73 lines
2.4 KiB
Rust
73 lines
2.4 KiB
Rust
use std::{
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collections::HashMap,
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hash::{BuildHasher, Hasher},
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};
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// Note: If needed, this will create a new HashMap without initial capacity.
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// /// This is the only way to create a HashMap with the U32HasherState
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// pub fn u32hashmap_new<V>() -> HashMap<u32, V, U32HasherState> {
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// HashMap::with_hasher(U32HasherState(()))
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// }
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/// This is the only way to create a HashMap with the U32HasherState and capacity
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pub fn u32hashmap_with_capacity<V>(capacity: usize) -> HashMap<u32, V, U32HasherState> {
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HashMap::with_capacity_and_hasher(capacity, U32HasherState(()))
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}
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/// A private unit type is contained so no one can make an instance of it.
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pub struct U32HasherState(());
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impl BuildHasher for U32HasherState {
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type Hasher = U32Hasher;
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fn build_hasher(&self) -> Self::Hasher {
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U32Hasher(0)
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}
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}
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/// This also can't be created outside this module due to private field.
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pub struct U32Hasher(u32);
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impl Hasher for U32Hasher {
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fn finish(&self) -> u64 {
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// Safety: Two u32s next to each other will make a u64
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bytemuck::cast([self.0, 0])
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}
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fn write(&mut self, bytes: &[u8]) {
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// Assert in debug builds (testing too) that only 4 byte keys (u32, i32, f32, etc.) run
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debug_assert!(bytes.len() == 4);
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// Safety: We know that the size of the key is 4 bytes
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// We also know that the only way to get an instance of HashMap using this "hasher"
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// is through the public functions in this module which set the key type to u32.
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self.0 = *bytemuck::from_bytes(bytes);
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}
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}
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#[cfg(test)]
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mod tests {
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use std::collections::HashMap;
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use super::U32HasherState;
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#[test]
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fn test_me() {
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let mut hm: HashMap<u32, String, U32HasherState> = HashMap::with_hasher(U32HasherState(()));
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hm.insert(0, String::from("0"));
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hm.insert(42, String::from("42"));
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hm.insert(256, String::from("256"));
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hm.insert(u32::MAX, String::from("MAX"));
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hm.insert(u32::MAX >> 2, String::from("MAX >> 2"));
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assert_eq!(hm.get(&0), Some(&String::from("0")));
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assert_eq!(hm.get(&42), Some(&String::from("42")));
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assert_eq!(hm.get(&256), Some(&String::from("256")));
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assert_eq!(hm.get(&u32::MAX), Some(&String::from("MAX")));
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assert_eq!(hm.get(&(u32::MAX >> 2)), Some(&String::from("MAX >> 2")));
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assert_eq!(hm.get(&(u32::MAX >> 4)), None);
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assert_eq!(hm.get(&3), None);
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assert_eq!(hm.get(&43), None);
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}
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}
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