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during lzw decoding, use a vector instead of a hashmap for the table

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since the keys for the previously used hashmap all corresponded with
an index, this can obviously just be a vector where the element index
corresponds to the lzw code. simple.
This commit is contained in:
Gered 2023-01-01 13:01:10 -05:00
parent a9610a5762
commit f902b16d3c
1 changed files with 9 additions and 7 deletions
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16
libretrogd/src/utils/lzwgif.rs
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@ -544,15 +544,16 @@ where
// needing the table to be able to hold buffers containing u16's instead of just u8's. // needing the table to be able to hold buffers containing u16's instead of just u8's.
// this does mean that the size of the table is always 2 less than the number of created codes. // this does mean that the size of the table is always 2 less than the number of created codes.
let mut table = HashMap::<LzwCode, Vec<u8>>::with_capacity(initial_table_size + 2); let mut table = vec![None; 1usize.wrapping_shl(MAX_BITS as u32)];
for i in 0..initial_table_size { for i in 0..initial_table_size {
table.insert(i as LzwCode, vec![i as u8]); table[i] = Some(vec![i as u8]);
} }
let mut max_code_value_for_bit_size = get_max_code_value_for_bits(current_bit_size); let mut max_code_value_for_bit_size = get_max_code_value_for_bits(current_bit_size);
let mut next_code = initial_table_size as LzwCode + 2; let mut next_code = initial_table_size as LzwCode + 2;
// read the next code which should actually be the first "interesting" value of the code stream // read the next code which should actually be the first "interesting" value of the code stream
code = match reader.read_code(src)? { code = match reader.read_code(src)? {
Some(code) if code > MAX_CODE_VALUE => return Err(LzwError::EncodingError(format!("Encountered code that is too large: {}", code))),
Some(code) if code == end_of_info_code => return Ok(()), Some(code) if code == end_of_info_code => return Ok(()),
Some(code) => code, Some(code) => code,
None => return Err(LzwError::EncodingError(String::from("Unexpected end of code stream"))), None => return Err(LzwError::EncodingError(String::from("Unexpected end of code stream"))),
@ -561,7 +562,7 @@ where
// ok, now we're able to get started! // ok, now we're able to get started!
// simply write out the table string associated with the first code // simply write out the table string associated with the first code
if let Some(string) = table.get(&code) { if let Some(string) = table.get(code as usize).unwrap() {
dest.write_all(string)?; dest.write_all(string)?;
} else { } else {
return Err(LzwError::EncodingError(format!("No table entry for code {}", code))); return Err(LzwError::EncodingError(format!("No table entry for code {}", code)));
@ -572,6 +573,7 @@ where
'inner: loop { 'inner: loop {
// grab the next code // grab the next code
code = match reader.read_code(src)? { code = match reader.read_code(src)? {
Some(code) if code > MAX_CODE_VALUE => return Err(LzwError::EncodingError(format!("Encountered code that is too large: {}", code))),
Some(code) if code == end_of_info_code => break 'outer, Some(code) if code == end_of_info_code => break 'outer,
Some(code) if code == clear_code => { Some(code) if code == clear_code => {
// reset the bit size and reader and then loop back to the outer loop which // reset the bit size and reader and then loop back to the outer loop which
@ -586,7 +588,7 @@ where
// note: prev_code should always be present since we looked it up in the table during a // note: prev_code should always be present since we looked it up in the table during a
// previous loop iteration ... // previous loop iteration ...
let prev_code_string = match table.get(&prev_code) { let prev_code_string = match table.get(prev_code as usize).unwrap() {
Some(prev_code_string) => prev_code_string, Some(prev_code_string) => prev_code_string,
None => { None => {
return Err(LzwError::EncodingError(format!("Previous code {} not found in table", prev_code))); return Err(LzwError::EncodingError(format!("Previous code {} not found in table", prev_code)));
@ -596,7 +598,7 @@ where
let new_code = next_code; let new_code = next_code;
next_code += 1; next_code += 1;
if let Some(string) = table.get(&code) { if let Some(string) = table.get(code as usize).unwrap() {
// write out the matching table string for the code just read // write out the matching table string for the code just read
dest.write_all(string)?; dest.write_all(string)?;
@ -604,7 +606,7 @@ where
let k = string.first().unwrap(); let k = string.first().unwrap();
let mut new_string = prev_code_string.clone(); let mut new_string = prev_code_string.clone();
new_string.push(*k); new_string.push(*k);
table.insert(new_code, new_string); table[new_code as usize] = Some(new_string);
} else { } else {
// code is not yet present in the table. // code is not yet present in the table.
// add prev_code string + the code we just read to the table and also write it out // add prev_code string + the code we just read to the table and also write it out
@ -612,7 +614,7 @@ where
let mut new_string = prev_code_string.clone(); let mut new_string = prev_code_string.clone();
new_string.push(*k); new_string.push(*k);
dest.write_all(&new_string)?; dest.write_all(&new_string)?;
table.insert(new_code, new_string); table[new_code as usize] = Some(new_string);
} }
if new_code == max_code_value_for_bit_size { if new_code == max_code_value_for_bit_size {