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optimize push_bits

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This commit is contained in:
Robin Appelman 2020-02-15 14:08:54 +01:00
commit 123223f479
3 changed files with 29 additions and 69 deletions
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61
src/writestream.rs
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@ -98,58 +98,23 @@ where
/// Push up to an usize worth of bits
fn push_bits(&mut self, bits: usize, count: usize) {
debug_assert!(count < USIZE_BITS - 8);
let bit_offset = self.bit_len & 7;
let byte_count = (count + 7) / 8;
let last_written_byte = self.bytes.pop().unwrap_or(0);
let merged_byte_count = (count + bit_offset + 7) / 8;
if bit_offset == 0 {
if E::is_le() {
self.bytes
.extend_from_slice(&bits.to_le_bytes()[0..byte_count])
} else {
let bytes = (bits << (USIZE_BITS - bit_offset - count)).to_be_bytes();
self.bytes.extend_from_slice(&bytes[0..byte_count])
}
self.bit_len += count;
if E::is_le() {
let merged = last_written_byte as usize | bits << bit_offset;
self.bytes
.extend_from_slice(&merged.to_le_bytes()[0..merged_byte_count]);
} else {
if E::is_le() {
let first_part_length = min(USIZE_SIZE - bit_offset, count);
let first_part = get_bits_from_usize::<E>(bits, 0, first_part_length) as u8;
let last_written_byte = self.bytes.pop().unwrap_or(0);
let merged_byte = last_written_byte | (first_part << bit_offset as u8);
self.bytes.push(merged_byte);
self.bit_len += first_part_length;
if first_part_length < count {
let second_part = get_bits_from_usize::<E>(
bits,
first_part_length,
count - first_part_length,
);
self.push_bits(second_part, count - first_part_length);
}
} else {
let first_part_length = min(USIZE_SIZE - bit_offset, count);
let first_part = get_bits_from_usize::<LittleEndian>(
bits,
count - first_part_length,
first_part_length,
) as u8;
let last_written_byte = self.bytes.pop().unwrap_or(0);
let merged_byte =
last_written_byte | first_part << (8 - bit_offset - first_part_length) as u8;
self.bytes.push(merged_byte);
self.bit_len += first_part_length;
if first_part_length < count {
let second_part =
get_bits_from_usize::<LittleEndian>(bits, 0, count - first_part_length);
self.push_bits(second_part, count - first_part_length);
}
}
let merged = ((last_written_byte as usize) << (USIZE_BITS - 8))
| bits << (USIZE_BITS - bit_offset - count);
self.bytes
.extend_from_slice(&merged.to_be_bytes()[0..merged_byte_count]);
}
self.bit_len += count;
}
/// Write a boolean into the buffer