Module `Uutf`

Non-blocking streaming Unicode codec.

Uutf is a non-blocking streaming codec to decode and encode the UTF-8, UTF-16, UTF-16LE and UTF-16BE encoding schemes. It can efficiently work character by character without blocking on IO. Decoders perform character position tracking and support newline normalization.

Functions are also provided to fold over the characters of UTF encoded OCaml string values and to directly encode characters in OCaml Buffer.t values.

See examples of use.

v1.0.2 — homepage

References

The Unicode Consortium. The Unicode Standard. (latest version)

Special Unicode characters

val u_bom : Stdlib.Uchar.t: u_bom is the byte order mark (BOM) character (U+FEFF).

val u_rep : Stdlib.Uchar.t: u_rep is the replacement character (U+FFFD).

Unicode encoding schemes

type encoding = [ | `UTF_16 | `UTF_16BE | `UTF_16LE | `UTF_8 ]: The type for Unicode encoding schemes.

type decoder_encoding = [ | encoding | `US_ASCII | `ISO_8859_1 ]: The type for encoding schemes decoded by Uutf. Unicode encoding schemes plus US-ASCII and ISO/IEC 8859-1 (latin-1).

val encoding_of_string : string -> decoder_encoding option: encoding_of_string s converts a (case insensitive) IANA character set name to an encoding.

val encoding_to_string : [< decoder_encoding ] -> string: encoding_to_string e is a IANA character set name for e.

Decode

type src = [ | `Channel of Stdlib.in_channel | `String of string | `Manual ]: The type for input sources. With a `Manual source the client must provide input with Manual.src.

type nln = [ | `ASCII of Stdlib.Uchar.t | `NLF of Stdlib.Uchar.t | `Readline of Stdlib.Uchar.t ]

The type for newline normalizations. The variant argument is the normalization character.

`ASCII, normalizes CR (U+000D), LF (U+000A) and CRLF (<U+000D, U+000A>).
`NLF, normalizes the Unicode newline function (NLF). This is NEL (U+0085) and the normalizations of `ASCII.
`Readline, normalizes for a Unicode readline function. This is FF (U+000C), LS (U+2028), PS (U+2029), and the normalizations of `NLF.

Used with an appropriate normalization character the `NLF and `Readline normalizations allow to implement all the different recommendations of Unicode's newline guidelines (section 5.8 in Unicode 9.0.0).

type decoder: The type for decoders.

val decoder : ?⁠nln:[< nln ] -> ?⁠encoding:[< decoder_encoding ] -> [< src ] -> decoder

decoder nln encoding src is a decoder that inputs from src.

Byte order mark. Byte order mark (BOM) constraints are application dependent and prone to misunderstandings (see the FAQ). Hence, Uutf decoders have a simple rule: an initial BOM is always removed from the input and not counted in character position tracking. The function decoder_removed_bom does however return true if a BOM was removed so that all the information can be recovered if needed.

For UTF-16BE and UTF-16LE the above rule is a violation of conformance D96 and D97 of the standard. Uutf favors the idea that if there's a BOM, decoding with `UTF_16 or the `UTF_16XX corresponding to the BOM should decode the same character sequence (this is not the case if you stick to the standard). The client can however regain conformance by consulting the result of decoder_removed_bom and take appropriate action.

Encoding. encoding specifies the decoded encoding scheme. If `UTF_16 is used the endianness is determined according to the standard: from a BOM if there is one, `UTF_16BE otherwise.

If encoding is unspecified it is guessed. The result of a guess can only be `UTF_8, `UTF_16BE or `UTF_16LE. The heuristic looks at the first three bytes of input (or less if impossible) and takes the first matching byte pattern in the table below.

xx = any byte
.. = any byte or no byte (input too small)
pp = positive byte
uu = valid UTF-8 first byte

Bytes    | Guess     | Rationale
---------+-----------+-----------------------------------------------
EF BB BF | `UTF_8    | UTF-8 BOM
FE FF .. | `UTF_16BE | UTF-16BE BOM
FF FE .. | `UTF_16LE | UTF-16LE BOM
00 pp .. | `UTF_16BE | ASCII UTF-16BE and U+0000 is often forbidden
pp 00 .. | `UTF_16LE | ASCII UTF-16LE and U+0000 is often forbidden
uu .. .. | `UTF_8    | ASCII UTF-8 or valid UTF-8 first byte.
xx xx .. | `UTF_16BE | Not UTF-8 => UTF-16, no BOM => UTF-16BE
.. .. .. | `UTF_8    | Single malformed UTF-8 byte or no input.

This heuristic is compatible both with BOM based recognitition and JSON-like encoding recognition that relies on ASCII being present at the beginning of the stream. Also, decoder_removed_bom will tell the client if the guess was BOM based.

Newline normalization. If nln is specified, the given newline normalization is performed, see nln. Otherwise all newlines are returned as found in the input.

Character position. The line number, column number, byte count and character count of the last decoded character (including `Malformed ones) are respectively returned by decoder_line, decoder_col, decoder_byte_count and decoder_count. Before the first call to Decode the line number is 1 and the column is 0. Each Decode returning `Uchar or `Malformed increments the column until a newline. On a newline, the line number is incremented and the column set to zero. For example the line is 2 and column 0 after the first newline was decoded. This can be understood as if Decode was moving an insertion point to the right in the data. A newline is anything normalized by `Readline, see nln.

Uutf assumes that each Unicode scalar value has a column width of 1. The same assumption may not be made by the display program (e.g. for emacs' compilation mode you need to set compilation-error-screen-columns to nil). The problem is in general difficult to solve without interaction or convention with the display program's rendering engine. Depending on the context better column increments can be implemented by using Uucp.Break.tty_width_hint or grapheme cluster boundaries (see Uuseg).

val decode : decoder -> [ `Await | `Uchar of Stdlib.Uchar.t | `End | `Malformed of string ]

decode d is:

`Await if d has a `Manual input source and awaits for more input. The client must use Manual.src to provide it.
`Uchar u if a Unicode scalar value u was decoded.
`End if the end of input was reached.
`Malformed bytes if the bytes sequence is malformed according to the decoded encoding scheme. If you are interested in a best-effort decoding you can still continue to decode after an error until the decoder synchronizes again on valid bytes. It may however be a good idea to signal the malformed characters by adding an u_rep character to the parsed data, see the examples.

Note. Repeated invocation always eventually returns `End, even in case of errors.

val decoder_encoding : decoder -> decoder_encoding

decoder_encoding d is d's the decoded encoding scheme of d.

Warning. If the decoder guesses the encoding or uses `UTF_16, rely on this value only after the first `Uchar was decoded.

val decoder_line : decoder -> int: decoder_line d is the line number of the last decoded (or malformed) character. See decoder for details.

val decoder_col : decoder -> int: decoder_col d is the column number of the last decoded (or malformed) character. See decoder for details.

val decoder_byte_count : decoder -> int: decoder_byte_count d is the number of bytes already decoded on d (including malformed ones). This is the last Decode's end byte offset counting from the beginning of the stream.

val decoder_count : decoder -> int: decoder_count d is the number of characters already decoded on d (including malformed ones). See decoder for details.

val decoder_removed_bom : decoder -> bool: decoder_removed_bom d is true iff an initial BOM was removed from the input stream. See decoder for details.

val decoder_src : decoder -> src: decoder_src d is d's input source.

val decoder_nln : decoder -> nln option: decoder_nln d returns d's newline normalization (if any).

val pp_decode : Stdlib.Format.formatter -> [< `Await | `Uchar of Stdlib.Uchar.t | `End | `Malformed of string ] -> unit: pp_decode ppf v prints an unspecified representation of v on ppf.

Encode

type dst = [ | `Channel of Stdlib.out_channel | `Buffer of Stdlib.Buffer.t | `Manual ]: The type for output destinations. With a `Manual destination the client must provide output storage with Manual.dst.

type encoder: The type for Unicode encoders.

val encoder : [< encoding ] -> [< dst ] -> encoder

encoder encoding dst is an encoder for encoding that outputs to dst.

Note. No initial BOM is encoded. If needed, this duty is left to the client.

val encode : encoder -> [< `Await | `End | `Uchar of Stdlib.Uchar.t ] -> [ `Ok | `Partial ]

encode e v is :

`Partial iff e has a `Manual destination and needs more output storage. The client must use Manual.dst to provide a new buffer and then call Encode with `Await until `Ok is returned.
`Ok when the encoder is ready to encode a new `Uchar or `End

For `Manual destination, encoding `End always returns `Partial, the client should continue as usual with `Await until `Ok is returned at which point Manual.dst_rem e is guaranteed to be the size of the last provided buffer (i.e. nothing was written).

Raises. Invalid_argument if an `Uchar or `End is encoded after a `Partial encode.

val encoder_encoding : encoder -> encoding: encoder_encoding e is e's encoding.

val encoder_dst : encoder -> dst: encoder_dst e is e's output destination.

Manual sources and destinations.

module Manual : sig ... end: Manual sources and destinations.

String folders and Buffer encoders

module String : sig ... end: Fold over the characters of UTF encoded OCaml string values.

module Buffer : sig ... end: UTF encode characters in OCaml Buffer.t values.

Examples

Read lines

The value of lines src is the list of lines in src as UTF-8 encoded OCaml strings. Line breaks are determined according to the recommendation R4 for a readline function in section 5.8 of Unicode 9.0.0. If a decoding error occurs we silently replace the malformed sequence by the replacement character u_rep and continue.

let lines ?encoding (src : [`Channel of in_channel | `String of string]) =
  let rec loop d buf acc = match Uutf.decode d with
  | `Uchar u ->
      begin match Uchar.to_int u with
      | 0x000A ->
          let line = Buffer.contents buf in
          Buffer.clear buf; loop d buf (line :: acc)
      | _ ->
          Uutf.Buffer.add_utf_8 buf u; loop d buf acc
      end
  | `End -> List.rev (Buffer.contents buf :: acc)
  | `Malformed _ -> Uutf.Buffer.add_utf_8 buf Uutf.u_rep; loop d buf acc
  | `Await -> assert false
  in
  let nln = `Readline (Uchar.of_int 0x000A) in
  loop (Uutf.decoder ~nln ?encoding src) (Buffer.create 512) []

Using the `Manual interface, lines_fd does the same but on a Unix file descriptor.

let lines_fd ?encoding (fd : Unix.file_descr) =
  let rec loop fd s d buf acc = match Uutf.decode d with
  | `Uchar u ->
      begin match Uchar.to_int u with
      | 0x000A ->
          let line = Buffer.contents buf in
          Buffer.clear buf; loop fd s d buf (line :: acc)
      | _ ->
          Uutf.Buffer.add_utf_8 buf u; loop fd s d buf acc
      end
  | `End -> List.rev (Buffer.contents buf :: acc)
  | `Malformed _ -> Uutf.Buffer.add_utf_8 buf Uutf.u_rep; loop fd s d buf acc
  | `Await ->
      let rec unix_read fd s j l = try Unix.read fd s j l with
      | Unix.Unix_error (Unix.EINTR, _, _) -> unix_read fd s j l
      in
      let rc = unix_read fd s 0 (Bytes.length s) in
      Uutf.Manual.src d s 0 rc; loop fd s d buf acc
  in
  let s = Bytes.create 65536 (* UNIX_BUFFER_SIZE in 4.0.0 *) in
  let nln = `Readline (Uchar.of_int 0x000A) in
  loop fd s (Uutf.decoder ~nln ?encoding `Manual) (Buffer.create 512) []

Recode

The result of recode src out_encoding dst has the characters of src written on dst with encoding out_encoding. If a decoding error occurs we silently replace the malformed sequence by the replacement character u_rep and continue. Note that we don't add an initial BOM to dst, recoding will thus loose the initial BOM src may have. Whether this is a problem or not depends on the context.

let recode ?nln ?encoding out_encoding
    (src : [`Channel of in_channel | `String of string])
    (dst : [`Channel of out_channel | `Buffer of Buffer.t])
  =
  let rec loop d e = match Uutf.decode d with
  | `Uchar _ as u -> ignore (Uutf.encode e u); loop d e
  | `End -> ignore (Uutf.encode e `End)
  | `Malformed _ -> ignore (Uutf.encode e (`Uchar Uutf.u_rep)); loop d e
  | `Await -> assert false
  in
  let d = Uutf.decoder ?nln ?encoding src in
  let e = Uutf.encoder out_encoding dst in
  loop d e

Using the `Manual interface, recode_fd does the same but between Unix file descriptors.

let recode_fd ?nln ?encoding out_encoding
    (fdi : Unix.file_descr)
    (fdo : Unix.file_descr)
  =
  let rec encode fd s e v = match Uutf.encode e v with `Ok -> ()
  | `Partial ->
      let rec unix_write fd s j l =
        let rec write fd s j l = try Unix.single_write fd s j l with
        | Unix.Unix_error (Unix.EINTR, _, _) -> write fd s j l
        in
        let wc = write fd s j l in
        if wc < l then unix_write fd s (j + wc) (l - wc) else ()
      in
      unix_write fd s 0 (Bytes.length s - Uutf.Manual.dst_rem e);
      Uutf.Manual.dst e s 0 (Bytes.length s);
      encode fd s e `Await
  in
  let rec loop fdi fdo ds es d e = match Uutf.decode d with
  | `Uchar _ as u -> encode fdo es e u; loop fdi fdo ds es d e
  | `End -> encode fdo es e `End
  | `Malformed _ -> encode fdo es e (`Uchar Uutf.u_rep); loop fdi fdo ds es d e
  | `Await ->
      let rec unix_read fd s j l = try Unix.read fd s j l with
      | Unix.Unix_error (Unix.EINTR, _, _) -> unix_read fd s j l
      in
      let rc = unix_read fdi ds 0 (Bytes.length ds) in
      Uutf.Manual.src d ds 0 rc; loop fdi fdo ds es d e
  in
  let ds = Bytes.create 65536 (* UNIX_BUFFER_SIZE in 4.0.0 *) in
  let es = Bytes.create 65536 (* UNIX_BUFFER_SIZE in 4.0.0 *) in
  let d = Uutf.decoder ?nln ?encoding `Manual in
  let e = Uutf.encoder out_encoding `Manual in
  Uutf.Manual.dst e es 0 (Bytes.length es);
  loop fdi fdo ds es d e