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//! See [the `phf` crate's documentation][phf] for details.
//!
use phf_shared::{HashKey, PhfHash};
use rand::distributions::Standard;
use rand::rngs::SmallRng;
use rand::{Rng, SeedableRng};
const DEFAULT_LAMBDA: usize = 5;
const FIXED_SEED: u64 = 1234567890;
pub struct HashState {
pub key: HashKey,
pub disps: Vec<(u32, u32)>,
pub map: Vec<usize>,
}
pub fn generate_hash<H: PhfHash>(entries: &[H]) -> HashState {
SmallRng::seed_from_u64(FIXED_SEED)
.sample_iter(Standard)
.find_map(|key| try_generate_hash(entries, key))
.expect("failed to solve PHF")
}
fn try_generate_hash<H: PhfHash>(entries: &[H], key: HashKey) -> Option<HashState> {
struct Bucket {
idx: usize,
keys: Vec<usize>,
}
let hashes: Vec<_> = entries
.iter()
.map(|entry| phf_shared::hash(entry, &key))
.collect();
let buckets_len = (hashes.len() + DEFAULT_LAMBDA - 1) / DEFAULT_LAMBDA;
let mut buckets = (0..buckets_len)
.map(|i| Bucket {
idx: i,
keys: vec![],
})
.collect::<Vec<_>>();
for (i, hash) in hashes.iter().enumerate() {
buckets[(hash.g % (buckets_len as u32)) as usize]
.keys
.push(i);
}
// Sort descending
buckets.sort_by(|a, b| a.keys.len().cmp(&b.keys.len()).reverse());
let table_len = hashes.len();
let mut map = vec![None; table_len];
let mut disps = vec![(0u32, 0u32); buckets_len];
// store whether an element from the bucket being placed is
// located at a certain position, to allow for efficient overlap
// checks. It works by storing the generation in each cell and
// each new placement-attempt is a new generation, so you can tell
// if this is legitimately full by checking that the generations
// are equal. (A u64 is far too large to overflow in a reasonable
// time for current hardware.)
let mut try_map = vec![0u64; table_len];
let mut generation = 0u64;
// the actual values corresponding to the markers above, as
// (index, key) pairs, for adding to the main map once we've
// chosen the right disps.
let mut values_to_add = vec![];
'buckets: for bucket in &buckets {
for d1 in 0..(table_len as u32) {
'disps: for d2 in 0..(table_len as u32) {
values_to_add.clear();
generation += 1;
for &key in &bucket.keys {
let idx = (phf_shared::displace(hashes[key].f1, hashes[key].f2, d1, d2)
% (table_len as u32)) as usize;
if map[idx].is_some() || try_map[idx] == generation {
continue 'disps;
}
try_map[idx] = generation;
values_to_add.push((idx, key));
}
// We've picked a good set of disps
disps[bucket.idx] = (d1, d2);
for &(idx, key) in &values_to_add {
map[idx] = Some(key);
}
continue 'buckets;
}
}
// Unable to find displacements for a bucket
return None;
}
Some(HashState {
key,
disps,
map: map.into_iter().map(|i| i.unwrap()).collect(),
})
}