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✏️ dbcs-codec.js

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"use strict";
var Buffer = require("safer-buffer").Buffer;

// Multibyte codec. In this scheme, a character is represented by 1 or more bytes.
// Our codec supports UTF-16 surrogates, extensions for GB18030 and unicode sequences.
// To save memory and loading time, we read table files only when requested.

exports._dbcs = DBCSCodec;

var UNASSIGNED = -1,
    GB18030_CODE = -2,
    SEQ_START  = -10,
    NODE_START = -1000,
    UNASSIGNED_NODE = new Array(0x100),
    DEF_CHAR = -1;

for (var i = 0; i < 0x100; i++)
    UNASSIGNED_NODE[i] = UNASSIGNED;

// Class DBCSCodec reads and initializes mapping tables.
function DBCSCodec(codecOptions, iconv) {
    this.encodingName = codecOptions.encodingName;
    if (!codecOptions)
        throw new Error("DBCS codec is called without the data.")
    if (!codecOptions.table)
        throw new Error("Encoding '" + this.encodingName + "' has no data.");

// Load tables.
    var mappingTable = codecOptions.table();

// Decode tables: MBCS -> Unicode.

// decodeTables is a trie, encoded as an array of arrays of integers. Internal arrays are trie nodes and all have len = 256.
    // Trie root is decodeTables[0].
    // Values: >=  0 -> unicode character code. can be > 0xFFFF
    //         == UNASSIGNED -> unknown/unassigned sequence.
    //         == GB18030_CODE -> this is the end of a GB18030 4-byte sequence.
    //         <= NODE_START -> index of the next node in our trie to process next byte.
    //         <= SEQ_START  -> index of the start of a character code sequence, in decodeTableSeq.
    this.decodeTables = [];
    this.decodeTables[0] = UNASSIGNED_NODE.slice(0); // Create root node.

// Sometimes a MBCS char corresponds to a sequence of unicode chars. We store them as arrays of integers here. 
    this.decodeTableSeq = [];

// Actual mapping tables consist of chunks. Use them to fill up decode tables.
    for (var i = 0; i < mappingTable.length; i++)
        this._addDecodeChunk(mappingTable[i]);

this.defaultCharUnicode = iconv.defaultCharUnicode;

// Encode tables: Unicode -> DBCS.

// `encodeTable` is array mapping from unicode char to encoded char. All its values are integers for performance.
    // Because it can be sparse, it is represented as array of buckets by 256 chars each. Bucket can be null.
    // Values: >=  0 -> it is a normal char. Write the value (if <=256 then 1 byte, if <=65536 then 2 bytes, etc.).
    //         == UNASSIGNED -> no conversion found. Output a default char.
    //         <= SEQ_START  -> it's an index in encodeTableSeq, see below. The character starts a sequence.
    this.encodeTable = [];
    
    // `encodeTableSeq` is used when a sequence of unicode characters is encoded as a single code. We use a tree of
    // objects where keys correspond to characters in sequence and leafs are the encoded dbcs values. A special DEF_CHAR key
    // means end of sequence (needed when one sequence is a strict subsequence of another).
    // Objects are kept separately from encodeTable to increase performance.
    this.encodeTableSeq = [];

// Some chars can be decoded, but need not be encoded.
    var skipEncodeChars = {};
    if (codecOptions.encodeSkipVals)
        for (var i = 0; i < codecOptions.encodeSkipVals.length; i++) {
            var val = codecOptions.encodeSkipVals[i];
            if (typeof val === 'number')
                skipEncodeChars[val] = true;
            else
                for (var j = val.from; j <= val.to; j++)
                    skipEncodeChars[j] = true;
        }
        
    // Use decode trie to recursively fill out encode tables.
    this._fillEncodeTable(0, 0, skipEncodeChars);

// Add more encoding pairs when needed.
    if (codecOptions.encodeAdd) {
        for (var uChar in codecOptions.encodeAdd)
            if (Object.prototype.hasOwnProperty.call(codecOptions.encodeAdd, uChar))
                this._setEncodeChar(uChar.charCodeAt(0), codecOptions.encodeAdd[uChar]);
    }

this.defCharSB  = this.encodeTable[0][iconv.defaultCharSingleByte.charCodeAt(0)];
    if (this.defCharSB === UNASSIGNED) this.defCharSB = this.encodeTable[0]['?'];
    if (this.defCharSB === UNASSIGNED) this.defCharSB = "?".charCodeAt(0);

// Load & create GB18030 tables when needed.
    if (typeof codecOptions.gb18030 === 'function') {
        this.gb18030 = codecOptions.gb18030(); // Load GB18030 ranges.

// Add GB18030 decode tables.
        var thirdByteNodeIdx = this.decodeTables.length;
        var thirdByteNode = this.decodeTables[thirdByteNodeIdx] = UNASSIGNED_NODE.slice(0);

var fourthByteNodeIdx = this.decodeTables.length;
        var fourthByteNode = this.decodeTables[fourthByteNodeIdx] = UNASSIGNED_NODE.slice(0);

for (var i = 0x81; i <= 0xFE; i++) {
            var secondByteNodeIdx = NODE_START - this.decodeTables[0][i];
            var secondByteNode = this.decodeTables[secondByteNodeIdx];
            for (var j = 0x30; j <= 0x39; j++)
                secondByteNode[j] = NODE_START - thirdByteNodeIdx;
        }
        for (var i = 0x81; i <= 0xFE; i++)
            thirdByteNode[i] = NODE_START - fourthByteNodeIdx;
        for (var i = 0x30; i <= 0x39; i++)
            fourthByteNode[i] = GB18030_CODE
    }        
}

DBCSCodec.prototype.encoder = DBCSEncoder;
DBCSCodec.prototype.decoder = DBCSDecoder;

// Decoder helpers
DBCSCodec.prototype._getDecodeTrieNode = function(addr) {
    var bytes = [];
    for (; addr > 0; addr >>= 8)
        bytes.push(addr & 0xFF);
    if (bytes.length == 0)
        bytes.push(0);

var node = this.decodeTables[0];
    for (var i = bytes.length-1; i > 0; i--) { // Traverse nodes deeper into the trie.
        var val = node[bytes[i]];

if (val == UNASSIGNED) { // Create new node.
            node[bytes[i]] = NODE_START - this.decodeTables.length;
            this.decodeTables.push(node = UNASSIGNED_NODE.slice(0));
        }
        else if (val <= NODE_START) { // Existing node.
            node = this.decodeTables[NODE_START - val];
        }
        else
            throw new Error("Overwrite byte in " + this.encodingName + ", addr: " + addr.toString(16));
    }
    return node;
}

DBCSCodec.prototype._addDecodeChunk = function(chunk) {
    // First element of chunk is the hex mbcs code where we start.
    var curAddr = parseInt(chunk[0], 16);

// Choose the decoding node where we'll write our chars.
    var writeTable = this._getDecodeTrieNode(curAddr);
    curAddr = curAddr & 0xFF;

// Write all other elements of the chunk to the table.
    for (var k = 1; k < chunk.length; k++) {
        var part = chunk[k];
        if (typeof part === "string") { // String, write as-is.
            for (var l = 0; l < part.length;) {
                var code = part.charCodeAt(l++);
                if (0xD800 <= code && code < 0xDC00) { // Decode surrogate
                    var codeTrail = part.charCodeAt(l++);
                    if (0xDC00 <= codeTrail && codeTrail < 0xE000)
                        writeTable[curAddr++] = 0x10000 + (code - 0xD800) * 0x400 + (codeTrail - 0xDC00);
                    else
                        throw new Error("Incorrect surrogate pair in "  + this.encodingName + " at chunk " + chunk[0]);
                }
                else if (0x0FF0 < code && code <= 0x0FFF) { // Character sequence (our own encoding used)
                    var len = 0xFFF - code + 2;
                    var seq = [];
                    for (var m = 0; m < len; m++)
                        seq.push(part.charCodeAt(l++)); // Simple variation: don't support surrogates or subsequences in seq.

writeTable[curAddr++] = SEQ_START - this.decodeTableSeq.length;
                    this.decodeTableSeq.push(seq);
                }
                else
                    writeTable[curAddr++] = code; // Basic char
            }
        } 
        else if (typeof part === "number") { // Integer, meaning increasing sequence starting with prev character.
            var charCode = writeTable[curAddr - 1] + 1;
            for (var l = 0; l < part; l++)
                writeTable[curAddr++] = charCode++;
        }
        else
            throw new Error("Incorrect type '" + typeof part + "' given in "  + this.encodingName + " at chunk " + chunk[0]);
    }
    if (curAddr > 0xFF)
        throw new Error("Incorrect chunk in "  + this.encodingName + " at addr " + chunk[0] + ": too long" + curAddr);
}

// Encoder helpers
DBCSCodec.prototype._getEncodeBucket = function(uCode) {
    var high = uCode >> 8; // This could be > 0xFF because of astral characters.
    if (this.encodeTable[high] === undefined)
        this.encodeTable[high] = UNASSIGNED_NODE.slice(0); // Create bucket on demand.
    return this.encodeTable[high];
}

DBCSCodec.prototype._setEncodeChar = function(uCode, dbcsCode) {
    var bucket = this._getEncodeBucket(uCode);
    var low = uCode & 0xFF;
    if (bucket[low] <= SEQ_START)
        this.encodeTableSeq[SEQ_START-bucket[low]][DEF_CHAR] = dbcsCode; // There's already a sequence, set a single-char subsequence of it.
    else if (bucket[low] == UNASSIGNED)
        bucket[low] = dbcsCode;
}

DBCSCodec.prototype._setEncodeSequence = function(seq, dbcsCode) {
    
    // Get the root of character tree according to first character of the sequence.
    var uCode = seq[0];
    var bucket = this._getEncodeBucket(uCode);
    var low = uCode & 0xFF;

var node;
    if (bucket[low] <= SEQ_START) {
        // There's already a sequence with  - use it.
        node = this.encodeTableSeq[SEQ_START-bucket[low]];
    }
    else {
        // There was no sequence object - allocate a new one.
        node = {};
        if (bucket[low] !== UNASSIGNED) node[DEF_CHAR] = bucket[low]; // If a char was set before - make it a single-char subsequence.
        bucket[low] = SEQ_START - this.encodeTableSeq.length;
        this.encodeTableSeq.push(node);
    }

// Traverse the character tree, allocating new nodes as needed.
    for (var j = 1; j < seq.length-1; j++) {
        var oldVal = node[uCode];
        if (typeof oldVal === 'object')
            node = oldVal;
        else {
            node = node[uCode] = {}
            if (oldVal !== undefined)
                node[DEF_CHAR] = oldVal
        }
    }

// Set the leaf to given dbcsCode.
    uCode = seq[seq.length-1];
    node[uCode] = dbcsCode;
}

DBCSCodec.prototype._fillEncodeTable = function(nodeIdx, prefix, skipEncodeChars) {
    var node = this.decodeTables[nodeIdx];
    for (var i = 0; i < 0x100; i++) {
        var uCode = node[i];
        var mbCode = prefix + i;
        if (skipEncodeChars[mbCode])
            continue;

if (uCode >= 0)
            this._setEncodeChar(uCode, mbCode);
        else if (uCode <= NODE_START)
            this._fillEncodeTable(NODE_START - uCode, mbCode << 8, skipEncodeChars);
        else if (uCode <= SEQ_START)
            this._setEncodeSequence(this.decodeTableSeq[SEQ_START - uCode], mbCode);
    }
}

// == Encoder ==================================================================

function DBCSEncoder(options, codec) {
    // Encoder state
    this.leadSurrogate = -1;
    this.seqObj = undefined;
    
    // Static data
    this.encodeTable = codec.encodeTable;
    this.encodeTableSeq = codec.encodeTableSeq;
    this.defaultCharSingleByte = codec.defCharSB;
    this.gb18030 = codec.gb18030;
}

DBCSEncoder.prototype.write = function(str) {
    var newBuf = Buffer.alloc(str.length * (this.gb18030 ? 4 : 3)),
        leadSurrogate = this.leadSurrogate,
        seqObj = this.seqObj, nextChar = -1,
        i = 0, j = 0;

while (true) {
        // 0. Get next character.
        if (nextChar === -1) {
            if (i == str.length) break;
            var uCode = str.charCodeAt(i++);
        }
        else {
            var uCode = nextChar;
            nextChar = -1;    
        }

// 1. Handle surrogates.
        if (0xD800 <= uCode && uCode < 0xE000) { // Char is one of surrogates.
            if (uCode < 0xDC00) { // We've got lead surrogate.
                if (leadSurrogate === -1) {
                    leadSurrogate = uCode;
                    continue;
                } else {
                    leadSurrogate = uCode;
                    // Double lead surrogate found.
                    uCode = UNASSIGNED;
                }
            } else { // We've got trail surrogate.
                if (leadSurrogate !== -1) {
                    uCode = 0x10000 + (leadSurrogate - 0xD800) * 0x400 + (uCode - 0xDC00);
                    leadSurrogate = -1;
                } else {
                    // Incomplete surrogate pair - only trail surrogate found.
                    uCode = UNASSIGNED;
                }
                
            }
        }
        else if (leadSurrogate !== -1) {
            // Incomplete surrogate pair - only lead surrogate found.
            nextChar = uCode; uCode = UNASSIGNED; // Write an error, then current char.
            leadSurrogate = -1;
        }

// 2. Convert uCode character.
        var dbcsCode = UNASSIGNED;
        if (seqObj !== undefined && uCode != UNASSIGNED) { // We are in the middle of the sequence
            var resCode = seqObj[uCode];
            if (typeof resCode === 'object') { // Sequence continues.
                seqObj = resCode;
                continue;

} else if (typeof resCode == 'number') { // Sequence finished. Write it.
                dbcsCode = resCode;

} else if (resCode == undefined) { // Current character is not part of the sequence.

// Try default character for this sequence
                resCode = seqObj[DEF_CHAR];
                if (resCode !== undefined) {
                    dbcsCode = resCode; // Found. Write it.
                    nextChar = uCode; // Current character will be written too in the next iteration.

} else {
                    // TODO: What if we have no default? (resCode == undefined)
                    // Then, we should write first char of the sequence as-is and try the rest recursively.
                    // Didn't do it for now because no encoding has this situation yet.
                    // Currently, just skip the sequence and write current char.
                }
            }
            seqObj = undefined;
        }
        else if (uCode >= 0) {  // Regular character
            var subtable = this.encodeTable[uCode >> 8];
            if (subtable !== undefined)
                dbcsCode = subtable[uCode & 0xFF];
            
            if (dbcsCode <= SEQ_START) { // Sequence start
                seqObj = this.encodeTableSeq[SEQ_START-dbcsCode];
                continue;
            }

if (dbcsCode == UNASSIGNED && this.gb18030) {
                // Use GB18030 algorithm to find character(s) to write.
                var idx = findIdx(this.gb18030.uChars, uCode);
                if (idx != -1) {
                    var dbcsCode = this.gb18030.gbChars[idx] + (uCode - this.gb18030.uChars[idx]);
                    newBuf[j++] = 0x81 + Math.floor(dbcsCode / 12600); dbcsCode = dbcsCode % 12600;
                    newBuf[j++] = 0x30 + Math.floor(dbcsCode / 1260); dbcsCode = dbcsCode % 1260;
                    newBuf[j++] = 0x81 + Math.floor(dbcsCode / 10); dbcsCode = dbcsCode % 10;
                    newBuf[j++] = 0x30 + dbcsCode;
                    continue;
                }
            }
        }

// 3. Write dbcsCode character.
        if (dbcsCode === UNASSIGNED)
            dbcsCode = this.defaultCharSingleByte;
        
        if (dbcsCode < 0x100) {
            newBuf[j++] = dbcsCode;
        }
        else if (dbcsCode < 0x10000) {
            newBuf[j++] = dbcsCode >> 8;   // high byte
            newBuf[j++] = dbcsCode & 0xFF; // low byte
        }
        else {
            newBuf[j++] = dbcsCode >> 16;
            newBuf[j++] = (dbcsCode >> 8) & 0xFF;
            newBuf[j++] = dbcsCode & 0xFF;
        }
    }

this.seqObj = seqObj;
    this.leadSurrogate = leadSurrogate;
    return newBuf.slice(0, j);
}

DBCSEncoder.prototype.end = function() {
    if (this.leadSurrogate === -1 && this.seqObj === undefined)
        return; // All clean. Most often case.

var newBuf = Buffer.alloc(10), j = 0;

if (this.seqObj) { // We're in the sequence.
        var dbcsCode = this.seqObj[DEF_CHAR];
        if (dbcsCode !== undefined) { // Write beginning of the sequence.
            if (dbcsCode < 0x100) {
                newBuf[j++] = dbcsCode;
            }
            else {
                newBuf[j++] = dbcsCode >> 8;   // high byte
                newBuf[j++] = dbcsCode & 0xFF; // low byte
            }
        } else {
            // See todo above.
        }
        this.seqObj = undefined;
    }

if (this.leadSurrogate !== -1) {
        // Incomplete surrogate pair - only lead surrogate found.
        newBuf[j++] = this.defaultCharSingleByte;
        this.leadSurrogate = -1;
    }
    
    return newBuf.slice(0, j);
}

// Export for testing
DBCSEncoder.prototype.findIdx = findIdx;

// == Decoder ==================================================================

function DBCSDecoder(options, codec) {
    // Decoder state
    this.nodeIdx = 0;
    this.prevBuf = Buffer.alloc(0);

// Static data
    this.decodeTables = codec.decodeTables;
    this.decodeTableSeq = codec.decodeTableSeq;
    this.defaultCharUnicode = codec.defaultCharUnicode;
    this.gb18030 = codec.gb18030;
}

DBCSDecoder.prototype.write = function(buf) {
    var newBuf = Buffer.alloc(buf.length*2),
        nodeIdx = this.nodeIdx, 
        prevBuf = this.prevBuf, prevBufOffset = this.prevBuf.length,
        seqStart = -this.prevBuf.length, // idx of the start of current parsed sequence.
        uCode;

if (prevBufOffset > 0) // Make prev buf overlap a little to make it easier to slice later.
        prevBuf = Buffer.concat([prevBuf, buf.slice(0, 10)]);
    
    for (var i = 0, j = 0; i < buf.length; i++) {
        var curByte = (i >= 0) ? buf[i] : prevBuf[i + prevBufOffset];

// Lookup in current trie node.
        var uCode = this.decodeTables[nodeIdx][curByte];

if (uCode >= 0) { 
            // Normal character, just use it.
        }
        else if (uCode === UNASSIGNED) { // Unknown char.
            // TODO: Callback with seq.
            //var curSeq = (seqStart >= 0) ? buf.slice(seqStart, i+1) : prevBuf.slice(seqStart + prevBufOffset, i+1 + prevBufOffset);
            i = seqStart; // Try to parse again, after skipping first byte of the sequence ('i' will be incremented by 'for' cycle).
            uCode = this.defaultCharUnicode.charCodeAt(0);
        }
        else if (uCode === GB18030_CODE) {
            var curSeq = (seqStart >= 0) ? buf.slice(seqStart, i+1) : prevBuf.slice(seqStart + prevBufOffset, i+1 + prevBufOffset);
            var ptr = (curSeq[0]-0x81)*12600 + (curSeq[1]-0x30)*1260 + (curSeq[2]-0x81)*10 + (curSeq[3]-0x30);
            var idx = findIdx(this.gb18030.gbChars, ptr);
            uCode = this.gb18030.uChars[idx] + ptr - this.gb18030.gbChars[idx];
        }
        else if (uCode <= NODE_START) { // Go to next trie node.
            nodeIdx = NODE_START - uCode;
            continue;
        }
        else if (uCode <= SEQ_START) { // Output a sequence of chars.
            var seq = this.decodeTableSeq[SEQ_START - uCode];
            for (var k = 0; k < seq.length - 1; k++) {
                uCode = seq[k];
                newBuf[j++] = uCode & 0xFF;
                newBuf[j++] = uCode >> 8;
            }
            uCode = seq[seq.length-1];
        }
        else
            throw new Error("iconv-lite internal error: invalid decoding table value " + uCode + " at " + nodeIdx + "/" + curByte);

// Write the character to buffer, handling higher planes using surrogate pair.
        if (uCode > 0xFFFF) { 
            uCode -= 0x10000;
            var uCodeLead = 0xD800 + Math.floor(uCode / 0x400);
            newBuf[j++] = uCodeLead & 0xFF;
            newBuf[j++] = uCodeLead >> 8;

uCode = 0xDC00 + uCode % 0x400;
        }
        newBuf[j++] = uCode & 0xFF;
        newBuf[j++] = uCode >> 8;

// Reset trie node.
        nodeIdx = 0; seqStart = i+1;
    }

this.nodeIdx = nodeIdx;
    this.prevBuf = (seqStart >= 0) ? buf.slice(seqStart) : prevBuf.slice(seqStart + prevBufOffset);
    return newBuf.slice(0, j).toString('ucs2');
}

DBCSDecoder.prototype.end = function() {
    var ret = '';

// Try to parse all remaining chars.
    while (this.prevBuf.length > 0) {
        // Skip 1 character in the buffer.
        ret += this.defaultCharUnicode;
        var buf = this.prevBuf.slice(1);

// Parse remaining as usual.
        this.prevBuf = Buffer.alloc(0);
        this.nodeIdx = 0;
        if (buf.length > 0)
            ret += this.write(buf);
    }

this.nodeIdx = 0;
    return ret;
}

// Binary search for GB18030. Returns largest i such that table[i] <= val.
function findIdx(table, val) {
    if (table[0] > val)
        return -1;

var l = 0, r = table.length;
    while (l < r-1) { // always table[l] <= val < table[r]
        var mid = l + Math.floor((r-l+1)/2);
        if (table[mid] <= val)
            l = mid;
        else
            r = mid;
    }
    return l;
}

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✏️ dbcs-codec.js