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

    pub fn distance_fast(x: &[u8], y: &[u8]) -> Result<u64, DistanceError>

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`.

This is 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)
    }

This function requires that `x` and `y` have the same 8-byte alignment (that is, `x.as_ptr() as usize % 8 == y.as_ptr() as usize % 8`). If not, `Err` is returned. If sub-optimal performance can be tolerated, consider using `distance` which incorporates a fallback to a slower but less restrictive algorithm.

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.

This is implemented using the same tree-merging approach as `weight`, see there for details.

# Panics

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

# Performance Comparison

length | `naive` (ns) | `distance_fast` (ns) | `naive`/`distance_fast`
--- | --- | --- | ---
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

# Examples

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

    // same alignment, but moderately complicated
    assert_eq!(hamming::distance_fast(&x[1..1000 - 8], &y[8 + 1..]), Ok(8 * (1000 - 8 - 1)));

    // differing alignments
    assert!(hamming::distance_fast(&x[1..], &y[..999]).is_err());

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