//! Fast, non-cryptographic hash used by rustc and Firefox.

use core::default::Default;
use core::hash::{BuildHasherDefault, Hasher};
use core::mem::size_of;
use core::ops::BitXor;

/// Type alias for a `HashBuilder` using the `fx` hash algorithm.
pub type FxHashBuilder = BuildHasherDefault<FxHasher>;

/// A speedy hash algorithm for use within rustc. The hashmap in liballoc
/// by default uses SipHash which isn't quite as speedy as we want. In the
/// compiler we're not really worried about DOS attempts, so we use a fast
/// non-cryptographic hash.
///
/// This is the same as the algorithm used by Firefox -- which is a homespun
/// one not based on any widely-known algorithm -- though modified to produce
/// 64-bit hash values instead of 32-bit hash values. It consistently
/// out-performs an FNV-based hash within rustc itself -- the collision rate is
/// similar or slightly worse than FNV, but the speed of the hash function
/// itself is much higher because it works on up to 8 bytes at a time.
pub struct FxHasher {
    hash: usize,
}

#[cfg(target_pointer_width = "32")]
const K: usize = 0x9e37_79b9;
#[cfg(target_pointer_width = "64")]
const K: usize = 0x517c_c1b7_2722_0a95;

impl Default for FxHasher {
    #[inline]
    fn default() -> Self {
        Self { hash: 0 }
    }
}

impl FxHasher {
    #[inline]
    fn add_to_hash(&mut self, i: usize) {
        self.hash = self.hash.rotate_left(5).bitxor(i).wrapping_mul(K);
    }
}

impl Hasher for FxHasher {
    #[inline]
    fn write(&mut self, mut bytes: &[u8]) {
        macro_rules! read_bytes {
            ($ty:ty, $src:expr) => {{
                assert!(size_of::<$ty>() <= $src.len());
                let mut data: $ty = 0;
                unsafe {
                    $src.as_ptr()
                        .copy_to_nonoverlapping(&mut data as *mut $ty as *mut u8, size_of::<$ty>());
                }
                data
            }};
        }

        let mut hash = Self { hash: self.hash };
        assert!(size_of::<usize>() <= 8);
        while bytes.len() >= size_of::<usize>() {
            hash.add_to_hash(read_bytes!(usize, bytes) as usize);
            bytes = &bytes[size_of::<usize>()..];
        }
        if (size_of::<usize>() > 4) && (bytes.len() >= 4) {
            hash.add_to_hash(read_bytes!(u32, bytes) as usize);
            bytes = &bytes[4..];
        }
        if (size_of::<usize>() > 2) && bytes.len() >= 2 {
            hash.add_to_hash(read_bytes!(u16, bytes) as usize);
            bytes = &bytes[2..];
        }
        if (size_of::<usize>() > 1) && !bytes.is_empty() {
            hash.add_to_hash(bytes[0] as usize);
        }
        self.hash = hash.hash;
    }

    #[inline]
    fn write_u8(&mut self, i: u8) {
        self.add_to_hash(i as usize);
    }

    #[inline]
    fn write_u16(&mut self, i: u16) {
        self.add_to_hash(i as usize);
    }

    #[inline]
    fn write_u32(&mut self, i: u32) {
        self.add_to_hash(i as usize);
    }

    #[cfg(target_pointer_width = "32")]
    #[inline]
    #[allow(clippy::cast_possible_truncation)]
    fn write_u64(&mut self, i: u64) {
        self.add_to_hash(i as usize);
        self.add_to_hash((i >> 32) as usize);
    }

    #[cfg(target_pointer_width = "64")]
    #[inline]
    #[allow(clippy::cast_possible_truncation)]
    fn write_u64(&mut self, i: u64) {
        self.add_to_hash(i as usize);
    }

    #[inline]
    fn write_usize(&mut self, i: usize) {
        self.add_to_hash(i);
    }

    #[inline]
    fn finish(&self) -> u64 {
        self.hash as u64
    }
}