1 //! Build support for precomputed constant hash tables.
2 //!
3 //! This module can generate constant hash tables using open addressing and quadratic probing.
4 //!
5 //! The hash tables are arrays that are guaranteed to:
6 //!
7 //! - Have a power-of-two size.
8 //! - Contain at least one empty slot.
9 
10 use std::iter;
11 
12 /// Compute an open addressed, quadratically probed hash table containing
13 /// `items`. The returned table is a list containing the elements of the
14 /// iterable `items` and `None` in unused slots.
15 pub fn generate_table<'cont, T, I: iter::Iterator<Item = &'cont T>, H: Fn(&T) -> usize>(
16     items: I,
17     num_items: usize,
18     hash_function: H,
19 ) -> Vec<Option<&'cont T>> {
20     let size = (1.20 * num_items as f64) as usize;
21 
22     // Probing code's stop condition relies on the table having one vacant entry at least.
23     let size = if size.is_power_of_two() {
24         size * 2
25     } else {
26         size.next_power_of_two()
27     };
28 
29     let mut table = vec![None; size];
30 
31     for i in items {
32         let mut h = hash_function(i) % size;
33         let mut s = 0;
34         while table[h].is_some() {
35             s += 1;
36             h = (h + s) % size;
37         }
38         table[h] = Some(i);
39     }
40 
41     table
42 }
43 
44 #[cfg(test)]
45 mod tests {
46     use super::generate_table;
47     use cranelift_codegen_shared::constant_hash::simple_hash;
48 
49     #[test]
50     fn test_generate_table() {
51         let v = vec!["Hello".to_string(), "world".to_string()];
52         let table = generate_table(v.iter(), v.len(), |s| simple_hash(&s));
53         assert_eq!(
54             table,
55             vec![
56                 None,
57                 Some(&"Hello".to_string()),
58                 Some(&"world".to_string()),
59                 None
60             ]
61         );
62     }
63 }
64