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some slight tweaks to the compressed format

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This commit is contained in:
Dennis Ranke
2021-12-27 16:58:39 +01:00
parent 7d280bd533
commit 08c86af06f
3 changed files with 23 additions and 10 deletions
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5
src/context_state.rs
View File

@@ -2,6 +2,7 @@ use crate::rans::{PROB_BITS, ONE_PROB};
const INIT_PROB: u16 = 1 << (PROB_BITS - 1);
const UPDATE_RATE: u32 = 4;
const UPDATE_ADD: u32 = 8;
#[derive(Clone)]
pub struct ContextState {
@@ -33,9 +34,9 @@ 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_RATE) as u16
old + ((ONE_PROB - old as u32 + UPDATE_ADD) >> UPDATE_RATE) as u16
} else {
old - (old >> UPDATE_RATE)
old - ((old + UPDATE_ADD as u16) >> UPDATE_RATE)
};
}
}

24
src/lz.rs
View File

@@ -18,15 +18,21 @@ impl Op {
encode_bit(coder, state, context_index, bit);
context_index = (context_index << 1) | bit as usize;
}
state.prev_was_match = false;
}
&Op::Match { offset, len } => {
encode_bit(coder, state, 0, true);
encode_bit(coder, state, 256, offset != state.last_offset);
if !state.prev_was_match {
encode_bit(coder, state, 256, offset != state.last_offset);
} else {
assert!(offset != state.last_offset);
}
if offset != state.last_offset {
encode_length(coder, state, 257, offset + 1);
state.last_offset = offset;
}
encode_length(coder, state, 257 + 64, len);
state.prev_was_match = true;
}
}
}
@@ -51,15 +57,16 @@ fn encode_length(
coder: &mut dyn EntropyCoder,
state: &mut CoderState,
context_start: usize,
value: u32,
mut value: u32,
) {
assert!(value >= 1);
let top_bit = u32::BITS - 1 - value.leading_zeros();
let mut context_index = context_start;
for i in 0..top_bit {
while value >= 2 {
encode_bit(coder, state, context_index, true);
encode_bit(coder, state, context_index + 1, (value >> i) & 1 != 0);
encode_bit(coder, state, context_index + 1, value & 1 != 0);
context_index += 2;
value >>= 1;
}
encode_bit(coder, state, context_index, false);
}
@@ -68,6 +75,7 @@ fn encode_length(
pub struct CoderState {
contexts: ContextState,
last_offset: u32,
prev_was_match: bool
}
impl CoderState {
@@ -75,6 +83,7 @@ impl CoderState {
CoderState {
contexts: ContextState::new(1 + 255 + 1 + 64 + 64),
last_offset: 0,
prev_was_match: false
}
}
@@ -88,6 +97,7 @@ pub fn unpack(packed_data: &[u8]) -> Vec<u8> {
let mut contexts = ContextState::new(1 + 255 + 1 + 64 + 64);
let mut result = vec![];
let mut offset = 0;
let mut prev_was_match = false;
fn decode_length(
decoder: &mut RansDecoder,
@@ -108,7 +118,7 @@ pub fn unpack(packed_data: &[u8]) -> Vec<u8> {
loop {
if decoder.decode_with_context(&mut contexts.context_mut(0)) {
if decoder.decode_with_context(&mut contexts.context_mut(256)) {
if prev_was_match || decoder.decode_with_context(&mut contexts.context_mut(256)) {
offset = decode_length(&mut decoder, &mut contexts, 257) - 1;
if offset == 0 {
break;
@@ -118,6 +128,7 @@ pub fn unpack(packed_data: &[u8]) -> Vec<u8> {
for _ in 0..length {
result.push(result[result.len() - offset]);
}
prev_was_match = true;
} else {
let mut context_index = 1;
let mut byte = 0;
@@ -127,6 +138,7 @@ pub fn unpack(packed_data: &[u8]) -> Vec<u8> {
byte |= (bit as u8) << i;
}
result.push(byte);
prev_was_match = false;
}
}

4
src/rans.rs
View File

@@ -1,7 +1,7 @@
use crate::context_state::Context;
const L_BITS: u32 = 16;
pub const PROB_BITS: u32 = 12;
const L_BITS: u32 = 12;
pub const PROB_BITS: u32 = 8;
pub const ONE_PROB: u32 = 1 << PROB_BITS;
pub trait EntropyCoder {