private PngHuffmanTables(PngDecodingDataStream stream) throws IOException {
int literals = PngLzBlockReader.FIRST_LENGTH_CODE + stream.getNextIdatBits(5);
int distances = PngLzBlockReader.FIRST_DISTANCE_CODE + stream.getNextIdatBits(5);
int codeLengthCodes = PngLzBlockReader.FIRST_CODE_LENGTH_CODE + stream.getNextIdatBits(4);
if (codeLengthCodes > PngLzBlockReader.LAST_CODE_LENGTH_CODE) {
stream.error();
}
/* Tricky, tricky, tricky. The length codes are stored in
* a very odd order. (For the order, see the definition of
* the static field lengthCodeOrder.) Also, the data may
* not contain values for all the codes. It may just contain
* values for the first X number of codes. The table should
* be of size <LengthCodeTableSize> regardless of the number
* of values actually given in the table.
*/
int[] lengthCodes = new int[LengthCodeTableSize];
for (int i = 0; i < codeLengthCodes; i++) {
lengthCodes[LengthCodeOrder[i]] = stream.getNextIdatBits(3);
}
PngHuffmanTable codeLengthsTable = new PngHuffmanTable(lengthCodes);
int[] literalLengths = readLengths(stream, literals, codeLengthsTable, LiteralTableSize);
int[] distanceLengths = readLengths(stream, distances, codeLengthsTable, DistanceTableSize);
literalTable = new PngHuffmanTable(literalLengths);
distanceTable = new PngHuffmanTable(distanceLengths);
}
private int[] readLengths(
PngDecodingDataStream stream, int numLengths, PngHuffmanTable lengthsTable, int tableSize)
throws IOException {
int[] lengths = new int[tableSize];
for (int index = 0; index < numLengths; ) {
int value = lengthsTable.getNextValue(stream);
if (value < 16) {
// Literal value
lengths[index] = value;
index++;
} else if (value == 16) {
// Repeat the previous code 3-6 times.
int count = stream.getNextIdatBits(2) + 3;
for (int i = 0; i < count; i++) {
lengths[index] = lengths[index - 1];
index++;
}
} else if (value == 17) {
// Repeat 0 3-10 times.
int count = stream.getNextIdatBits(3) + 3;
for (int i = 0; i < count; i++) {
lengths[index] = 0;
index++;
}
} else if (value == 18) {
// Repeat 0 11-138 times.
int count = stream.getNextIdatBits(7) + 11;
for (int i = 0; i < count; i++) {
lengths[index] = 0;
index++;
}
} else {
stream.error();
}
}
return lengths;
}
int getNextValue(PngDecodingDataStream stream) throws IOException {
int code = stream.getNextIdatBit();
int codelength = 0;
// Here we are taking advantage of the fact that 1 bits are used as
// a prefix to the longer codeValues.
while (codelength < MAX_CODE_LENGTH && code > codeLengthInfo[codelength].max) {
code = ((code << 1) | stream.getNextIdatBit());
codelength++;
}
if (codelength >= MAX_CODE_LENGTH) stream.error();
// Now we have a Huffman code of length (codelength + 1) that
// is somewhere in the range
// minCodesByLength[codelength]..maxCodesByLength[codelength].
// This code is the (offset + 1)'th code of (codelength + 1);
int offset = code - codeLengthInfo[codelength].min;
// indexesByLength[codelength] is the first code of length (codelength + 1)
// so now we can look up the value for the Huffman code in the table.
int index = codeLengthInfo[codelength].baseIndex + offset;
return codeValues[index];
}
