okhttp/src/main/java/com/squareup/okhttp/internal/spdy/Huffman.java

/*
 * Copyright 2013 Twitter, Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.squareup.okhttp.internal.spdy;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStream;

/**
 * This class was originally composed from the following classes in
 * <a href="https://github.com/twitter/hpack">Twitter Hpack</a>.
 * <ul>
 * <li>{@code com.twitter.hpack.HuffmanEncoder}</li>
 * <li>{@code com.twitter.hpack.HuffmanDecoder}</li>
 * <li>{@code com.twitter.hpack.HpackUtil}</li>
 * </ul>
 */
class Huffman {

  // Appendix C: Huffman Codes
  // http://tools.ietf.org/html/draft-ietf-httpbis-header-compression-12#appendix-B
  private static final int[] CODES = {
      0x1ff8, 0x7fffd8, 0xfffffe2, 0xfffffe3, 0xfffffe4, 0xfffffe5, 0xfffffe6, 0xfffffe7, 0xfffffe8,
      0xffffea, 0x3ffffffc, 0xfffffe9, 0xfffffea, 0x3ffffffd, 0xfffffeb, 0xfffffec, 0xfffffed,
      0xfffffee, 0xfffffef, 0xffffff0, 0xffffff1, 0xffffff2, 0x3ffffffe, 0xffffff3, 0xffffff4,
      0xffffff5, 0xffffff6, 0xffffff7, 0xffffff8, 0xffffff9, 0xffffffa, 0xffffffb, 0x14, 0x3f8,
      0x3f9, 0xffa, 0x1ff9, 0x15, 0xf8, 0x7fa, 0x3fa, 0x3fb, 0xf9, 0x7fb, 0xfa, 0x16, 0x17, 0x18,
      0x0, 0x1, 0x2, 0x19, 0x1a, 0x1b, 0x1c, 0x1d, 0x1e, 0x1f, 0x5c, 0xfb, 0x7ffc, 0x20, 0xffb,
      0x3fc, 0x1ffa, 0x21, 0x5d, 0x5e, 0x5f, 0x60, 0x61, 0x62, 0x63, 0x64, 0x65, 0x66, 0x67, 0x68,
      0x69, 0x6a, 0x6b, 0x6c, 0x6d, 0x6e, 0x6f, 0x70, 0x71, 0x72, 0xfc, 0x73, 0xfd, 0x1ffb, 0x7fff0,
      0x1ffc, 0x3ffc, 0x22, 0x7ffd, 0x3, 0x23, 0x4, 0x24, 0x5, 0x25, 0x26, 0x27, 0x6, 0x74, 0x75,
      0x28, 0x29, 0x2a, 0x7, 0x2b, 0x76, 0x2c, 0x8, 0x9, 0x2d, 0x77, 0x78, 0x79, 0x7a, 0x7b, 0x7ffe,
      0x7fc, 0x3ffd, 0x1ffd, 0xffffffc, 0xfffe6, 0x3fffd2, 0xfffe7, 0xfffe8, 0x3fffd3, 0x3fffd4,
      0x3fffd5, 0x7fffd9, 0x3fffd6, 0x7fffda, 0x7fffdb, 0x7fffdc, 0x7fffdd, 0x7fffde, 0xffffeb,
      0x7fffdf, 0xffffec, 0xffffed, 0x3fffd7, 0x7fffe0, 0xffffee, 0x7fffe1, 0x7fffe2, 0x7fffe3,
      0x7fffe4, 0x1fffdc, 0x3fffd8, 0x7fffe5, 0x3fffd9, 0x7fffe6, 0x7fffe7, 0xffffef, 0x3fffda,
      0x1fffdd, 0xfffe9, 0x3fffdb, 0x3fffdc, 0x7fffe8, 0x7fffe9, 0x1fffde, 0x7fffea, 0x3fffdd,
      0x3fffde, 0xfffff0, 0x1fffdf, 0x3fffdf, 0x7fffeb, 0x7fffec, 0x1fffe0, 0x1fffe1, 0x3fffe0,
      0x1fffe2, 0x7fffed, 0x3fffe1, 0x7fffee, 0x7fffef, 0xfffea, 0x3fffe2, 0x3fffe3, 0x3fffe4,
      0x7ffff0, 0x3fffe5, 0x3fffe6, 0x7ffff1, 0x3ffffe0, 0x3ffffe1, 0xfffeb, 0x7fff1, 0x3fffe7,
      0x7ffff2, 0x3fffe8, 0x1ffffec, 0x3ffffe2, 0x3ffffe3, 0x3ffffe4, 0x7ffffde, 0x7ffffdf,
      0x3ffffe5, 0xfffff1, 0x1ffffed, 0x7fff2, 0x1fffe3, 0x3ffffe6, 0x7ffffe0, 0x7ffffe1, 0x3ffffe7,
      0x7ffffe2, 0xfffff2, 0x1fffe4, 0x1fffe5, 0x3ffffe8, 0x3ffffe9, 0xffffffd, 0x7ffffe3,
      0x7ffffe4, 0x7ffffe5, 0xfffec, 0xfffff3, 0xfffed, 0x1fffe6, 0x3fffe9, 0x1fffe7, 0x1fffe8,
      0x7ffff3, 0x3fffea, 0x3fffeb, 0x1ffffee, 0x1ffffef, 0xfffff4, 0xfffff5, 0x3ffffea, 0x7ffff4,
      0x3ffffeb, 0x7ffffe6, 0x3ffffec, 0x3ffffed, 0x7ffffe7, 0x7ffffe8, 0x7ffffe9, 0x7ffffea,
      0x7ffffeb, 0xffffffe, 0x7ffffec, 0x7ffffed, 0x7ffffee, 0x7ffffef, 0x7fffff0, 0x3ffffee
  };

  private static final byte[] CODE_LENGTHS = {
      13, 23, 28, 28, 28, 28, 28, 28, 28, 24, 30, 28, 28, 30, 28, 28, 28, 28, 28, 28, 28, 28, 30,
      28, 28, 28, 28, 28, 28, 28, 28, 28, 6, 10, 10, 12, 13, 6, 8, 11, 10, 10, 8, 11, 8, 6, 6, 6, 5,
      5, 5, 6, 6, 6, 6, 6, 6, 6, 7, 8, 15, 6, 12, 10, 13, 6, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7, 7,
      7, 7, 7, 7, 7, 7, 7, 7, 7, 8, 7, 8, 13, 19, 13, 14, 6, 15, 5, 6, 5, 6, 5, 6, 6, 6, 5, 7, 7, 6,
      6, 6, 5, 6, 7, 6, 5, 5, 6, 7, 7, 7, 7, 7, 15, 11, 14, 13, 28, 20, 22, 20, 20, 22, 22, 22, 23,
      22, 23, 23, 23, 23, 23, 24, 23, 24, 24, 22, 23, 24, 23, 23, 23, 23, 21, 22, 23, 22, 23, 23,
      24, 22, 21, 20, 22, 22, 23, 23, 21, 23, 22, 22, 24, 21, 22, 23, 23, 21, 21, 22, 21, 23, 22,
      23, 23, 20, 22, 22, 22, 23, 22, 22, 23, 26, 26, 20, 19, 22, 23, 22, 25, 26, 26, 26, 27, 27,
      26, 24, 25, 19, 21, 26, 27, 27, 26, 27, 24, 21, 21, 26, 26, 28, 27, 27, 27, 20, 24, 20, 21,
      22, 21, 21, 23, 22, 22, 25, 25, 24, 24, 26, 23, 26, 27, 26, 26, 27, 27, 27, 27, 27, 28, 27,
      27, 27, 27, 27, 26
  };

  private static final Huffman INSTANCE = new Huffman();

  public static Huffman get() {
    return INSTANCE;
  }

  private final Node root = new Node();

  private Huffman() {
    buildTree();
  }

  void encode(byte[] data, OutputStream out) throws IOException {
    long current = 0;
    int n = 0;

    for (int i = 0; i < data.length; i++) {
      int b = data[i] & 0xFF;
      int code = CODES[b];
      int nbits = CODE_LENGTHS[b];

      current <<= nbits;
      current |= code;
      n += nbits;

      while (n >= 8) {
        n -= 8;
        out.write(((int) (current >> n)));
      }
    }

    if (n > 0) {
      current <<= (8 - n);
      current |= (0xFF >>> n);
      out.write((int) current);
    }
  }

  int encodedLength(byte[] bytes) {
    long len = 0;

    for (int i = 0; i < bytes.length; i++) {
      int b = bytes[i] & 0xFF;
      len += CODE_LENGTHS[b];
    }

    return (int) ((len + 7) >> 3);
  }

  byte[] decode(byte[] buf) throws IOException {
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    Node node = root;
    int current = 0;
    int nbits = 0;
    for (int i = 0; i < buf.length; i++) {
      int b = buf[i] & 0xFF;
      current = (current << 8) | b;
      nbits += 8;
      while (nbits >= 8) {
        int c = (current >>> (nbits - 8)) & 0xFF;
        node = node.children[c];
        if (node.children == null) {
          // terminal node
          baos.write(node.symbol);
          nbits -= node.terminalBits;
          node = root;
        } else {
          // non-terminal node
          nbits -= 8;
        }
      }
    }

    while (nbits > 0) {
      int c = (current << (8 - nbits)) & 0xFF;
      node = node.children[c];
      if (node.children != null || node.terminalBits > nbits) {
        break;
      }
      baos.write(node.symbol);
      nbits -= node.terminalBits;
      node = root;
    }

    return baos.toByteArray();
  }

  private void buildTree() {
    for (int i = 0; i < CODE_LENGTHS.length; i++) {
      addCode(i, CODES[i], CODE_LENGTHS[i]);
    }
  }

  private void addCode(int sym, int code, byte len) {
    Node terminal = new Node(sym, len);

    Node current = root;
    while (len > 8) {
      len -= 8;
      int i = ((code >>> len) & 0xFF);
      if (current.children == null) {
        throw new IllegalStateException("invalid dictionary: prefix not unique");
      }
      if (current.children[i] == null) {
        current.children[i] = new Node();
      }
      current = current.children[i];
    }

    int shift = 8 - len;
    int start = (code << shift) & 0xFF;
    int end = 1 << shift;
    for (int i = start; i < start + end; i++) {
      current.children[i] = terminal;
    }
  }

  private static final class Node {

    // Null if terminal.
    private final Node[] children;

    // Terminal nodes have a symbol.
    private final int symbol;

    // Number of bits represented in the terminal node.
    private final int terminalBits;

    /** Construct an internal node. */
    Node() {
      this.children = new Node[256];
      this.symbol = 0; // Not read.
      this.terminalBits = 0; // Not read.
    }

    /**
     * Construct a terminal node.
     *
     * @param symbol symbol the node represents
     * @param bits length of Huffman code in bits
     */
    Node(int symbol, int bits) {
      this.children = null;
      this.symbol = symbol;
      int b = bits & 0x07;
      this.terminalBits = b == 0 ? 8 : b;
    }
  }
}