/* Copyright 2015 Google Inc. All Rights Reserved.

   https://github.com/google/brotli/blob/master/LICENSE

   Distributed under MIT license.
   See file LICENSE for detail or copy at https://opensource.org/licenses/MIT
*/

package net.sourceforge.plantuml.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 >>= 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) {
		do {
			end -= step;
			table[offset + end] = item;
		} while (end > 0);
	}

	/**
	 * @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 left = 1 << (len - rootBits);
		while (len < MAX_LENGTH) {
			left -= count[len];
			if (left <= 0) {
				break;
			}
			len++;
			left <<= 1;
		}
		return len - rootBits;
	}

	/**
	 * Builds Huffman lookup table assuming code lengths are in symbol order.
	 */
	static void buildHuffmanTable(int[] rootTable, int tableOffset, int rootBits, int[] codeLengths,
			int codeLengthsSize) {
		int key; // Reversed prefix code.
		int[] sorted = new int[codeLengthsSize]; // Symbols sorted by code length.
		// TODO: fill with zeroes?
		int[] count = new int[MAX_LENGTH + 1]; // Number of codes of each length.
		int[] offset = new int[MAX_LENGTH + 1]; // Offsets in sorted table for each length.
		int symbol;

		// Build histogram of code lengths.
		for (symbol = 0; symbol < codeLengthsSize; symbol++) {
			count[codeLengths[symbol]]++;
		}

		// 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 (symbol = 0; symbol < codeLengthsSize; symbol++) {
			if (codeLengths[symbol] != 0) {
				sorted[offset[codeLengths[symbol]]++] = symbol;
			}
		}

		int tableBits = rootBits;
		int tableSize = 1 << tableBits;
		int totalSize = tableSize;

		// Special case code with only one value.
		if (offset[MAX_LENGTH] == 1) {
			for (key = 0; key < totalSize; key++) {
				rootTable[tableOffset + key] = sorted[0];
			}
			return;
		}

		// Fill in root table.
		key = 0;
		symbol = 0;
		for (int len = 1, step = 2; len <= rootBits; len++, step <<= 1) {
			for (; count[len] > 0; count[len]--) {
				replicateValue(rootTable, tableOffset + key, step, tableSize, len << 16 | sorted[symbol++]);
				key = getNextKey(key, len);
			}
		}

		// Fill in 2nd level tables and add pointers to root table.
		int mask = totalSize - 1;
		int low = -1;
		int currentOffset = tableOffset;
		for (int len = rootBits + 1, step = 2; len <= MAX_LENGTH; len++, step <<= 1) {
			for (; count[len] > 0; count[len]--) {
				if ((key & mask) != low) {
					currentOffset += tableSize;
					tableBits = nextTableBitSize(count, len, rootBits);
					tableSize = 1 << tableBits;
					totalSize += tableSize;
					low = key & mask;
					rootTable[tableOffset + low] = (tableBits + rootBits) << 16 | (currentOffset - tableOffset - low);
				}
				replicateValue(rootTable, currentOffset + (key >> rootBits), step, tableSize,
						(len - rootBits) << 16 | sorted[symbol++]);
				key = getNextKey(key, len);
			}
		}
	}
}
