# hamming::distance - Rust
API documentation for the Rust `distance` fn in crate `hamming`.
Keywords: rust, rustlang, rust-lang, distance

# Function hamming::distance [−] [src]

    pub fn distance(x: &[u8], y: &[u8]) -> u64

Computes the bitwise Hamming distance between `x` and `y`, that is, the number of bits where `x` and `y` differ, or, the number of set bits in the xor of `x` and `y`.

When `x` and `y` have the same 8-byte alignment, this uses `distance_fast`, a highly optimised version of the following naive version:

    fn naive(x: &[u8], y: &[u8]) -> u64 {
        x.iter().zip(y).fold(0, |a, (b, c)| a + (*b ^ *c).count_ones() as u64)
    }

If alignments differ, a slower but less restrictive algorithm is used.

It is essentially guaranteed that `x` and `y` will have the same 8-byte alignment if they are both just `Vec<u8>`s of non-trivial length (e.g. larger than 8) as in the example below.

# Panics

`x` and `y` must have the same length, or else `distance` panics.

# Performance Comparison

length | `naive` (ns) | `distance` (ns) | `naive`/`distance`
--- | --- | --- | ---
1 | 5 | 6 | 0.83
10 | 44 | 45 | 0.97
100 | 461 | 473 | 0.97
1,000 | 4,510 | 397 | 11
10,000 | 46,700 | 2,740 | 17
100,000 | 45,600 | 20,400 | 22
1,000,000 | 4,590,000 | 196,000 | 23

The benchmarks ensured that `x` and `y` had the same alignment.

# Examples

    let x = vec![0xFF; 1000];
    let y = vec![0; 1000];
    assert_eq!(hamming::distance(&x, &y), 8 * 1000);

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