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#1743 in Text processing

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Encoding 0.3.0-dev

Encoding on Travis CI

Character encoding support for Rust. (also known as rust-encoding) It is based on WHATWG Encoding Standard, and also provides an advanced interface for error detection and recovery.

This documentation is for the development version (0.3). Please see the stable documentation for 0.2.x versions.

Complete Documentation (stable)


Put this in your Cargo.toml:

encoding = "0.3"

Then put this in your crate root:

extern crate encoding;

Data Table

By default, Encoding comes with ~480 KB of data table ("indices"). This allows Encoding to encode and decode legacy encodings efficiently, but this might not be desirable for some applications.

Encoding provides the no-optimized-legacy-encoding Cargo feature to reduce the size of encoding tables (to ~185 KB) at the expense of encoding performance (typically 5x to 20x slower). The decoding performance remains identical. This feature is strongly intended for end users. Do not try to enable this feature from library crates, ever.

For finer-tuned optimization, see src/index/gen_index.py for custom table generation.


To encode a string:

use encoding::{Encoding, EncoderTrap};
use encoding::all::ISO_8859_1;

assert_eq!(ISO_8859_1.encode("caf\u{e9}", EncoderTrap::Strict),

To encode a string with unrepresentable characters:

use encoding::{Encoding, EncoderTrap};
use encoding::all::ISO_8859_2;

assert!(ISO_8859_2.encode("Acme\u{a9}", EncoderTrap::Strict).is_err());
assert_eq!(ISO_8859_2.encode("Acme\u{a9}", EncoderTrap::Replace),
assert_eq!(ISO_8859_2.encode("Acme\u{a9}", EncoderTrap::Ignore),
assert_eq!(ISO_8859_2.encode("Acme\u{a9}", EncoderTrap::NcrEscape),

To decode a byte sequence:

use encoding::{Encoding, DecoderTrap};
use encoding::all::ISO_8859_1;

assert_eq!(ISO_8859_1.decode(&[99,97,102,233], DecoderTrap::Strict),

To decode a byte sequence with invalid sequences:

use encoding::{Encoding, DecoderTrap};
use encoding::all::ISO_8859_6;

assert!(ISO_8859_6.decode(&[65,99,109,101,169], DecoderTrap::Strict).is_err());
assert_eq!(ISO_8859_6.decode(&[65,99,109,101,169], DecoderTrap::Replace),
assert_eq!(ISO_8859_6.decode(&[65,99,109,101,169], DecoderTrap::Ignore),

To encode or decode the input into the already allocated buffer:

use encoding::{Encoding, EncoderTrap, DecoderTrap};
use encoding::all::{ISO_8859_2, ISO_8859_6};

let mut bytes = Vec::new();
let mut chars = String::new();

assert!(ISO_8859_2.encode_to("Acme\u{a9}", EncoderTrap::Ignore, &mut bytes).is_ok());
assert!(ISO_8859_6.decode_to(&[65,99,109,101,169], DecoderTrap::Replace, &mut chars).is_ok());

assert_eq!(bytes, [65,99,109,101]);
assert_eq!(chars, "Acme\u{fffd}");

A practical example of custom encoder traps:

use encoding::{Encoding, ByteWriter, EncoderTrap, DecoderTrap};
use encoding::types::RawEncoder;
use encoding::all::ASCII;

// hexadecimal numeric character reference replacement
fn hex_ncr_escape(_encoder: &mut RawEncoder, input: &str, output: &mut ByteWriter) -> bool {
    let escapes: Vec<String> =
        input.chars().map(|ch| format!("&#x{:x};", ch as isize)).collect();
    let escapes = escapes.concat();
static HEX_NCR_ESCAPE: EncoderTrap = EncoderTrap::Call(hex_ncr_escape);

let orig = "Hello, 世界!".to_string();
let encoded = ASCII.encode(&orig, HEX_NCR_ESCAPE).unwrap();
assert_eq!(ASCII.decode(&encoded, DecoderTrap::Strict),
           Ok("Hello, &#x4e16;&#x754c;!".to_string()));

Getting the encoding from the string label, as specified in WHATWG Encoding standard:

use encoding::{Encoding, DecoderTrap};
use encoding::label::encoding_from_whatwg_label;
use encoding::all::WINDOWS_949;

let euckr = encoding_from_whatwg_label("euc-kr").unwrap();
assert_eq!(euckr.name(), "windows-949");
assert_eq!(euckr.whatwg_name(), Some("euc-kr")); // for the sake of compatibility
let broken = &[0xbf, 0xec, 0xbf, 0xcd, 0xff, 0xbe, 0xd3];
assert_eq!(euckr.decode(broken, DecoderTrap::Replace),

// corresponding Encoding native API:
assert_eq!(WINDOWS_949.decode(broken, DecoderTrap::Replace),

Types and Stuffs

There are three main entry points to Encoding.

Encoding is a single character encoding. It contains encode and decode methods for converting String to Vec<u8> and vice versa. For the error handling, they receive traps (EncoderTrap and DecoderTrap respectively) which replace any error with some string (e.g. U+FFFD) or sequence (e.g. ?). You can also use EncoderTrap::Strict and DecoderTrap::Strict traps to stop on an error.

There are two ways to get Encoding:

  • encoding::all has static items for every supported encoding. You should use them when the encoding would not change or only handful of them are required. Combined with link-time optimization, any unused encoding would be discarded from the binary.
  • encoding::label has functions to dynamically get an encoding from given string ("label"). They will return a static reference to the encoding, which type is also known as EncodingRef. It is useful when a list of required encodings is not available in advance, but it will result in the larger binary and missed optimization opportunities.

RawEncoder is an experimental incremental encoder. At each step of raw_feed, it receives a slice of string and emits any encoded bytes to a generic ByteWriter (normally Vec<u8>). It will stop at the first error if any, and would return a CodecError struct in that case. The caller is responsible for calling raw_finish at the end of encoding process.

RawDecoder is an experimental incremental decoder. At each step of raw_feed, it receives a slice of byte sequence and emits any decoded characters to a generic StringWriter (normally String). Otherwise it is identical to RawEncoders.

One should prefer Encoding::{encode,decode} as a primary interface. RawEncoder and RawDecoder is experimental and can change substantially. See the additional documents on encoding::types module for more information on them.

Supported Encodings

Encoding covers all encodings specified by WHATWG Encoding Standard and some more:

  • 7-bit strict ASCII (ascii)
  • UTF-8 (utf-8)
  • UTF-16 in little endian (utf-16 or utf-16le) and big endian (utf-16be)
  • All single byte encoding in WHATWG Encoding Standard:
    • IBM code page 866
    • ISO 8859-{2,3,4,5,6,7,8,10,13,14,15,16}
    • KOI8-R, KOI8-U
    • MacRoman (macintosh), Macintosh Cyrillic encoding (x-mac-cyrillic)
    • Windows code pages 874, 1250, 1251, 1252 (instead of ISO 8859-1), 1253, 1254 (instead of ISO 8859-9), 1255, 1256, 1257, 1258
  • All multi byte encodings in WHATWG Encoding Standard:
    • Windows code page 949 (euc-kr, since the strict EUC-KR is hardly used)
    • EUC-JP and Windows code page 932 (shift_jis, since it's the most widespread extension to Shift_JIS)
    • ISO-2022-JP with asymmetric JIS X 0212 support (Note: this is not yet up to date to the current standard)
    • GBK
    • GB 18030
    • Big5-2003 with HKSCS-2008 extensions
  • Encodings that were originally specified by WHATWG Encoding Standard:
    • HZ
  • ISO 8859-1 (distinct from Windows code page 1252)

Parenthesized names refer to the encoding's primary name assigned by WHATWG Encoding Standard.

Many legacy character encodings lack the proper specification, and even those that have a specification are highly dependent of the actual implementation. Consequently one should be careful when picking a desired character encoding. The only standards reliable in this regard are WHATWG Encoding Standard and vendor-provided mappings from the Unicode consortium. Whenever in doubt, look at the source code and specifications for detailed explanations.