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alternate LUT-base reverse-bits code

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Not faster on Unix/x86. Left the code for reference though.

Change-Id: I7d419e9835e7746b85b8f8cc6f2af92db0eb8b03
This commit is contained in:
skal 2013-06-20 19:58:11 +02:00
parent 24cc307ae3
commit 1b631e291a
1 changed files with 24 additions and 2 deletions
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26
src/utils/huffman.c
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@ -22,6 +22,10 @@
extern "C" {
#endif
// Uncomment the following to use look-up table for ReverseBits()
// (might be faster on some platform)
// #define USE_LUT_REVERSE_BITS
#define NON_EXISTENT_SYMBOL (-1)
static void TreeNodeInit(HuffmanTreeNode* const node) {
@ -119,9 +123,12 @@ int HuffmanCodeLengthsToCodes(const int* const code_lengths,
return 1;
}
static int ReverseBits(int bits, int num_bits) {
#ifndef USE_LUT_REVERSE_BITS
static int ReverseBitsShort(int bits, int num_bits) {
int retval = 0;
int i;
assert(num_bits <= 8); // Not a hard requirement, just for coherency.
for (i = 0; i < num_bits; ++i) {
retval <<= 1;
retval |= bits & 1;
@ -130,12 +137,27 @@ static int ReverseBits(int bits, int num_bits) {
return retval;
}
#else
static const uint8_t kReversedBits[16] = { // Pre-reversed 4-bit values.
0x0, 0x8, 0x4, 0xc, 0x2, 0xa, 0x6, 0xe,
0x1, 0x9, 0x5, 0xd, 0x3, 0xb, 0x7, 0xf
};
static int ReverseBitsShort(int bits, int num_bits) {
const uint8_t v = (kReversedBits[bits & 0xf] << 4) | kReversedBits[bits >> 4];
assert(num_bits <= 8);
return v >> (8 - num_bits);
}
#endif
static int TreeAddSymbol(HuffmanTree* const tree,
int symbol, int code, int code_length) {
HuffmanTreeNode* node = tree->root_;
const HuffmanTreeNode* const max_node = tree->root_ + tree->max_nodes_;
if (code_length <= HUFF_LUT_BITS) {
const int base_code = ReverseBits(code, code_length);
const int base_code = ReverseBitsShort(code, code_length);
int i;
for (i = 0; i < (1 << (HUFF_LUT_BITS - code_length)); ++i) {
const int idx = base_code | (i << code_length);