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implement simplified prob update, update unpack.c

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This commit is contained in:
Dennis Ranke
2022-09-10 12:01:42 +02:00
parent 36cb6d77b5
commit f1f1c64a76
2 changed files with 20 additions and 18 deletions
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25
c_unpacker/unpack.c
View File

@@ -19,8 +19,8 @@ int upkr_decode_bit(int context_index) {
upkr_current_byte = *upkr_data_ptr++;
upkr_bits_left = 8;
}
upkr_state = (upkr_state << 1) + (upkr_current_byte & 1);
upkr_current_byte >>= 1;
upkr_state = (upkr_state << 1) + (upkr_current_byte >> 7);
upkr_current_byte <<= 1;
--upkr_bits_left;
}
#else
@@ -30,19 +30,18 @@ int upkr_decode_bit(int context_index) {
#endif
int prob = upkr_probs[context_index];
int bit = (upkr_state & 255) < prob ? 1 : 0;
int bit = (upkr_state & 255) >= prob ? 1 : 0;
int tmp = prob;
if(!bit) {
tmp = 256 - tmp;
int prob_offset = 16;
int state_offset = 0;
int state_scale = prob;
if(bit) {
state_offset = -prob;
state_scale = 256 - prob;
prob_offset = 0;
}
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_state = state_offset + state_scale * (upkr_state >> 8) + (upkr_state & 255);
upkr_probs[context_index] = prob_offset + prob - ((prob + 8) >> 4);
return bit;
}

13
src/context_state.rs
View File

@@ -1,8 +1,8 @@
use crate::rans::{ONE_PROB, PROB_BITS};
const INIT_PROB: u16 = 1 << (PROB_BITS - 1);
const UPDATE_RATE: u32 = 4;
const UPDATE_ADD: u32 = 8;
const UPDATE_RATE: i32 = 4;
const UPDATE_ADD: i32 = 8;
#[derive(Clone)]
pub struct ContextState {
@@ -33,10 +33,13 @@ impl<'a> Context<'a> {
pub fn update(&mut self, bit: bool) {
let old = self.state.contexts[self.index];
self.state.contexts[self.index] = if !bit {
old + ((ONE_PROB - old as u32 + UPDATE_ADD) >> UPDATE_RATE) as u8
let offset = if !bit {
ONE_PROB as i32 >> UPDATE_RATE
} else {
old - ((old as u32 + UPDATE_ADD) >> UPDATE_RATE) as u8
0
};
self.state.contexts[self.index] =
(offset + old as i32 - ((old as i32 + UPDATE_ADD) >> UPDATE_RATE)) as u8;
}
}