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fix lzw encoding code bit increment and max code value checks

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this fixes it so that the full 12 bits for codes will be used instead
of only 11. whoops
This commit is contained in:
Gered 2022-12-30 10:51:30 -05:00
parent 0c1f50e63e
commit 803b31d8c8
1 changed files with 11 additions and 13 deletions
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24
libretrogd/src/utils/lzw.rs
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@ -33,6 +33,7 @@ const GIF_MAX_SUB_CHUNK_SIZE: usize = 255;
const GIF_MAX_CODE_SIZE_BITS: usize = 8;
const MIN_BITS: usize = 2;
const MAX_BITS: usize = 12;
const MAX_CODE_VALUE: LzwCode = (1 as LzwCode).wrapping_shl(MAX_BITS as u32) - 1;
fn is_valid_min_code_size_bits(min_code_size_bits: usize) -> bool {
min_code_size_bits >= MIN_BITS && min_code_size_bits <= MAX_BITS
@ -446,21 +447,11 @@ where
// we have a match, so lets just keep collecting bytes in our buffer ...
buffer.push_value(byte as u16);
} else {
let mut was_bit_size_increased = false;
let mut is_reset_needed = false;
// no match in the table, so we need to create a new code in the table for this
// string of bytes (buffer + byte) and also emit the code for _just_ the buffer string
let new_code = next_code;
next_code += 1;
if new_code > max_code_value_for_bit_size {
current_bit_size += 1;
if current_bit_size >= MAX_BITS {
is_reset_needed = true;
}
max_code_value_for_bit_size = get_max_code_value_for_bits(current_bit_size);
was_bit_size_increased = true;
}
table.insert(buffer_plus_byte, new_code);
@ -470,11 +461,18 @@ where
return Err(LzwError::EncodingError(format!("Expected to find code in table for buffer {:?} but none was found", buffer)));
}
if was_bit_size_increased {
// bump up to the next bit size once we've seen enough codes to necessitate it ...
// note that this just means codes that exist in the table, not _necessarily_ codes
// which have actually been written out yet ...
if new_code > max_code_value_for_bit_size {
current_bit_size += 1;
max_code_value_for_bit_size = get_max_code_value_for_bits(current_bit_size);
writer.increase_bit_size()?;
}
if is_reset_needed {
// reset the table and code bit sizes once we've seen enough codes to fill all our
// allowed bits. again, this is just based on codes that exist in the table!
if new_code == MAX_CODE_VALUE {
// we reached the maximum code bit size, time to re-initialize the code table
table = HashMap::with_capacity(initial_table_size + 2);
for i in 0..initial_table_size {