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diff --git a/src/lib.rs b/src/lib.rs
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+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Fast, non-cryptographic hash used by rustc and Firefox.
+//!
+//! # Example
+//!
+//! ```rust
+//! # #[cfg(feature = "std")]
+//! # fn main() {
+//! use rustc_hash::FxHashMap;
+//! let mut map: FxHashMap<u32, u32> = FxHashMap::default();
+//! map.insert(22, 44);
+//! # }
+//! # #[cfg(not(feature = "std"))]
+//! # fn main() { }
+//! ```
+
+#![no_std]
+
+#[cfg(feature = "std")]
+extern crate std;
+
+use core::convert::TryInto;
+use core::default::Default;
+#[cfg(feature = "std")]
+use core::hash::BuildHasherDefault;
+use core::hash::Hasher;
+use core::mem::size_of;
+use core::ops::BitXor;
+#[cfg(feature = "std")]
+use std::collections::{HashMap, HashSet};
+
+/// Type alias for a hashmap using the `fx` hash algorithm.
+#[cfg(feature = "std")]
+pub type FxHashMap<K, V> = HashMap<K, V, BuildHasherDefault<FxHasher>>;
+
+/// Type alias for a hashmap using the `fx` hash algorithm.
+#[cfg(feature = "std")]
+pub type FxHashSet<V> = HashSet<V, 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 = 0x9e3779b9;
+#[cfg(target_pointer_width = "64")]
+const K: usize = 0x517cc1b727220a95;
+
+impl Default for FxHasher {
+    #[inline]
+    fn default() -> FxHasher {
+        FxHasher { 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]) {
+        #[cfg(target_pointer_width = "32")]
+        let read_usize = |bytes: &[u8]| u32::from_ne_bytes(bytes[..4].try_into().unwrap());
+        #[cfg(target_pointer_width = "64")]
+        let read_usize = |bytes: &[u8]| u64::from_ne_bytes(bytes[..8].try_into().unwrap());
+
+        let mut hash = FxHasher { hash: self.hash };
+        assert!(size_of::<usize>() <= 8);
+        while bytes.len() >= size_of::<usize>() {
+            hash.add_to_hash(read_usize(bytes) as usize);
+            bytes = &bytes[size_of::<usize>()..];
+        }
+        if (size_of::<usize>() > 4) && (bytes.len() >= 4) {
+            hash.add_to_hash(u32::from_ne_bytes(bytes[..4].try_into().unwrap()) as usize);
+            bytes = &bytes[4..];
+        }
+        if (size_of::<usize>() > 2) && bytes.len() >= 2 {
+            hash.add_to_hash(u16::from_ne_bytes(bytes[..2].try_into().unwrap()) as usize);
+            bytes = &bytes[2..];
+        }
+        if (size_of::<usize>() > 1) && bytes.len() >= 1 {
+            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]
+    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]
+    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
+    }
+}