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alternative way to write state/prob update

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This commit is contained in:
Dennis Ranke
2022-09-19 18:33:02 +02:00
parent 5c7aee046a
commit cc41feb5cd
3 changed files with 48 additions and 10 deletions
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8
asm_unpackers/Makefile
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@@ -17,6 +17,14 @@ test_armv6m: build/unpack_armv6m
build/unpack_armv6m.bin: unpack_armv6m.S
arm-none-eabi-gcc -march=armv6-m -c -o build/unpack_armv6m.o $?
arm-none-eabi-objcopy -O binary --only-section=.text build/unpack_armv6m.o $@
build/unpack_c: ../c_unpacker/main.c ../c_unpacker/unpack.c
mkdir -p build
gcc -g -o $@ $^
test_c: build/unpack_c
$< test_data.upk /tmp/out.bin
cmp test_data.bin /tmp/out.bin
sizes: build/unpack_armv6m.bin
ls -l build/*.bin

33
c_unpacker/decode_bit_alt.c Normal file
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@@ -0,0 +1,33 @@
int upkr_decode_bit(int context_index) {
#ifdef UPKR_BITSTREAM
while(upkr_state < 32768) {
if(upkr_bits_left == 0) {
upkr_current_byte = *upkr_data_ptr++;
upkr_bits_left = 8;
}
upkr_state = (upkr_state << 1) + (upkr_current_byte & 1);
upkr_current_byte >>= 1;
--upkr_bits_left;
}
#else
while(upkr_state < 4096) {
upkr_state = (upkr_state << 8) | *upkr_data_ptr++;
}
#endif
int prob = upkr_probs[context_index];
int bit = (upkr_state & 255) < prob ? 1 : 0;
if(bit) {
prob = 256 - prob;
}
upkr_state -= prob * ((upkr_state >> 8) + (bit ^ 1));
prob -= (prob + 8) >> 4;
if(bit) {
prob = -prob;
}
upkr_probs[context_index] = prob;
return bit;
}

17
c_unpacker/unpack.c
View File

@@ -32,17 +32,14 @@ int upkr_decode_bit(int context_index) {
int prob = upkr_probs[context_index];
int bit = (upkr_state & 255) < prob ? 1 : 0;
int tmp = prob;
if(!bit) {
tmp = 256 - tmp;
if(bit) {
upkr_state = prob * (upkr_state >> 8) + (upkr_state & 255);
prob += (256 - prob + 8) >> 4;
} else {
upkr_state = (256 - prob) * (upkr_state >> 8) + (upkr_state & 255) - prob;
prob -= (prob + 8) >> 4;
}
upkr_state = tmp * (upkr_state >> 8) + (upkr_state & 255);
tmp += (256 - tmp + 8) >> 4;
if(!bit) {
upkr_state -= prob;
tmp = 256 - tmp;
}
upkr_probs[context_index] = tmp;
upkr_probs[context_index] = prob;
return bit;
}