use std::hash::Hasher;
use std::mem;
use std::ptr;
-use std::slice;
#[cfg(test)]
mod tests;
}};
}
-/// Loads an u64 using up to 7 bytes of a byte slice.
+/// Loads a u64 using up to 7 bytes of a byte slice. It looks clumsy but the
+/// `copy_nonoverlapping` calls that occur (via `load_int_le!`) all have fixed
+/// sizes and avoid calling `memcpy`, which is good for speed.
///
/// Unsafe because: unchecked indexing at start..start+len
#[inline]
let mut i = 0; // current byte index (from LSB) in the output u64
let mut out = 0;
if i + 3 < len {
- out = u64::from(load_int_le!(buf, start + i, u32));
+ out = load_int_le!(buf, start + i, u32) as u64;
i += 4;
}
if i + 1 < len {
- out |= u64::from(load_int_le!(buf, start + i, u16)) << (i * 8);
+ out |= (load_int_le!(buf, start + i, u16) as u64) << (i * 8);
i += 2
}
if i < len {
- out |= u64::from(*buf.get_unchecked(start + i)) << (i * 8);
+ out |= (*buf.get_unchecked(start + i) as u64) << (i * 8);
i += 1;
}
debug_assert_eq!(i, len);
self.state.v1 ^= 0xee;
}
- // Specialized write function that is only valid for buffers with len <= 8.
- // It's used to force inlining of write_u8 and write_usize, those would normally be inlined
- // except for composite types (that includes slices and str hashing because of delimiter).
- // Without this extra push the compiler is very reluctant to inline delimiter writes,
- // degrading performance substantially for the most common use cases.
+ // A specialized write function for values with size <= 8.
+ //
+ // The hashing of multi-byte integers depends on endianness. E.g.:
+ // - little-endian: `write_u32(0xDDCCBBAA)` == `write([0xAA, 0xBB, 0xCC, 0xDD])`
+ // - big-endian: `write_u32(0xDDCCBBAA)` == `write([0xDD, 0xCC, 0xBB, 0xAA])`
+ //
+ // This function does the right thing for little-endian hardware. On
+ // big-endian hardware `x` must be byte-swapped first to give the right
+ // behaviour. After any byte-swapping, the input must be zero-extended to
+ // 64-bits. The caller is responsible for the byte-swapping and
+ // zero-extension.
#[inline]
- fn short_write(&mut self, msg: &[u8]) {
- debug_assert!(msg.len() <= 8);
- let length = msg.len();
- self.length += length;
+ fn short_write<T>(&mut self, _x: T, x: u64) {
+ let size = mem::size_of::<T>();
+ self.length += size;
+
+ // The original number must be zero-extended, not sign-extended.
+ debug_assert!(if size < 8 { x >> (8 * size) == 0 } else { true });
+ // The number of bytes needed to fill `self.tail`.
let needed = 8 - self.ntail;
- let fill = cmp::min(length, needed);
- if fill == 8 {
- self.tail = unsafe { load_int_le!(msg, 0, u64) };
- } else {
- self.tail |= unsafe { u8to64_le(msg, 0, fill) } << (8 * self.ntail);
- if length < needed {
- self.ntail += length;
- return;
- }
+
+ // SipHash parses the input stream as 8-byte little-endian integers.
+ // Inputs are put into `self.tail` until 8 bytes of data have been
+ // collected, and then that word is processed.
+ //
+ // For example, imagine that `self.tail` is 0x0000_00EE_DDCC_BBAA,
+ // `self.ntail` is 5 (because 5 bytes have been put into `self.tail`),
+ // and `needed` is therefore 3.
+ //
+ // - Scenario 1, `self.write_u8(0xFF)`: we have already zero-extended
+ // the input to 0x0000_0000_0000_00FF. We now left-shift it five
+ // bytes, giving 0x0000_FF00_0000_0000. We then bitwise-OR that value
+ // into `self.tail`, resulting in 0x0000_FFEE_DDCC_BBAA.
+ // (Zero-extension of the original input is critical in this scenario
+ // because we don't want the high two bytes of `self.tail` to be
+ // touched by the bitwise-OR.) `self.tail` is not yet full, so we
+ // return early, after updating `self.ntail` to 6.
+ //
+ // - Scenario 2, `self.write_u32(0xIIHH_GGFF)`: we have already
+ // zero-extended the input to 0x0000_0000_IIHH_GGFF. We now
+ // left-shift it five bytes, giving 0xHHGG_FF00_0000_0000. We then
+ // bitwise-OR that value into `self.tail`, resulting in
+ // 0xHHGG_FFEE_DDCC_BBAA. `self.tail` is now full, and we can use it
+ // to update `self.state`. (As mentioned above, this assumes a
+ // little-endian machine; on a big-endian machine we would have
+ // byte-swapped 0xIIHH_GGFF in the caller, giving 0xFFGG_HHII, and we
+ // would then end up bitwise-ORing 0xGGHH_II00_0000_0000 into
+ // `self.tail`).
+ //
+ self.tail |= x << (8 * self.ntail);
+ if size < needed {
+ self.ntail += size;
+ return;
}
+
+ // `self.tail` is full, process it.
self.state.v3 ^= self.tail;
Sip24Rounds::c_rounds(&mut self.state);
self.state.v0 ^= self.tail;
- // Buffered tail is now flushed, process new input.
- self.ntail = length - needed;
- self.tail = unsafe { u8to64_le(msg, needed, self.ntail) };
- }
-
- #[inline(always)]
- fn short_write_gen<T>(&mut self, x: T) {
- let bytes =
- unsafe { slice::from_raw_parts(&x as *const T as *const u8, mem::size_of::<T>()) };
- self.short_write(bytes);
+ // Continuing scenario 2: we have one byte left over from the input. We
+ // set `self.ntail` to 1 and `self.tail` to `0x0000_0000_IIHH_GGFF >>
+ // 8*3`, which is 0x0000_0000_0000_00II. (Or on a big-endian machine
+ // the prior byte-swapping would leave us with 0x0000_0000_0000_00FF.)
+ //
+ // The `if` is needed to avoid shifting by 64 bits, which Rust
+ // complains about.
+ self.ntail = size - needed;
+ self.tail = if needed < 8 { x >> (8 * needed) } else { 0 };
}
#[inline]
impl Hasher for SipHasher128 {
#[inline]
fn write_u8(&mut self, i: u8) {
- self.short_write_gen(i);
+ self.short_write(i, i as u64);
}
#[inline]
fn write_u16(&mut self, i: u16) {
- self.short_write_gen(i);
+ self.short_write(i, i.to_le() as u64);
}
#[inline]
fn write_u32(&mut self, i: u32) {
- self.short_write_gen(i);
+ self.short_write(i, i.to_le() as u64);
}
#[inline]
fn write_u64(&mut self, i: u64) {
- self.short_write_gen(i);
+ self.short_write(i, i.to_le() as u64);
}
#[inline]
fn write_usize(&mut self, i: usize) {
- self.short_write_gen(i);
+ self.short_write(i, i.to_le() as u64);
}
#[inline]
fn write_i8(&mut self, i: i8) {
- self.short_write_gen(i);
+ self.short_write(i, i as u8 as u64);
}
#[inline]
fn write_i16(&mut self, i: i16) {
- self.short_write_gen(i);
+ self.short_write(i, (i as u16).to_le() as u64);
}
#[inline]
fn write_i32(&mut self, i: i32) {
- self.short_write_gen(i);
+ self.short_write(i, (i as u32).to_le() as u64);
}
#[inline]
fn write_i64(&mut self, i: i64) {
- self.short_write_gen(i);
+ self.short_write(i, (i as u64).to_le() as u64);
}
#[inline]
fn write_isize(&mut self, i: isize) {
- self.short_write_gen(i);
+ self.short_write(i, (i as usize).to_le() as u64);
}
#[inline]
projects / rust.git / blobdiff