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more efficient way of getting a usize from byte slice

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Robin Appelman 2019-02-23 15:55:06 +01:00
commit 1db817c70c
1 changed files with 21 additions and 21 deletions
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14
src/buffer.rs
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@ -1,6 +1,6 @@
use crate::{ReadError, Result};
use crate::endianness::Endianness; use crate::endianness::Endianness;
use crate::is_signed::IsSigned; use crate::is_signed::IsSigned;
use crate::{ReadError, Result};
use num_traits::{Float, PrimInt}; use num_traits::{Float, PrimInt};
use std::cmp::min; use std::cmp::min;
use std::marker::PhantomData; use std::marker::PhantomData;
@ -160,7 +160,6 @@ where
self.byte_len self.byte_len
} }
#[inline]
fn read_usize(&self, position: usize, count: usize) -> Result<usize> { fn read_usize(&self, position: usize, count: usize) -> Result<usize> {
if position + count > self.bit_len { if position + count > self.bit_len {
return Err(ReadError::NotEnoughData { return Err(ReadError::NotEnoughData {
@ -173,14 +172,15 @@ where
} else { } else {
min(position / 8, self.byte_len - USIZE_SIZE) min(position / 8, self.byte_len - USIZE_SIZE)
}; };
//let byte_index = position / 8;
let bit_offset = position - byte_index * 8; let bit_offset = position - byte_index * 8;
let slice = &self.bytes[byte_index..byte_index + USIZE_SIZE]; let raw_container: &usize = unsafe {
let bytes: [u8; USIZE_SIZE] = unsafe { *(slice.as_ptr() as *const [u8; USIZE_SIZE]) }; // this is only safe for us because we're sure that there is enough data in the slice
std::mem::transmute(self.bytes.as_ptr().offset(byte_index as isize))
};
let container = if E::is_le() { let container = if E::is_le() {
usize::from_le_bytes(bytes) usize::from_le(*raw_container)
} else { } else {
usize::from_be_bytes(bytes) usize::from_be(*raw_container)
}; };
let shifted = if E::is_le() { let shifted = if E::is_le() {
container >> bit_offset container >> bit_offset