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read unaligned bytes optimization

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This commit is contained in:
Robin Appelman 2019-12-25 23:18:35 +01:00
commit 01a8edd782
1 changed files with 14 additions and 16 deletions
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30
src/buffer.rs
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@ -350,7 +350,9 @@ where
}
}
if position & 7 == 0 {
let shift = position & 7;
if shift == 0 {
let byte_pos = position / 8;
return Ok(self.bytes[byte_pos..byte_pos + byte_count].to_vec());
}
@ -358,23 +360,19 @@ where
let mut data = Vec::with_capacity(byte_count);
let mut byte_left = byte_count;
let max_read = size_of::<usize>() - 1;
let mut read_pos = position;
let mut read_pos = position / 8;
while byte_left > 0 {
let read = min(byte_left, max_read);
let raw_bytes = self.read_usize(read_pos, read * 8);
let bytes: [u8; USIZE_SIZE] = if E::is_le() {
raw_bytes.to_le_bytes()
} else {
raw_bytes.to_be_bytes()
};
let usable_bytes = if E::is_le() {
&bytes[0..read]
} else {
&bytes[USIZE_SIZE - read..USIZE_SIZE]
};
let raw_bytes: [u8; USIZE_SIZE] = self.read_usize_bytes(read_pos);
let raw_usize: usize = usize::from_le_bytes(raw_bytes);
let shifted = raw_usize >> shift;
let bytes: [u8; USIZE_SIZE] = shifted.to_le_bytes();
let read_bytes = min(byte_left, USIZE_SIZE - 1);
let usable_bytes = &bytes[0..read_bytes];
data.extend_from_slice(usable_bytes);
byte_left -= read;
read_pos += read * 8;
read_pos += read_bytes;
byte_left -= read_bytes;
}
Ok(data)
}