    overlap.

    Raise [Invalid_argument] if [srcoff] and [len] do not
    designate a valid range of [src], or if [dstoff] and [len]
    do not designate a valid range of [dst]. *)

val blit_string : string -> int -> bytes -> int -> int -> unit
(** [blit src srcoff dst dstoff len] copies [len] bytes from string
    [src], starting at index [srcoff], to byte sequence [dst],
    starting at index [dstoff].

    Raise [Invalid_argument] if [srcoff] and [len] do not
    designate a valid range of [src], or if [dstoff] and [len]
    do not designate a valid range of [dst]. *)

val concat : bytes -> bytes list -> bytes
* [ concat sep sl ] concatenates the list of byte sequences [ sl ] , 
     inserting the separator byte sequence [ sep ] between each , and 
     returns the result as a new byte sequence . 

     Raise [ Invalid_argument ] if the result is longer than 
     { !  } bytes .
    inserting the separator byte sequence [sep] between each, and
    returns the result as a new byte sequence.

    Raise [Invalid_argument] if the result is longer than
    {!Sys.max_string_length} bytes. *)

val cat : bytes -> bytes -> bytes
* [ cat s1 s2 ] concatenates [ s1 ] and [ s2 ] and returns the result 
      as new byte sequence . 

     Raise [ Invalid_argument ] if the result is longer than 
     { !  } bytes .
     as new byte sequence.

    Raise [Invalid_argument] if the result is longer than
    {!Sys.max_string_length} bytes. *)

val iter : (char -> unit) -> bytes -> unit
* [ iter f s ] applies function [ f ] in turn to all the bytes of [ s ] . 
     It is equivalent to [ f ( get s 0 ) ; f ( get s 1 ) ; ... ; f ( get s 
     ( length s - 1 ) ) ; ( ) ] .
    It is equivalent to [f (get s 0); f (get s 1); ...; f (get s
    (length s - 1)); ()]. *)

val iteri : (int -> char -> unit) -> bytes -> unit
* Same as { ! Bytes.iter } , but the function is applied to the index of 
     the byte as first argument and the byte itself as second 
     argument .
    the byte as first argument and the byte itself as second
    argument. *)

val map : (char -> char) -> bytes -> bytes
(** [map f s] applies function [f] in turn to all the bytes of [s]
    (in increasing index order) and stores the resulting bytes in
    a new sequence that is returned as the result. *)

val mapi : (int -> char -> char) -> bytes -> bytes
(** [mapi f s] calls [f] with each character of [s] and its
    index (in increasing index order) and stores the resulting bytes
    in a new sequence that is returned as the result. *)

val trim : bytes -> bytes
* Return a copy of the argument , without leading and trailing 
     whitespace . The bytes regarded as whitespace are the ASCII 
     characters [ ' ' ] , [ ' \012 ' ] , [ ' \n ' ] , [ ' \r ' ] , and [ ' \t ' ] .
    whitespace. The bytes regarded as whitespace are the ASCII
    characters [' '], ['\012'], ['\n'], ['\r'], and ['\t']. *)

val escaped : bytes -> bytes
* Return a copy of the argument , with special characters represented 
     by escape sequences , following the lexical conventions of OCaml . 
     All characters outside the ASCII printable range ( 32 .. 126 ) are 
     escaped , as well as backslash and double - quote . 

     Raise [ Invalid_argument ] if the result is longer than 
     { !  } bytes .
    by escape sequences, following the lexical conventions of OCaml.
    All characters outside the ASCII printable range (32..126) are
    escaped, as well as backslash and double-quote.

    Raise [Invalid_argument] if the result is longer than
    {!Sys.max_string_length} bytes. *)

val index : bytes -> char -> int
* [ index s c ] returns the index of the first occurrence of byte [ c ] 
     in [ s ] . 

     Raise [ Not_found ] if [ c ] does not occur in [ s ] .
    in [s].

    Raise [Not_found] if [c] does not occur in [s]. *)

val index_opt: bytes -> char -> int option
* [  s c ] returns the index of the first occurrence of byte [ c ] 
     in [ s ] or [ None ] if [ c ] does not occur in [ s ] . 
     @since 4.05
    in [s] or [None] if [c] does not occur in [s].
    @since 4.05 *)

val rindex : bytes -> char -> int
(** [rindex s c] returns the index of the last occurrence of byte [c]
    in [s].

    Raise [Not_found] if [c] does not occur in [s]. *)

val rindex_opt: bytes -> char -> int option
* [ rindex_opt s c ] returns the index of the last occurrence of byte [ c ] 
     in [ s ] or [ None ] if [ c ] does not occur in [ s ] . 
     @since 4.05
    in [s] or [None] if [c] does not occur in [s].
    @since 4.05 *)

val index_from : bytes -> int -> char -> int
* [ index_from s i c ] returns the index of the first occurrence of 
     byte [ c ] in [ s ] after position [ i ] .   [ Bytes.index s c ] is 
     equivalent to [ Bytes.index_from s 0 c ] . 

     Raise [ Invalid_argument ] if [ i ] is not a valid position in [ s ] . 
     Raise [ Not_found ] if [ c ] does not occur in [ s ] after position [ i ] .
    byte [c] in [s] after position [i].  [Bytes.index s c] is
    equivalent to [Bytes.index_from s 0 c].

    Raise [Invalid_argument] if [i] is not a valid position in [s].
    Raise [Not_found] if [c] does not occur in [s] after position [i]. *)

val index_from_opt: bytes -> int -> char -> int option
* [ index_from _ opts i c ] returns the index of the first occurrence of 
     byte [ c ] in [ s ] after position [ i ] or [ None ] if [ c ] does not occur in [ s ] after position [ i ] . 
     [ Bytes.index_opt s c ] is equivalent to [ Bytes.index_from_opt s 0 c ] . 

     Raise [ Invalid_argument ] if [ i ] is not a valid position in [ s ] . 
     @since 4.05
    byte [c] in [s] after position [i] or [None] if [c] does not occur in [s] after position [i].
    [Bytes.index_opt s c] is equivalent to [Bytes.index_from_opt s 0 c].

    Raise [Invalid_argument] if [i] is not a valid position in [s].
    @since 4.05 *)

val rindex_from : bytes -> int -> char -> int
* [ rindex_from s i c ] returns the index of the last occurrence of 
     byte [ c ] in [ s ] before position [ i+1 ] .   [ rindex s c ] is equivalent 
     to [ rindex_from s ( Bytes.length s - 1 ) c ] . 

     Raise [ Invalid_argument ] if [ i+1 ] is not a valid position in [ s ] . 
     Raise [ Not_found ] if [ c ] does not occur in [ s ] before position [ i+1 ] .
    byte [c] in [s] before position [i+1].  [rindex s c] is equivalent
    to [rindex_from s (Bytes.length s - 1) c].

    Raise [Invalid_argument] if [i+1] is not a valid position in [s].
    Raise [Not_found] if [c] does not occur in [s] before position [i+1]. *)

val rindex_from_opt: bytes -> int -> char -> int option
* [ rindex_from_opt s i c ] returns the index of the last occurrence 
     of byte [ c ] in [ s ] before position [ i+1 ] or [ None ] if [ c ] does not 
     occur in [ s ] before position [ i+1 ] .   [ rindex_opt s c ] is equivalent to 
     [ rindex_from s ( Bytes.length s - 1 ) c ] . 

     Raise [ Invalid_argument ] if [ i+1 ] is not a valid position in [ s ] . 
     @since 4.05
    of byte [c] in [s] before position [i+1] or [None] if [c] does not
    occur in [s] before position [i+1].  [rindex_opt s c] is equivalent to
    [rindex_from s (Bytes.length s - 1) c].

    Raise [Invalid_argument] if [i+1] is not a valid position in [s].
    @since 4.05 *)

val contains : bytes -> char -> bool
(** [contains s c] tests if byte [c] appears in [s]. *)

val contains_from : bytes -> int -> char -> bool
(** [contains_from s start c] tests if byte [c] appears in [s] after
    position [start].  [contains s c] is equivalent to [contains_from
    s 0 c].

    Raise [Invalid_argument] if [start] is not a valid position in [s]. *)

val rcontains_from : bytes -> int -> char -> bool
* [ rcontains_from s stop c ] tests if byte [ c ] appears in [ s ] before 
     position [ stop+1 ] . 

     Raise [ Invalid_argument ] if [ stop < 0 ] or [ stop+1 ] is not a valid 
     position in [ s ] .
    position [stop+1].

    Raise [Invalid_argument] if [stop < 0] or [stop+1] is not a valid
    position in [s]. *)

val uppercase : bytes -> bytes
  [@@ocaml.deprecated "Use Bytes.uppercase_ascii instead."]
(** Return a copy of the argument, with all lowercase letters
   translated to uppercase, including accented letters of the ISO
   Latin-1 (8859-1) character set.
   @deprecated Functions operating on Latin-1 character set are deprecated. *)

val lowercase : bytes -> bytes
  [@@ocaml.deprecated "Use Bytes.lowercase_ascii instead."]
(** Return a copy of the argument, with all uppercase letters
   translated to lowercase, including accented letters of the ISO
   Latin-1 (8859-1) character set.
   @deprecated Functions operating on Latin-1 character set are deprecated. *)

val capitalize : bytes -> bytes
  [@@ocaml.deprecated "Use Bytes.capitalize_ascii instead."]
* Return a copy of the argument , with the first character set to uppercase , 
    using the ISO Latin-1 ( 8859 - 1 ) character set .. 
    @deprecated Functions operating on Latin-1 character set are deprecated .
   using the ISO Latin-1 (8859-1) character set..
   @deprecated Functions operating on Latin-1 character set are deprecated. *)

val uncapitalize : bytes -> bytes
  [@@ocaml.deprecated "Use Bytes.uncapitalize_ascii instead."]
* Return a copy of the argument , with the first character set to lowercase , 
    using the ISO Latin-1 ( 8859 - 1 ) character set .. 
    @deprecated Functions operating on Latin-1 character set are deprecated .
   using the ISO Latin-1 (8859-1) character set..
   @deprecated Functions operating on Latin-1 character set are deprecated. *)

val uppercase_ascii : bytes -> bytes
* Return a copy of the argument , with all lowercase letters 
    translated to uppercase , using the US - ASCII character set . 
    @since 4.03.0
   translated to uppercase, using the US-ASCII character set.
   @since 4.03.0 *)

val lowercase_ascii : bytes -> bytes
* Return a copy of the argument , with all uppercase letters 
    translated to lowercase , using the US - ASCII character set . 
    @since 4.03.0
   translated to lowercase, using the US-ASCII character set.
   @since 4.03.0 *)

val capitalize_ascii : bytes -> bytes
* Return a copy of the argument , with the first character set to uppercase , 
    using the US - ASCII character set . 
    @since 4.03.0
   using the US-ASCII character set.
   @since 4.03.0 *)

val uncapitalize_ascii : bytes -> bytes
* Return a copy of the argument , with the first character set to lowercase , 
    using the US - ASCII character set . 
    @since 4.03.0
   using the US-ASCII character set.
   @since 4.03.0 *)

type t = bytes
(** An alias for the type of byte sequences. *)

val compare: t -> t -> int
(** The comparison function for byte sequences, with the same
    specification as {!Pervasives.compare}.  Along with the type [t],
    this function [compare] allows the module [Bytes] to be passed as
    argument to the functors {!Set.Make} and {!Map.Make}. *)

val equal: t -> t -> bool
* The equality function for byte sequences . 
     @since 4.03.0
    @since 4.03.0 *)

* { 3 Unsafe conversions ( for advanced users ) } 

     This section describes unsafe , low - level conversion functions 
     between [ bytes ] and [ string ] . They do not copy the internal data ; 
     used improperly , they can break the immutability invariant on 
     strings provided by the [ -safe - string ] option . They are available for 
     expert library authors , but for most purposes you should use the 
     always - correct { ! Bytes.to_string } and { ! Bytes.of_string } instead . 


    This section describes unsafe, low-level conversion functions
    between [bytes] and [string]. They do not copy the internal data;
    used improperly, they can break the immutability invariant on
    strings provided by the [-safe-string] option. They are available for
    expert library authors, but for most purposes you should use the
    always-correct {!Bytes.to_string} and {!Bytes.of_string} instead.
*)

val unsafe_to_string : bytes -> string
* Unsafely convert a byte sequence into a string . 

     To reason about the use of [ unsafe_to_string ] , it is convenient to 
     consider an " ownership " discipline . A piece of code that 
     manipulates some data " owns " it ; there are several disjoint ownership 
     modes , including : 
     - Unique ownership : the data may be accessed and mutated 
     - Shared ownership : the data has several owners , that may only 
       access it , not mutate it . 

     Unique ownership is linear : passing the data to another piece of 
     code means giving up ownership ( we can not write the 
     data again ) . A unique owner may decide to make the data shared 
     ( giving up mutation rights on it ) , but shared data may not become 
     uniquely - owned again . 

    [ unsafe_to_string s ] can only be used when the caller owns the byte 
    sequence [ s ] -- either uniquely or as shared immutable data . The 
    caller gives up ownership of [ s ] , and gains ownership of the 
    returned string . 

    There are two valid use - cases that respect this ownership 
    discipline : 

    1 . Creating a string by initializing and mutating a byte sequence 
    that is never changed after initialization is performed . 

    { [ 
 let    : string = 
   let s = Bytes.create len in 
   for i = 0 to len - 1 do Bytes.set s i ( f i ) done ; 
   Bytes.unsafe_to_string s 
    ] } 

    This function is safe because the byte sequence [ s ] will never be 
    accessed or mutated after [ unsafe_to_string ] is called . The 
    [ string_init ] code gives up ownership of [ s ] , and returns the 
    ownership of the resulting string to its caller . 

    Note that it would be unsafe if [ s ] was passed as an additional 
    parameter to the function [ f ] as it could escape this way and be 
    mutated in the future -- [ string_init ] would give up ownership of 
    [ s ] to pass it to [ f ] , and could not call [ unsafe_to_string ] 
    safely . 

    We have provided the { ! String.init } , { ! String.map } and 
    { ! String.mapi } functions to cover most cases of building 
    new strings . You should prefer those over [ to_string ] or 
    [ unsafe_to_string ] whenever applicable . 

    2 . Temporarily giving ownership of a byte sequence to a function 
    that expects a uniquely owned string and returns ownership back , so 
    that we can mutate the sequence again after the call ended . 

    { [ 
 let bytes_length ( s : bytes ) = 
   String.length ( Bytes.unsafe_to_string s ) 
    ] } 

    In this use - case , we do not promise that [ s ] will never be mutated 
    after the call to [ bytes_length s ] . The { !  } function 
    temporarily borrows unique ownership of the byte sequence 
    ( and sees it as a [ string ] ) , but returns this ownership back to 
    the caller , which may assume that [ s ] is still a valid byte 
    sequence after the call . Note that this is only correct because we 
    know that { !  } does not capture its argument -- it could 
    escape by a side - channel such as a memoization combinator . 

    The caller may not mutate [ s ] while the string is borrowed ( it has 
    temporarily given up ownership ) . This affects concurrent programs , 
    but also higher - order functions : if { !  } returned 
    a closure to be called later , [ s ] should not be mutated until this 
    closure is fully applied and returns ownership . 


    To reason about the use of [unsafe_to_string], it is convenient to
    consider an "ownership" discipline. A piece of code that
    manipulates some data "owns" it; there are several disjoint ownership
    modes, including:
    - Unique ownership: the data may be accessed and mutated
    - Shared ownership: the data has several owners, that may only
      access it, not mutate it.

    Unique ownership is linear: passing the data to another piece of
    code means giving up ownership (we cannot write the
    data again). A unique owner may decide to make the data shared
    (giving up mutation rights on it), but shared data may not become
    uniquely-owned again.

   [unsafe_to_string s] can only be used when the caller owns the byte
   sequence [s] -- either uniquely or as shared immutable data. The
   caller gives up ownership of [s], and gains ownership of the
   returned string.

   There are two valid use-cases that respect this ownership
   discipline:

   1. Creating a string by initializing and mutating a byte sequence
   that is never changed after initialization is performed.

   {[
let string_init len f : string =
  let s = Bytes.create len in
  for i = 0 to len - 1 do Bytes.set s i (f i) done;
  Bytes.unsafe_to_string s
   ]}

   This function is safe because the byte sequence [s] will never be
   accessed or mutated after [unsafe_to_string] is called. The
   [string_init] code gives up ownership of [s], and returns the
   ownership of the resulting string to its caller.

   Note that it would be unsafe if [s] was passed as an additional
   parameter to the function [f] as it could escape this way and be
   mutated in the future -- [string_init] would give up ownership of
   [s] to pass it to [f], and could not call [unsafe_to_string]
   safely.

   We have provided the {!String.init}, {!String.map} and
   {!String.mapi} functions to cover most cases of building
   new strings. You should prefer those over [to_string] or
   [unsafe_to_string] whenever applicable.

   2. Temporarily giving ownership of a byte sequence to a function
   that expects a uniquely owned string and returns ownership back, so
   that we can mutate the sequence again after the call ended.

   {[
let bytes_length (s : bytes) =
  String.length (Bytes.unsafe_to_string s)
   ]}

   In this use-case, we do not promise that [s] will never be mutated
   after the call to [bytes_length s]. The {!String.length} function
   temporarily borrows unique ownership of the byte sequence
   (and sees it as a [string]), but returns this ownership back to
   the caller, which may assume that [s] is still a valid byte
   sequence after the call. Note that this is only correct because we
   know that {!String.length} does not capture its argument -- it could
   escape by a side-channel such as a memoization combinator.

   The caller may not mutate [s] while the string is borrowed (it has
   temporarily given up ownership). This affects concurrent programs,
   but also higher-order functions: if {!String.length} returned
   a closure to be called later, [s] should not be mutated until this
   closure is fully applied and returns ownership.
*)

val unsafe_of_string : string -> bytes
* Unsafely convert a shared string to a byte sequence that should 
     not be mutated . 

     The same ownership discipline that makes [ unsafe_to_string ] 
     correct applies to [ unsafe_of_string ] : you may use it if you were 
     the owner of the [ string ] value , and you will own the return 
     [ bytes ] in the same mode . 

     In practice , unique ownership of string values is extremely 
     difficult to reason about correctly . You should always assume 
     strings are shared , never uniquely owned . 

     For example , string literals are implicitly shared by the 
     compiler , so you never uniquely own them . 

     { [ 
 let incorrect = Bytes.unsafe_of_string " hello " 
 let s = Bytes.of_string " hello " 
     ] } 

     The first declaration is incorrect , because the string literal 
     [ " hello " ] could be shared by the compiler with other parts of the 
     program , and mutating [ incorrect ] is a bug . You must always use 
     the second version , which performs a copy and is thus correct . 

     Assuming unique ownership of strings that are not string 
     literals , but are ( partly ) built from string literals , is also 
     incorrect . For example , mutating [ unsafe_of_string ( " foo " ^ s ) ] 
     could mutate the shared string [ " foo " ] -- assuming a rope - like 
     representation of strings . More generally , functions operating on 
     strings will assume shared ownership , they do not preserve unique 
     ownership . It is thus incorrect to assume unique ownership of the 
     result of [ unsafe_of_string ] . 

     The only case we have reasonable confidence is safe is if the 
     produced [ bytes ] is shared -- used as an immutable byte 
     sequence . This is possibly useful for incremental migration of 
     low - level programs that manipulate immutable sequences of bytes 
     ( for example { ! Marshal.from_bytes } ) and previously used the 
     [ string ] type for this purpose . 

    not be mutated.

    The same ownership discipline that makes [unsafe_to_string]
    correct applies to [unsafe_of_string]: you may use it if you were
    the owner of the [string] value, and you will own the return
    [bytes] in the same mode.

    In practice, unique ownership of string values is extremely
    difficult to reason about correctly. You should always assume
    strings are shared, never uniquely owned.

    For example, string literals are implicitly shared by the
    compiler, so you never uniquely own them.

    {[
let incorrect = Bytes.unsafe_of_string "hello"
let s = Bytes.of_string "hello"
    ]}

    The first declaration is incorrect, because the string literal
    ["hello"] could be shared by the compiler with other parts of the
    program, and mutating [incorrect] is a bug. You must always use
    the second version, which performs a copy and is thus correct.

    Assuming unique ownership of strings that are not string
    literals, but are (partly) built from string literals, is also
    incorrect. For example, mutating [unsafe_of_string ("foo" ^ s)]
    could mutate the shared string ["foo"] -- assuming a rope-like
    representation of strings. More generally, functions operating on
    strings will assume shared ownership, they do not preserve unique
    ownership. It is thus incorrect to assume unique ownership of the
    result of [unsafe_of_string].

    The only case we have reasonable confidence is safe is if the
    produced [bytes] is shared -- used as an immutable byte
    sequence. This is possibly useful for incremental migration of
    low-level programs that manipulate immutable sequences of bytes
    (for example {!Marshal.from_bytes}) and previously used the
    [string] type for this purpose.
*)

* { 6 Iterators }

val to_seq : t -> char Seq.t
* Iterate on the string , in increasing index order . Modifications of the 
     string during iteration will be reflected in the iterator . 
     @since 4.07
    string during iteration will be reflected in the iterator.
    @since 4.07 *)

val to_seqi : t -> (int * char) Seq.t
* Iterate on the string , in increasing order , yielding indices along chars 
     @since 4.07
    @since 4.07 *)

val of_seq : char Seq.t -> t
* Create a string from the generator 
     @since 4.07
    @since 4.07 *)

(**/**)

(* The following is for system use only. Do not call directly. *)

external unsafe_get : bytes -> int -> char = "%bytes_unsafe_get"
external unsafe_set : bytes -> int -> char -> unit = "%bytes_unsafe_set"
external unsafe_blit :
  bytes -> int -> bytes -> int -> int -> unit
  = "caml_blit_bytes" [@@noalloc]
external unsafe_fill :
  bytes -> int -> int -> char -> unit = "caml_fill_bytes" [@@noalloc]

************************************************************************
                                                                        
                                 OCaml                                  
                                                                        
                                                                        
     en Automatique.                                                    
                                                                        
   All rights reserved.  This file is distributed under the terms of    
   special exception on linking described in the file LICENSE.          
                                                                        
************************************************************************
* Return the length (number of bytes) of the argument. 
* [get s n] returns the byte at index [n] in argument [s].

    Raise [Invalid_argument] if [n] is not a valid index in [s]. 
* [set s n c] modifies [s] in place, replacing the byte at index [n]
    with [c].

    Raise [Invalid_argument] if [n] is not a valid index in [s]. 
* Return a new byte sequence that contains the same bytes as the
    argument. 
* Return a new byte sequence that contains the same bytes as the
    given string. 
* Return a new string that contains the same bytes as the given byte
    sequence. 
* [sub s start len] returns a new byte sequence of length [len],
    containing the subsequence of [s] that starts at position [start]
    and has length [len].

    Raise [Invalid_argument] if [start] and [len] do not designate a
    valid range of [s]. 
* Same as [sub] but return a string instead of a byte sequence. 
* [fill s start len c] modifies [s] in place, replacing [len]
    characters with [c], starting at [start].

    Raise [Invalid_argument] if [start] and [len] do not designate a
    valid range of [s]. 
* [blit src srcoff dst dstoff len] copies [len] bytes from sequence
    [src], starting at index [srcoff], to sequence [dst], starting at
    index [dstoff]. It works correctly even if [src] and [dst] are the
    same byte sequence, and the source and destination intervals
    overlap.

    Raise [Invalid_argument] if [srcoff] and [len] do not
    designate a valid range of [src], or if [dstoff] and [len]
    do not designate a valid range of [dst]. 
* [blit src srcoff dst dstoff len] copies [len] bytes from string
    [src], starting at index [srcoff], to byte sequence [dst],
    starting at index [dstoff].

    Raise [Invalid_argument] if [srcoff] and [len] do not
    designate a valid range of [src], or if [dstoff] and [len]
    do not designate a valid range of [dst]. 
* [map f s] applies function [f] in turn to all the bytes of [s]
    (in increasing index order) and stores the resulting bytes in
    a new sequence that is returned as the result. 
* [mapi f s] calls [f] with each character of [s] and its
    index (in increasing index order) and stores the resulting bytes
    in a new sequence that is returned as the result. 
* [rindex s c] returns the index of the last occurrence of byte [c]
    in [s].

    Raise [Not_found] if [c] does not occur in [s]. 
* [contains s c] tests if byte [c] appears in [s]. 
* [contains_from s start c] tests if byte [c] appears in [s] after
    position [start].  [contains s c] is equivalent to [contains_from
    s 0 c].

    Raise [Invalid_argument] if [start] is not a valid position in [s]. 
* Return a copy of the argument, with all lowercase letters
   translated to uppercase, including accented letters of the ISO
   Latin-1 (8859-1) character set.
   @deprecated Functions operating on Latin-1 character set are deprecated. 
* Return a copy of the argument, with all uppercase letters
   translated to lowercase, including accented letters of the ISO
   Latin-1 (8859-1) character set.
   @deprecated Functions operating on Latin-1 character set are deprecated. 
* An alias for the type of byte sequences. 
* The comparison function for byte sequences, with the same
    specification as {!Pervasives.compare}.  Along with the type [t],
    this function [compare] allows the module [Bytes] to be passed as
    argument to the functors {!Set.Make} and {!Map.Make}. 
*/*
 The following is for system use only. Do not call directly. 
                , projet Cristal , INRIA Rocquencourt           
    Copyright 1996 Institut National de Recherche en Informatique et     
    the GNU Lesser General Public License version 2.1 , with the          

* Byte sequence operations . 

    A byte sequence is a mutable data structure that contains a 
    fixed - length sequence of bytes . Each byte can be indexed in 
    constant time for reading or writing . 

    Given a byte sequence [ s ] of length [ l ] , we can access each of the 
    [ l ] bytes of [ s ] via its index in the sequence . Indexes start at 
    [ 0 ] , and we will call an index valid in [ s ] if it falls within the 
    range [ [ 0 ... l-1 ] ] ( inclusive ) . A position is the point between two 
    bytes or at the beginning or end of the sequence .   We call a 
    position valid in [ s ] if it falls within the range [ [ 0 ... l ] ] 
    ( inclusive ) . Note that the byte at index [ n ] is between positions 
    [ n ] and [ n+1 ] . 

    Two parameters [ start ] and [ len ] are said to designate a valid 
    range of [ s ] if [ len > = 0 ] and [ start ] and [ start+len ] are valid 
    positions in [ s ] . 

    Byte sequences can be modified in place , for instance via the [ set ] 
    and [ blit ] functions described below .   See also strings ( module 
    { ! String } ) , which are almost the same data structure , but can not be 
    modified in place . 

    Bytes are represented by the OCaml type [ char ] . 

    @since 4.02.0 
 

   A byte sequence is a mutable data structure that contains a
   fixed-length sequence of bytes. Each byte can be indexed in
   constant time for reading or writing.

   Given a byte sequence [s] of length [l], we can access each of the
   [l] bytes of [s] via its index in the sequence. Indexes start at
   [0], and we will call an index valid in [s] if it falls within the
   range [[0...l-1]] (inclusive). A position is the point between two
   bytes or at the beginning or end of the sequence.  We call a
   position valid in [s] if it falls within the range [[0...l]]
   (inclusive). Note that the byte at index [n] is between positions
   [n] and [n+1].

   Two parameters [start] and [len] are said to designate a valid
   range of [s] if [len >= 0] and [start] and [start+len] are valid
   positions in [s].

   Byte sequences can be modified in place, for instance via the [set]
   and [blit] functions described below.  See also strings (module
   {!String}), which are almost the same data structure, but cannot be
   modified in place.

   Bytes are represented by the OCaml type [char].

   @since 4.02.0
 *)

external length : bytes -> int = "%bytes_length"

external get : bytes -> int -> char = "%bytes_safe_get"

external set : bytes -> int -> char -> unit = "%bytes_safe_set"

external create : int -> bytes = "caml_create_bytes"
* [ create n ] returns a new byte sequence of length [ n ] . The 
     sequence is uninitialized and contains arbitrary bytes . 

     Raise [ Invalid_argument ] if [ n < 0 ] or [ n > ] { !  } .
    sequence is uninitialized and contains arbitrary bytes.

    Raise [Invalid_argument] if [n < 0] or [n > ]{!Sys.max_string_length}. *)

val make : int -> char -> bytes
* [ make n c ] returns a new byte sequence of length [ n ] , filled with 
     the byte [ c ] . 

     Raise [ Invalid_argument ] if [ n < 0 ] or [ n > ] { !  } .
    the byte [c].

    Raise [Invalid_argument] if [n < 0] or [n > ]{!Sys.max_string_length}. *)

val init : int -> (int -> char) -> bytes
* [ Bytes.init n f ] returns a fresh byte sequence of length [ n ] , with 
     character [ i ] initialized to the result of [ f i ] ( in increasing 
     index order ) . 

     Raise [ Invalid_argument ] if [ n < 0 ] or [ n > ] { !  } .
    character [i] initialized to the result of [f i] (in increasing
    index order).

    Raise [Invalid_argument] if [n < 0] or [n > ]{!Sys.max_string_length}. *)

val empty : bytes
* A byte sequence of size 0 .

val copy : bytes -> bytes

val of_string : string -> bytes

val to_string : bytes -> string

val sub : bytes -> int -> int -> bytes

val sub_string : bytes -> int -> int -> string

val extend : bytes -> int -> int -> bytes
* [ extend s left right ] returns a new byte sequence that contains 
     the bytes of [ s ] , with [ left ] uninitialized bytes prepended and 
     [ right ] uninitialized bytes appended to it . If [ left ] or [ right ] 
     is negative , then bytes are removed ( instead of appended ) from 
     the corresponding side of [ s ] . 

     Raise [ Invalid_argument ] if the result length is negative or 
     longer than { !  } bytes .
    the bytes of [s], with [left] uninitialized bytes prepended and
    [right] uninitialized bytes appended to it. If [left] or [right]
    is negative, then bytes are removed (instead of appended) from
    the corresponding side of [s].

    Raise [Invalid_argument] if the result length is negative or
    longer than {!Sys.max_string_length} bytes. *)

val fill : bytes -> int -> int -> char -> unit

val blit : bytes -> int -> bytes -> int -> int -> unit

val blit_string : string -> int -> bytes -> int -> int -> unit

val concat : bytes -> bytes list -> bytes
* [ concat sep sl ] concatenates the list of byte sequences [ sl ] , 
     inserting the separator byte sequence [ sep ] between each , and 
     returns the result as a new byte sequence . 

     Raise [ Invalid_argument ] if the result is longer than 
     { !  } bytes .
    inserting the separator byte sequence [sep] between each, and
    returns the result as a new byte sequence.

    Raise [Invalid_argument] if the result is longer than
    {!Sys.max_string_length} bytes. *)

val cat : bytes -> bytes -> bytes
* [ cat s1 s2 ] concatenates [ s1 ] and [ s2 ] and returns the result 
      as new byte sequence . 

     Raise [ Invalid_argument ] if the result is longer than 
     { !  } bytes .
     as new byte sequence.

    Raise [Invalid_argument] if the result is longer than
    {!Sys.max_string_length} bytes. *)

val iter : (char -> unit) -> bytes -> unit
* [ iter f s ] applies function [ f ] in turn to all the bytes of [ s ] . 
     It is equivalent to [ f ( get s 0 ) ; f ( get s 1 ) ; ... ; f ( get s 
     ( length s - 1 ) ) ; ( ) ] .
    It is equivalent to [f (get s 0); f (get s 1); ...; f (get s
    (length s - 1)); ()]. *)

val iteri : (int -> char -> unit) -> bytes -> unit
* Same as { ! Bytes.iter } , but the function is applied to the index of 
     the byte as first argument and the byte itself as second 
     argument .
    the byte as first argument and the byte itself as second
    argument. *)

val map : (char -> char) -> bytes -> bytes

val mapi : (int -> char -> char) -> bytes -> bytes

val trim : bytes -> bytes
* Return a copy of the argument , without leading and trailing 
     whitespace . The bytes regarded as whitespace are the ASCII 
     characters [ ' ' ] , [ ' \012 ' ] , [ ' \n ' ] , [ ' \r ' ] , and [ ' \t ' ] .
    whitespace. The bytes regarded as whitespace are the ASCII
    characters [' '], ['\012'], ['\n'], ['\r'], and ['\t']. *)

val escaped : bytes -> bytes
* Return a copy of the argument , with special characters represented 
     by escape sequences , following the lexical conventions of OCaml . 
     All characters outside the ASCII printable range ( 32 .. 126 ) are 
     escaped , as well as backslash and double - quote . 

     Raise [ Invalid_argument ] if the result is longer than 
     { !  } bytes .
    by escape sequences, following the lexical conventions of OCaml.
    All characters outside the ASCII printable range (32..126) are
    escaped, as well as backslash and double-quote.

    Raise [Invalid_argument] if the result is longer than
    {!Sys.max_string_length} bytes. *)

val index : bytes -> char -> int
* [ index s c ] returns the index of the first occurrence of byte [ c ] 
     in [ s ] . 

     Raise [ Not_found ] if [ c ] does not occur in [ s ] .
    in [s].

    Raise [Not_found] if [c] does not occur in [s]. *)

val index_opt: bytes -> char -> int option
* [  s c ] returns the index of the first occurrence of byte [ c ] 
     in [ s ] or [ None ] if [ c ] does not occur in [ s ] . 
     @since 4.05
    in [s] or [None] if [c] does not occur in [s].
    @since 4.05 *)

val rindex : bytes -> char -> int

val rindex_opt: bytes -> char -> int option
* [ rindex_opt s c ] returns the index of the last occurrence of byte [ c ] 
     in [ s ] or [ None ] if [ c ] does not occur in [ s ] . 
     @since 4.05
    in [s] or [None] if [c] does not occur in [s].
    @since 4.05 *)

val index_from : bytes -> int -> char -> int
* [ index_from s i c ] returns the index of the first occurrence of 
     byte [ c ] in [ s ] after position [ i ] .   [ Bytes.index s c ] is 
     equivalent to [ Bytes.index_from s 0 c ] . 

     Raise [ Invalid_argument ] if [ i ] is not a valid position in [ s ] . 
     Raise [ Not_found ] if [ c ] does not occur in [ s ] after position [ i ] .
    byte [c] in [s] after position [i].  [Bytes.index s c] is
    equivalent to [Bytes.index_from s 0 c].

    Raise [Invalid_argument] if [i] is not a valid position in [s].
    Raise [Not_found] if [c] does not occur in [s] after position [i]. *)

val index_from_opt: bytes -> int -> char -> int option
* [ index_from _ opts i c ] returns the index of the first occurrence of 
     byte [ c ] in [ s ] after position [ i ] or [ None ] if [ c ] does not occur in [ s ] after position [ i ] . 
     [ Bytes.index_opt s c ] is equivalent to [ Bytes.index_from_opt s 0 c ] . 

     Raise [ Invalid_argument ] if [ i ] is not a valid position in [ s ] . 
     @since 4.05
    byte [c] in [s] after position [i] or [None] if [c] does not occur in [s] after position [i].
    [Bytes.index_opt s c] is equivalent to [Bytes.index_from_opt s 0 c].

    Raise [Invalid_argument] if [i] is not a valid position in [s].
    @since 4.05 *)

val rindex_from : bytes -> int -> char -> int
* [ rindex_from s i c ] returns the index of the last occurrence of 
     byte [ c ] in [ s ] before position [ i+1 ] .   [ rindex s c ] is equivalent 
     to [ rindex_from s ( Bytes.length s - 1 ) c ] . 

     Raise [ Invalid_argument ] if [ i+1 ] is not a valid position in [ s ] . 
     Raise [ Not_found ] if [ c ] does not occur in [ s ] before position [ i+1 ] .
    byte [c] in [s] before position [i+1].  [rindex s c] is equivalent
    to [rindex_from s (Bytes.length s - 1) c].

    Raise [Invalid_argument] if [i+1] is not a valid position in [s].
    Raise [Not_found] if [c] does not occur in [s] before position [i+1]. *)

val rindex_from_opt: bytes -> int -> char -> int option
* [ rindex_from_opt s i c ] returns the index of the last occurrence 
     of byte [ c ] in [ s ] before position [ i+1 ] or [ None ] if [ c ] does not 
     occur in [ s ] before position [ i+1 ] .   [ rindex_opt s c ] is equivalent to 
     [ rindex_from s ( Bytes.length s - 1 ) c ] . 

     Raise [ Invalid_argument ] if [ i+1 ] is not a valid position in [ s ] . 
     @since 4.05
    of byte [c] in [s] before position [i+1] or [None] if [c] does not
    occur in [s] before position [i+1].  [rindex_opt s c] is equivalent to
    [rindex_from s (Bytes.length s - 1) c].

    Raise [Invalid_argument] if [i+1] is not a valid position in [s].
    @since 4.05 *)

val contains : bytes -> char -> bool

val contains_from : bytes -> int -> char -> bool

val rcontains_from : bytes -> int -> char -> bool
* [ rcontains_from s stop c ] tests if byte [ c ] appears in [ s ] before 
     position [ stop+1 ] . 

     Raise [ Invalid_argument ] if [ stop < 0 ] or [ stop+1 ] is not a valid 
     position in [ s ] .
    position [stop+1].

    Raise [Invalid_argument] if [stop < 0] or [stop+1] is not a valid
    position in [s]. *)

val uppercase : bytes -> bytes
  [@@ocaml.deprecated "Use Bytes.uppercase_ascii instead."]

val lowercase : bytes -> bytes
  [@@ocaml.deprecated "Use Bytes.lowercase_ascii instead."]

val capitalize : bytes -> bytes
  [@@ocaml.deprecated "Use Bytes.capitalize_ascii instead."]
* Return a copy of the argument , with the first character set to uppercase , 
    using the ISO Latin-1 ( 8859 - 1 ) character set .. 
    @deprecated Functions operating on Latin-1 character set are deprecated .
   using the ISO Latin-1 (8859-1) character set..
   @deprecated Functions operating on Latin-1 character set are deprecated. *)

val uncapitalize : bytes -> bytes
  [@@ocaml.deprecated "Use Bytes.uncapitalize_ascii instead."]
* Return a copy of the argument , with the first character set to lowercase , 
    using the ISO Latin-1 ( 8859 - 1 ) character set .. 
    @deprecated Functions operating on Latin-1 character set are deprecated .
   using the ISO Latin-1 (8859-1) character set..
   @deprecated Functions operating on Latin-1 character set are deprecated. *)

val uppercase_ascii : bytes -> bytes
* Return a copy of the argument , with all lowercase letters 
    translated to uppercase , using the US - ASCII character set . 
    @since 4.03.0
   translated to uppercase, using the US-ASCII character set.
   @since 4.03.0 *)

val lowercase_ascii : bytes -> bytes
* Return a copy of the argument , with all uppercase letters 
    translated to lowercase , using the US - ASCII character set . 
    @since 4.03.0
   translated to lowercase, using the US-ASCII character set.
   @since 4.03.0 *)

val capitalize_ascii : bytes -> bytes
* Return a copy of the argument , with the first character set to uppercase , 
    using the US - ASCII character set . 
    @since 4.03.0
   using the US-ASCII character set.
   @since 4.03.0 *)

val uncapitalize_ascii : bytes -> bytes
* Return a copy of the argument , with the first character set to lowercase , 
    using the US - ASCII character set . 
    @since 4.03.0
   using the US-ASCII character set.
   @since 4.03.0 *)

type t = bytes

val compare: t -> t -> int

val equal: t -> t -> bool
* The equality function for byte sequences . 
     @since 4.03.0
    @since 4.03.0 *)

* { 3 Unsafe conversions ( for advanced users ) } 

     This section describes unsafe , low - level conversion functions 
     between [ bytes ] and [ string ] . They do not copy the internal data ; 
     used improperly , they can break the immutability invariant on 
     strings provided by the [ -safe - string ] option . They are available for 
     expert library authors , but for most purposes you should use the 
     always - correct { ! Bytes.to_string } and { ! Bytes.of_string } instead . 


    This section describes unsafe, low-level conversion functions
    between [bytes] and [string]. They do not copy the internal data;
    used improperly, they can break the immutability invariant on
    strings provided by the [-safe-string] option. They are available for
    expert library authors, but for most purposes you should use the
    always-correct {!Bytes.to_string} and {!Bytes.of_string} instead.
*)

val unsafe_to_string : bytes -> string
* Unsafely convert a byte sequence into a string . 

     To reason about the use of [ unsafe_to_string ] , it is convenient to 
     consider an " ownership " discipline . A piece of code that 
     manipulates some data " owns " it ; there are several disjoint ownership 
     modes , including : 
     - Unique ownership : the data may be accessed and mutated 
     - Shared ownership : the data has several owners , that may only 
       access it , not mutate it . 

     Unique ownership is linear : passing the data to another piece of 
     code means giving up ownership ( we can not write the 
     data again ) . A unique owner may decide to make the data shared 
     ( giving up mutation rights on it ) , but shared data may not become 
     uniquely - owned again . 

    [ unsafe_to_string s ] can only be used when the caller owns the byte 
    sequence [ s ] -- either uniquely or as shared immutable data . The 
    caller gives up ownership of [ s ] , and gains ownership of the 
    returned string . 

    There are two valid use - cases that respect this ownership 
    discipline : 

    1 . Creating a string by initializing and mutating a byte sequence 
    that is never changed after initialization is performed . 

    { [ 
 let    : string = 
   let s = Bytes.create len in 
   for i = 0 to len - 1 do Bytes.set s i ( f i ) done ; 
   Bytes.unsafe_to_string s 
    ] } 

    This function is safe because the byte sequence [ s ] will never be 
    accessed or mutated after [ unsafe_to_string ] is called . The 
    [ string_init ] code gives up ownership of [ s ] , and returns the 
    ownership of the resulting string to its caller . 

    Note that it would be unsafe if [ s ] was passed as an additional 
    parameter to the function [ f ] as it could escape this way and be 
    mutated in the future -- [ string_init ] would give up ownership of 
    [ s ] to pass it to [ f ] , and could not call [ unsafe_to_string ] 
    safely . 

    We have provided the { ! String.init } , { ! String.map } and 
    { ! String.mapi } functions to cover most cases of building 
    new strings . You should prefer those over [ to_string ] or 
    [ unsafe_to_string ] whenever applicable . 

    2 . Temporarily giving ownership of a byte sequence to a function 
    that expects a uniquely owned string and returns ownership back , so 
    that we can mutate the sequence again after the call ended . 

    { [ 
 let bytes_length ( s : bytes ) = 
   String.length ( Bytes.unsafe_to_string s ) 
    ] } 

    In this use - case , we do not promise that [ s ] will never be mutated 
    after the call to [ bytes_length s ] . The { !  } function 
    temporarily borrows unique ownership of the byte sequence 
    ( and sees it as a [ string ] ) , but returns this ownership back to 
    the caller , which may assume that [ s ] is still a valid byte 
    sequence after the call . Note that this is only correct because we 
    know that { !  } does not capture its argument -- it could 
    escape by a side - channel such as a memoization combinator . 

    The caller may not mutate [ s ] while the string is borrowed ( it has 
    temporarily given up ownership ) . This affects concurrent programs , 
    but also higher - order functions : if { !  } returned 
    a closure to be called later , [ s ] should not be mutated until this 
    closure is fully applied and returns ownership . 


    To reason about the use of [unsafe_to_string], it is convenient to
    consider an "ownership" discipline. A piece of code that
    manipulates some data "owns" it; there are several disjoint ownership
    modes, including:
    - Unique ownership: the data may be accessed and mutated
    - Shared ownership: the data has several owners, that may only
      access it, not mutate it.

    Unique ownership is linear: passing the data to another piece of
    code means giving up ownership (we cannot write the
    data again). A unique owner may decide to make the data shared
    (giving up mutation rights on it), but shared data may not become
    uniquely-owned again.

   [unsafe_to_string s] can only be used when the caller owns the byte
   sequence [s] -- either uniquely or as shared immutable data. The
   caller gives up ownership of [s], and gains ownership of the
   returned string.

   There are two valid use-cases that respect this ownership
   discipline:

   1. Creating a string by initializing and mutating a byte sequence
   that is never changed after initialization is performed.

   {[
let string_init len f : string =
  let s = Bytes.create len in
  for i = 0 to len - 1 do Bytes.set s i (f i) done;
  Bytes.unsafe_to_string s
   ]}

   This function is safe because the byte sequence [s] will never be
   accessed or mutated after [unsafe_to_string] is called. The
   [string_init] code gives up ownership of [s], and returns the
   ownership of the resulting string to its caller.

   Note that it would be unsafe if [s] was passed as an additional
   parameter to the function [f] as it could escape this way and be
   mutated in the future -- [string_init] would give up ownership of
   [s] to pass it to [f], and could not call [unsafe_to_string]
   safely.

   We have provided the {!String.init}, {!String.map} and
   {!String.mapi} functions to cover most cases of building
   new strings. You should prefer those over [to_string] or
   [unsafe_to_string] whenever applicable.

   2. Temporarily giving ownership of a byte sequence to a function
   that expects a uniquely owned string and returns ownership back, so
   that we can mutate the sequence again after the call ended.

   {[
let bytes_length (s : bytes) =
  String.length (Bytes.unsafe_to_string s)
   ]}

   In this use-case, we do not promise that [s] will never be mutated
   after the call to [bytes_length s]. The {!String.length} function
   temporarily borrows unique ownership of the byte sequence
   (and sees it as a [string]), but returns this ownership back to
   the caller, which may assume that [s] is still a valid byte
   sequence after the call. Note that this is only correct because we
   know that {!String.length} does not capture its argument -- it could
   escape by a side-channel such as a memoization combinator.

   The caller may not mutate [s] while the string is borrowed (it has
   temporarily given up ownership). This affects concurrent programs,
   but also higher-order functions: if {!String.length} returned
   a closure to be called later, [s] should not be mutated until this
   closure is fully applied and returns ownership.
*)

val unsafe_of_string : string -> bytes
* Unsafely convert a shared string to a byte sequence that should 
     not be mutated . 

     The same ownership discipline that makes [ unsafe_to_string ] 
     correct applies to [ unsafe_of_string ] : you may use it if you were 
     the owner of the [ string ] value , and you will own the return 
     [ bytes ] in the same mode . 

     In practice , unique ownership of string values is extremely 
     difficult to reason about correctly . You should always assume 
     strings are shared , never uniquely owned . 

     For example , string literals are implicitly shared by the 
     compiler , so you never uniquely own them . 

     { [ 
 let incorrect = Bytes.unsafe_of_string " hello " 
 let s = Bytes.of_string " hello " 
     ] } 

     The first declaration is incorrect , because the string literal 
     [ " hello " ] could be shared by the compiler with other parts of the 
     program , and mutating [ incorrect ] is a bug . You must always use 
     the second version , which performs a copy and is thus correct . 

     Assuming unique ownership of strings that are not string 
     literals , but are ( partly ) built from string literals , is also 
     incorrect . For example , mutating [ unsafe_of_string ( " foo " ^ s ) ] 
     could mutate the shared string [ " foo " ] -- assuming a rope - like 
     representation of strings . More generally , functions operating on 
     strings will assume shared ownership , they do not preserve unique 
     ownership . It is thus incorrect to assume unique ownership of the 
     result of [ unsafe_of_string ] . 

     The only case we have reasonable confidence is safe is if the 
     produced [ bytes ] is shared -- used as an immutable byte 
     sequence . This is possibly useful for incremental migration of 
     low - level programs that manipulate immutable sequences of bytes 
     ( for example { ! Marshal.from_bytes } ) and previously used the 
     [ string ] type for this purpose . 

    not be mutated.

    The same ownership discipline that makes [unsafe_to_string]
    correct applies to [unsafe_of_string]: you may use it if you were
    the owner of the [string] value, and you will own the return
    [bytes] in the same mode.

    In practice, unique ownership of string values is extremely
    difficult to reason about correctly. You should always assume
    strings are shared, never uniquely owned.

    For example, string literals are implicitly shared by the
    compiler, so you never uniquely own them.

    {[
let incorrect = Bytes.unsafe_of_string "hello"
let s = Bytes.of_string "hello"
    ]}

    The first declaration is incorrect, because the string literal
    ["hello"] could be shared by the compiler with other parts of the
    program, and mutating [incorrect] is a bug. You must always use
    the second version, which performs a copy and is thus correct.

    Assuming unique ownership of strings that are not string
    literals, but are (partly) built from string literals, is also
    incorrect. For example, mutating [unsafe_of_string ("foo" ^ s)]
    could mutate the shared string ["foo"] -- assuming a rope-like
    representation of strings. More generally, functions operating on
    strings will assume shared ownership, they do not preserve unique
    ownership. It is thus incorrect to assume unique ownership of the
    result of [unsafe_of_string].

    The only case we have reasonable confidence is safe is if the
    produced [bytes] is shared -- used as an immutable byte
    sequence. This is possibly useful for incremental migration of
    low-level programs that manipulate immutable sequences of bytes
    (for example {!Marshal.from_bytes}) and previously used the
    [string] type for this purpose.
*)

* { 6 Iterators }

val to_seq : t -> char Seq.t
* Iterate on the string , in increasing index order . Modifications of the 
     string during iteration will be reflected in the iterator . 
     @since 4.07
    string during iteration will be reflected in the iterator.
    @since 4.07 *)

val to_seqi : t -> (int * char) Seq.t
* Iterate on the string , in increasing order , yielding indices along chars 
     @since 4.07
    @since 4.07 *)

val of_seq : char Seq.t -> t
* Create a string from the generator 
     @since 4.07
    @since 4.07 *)



external unsafe_get : bytes -> int -> char = "%bytes_unsafe_get"
external unsafe_set : bytes -> int -> char -> unit = "%bytes_unsafe_set"
external unsafe_blit :
  bytes -> int -> bytes -> int -> int -> unit
  = "caml_blit_bytes" [@@noalloc]
external unsafe_fill :
  bytes -> int -> int -> char -> unit = "caml_fill_bytes" [@@noalloc]
