/* Copyright 2015 Google Inc. All Rights Reserved.
   
      Distributed under MIT license.
      See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
   */
   
   package org.htmlunit.util.brotli;
   
   /**
   * Utilities for building Huffman decoding tables.
   */
  final class Huffman {
  
    private static final int MAX_LENGTH = 15;
  
    /**
     * Returns reverse(reverse(key, len) + 1, len).
     *
     * <p> reverse(key, len) is the bit-wise reversal of the len least significant bits of key.
     */
    private static int getNextKey(int key, int len) {
      int step = 1 << (len - 1);
      while ((key & step) != 0) {
        step = step >> 1;
      }
      return (key & (step - 1)) + step;
    }
  
    /**
     * Stores {@code item} in {@code table[0], table[step], table[2 * step] .., table[end]}.
     *
     * <p> Assumes that end is an integer multiple of step.
     */
    private static void replicateValue(int[] table, int offset, int step, int end, int item) {
      int pos = end;
      while (pos > 0) {
        pos -= step;
        table[offset + pos] = item;
      }
    }
  
    /**
     * @param count histogram of bit lengths for the remaining symbols,
     * @param len code length of the next processed symbol.
     * @return table width of the next 2nd level table.
     */
    private static int nextTableBitSize(int[] count, int len, int rootBits) {
      int bits = len;
      int left = 1 << (bits - rootBits);
      while (bits < MAX_LENGTH) {
        left -= count[bits];
        if (left <= 0) {
          break;
        }
        bits++;
        left = left << 1;
      }
      return bits - rootBits;
    }
  
    /**
     * Builds Huffman lookup table assuming code lengths are in symbol order.
     *
     * @return number of slots used by resulting Huffman table
     */
    static int buildHuffmanTable(int[] tableGroup, int tableIdx, int rootBits, int[] codeLengths,
        int codeLengthsSize) {
      final int tableOffset = tableGroup[tableIdx];
      final int[] sorted = new int[codeLengthsSize]; // Symbols sorted by code length.
      // TODO(eustas): fill with zeroes?
      final int[] count = new int[MAX_LENGTH + 1]; // Number of codes of each length.
      final int[] offset = new int[MAX_LENGTH + 1]; // Offsets in sorted table for each length.
  
      // Build histogram of code lengths.
      for (int sym = 0; sym < codeLengthsSize; ++sym) {
        count[codeLengths[sym]]++;
      }
  
      // Generate offsets into sorted symbol table by code length.
      offset[1] = 0;
      for (int len = 1; len < MAX_LENGTH; ++len) {
        offset[len + 1] = offset[len] + count[len];
      }
  
      // Sort symbols by length, by symbol order within each length.
      for (int sym = 0; sym < codeLengthsSize; ++sym) {
        if (codeLengths[sym] != 0) {
          sorted[offset[codeLengths[sym]]++] = sym;
        }
      }
  
      int tableBits = rootBits;
      int tableSize = 1 << tableBits;
      int totalSize = tableSize;
  
      // Special case code with only one value.
      if (offset[MAX_LENGTH] == 1) {
        for (int k = 0; k < totalSize; ++k) {
          tableGroup[tableOffset + k] = sorted[0];
       }
       return totalSize;
     }
 
     // Fill in root table.
     int key = 0;  // Reversed prefix code.
     int symbol = 0;
     int step = 1;
     for (int len = 1; len <= rootBits; ++len) {
       step = step << 1;
       while (count[len] > 0) {
         replicateValue(tableGroup, tableOffset + key, step, tableSize,
             len << 16 | sorted[symbol++]);
         key = getNextKey(key, len);
         count[len]--;
       }
     }
 
     // Fill in 2nd level tables and add pointers to root table.
     final int mask = totalSize - 1;
     int low = -1;
     int currentOffset = tableOffset;
     step = 1;
     for (int len = rootBits + 1; len <= MAX_LENGTH; ++len) {
       step = step << 1;
       while (count[len] > 0) {
         if ((key & mask) != low) {
           currentOffset += tableSize;
           tableBits = nextTableBitSize(count, len, rootBits);
           tableSize = 1 << tableBits;
           totalSize += tableSize;
           low = key & mask;
           tableGroup[tableOffset + low] =
               (tableBits + rootBits) << 16 | (currentOffset - tableOffset - low);
         }
         replicateValue(tableGroup, currentOffset + (key >> rootBits), step, tableSize,
             (len - rootBits) << 16 | sorted[symbol++]);
         key = getNextKey(key, len);
         count[len]--;
       }
     }
     return totalSize;
   }
 }