gif-encoder-2/src/LZWEncoder.js

GIF generator for Node.js

/*
  LZWEncoder.js

  Authors
  Kevin Weiner (original Java version - kweiner@fmsware.com)
  Thibault Imbert (AS3 version - bytearray.org)
  Johan Nordberg (JS version - code@johan-nordberg.com)

  Acknowledgements
  GIFCOMPR.C - GIF Image compression routines
  Lempel-Ziv compression based on 'compress'. GIF modifications by
  David Rowley (mgardi@watdcsu.waterloo.edu)
  GIF Image compression - modified 'compress'
  Based on: compress.c - File compression ala IEEE Computer, June 1984.
  By Authors: Spencer W. Thomas (decvax!harpo!utah-cs!utah-gr!thomas)
  Jim McKie (decvax!mcvax!jim)
  Steve Davies (decvax!vax135!petsd!peora!srd)
  Ken Turkowski (decvax!decwrl!turtlevax!ken)
  James A. Woods (decvax!ihnp4!ames!jaw)
  Joe Orost (decvax!vax135!petsd!joe)
*/

var EOF = -1
var BITS = 12
var HSIZE = 5003 // 80% occupancy
var masks = [
  0x0000,
  0x0001,
  0x0003,
  0x0007,
  0x000f,
  0x001f,
  0x003f,
  0x007f,
  0x00ff,
  0x01ff,
  0x03ff,
  0x07ff,
  0x0fff,
  0x1fff,
  0x3fff,
  0x7fff,
  0xffff
]

function LZWEncoder(width, height, pixels, colorDepth) {
  var initCodeSize = Math.max(2, colorDepth)

  var accum = new Uint8Array(256)
  var htab = new Int32Array(HSIZE)
  var codetab = new Int32Array(HSIZE)

  var cur_accum,
    cur_bits = 0
  var a_count
  var free_ent = 0 // first unused entry
  var maxcode

  // block compression parameters -- after all codes are used up,
  // and compression rate changes, start over.
  var clear_flg = false

  // Algorithm: use open addressing double hashing (no chaining) on the
  // prefix code / next character combination. We do a variant of Knuth's
  // algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
  // secondary probe. Here, the modular division first probe is gives way
  // to a faster exclusive-or manipulation. Also do block compression with
  // an adaptive reset, whereby the code table is cleared when the compression
  // ratio decreases, but after the table fills. The variable-length output
  // codes are re-sized at this point, and a special CLEAR code is generated
  // for the decompressor. Late addition: construct the table according to
  // file size for noticeable speed improvement on small files. Please direct
  // questions about this implementation to ames!jaw.
  var g_init_bits, ClearCode, EOFCode

  // Add a character to the end of the current packet, and if it is 254
  // characters, flush the packet to disk.
  function char_out(c, outs) {
    accum[a_count++] = c
    if (a_count >= 254) flush_char(outs)
  }

  // Clear out the hash table
  // table clear for block compress
  function cl_block(outs) {
    cl_hash(HSIZE)
    free_ent = ClearCode + 2
    clear_flg = true
    output(ClearCode, outs)
  }

  // Reset code table
  function cl_hash(hsize) {
    for (var i = 0; i < hsize; ++i) htab[i] = -1
  }

  function compress(init_bits, outs) {
    var fcode, c, i, ent, disp, hsize_reg, hshift

    // Set up the globals: g_init_bits - initial number of bits
    g_init_bits = init_bits

    // Set up the necessary values
    clear_flg = false
    n_bits = g_init_bits
    maxcode = MAXCODE(n_bits)

    ClearCode = 1 << (init_bits - 1)
    EOFCode = ClearCode + 1
    free_ent = ClearCode + 2

    a_count = 0 // clear packet

    ent = nextPixel()

    hshift = 0
    for (fcode = HSIZE; fcode < 65536; fcode *= 2) ++hshift
    hshift = 8 - hshift // set hash code range bound
    hsize_reg = HSIZE
    cl_hash(hsize_reg) // clear hash table

    output(ClearCode, outs)

    outer_loop: while ((c = nextPixel()) != EOF) {
      fcode = (c << BITS) + ent
      i = (c << hshift) ^ ent // xor hashing
      if (htab[i] === fcode) {
        ent = codetab[i]
        continue
      } else if (htab[i] >= 0) {
        // non-empty slot
        disp = hsize_reg - i // secondary hash (after G. Knott)
        if (i === 0) disp = 1
        do {
          if ((i -= disp) < 0) i += hsize_reg
          if (htab[i] === fcode) {
            ent = codetab[i]
            continue outer_loop
          }
        } while (htab[i] >= 0)
      }
      output(ent, outs)
      ent = c
      if (free_ent < 1 << BITS) {
        codetab[i] = free_ent++ // code -> hashtable
        htab[i] = fcode
      } else {
        cl_block(outs)
      }
    }

    // Put out the final code.
    output(ent, outs)
    output(EOFCode, outs)
  }

  function encode(outs) {
    outs.writeByte(initCodeSize) // write "initial code size" byte
    remaining = width * height // reset navigation variables
    curPixel = 0
    compress(initCodeSize + 1, outs) // compress and write the pixel data
    outs.writeByte(0) // write block terminator
  }

  // Flush the packet to disk, and reset the accumulator
  function flush_char(outs) {
    if (a_count > 0) {
      outs.writeByte(a_count)
      outs.writeBytes(accum, 0, a_count)
      a_count = 0
    }
  }

  function MAXCODE(n_bits) {
    return (1 << n_bits) - 1
  }

  // Return the next pixel from the image
  function nextPixel() {
    if (remaining === 0) return EOF
    --remaining
    var pix = pixels[curPixel++]
    return pix & 0xff
  }

  function output(code, outs) {
    cur_accum &= masks[cur_bits]

    if (cur_bits > 0) cur_accum |= code << cur_bits
    else cur_accum = code

    cur_bits += n_bits

    while (cur_bits >= 8) {
      char_out(cur_accum & 0xff, outs)
      cur_accum >>= 8
      cur_bits -= 8
    }

    // If the next entry is going to be too big for the code size,
    // then increase it, if possible.
    if (free_ent > maxcode || clear_flg) {
      if (clear_flg) {
        maxcode = MAXCODE((n_bits = g_init_bits))
        clear_flg = false
      } else {
        ++n_bits
        if (n_bits == BITS) maxcode = 1 << BITS
        else maxcode = MAXCODE(n_bits)
      }
    }

    if (code == EOFCode) {
      // At EOF, write the rest of the buffer.
      while (cur_bits > 0) {
        char_out(cur_accum & 0xff, outs)
        cur_accum >>= 8
        cur_bits -= 8
      }
      flush_char(outs)
    }
  }

  this.encode = encode
}

module.exports = LZWEncoder