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StringEncoding: Difference between revisions

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{{obsolete|spec=[https://encoding.spec.whatwg.org/#api Encoding Standard: API]}}
Proposed Text Encoding Web API for Typed Arrays
Proposed Text Encoding Web API for Typed Arrays


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== Open Issues ==
== Open Issues ==


* Encoding errors
* Should the <var>encoding</var> attribute return the <var>Name</var> of the encoding or the name that was passed in?
** Define options dict for <code>TextEncoder()</code> that allows selecting between throw vs. replacement character / replacement callback?
 
* The [http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html Encoding specification] defines the byte order mark as more authoritative than an explicit encoding label when decoding content.
== Notes to Implementers ==
** Is that desirable here?
 
** What does it mean to e.g. write <code>TextDecoder('iso-2022-kr').decode(str)</code> if string turns out to be UTF-8?
* Streaming decode/encode requires retaining partial buffers between calls.
** What about streaming and other re-uses of the same decoder object?
* Streaming decode/encode requires keeping buffers between calls.
** Some encode/decode algorithms require adjusting the <var>code point pointer</var> or <var>byte pointer</var> by a negative amount. This could occur across "chunk" boundaries. This implies that when the internal <var>streaming</var> flag is set on an encoder/decoder that the last N elements of the stream are saved for the next call and used as a prefix for the stream. N is defined by the specific encoding algorithm.
** Some encode/decode algorithms require adjusting the <var>code point pointer</var> or <var>byte pointer</var> by a negative amount. This could occur across "chunk" boundaries. This implies that when the internal <var>streaming</var> flag is set on an encoder/decoder that the last N elements of the stream are saved for the next call and used as a prefix for the stream. N is defined by the specific encoding algorithm.
** ''This is more of a note to implementers than an issue, but it may merit further discussion.''
** This is not yet implemented in the [http://code.google.com/p/stringencoding/ ECMAScript shim]
* Move the algorithm to <b><i>convert a sequence of Unicode characters to a DOMString</i></b> into [http://dev.w3.org/2006/webapi/WebIDL/#idl-DOMString WebIDL].


== Resolved Issues ==
== Resolved Issues ==
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<dt>Should this be a standalone API (as written), or live on e.g. DataView, or on e.g. String?
<dt>Should this be a standalone API (as written), or live on e.g. DataView, or on e.g. String?
<dd>There seems to be pretty strong consensus that streaming, stateful coding is high priority and that an object-oriented API is cleanest, and shoe-horning those onto existing objects would be messy.
<dd>There seems to be pretty strong consensus that streaming, stateful coding is high priority and that an object-oriented API is cleanest, and shoe-horning those onto existing objects would be messy.
<dt>Should legacy encodings be supported?
<dd>Resolution: Consensus on the [http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2012-August/036825.html WHATWG mailing list] - support legacy encodings for decode, and only 'utf-8', 'utf-16' and 'utf-16be' for encode.
<dt>What do to on encoding errors? If non-UTF encodings are supported then we may want to allow substitution (e.g. ASCII '?') or a script callback (for arbitrary escaping).
<dd>Resolution: not for "v1" (see above)
<dt>How are byte order marks handled?
<dd>BOM is respected if and only if the requested coding is a case-insensitive match for "<code>utf-16</code>"
</dl>
</dl>


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};
};


[Constructor,
[Constructor(optional DOMString encoding)]
Constructor(DOMString encoding)]
interface TextEncoder {
interface TextEncoder {
   readonly attribute DOMString encoding;
   readonly attribute DOMString encoding;
   ArrayBufferView encode(DOMString? string, optional TextEncodeOptions options);
   Uint8Array encode(DOMString? string, optional TextEncodeOptions options);
};
};
</pre>
</pre>
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The constructor runs the following steps:
The constructor runs the following steps:
# If the constructor is called with no arguments, let <var>label</var> be the "<code>utf-8</code>". Otherwise, let <var>label</var> be the value of the <var>encoding</var> argument.
# If the constructor is called with no arguments, let <var>label</var> be the "<code>utf-8</code>". Otherwise, let <var>label</var> be the value of the <var>encoding</var> argument.
# Run the '''steps to get an encoding''' from [http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html Encoding], with <var>label</var> as <var>label</var>.
# Run the '''steps to get an encoding''' from [http://encoding.spec.whatwg.org/ Encoding], with <var>label</var> as <var>label</var>.
#* If the steps result in failure, throw an "<code>EncodingError</code>" exception and terminate these steps.
#* If the steps result in failure, throw an "<code>EncodingError</code>" exception and terminate these steps.
#* Otherwise, if the <var>Name</var> of the returned encoding is not one of "<code>utf-8</code>", "<code>utf-16</code>", or "<code>utf-16be</code>" throw an "<code>EncodingError</code>" exception and terminate these steps.
#* Otherwise, set the encoder object's internal <var>encoding</var> property to the returned encoding.
#* Otherwise, set the encoder object's internal <var>encoding</var> property to the returned encoding.
# Initialize the internal <var>streaming</var> flag of the encoder object to false.  
# Initialize the internal <var>streaming</var> flag of the encoder object to false.  
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<dl>
<dl>
<dt><code>encoding</code> of type DOMString, readonly
<dt><code>encoding</code> of type DOMString, readonly
<dd>Returns the <var>Name</var> of the encoder object's <var>encoding</var>, per [http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html Encoding].  
<dd>Returns the <var>Name</var> of the encoder object's <var>encoding</var>, per [http://encoding.spec.whatwg.org/ Encoding].  


:Note that this may differ from the name of the encoding specified during the call to the constructor. For example, if the constructor is called with <var>encoding</var> of <code>"ascii"</code> the <var>encoding</var> attribute of the ''encoder object'' would have the value <code>"windows-1252"</code> as <code>"ascii"</code> is a label for that encoding.
:Note that this may differ from the name of the encoding specified during the call to the constructor. For example, if the constructor is called with <var>encoding</var> of <code>"ascii"</code> the <var>encoding</var> attribute of the ''encoder object'' would have the value <code>"windows-1252"</code> as <code>"ascii"</code> is a label for that encoding.
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#* The output of the the algorithm is a <var>sequence of emitted bytes</var>.
#* The output of the the algorithm is a <var>sequence of emitted bytes</var>.
# Returns a <code>Unit8Array</code> object wrapping an <code>ArrayBuffer</code> containing the <var>sequence of emitted bytes</var> by encoder algorithm.
# Returns a <code>Unit8Array</code> object wrapping an <code>ArrayBuffer</code> containing the <var>sequence of emitted bytes</var> by encoder algorithm.
:''NOTE: Because only UTF encodings are supported, and because of the use of the [http://dev.w3.org/2006/webapi/WebIDL/#dfn-obtain-unicode steps to convert a DOMString to a sequence of Unicode characters] and thus Unicode code points, no input can cause the encoding process to emit an encoder error.''
</dl>
</dl>


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};
};


[Constructor,
[Constructor(optional DOMString encoding, optional TextDecoderOptions options)]
Constructor(optional DOMString encoding, optional TextDecoderOptions options)]
interface TextDecoder {
interface TextDecoder {
   readonly attribute DOMString encoding;
   readonly attribute DOMString encoding;
Line 125: Line 136:


The constructor runs the following steps:
The constructor runs the following steps:
# The constructor creates a ''decoder object''. It has the internal properties <var>encoding</var>, a <var>fatal</var> flag which is initially unset, and a <var>streaming</var> flag which is initially unset.
# The constructor creates a ''decoder object''. It has the internal properties <var>encoding</var>, a <var>fatal</var> flag which is initially unset,  a <var>streaming</var> flag which is initially unset, an <var>offset</var> pointer which is initially <code>0</code>, and a <var>useBOM</var> flag which is initially unset.
# If called without an <var>encoding</var> argument, let <var>label</var> be the "<code>utf-8</code>". Otherwise, let <var>label</var> be the value of the <var>encoding</var> argument.
# If called without an <var>encoding</var> argument, let <var>label</var> be the "<code>utf-8</code>". Otherwise, let <var>label</var> be the value of the <var>encoding</var> argument.
# Run the '''steps to get an encoding''' from [http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html Encoding], with <var>label</var> as <var>label</var>.  
# If <var>label</var> is a case-insensitive match for "<code>utf-16</code>" then set the internal <var>useBOM</var> flag.
# Run the '''steps to get an encoding''' from [http://encoding.spec.whatwg.org/ Encoding], with <var>label</var> as <var>label</var>.  
## If the steps result in failure, throw a "<code>EncodingError</code>" exception and terminate these steps.
## If the steps result in failure, throw a "<code>EncodingError</code>" exception and terminate these steps.
## Otherwise, set the ''decoder object's'' internal <var>encoding</var> property to the returned encoding.
## Otherwise, set the ''decoder object's'' internal <var>encoding</var> property to the returned encoding.
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<dt><code>encoding</code> of type DOMString, readonly
<dt><code>encoding</code> of type DOMString, readonly
<dd>
<dd>
Returns the <var>Name</var> of the decoder object's <var>encoding</var>, per [http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html Encoding].
Returns the <var>Name</var> of the decoder object's <var>encoding</var>, per [http://encoding.spec.whatwg.org/ Encoding].


:Note that this may differ from the name of the encoding specified during the call to the constructor. For example, if the constructor is called with <var>encoding</var> of <code>"ascii"</code> the <var>encoding</var> attribute of the ''decoder object'' would have the value <code>"windows-1252"</code> as <code>"ascii"</code> is a label for that encoding.
:Note that this may differ from the name of the encoding specified during the call to the constructor. For example, if the constructor is called with <var>encoding</var> of <code>"ascii"</code> the <var>encoding</var> attribute of the ''decoder object'' would have the value <code>"windows-1252"</code> as <code>"ascii"</code> is a label for that encoding.
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The <code>decode</code> method runs the following steps:
The <code>decode</code> method runs the following steps:


# If the internal <var>streaming</var> flag of the ''decoder object'' is not set, then reset the <var>encoding algorithm state</var> to the default values for encoding <var>encoding</var>. Otherwise, the <var>encoding algorithm state</var> is re-used from the previous call to <code>decode</code> on this object.  
# If <var>view</var> is not specified, let <var>view</var> be a <code>Uint8Array</code> of length <code>0</code>.
# If the <var>options</var> parameter is specified and the <var>stream</var> option is '''true''', then the internal <var>streaming</var> flag is set; otherwise the internal <var>streaming</var> flag is cleared.
# If the internal <var>streaming</var> flag of the ''decoder object'' is not set, then reset the <var>encoding algorithm state</var> to the default values for encoding <var>encoding</var> and set <var>offset</var> to <code>0</code>. Otherwise, the <var>encoding algorithm state</var> is re-used from the previous call to <code>decode</code> on this object.
# Run the decoder algorithm of the ''decoder object's'' encoder
# If the <var>options</var> parameter is specified and the <var>stream</var> option is '''true''', then the internal <var>streaming</var> flag is set. Otherwise the internal <var>streaming</var> flag is cleared.
#* The input to the algorithm is a <var>byte stream</var>. The <var>byte stream</var> is provided by the bytes in <var>view.buffer</var> starting at offset <var>view.byteOffset</var>. A maximum of <var>view.byteLength</var> bytes are yielded by the stream from <var>view.buffer</var>. If <var>view</var> is not specified, the stream is empty.
# Run or resume the decoder algorithm of the ''decoder object's'' encoder, with the following additions:
#* If the <var>options</var> parameter not specified or the '''stream''' option is '''false''', then after <var>view.byteLength</var> bytes are yielded by the stream the '''EOF byte''' is yielded.
#* The input to the algorithm is a <var>byte stream</var>.
#** If <var>offset</var> is greater than <code>0</code>, the bytes in <var>byte stream</var> for positions less than <var>offset</var> are provided by buffer(s) passed in to previous calls to <code>decode()</code>.
#** The bytes in <var>byte stream</var> for positions <var>offset</var> through <code><var>offset</var> + <var>view.byteLength</var> - 1</code> are provided by the bytes in <var>view.buffer</var> starting at offset <var>view.byteOffset</var>.
#** When accessing the byte in <var>byte stream</var> at position <code><var>offset</var> + <var>view.byteLength</var></code>:
#*** If the internal <var>streaming</var> flag is not set, then yield the '''EOF byte'''.
#*** Otherwise, set <var>offset</var> to <code><var>offset</var> + <var>view.byteLength</var></code> and suspend the steps of the decoder algorithm until a subsequent call to <code>decode()</code>
#* If encoding is one of "<code>utf-8</code>", "<code>utf-16</code>" or "<code>utf-16be</code>" and <var>offset</var> is <code>0</code>, then prior to running the steps of the decoder algorithm:
#** If the internal <var>useBOM</var> flag is set, then:
#*** If less than two bytes are present in the stream and the internal <var>streaming</var> flag is true, suspend this algorithm and return the empty string.
#*** If the next two bytes of the stream are '''0xFF 0xFE''' then set <var>offset</var> to <code>2</code>, and clear the <var>utf-16be</var> flag of the decoder state.
#*** If the next two bytes of the stream are '''0xFE 0xFF''' then set <var>offset</var> to <code>2</code>, and set the <var>utf-16be</var> flag of the decoder state.
#** If encoding is "<code>utf-8</code>", then:
#*** If less than three bytes are present in the stream and the internal <var>streaming</var> flag is true, suspend this algorithm and return the empty string.
#*** If the next three bytes of the stream are '''0xEF 0xBB 0xBF''' then set <var>offset</var> to <code>3</code>.
#** If encoding is "<code>utf-16</code>", then:
#*** If less than two bytes are present in the stream and the internal <var>streaming</var> flag is true, suspend this algorithm and return the empty string.
#*** If the next two bytes of the stream are '''0xFF 0xFE''' then set <var>offset</var> to <code>2</code>.
#** If encoding is "<code>utf-16be</code>", then:
#*** If less than two bytes are present in the stream and the internal <var>streaming</var> flag is true, suspend this algorithm and return the empty string.
#*** If the next two bytes of the stream are '''0xFE 0xFF''' then set <var>offset</var> to <code>2</code>.
#* If the internal <var>fatal</var> flag of the ''decoder object'' is set, then a '''decoder error''' causes an <code>DOMException</code> of type <code>EncodingError</code> to be thrown rather than emitting a fallback code point.
#* If the internal <var>fatal</var> flag of the ''decoder object'' is set, then a '''decoder error''' causes an <code>DOMException</code> of type <code>EncodingError</code> to be thrown rather than emitting a fallback code point.
#* The output of the algorithm is a sequence of <var>emitted code points</var>.
#* The output of the algorithm is a <var>sequence of emitted code points</var>.
# Return a <code>DOMString</code> by encoding the <var>sequence of emitted code points</var> following the [[#Steps to convert a sequence of Unicode characters to a DOMString|steps to convert a sequence of Unicode characters to a DOMString]].
# Return an IDL <code>DOMString</code> value that represents the sequence of code units resulting from encoding the <var>sequence of emitted code points</var> as UTF-16.
 
:''ISSUE: Need to handle BOMs. [http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html Encoding] specifies this by using the caller-specified encoding as a suggestion, and consuming the BOM as a part of selecting the real encoder where BOM takes precedence. At the very least a matching BOM should be ignored. A mismatching BOM could throw, be a decoding error, or could actually switch the decoder for this stream (call or sequence of calls), possibly if-and-only-if the constructor was called without an encoding.''


</dl>
</dl>
== Algorithms ==
=== Steps to convert a sequence of Unicode characters to a DOMString ===
:TODO: Move the following to [http://dev.w3.org/2006/webapi/WebIDL/#idl-DOMString WebIDL]
The following algorithm defines a way to <b><i>convert a sequence of Unicode characters to a DOMString</i></b>:
# Let <var>U</var><sub>0...<var>n</var>-1</sub> be the sequence of Unicode characters
# Initialize <var>i</var> to 0
# Initialize <var>S</var> to be an empty sequence of [http://dev.w3.org/2006/webapi/WebIDL/#dfn-code-unit code units]
# While <var>i</var> < <var>n</var>
## Let <var>c</var> be the <b>code point</b> of the Unicode character in <var>U</var> at index <var>i</var>
## If <var>c</var> &ge; 2<sup>16</sup>, then:
### Append to <var>S</var> a [http://dev.w3.org/2006/webapi/WebIDL/#dfn-code-unit code unit] equal to (<var>c</var> - 2<sup>16</sup>) / 2<sup>10</sup> + 0xD800, where "/" represents integer division.
### Append to <var>S</var> a [http://dev.w3.org/2006/webapi/WebIDL/#dfn-code-unit code unit] equal to (<var>c</var> - 2<sup>16</sup>) % 2<sup>10</sup> + 0xDC00, where "%" represents the remainder of an integer division.
## Otherwise, append to <var>S</var> a [http://dev.w3.org/2006/webapi/WebIDL/#dfn-code-unit code unit] equal to <var>c</var>.
## Set <var>i</var> to <var>i</var>+1
# Return the IDL [http://dev.w3.org/2006/webapi/WebIDL/#idl-DOMString DOMString] value that represents sequence of code units <var>S</var>.


== Examples ==
== Examples ==
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== Encodings ==
== Encodings ==


Encodings are defined and implemented per [http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html Encoding]. This implicitly includes the steps to <em>get an encoding</em> from a string, and the logic for label matching and case-insensitivity.
Encodings are defined and implemented per [http://encoding.spec.whatwg.org/ Encoding]. This implicitly includes the steps to <em>get an encoding</em> from a string, and the logic for label matching and case-insensitivity.


User agents MUST NOT support any other encodings or labels than those defined in [http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html Encoding], and MUST support all encodings and labels defined in that specification, with the additions defined below.
User agents MUST NOT support any other encodings or labels than those defined in [http://encoding.spec.whatwg.org/ Encoding], and MUST support all encodings and labels defined in that specification, with the additions defined below.


:''NOTE: In [http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html Encoding], "ascii" is a label for '''windows-1252'''; there is no 7-bit-or-raise-exception encoding. Applications that are required to restrict the content of decoded strings should implement validation after decoding.''
:''NOTE: In [http://encoding.spec.whatwg.org/ Encoding], "ascii" is a label for '''windows-1252'''; there is no 7-bit-or-raise-exception encoding. Applications that are required to restrict the content of decoded strings should implement validation after decoding.''


:''NOTE: Unicode normalization forms are outside the scope of this specification. No normalization is done prior to encoding or after decoding.''
:''NOTE: Unicode normalization forms are outside the scope of this specification. No normalization is done prior to encoding or after decoding.''


:''NOTE: Handling of encoding-specific issues, e.g. over-long UTF-8 encodings, byte order marks, unmatched surrogate pairs, and so on is defined by [http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html Encoding].''
:''NOTE: Handling of encoding-specific issues, e.g. over-long UTF-8 encodings, byte order marks, unmatched surrogate pairs, and so on is defined by [http://encoding.spec.whatwg.org/ Encoding].''


== References ==
== References ==


* WebIDL http://dev.w3.org/2006/webapi/WebIDL
* WebIDL http://dev.w3.org/2006/webapi/WebIDL
* Encoding http://dvcs.w3.org/hg/encoding/raw-file/tip/Overview.html
* Encoding http://encoding.spec.whatwg.org/




Line 269: Line 279:
* Ryan Dahl
* Ryan Dahl
* Anne van Kesteren
* Anne van Kesteren
* Cameron McCormack


== Appendix ==
== Appendix ==


A "shim" implementation in JavaScript (that may not fully match the current version of the spec) plus some initial unit tests can be found at:
A "shim" implementation in ECMAScript (that may not fully match the current version of the spec) plus some initial unit tests can be found at:


:http://code.google.com/p/stringencoding/
:http://code.google.com/p/stringencoding/
[[Category:Proposals]]

Latest revision as of 15:16, 27 September 2014

This document is obsolete.

For the current specification, see: Encoding Standard: API


Proposed Text Encoding Web API for Typed Arrays

Editors

  • Joshua Bell (Google, Inc)

Abstract

This specification defines an API for encoding strings to binary data, and decoding strings from binary data.

NOTE: This specification intentionally does not address the opposite scenario of encoding binary data as strings and decoding binary data from strings, for example using Base64 encoding.

Discussion on this topic has so far taken place on the [email protected] mailing list. See http://www.khronos.org/webgl/public-mailing-list/archives/1111/msg00017.html for the initial discussion thread.

Discussion has since moved to the WHATWG spec discussion mailing list. See http://lists.whatwg.org/htdig.cgi/whatwg-whatwg.org/2012-March/035038.html for the latest discussion thread.

Open Issues

  • Should the encoding attribute return the Name of the encoding or the name that was passed in?

Notes to Implementers

  • Streaming decode/encode requires retaining partial buffers between calls.
    • Some encode/decode algorithms require adjusting the code point pointer or byte pointer by a negative amount. This could occur across "chunk" boundaries. This implies that when the internal streaming flag is set on an encoder/decoder that the last N elements of the stream are saved for the next call and used as a prefix for the stream. N is defined by the specific encoding algorithm.
    • This is not yet implemented in the ECMAScript shim

Resolved Issues

Encode as many characters as possible into a fixed-size buffer for transmission, and repeat starting with next unencoded character
Resolution: not for "v1" - can be implemented using this API, and breaking the string down by "character" is unlikely to be as obvious as it sounds - surrogate pairs, combining sequences, etc
Support two versions of encode; one which takes target buffer, and one which creates/returns a right-sized buffer
Resolution: See above, and wait for developer feedback.
Allow arbitrary end byte sequences (e.g. 0xFF for UTF-8 strings)
Resolution: Add indexOf to ArrayBufferView
Remove binary encoding? (a proposed 8-bit-clean encoding for interop with legacy binary data stored in ECMAScript strings)
The only real use is with atob()/btoa(); a better API would be Base64 directly in/out of Typed Arrays. Consensus on WHATWG is to add better APIs e.g.
partial interface ArrayBufferView {
    DOMString toBase64();
};

partial interface ArrayBuffer {
    static ArrayBuffer fromBase64(DOMString string);
};
Should this be a standalone API (as written), or live on e.g. DataView, or on e.g. String?
There seems to be pretty strong consensus that streaming, stateful coding is high priority and that an object-oriented API is cleanest, and shoe-horning those onto existing objects would be messy.
Should legacy encodings be supported?
Resolution: Consensus on the WHATWG mailing list - support legacy encodings for decode, and only 'utf-8', 'utf-16' and 'utf-16be' for encode.
What do to on encoding errors? If non-UTF encodings are supported then we may want to allow substitution (e.g. ASCII '?') or a script callback (for arbitrary escaping).
Resolution: not for "v1" (see above)
How are byte order marks handled?
BOM is respected if and only if the requested coding is a case-insensitive match for "utf-16"

API

TextEncoder

WebIDL

dictionary TextEncodeOptions {
  boolean stream = false;
};

[Constructor(optional DOMString encoding)]
interface TextEncoder {
  readonly attribute DOMString encoding;
  Uint8Array encode(DOMString? string, optional TextEncodeOptions options);
};

The constructor runs the following steps:

  1. If the constructor is called with no arguments, let label be the "utf-8". Otherwise, let label be the value of the encoding argument.
  2. Run the steps to get an encoding from Encoding, with label as label.
    • If the steps result in failure, throw an "EncodingError" exception and terminate these steps.
    • Otherwise, if the Name of the returned encoding is not one of "utf-8", "utf-16", or "utf-16be" throw an "EncodingError" exception and terminate these steps.
    • Otherwise, set the encoder object's internal encoding property to the returned encoding.
  3. Initialize the internal streaming flag of the encoder object to false.
  4. Initialize the internal encoding algorithm state to the default values for the encoding encoding.
encoding of type DOMString, readonly
Returns the Name of the encoder object's encoding, per Encoding.
Note that this may differ from the name of the encoding specified during the call to the constructor. For example, if the constructor is called with encoding of "ascii" the encoding attribute of the encoder object would have the value "windows-1252" as "ascii" is a label for that encoding.
encode
The encode method runs these steps:
  1. If the internal streaming flag is not set, then reset the encoding algorithm state to the default values for encoding. Otherwise, the encoding algorithm state is re-used from the previous call to encode on this object.
  2. If the options parameter is specified and the stream option is true, then the internal streaming flag is set; otherwise the internal streaming flag is cleared.
  3. Run the steps of the encoding algorithm:
    • The input to the algorithm is a stream of code points. The stream is composed of the Unicode code points for the Unicode characters produced by following the steps to convert a DOMString to a sequence of Unicode characters in WebIDL with string as the input. If string is null, the stream of code points is empty.
    • If the options parameter not specified or the stream option is false, then after final code point is yielded by the stream then the EOF code point is yielded.
    • The output of the the algorithm is a sequence of emitted bytes.
  4. Returns a Unit8Array object wrapping an ArrayBuffer containing the sequence of emitted bytes by encoder algorithm.
NOTE: Because only UTF encodings are supported, and because of the use of the steps to convert a DOMString to a sequence of Unicode characters and thus Unicode code points, no input can cause the encoding process to emit an encoder error.

TextDecoder

WebIDL

dictionary TextDecoderOptions {
  boolean fatal = false;
};

dictionary TextDecodeOptions {
  boolean stream = false;
};

[Constructor(optional DOMString encoding, optional TextDecoderOptions options)]
interface TextDecoder {
  readonly attribute DOMString encoding;
  DOMString decode(optional ArrayBufferView view, optional TextDecodeOptions options);
};

The constructor runs the following steps:

  1. The constructor creates a decoder object. It has the internal properties encoding, a fatal flag which is initially unset, a streaming flag which is initially unset, an offset pointer which is initially 0, and a useBOM flag which is initially unset.
  2. If called without an encoding argument, let label be the "utf-8". Otherwise, let label be the value of the encoding argument.
  3. If label is a case-insensitive match for "utf-16" then set the internal useBOM flag.
  4. Run the steps to get an encoding from Encoding, with label as label.
    1. If the steps result in failure, throw a "EncodingError" exception and terminate these steps.
    2. Otherwise, set the decoder object's internal encoding property to the returned encoding.
  5. If the constructor is called with an options argument, and the fatal property of the dictionary is set, set the internal fatal flag of the decoder object.
  6. Initialize the internal encoding algorithm state to the default values for the encoding encoding.
  7. Return the decoder object.
encoding of type DOMString, readonly
Returns the Name of the decoder object's encoding, per Encoding.
Note that this may differ from the name of the encoding specified during the call to the constructor. For example, if the constructor is called with encoding of "ascii" the encoding attribute of the decoder object would have the value "windows-1252" as "ascii" is a label for that encoding.
decode
The decode method runs the following steps:
  1. If view is not specified, let view be a Uint8Array of length 0.
  2. If the internal streaming flag of the decoder object is not set, then reset the encoding algorithm state to the default values for encoding encoding and set offset to 0. Otherwise, the encoding algorithm state is re-used from the previous call to decode on this object.
  3. If the options parameter is specified and the stream option is true, then the internal streaming flag is set. Otherwise the internal streaming flag is cleared.
  4. Run or resume the decoder algorithm of the decoder object's encoder, with the following additions:
    • The input to the algorithm is a byte stream.
      • If offset is greater than 0, the bytes in byte stream for positions less than offset are provided by buffer(s) passed in to previous calls to decode().
      • The bytes in byte stream for positions offset through offset + view.byteLength - 1 are provided by the bytes in view.buffer starting at offset view.byteOffset.
      • When accessing the byte in byte stream at position offset + view.byteLength:
        • If the internal streaming flag is not set, then yield the EOF byte.
        • Otherwise, set offset to offset + view.byteLength and suspend the steps of the decoder algorithm until a subsequent call to decode()
    • If encoding is one of "utf-8", "utf-16" or "utf-16be" and offset is 0, then prior to running the steps of the decoder algorithm:
      • If the internal useBOM flag is set, then:
        • If less than two bytes are present in the stream and the internal streaming flag is true, suspend this algorithm and return the empty string.
        • If the next two bytes of the stream are 0xFF 0xFE then set offset to 2, and clear the utf-16be flag of the decoder state.
        • If the next two bytes of the stream are 0xFE 0xFF then set offset to 2, and set the utf-16be flag of the decoder state.
      • If encoding is "utf-8", then:
        • If less than three bytes are present in the stream and the internal streaming flag is true, suspend this algorithm and return the empty string.
        • If the next three bytes of the stream are 0xEF 0xBB 0xBF then set offset to 3.
      • If encoding is "utf-16", then:
        • If less than two bytes are present in the stream and the internal streaming flag is true, suspend this algorithm and return the empty string.
        • If the next two bytes of the stream are 0xFF 0xFE then set offset to 2.
      • If encoding is "utf-16be", then:
        • If less than two bytes are present in the stream and the internal streaming flag is true, suspend this algorithm and return the empty string.
        • If the next two bytes of the stream are 0xFE 0xFF then set offset to 2.
    • If the internal fatal flag of the decoder object is set, then a decoder error causes an DOMException of type EncodingError to be thrown rather than emitting a fallback code point.
    • The output of the algorithm is a sequence of emitted code points.
  5. Return an IDL DOMString value that represents the sequence of code units resulting from encoding the sequence of emitted code points as UTF-16.

Examples

Example #1 - encoding strings

The following example uses the API to encode an array of strings into a ArrayBuffer. The result is a Uint8Array containing the number of strings (as a Uint32), followed by the length of the first string (as a Uint32), the UTF-8 encoded string data, the length of the second string (as a Uint32), the string data, and so on.

function encodeArrayOfStrings(strings, encoding) {
  var encoder, encoded, len, i, bytes, view, offset;

  encoder = TextEncoder(encoding);
  encoded = [];

  len = Uint32Array.BYTES_PER_ELEMENT;
  for (i = 0; i < strings.length; i += 1) {
    len += Uint32Array.BYTES_PER_ELEMENT;
    encoded[i] = TextEncoder(encoding).encode(strings[i]);
    len += encoded[i].byteLength;
  }

  bytes = new Uint8Array(len);
  view = new DataView(bytes.buffer);
  offset = 0;

  view.setUint32(offset, strings.length);
  offset += Uint32Array.BYTES_PER_ELEMENT;
  for (i = 0; i < encoded.length; i += 1) {
    len = encoded[i].byteLength;
    view.setUint32(offset, len);
    offset += Uint32Array.BYTES_PER_ELEMENT;
    bytes.set(encoded[i], offset);
    offset += len;
  }
  return bytes.buffer;
}

Example #2 - decoding strings

The following example decodes an ArrayBuffer containing data encoded in the format produced by the previous example back into an array of strings.

function decodeArrayOfStrings(buffer, encoding) {
  var decoder, view, offset, num_strings, strings, i, len;

  decoder = TextDecoder(encoding);
  view = new DataView(buffer);
  offset = 0;
  strings = [];

  num_strings = view.getUint32(offset);
  offset += Uint32Array.BYTES_PER_ELEMENT;
  for (i = 0; i < num_strings; i += 1) {
    len = view.getUint32(offset);
    offset += Uint32Array.BYTES_PER_ELEMENT;
    strings[i] = decoder.decode(
      new DataView(view.buffer, offset, len));
    offset += len;
  }
  return strings;
}

Encodings

Encodings are defined and implemented per Encoding. This implicitly includes the steps to get an encoding from a string, and the logic for label matching and case-insensitivity.

User agents MUST NOT support any other encodings or labels than those defined in Encoding, and MUST support all encodings and labels defined in that specification, with the additions defined below.

NOTE: In Encoding, "ascii" is a label for windows-1252; there is no 7-bit-or-raise-exception encoding. Applications that are required to restrict the content of decoded strings should implement validation after decoding.
NOTE: Unicode normalization forms are outside the scope of this specification. No normalization is done prior to encoding or after decoding.
NOTE: Handling of encoding-specific issues, e.g. over-long UTF-8 encodings, byte order marks, unmatched surrogate pairs, and so on is defined by Encoding.

References


Acknowledgements

  • Alan Chaney
  • Ben Noordhuis
  • Glenn Maynard
  • John Tamplin
  • Kenneth Russell (Google, Inc)
  • Robert Mustacchi
  • Ryan Dahl
  • Anne van Kesteren
  • Cameron McCormack

Appendix

A "shim" implementation in ECMAScript (that may not fully match the current version of the spec) plus some initial unit tests can be found at:

http://code.google.com/p/stringencoding/