Offset slice benchmark

offset.go

package main

import (
	"fmt"
	"testing"
)

// An offset slice seems to be the only way to implement the C idiom to
// an offset (by o) a view (v) on an array (a):
//  int *v, o;
//  int [n]a;
//  v = a - o;
//  // now v[i] is a view on a[i-o]
type offsetSlice struct {
	offset int
	slice  []int
}

func (o *offsetSlice) at(i int) int { return o.slice[i-o.offset] }
func (o *offsetSlice) set(i, v int) { o.slice[i-o.offset] = v }

func directSet(b *testing.B, o *offsetSlice, l int) {
	for i := 0; i < b.N; i++ {
		for j := 0; j < 1e3; j++ {
			o.slice[l-o.offset] = i
		}
	}
}

func methodSet(b *testing.B, o *offsetSlice, l int) {
	for i := 0; i < b.N; i++ {
		for j := 0; j < 1e3; j++ {
			o.set(l, i)
		}
	}
}

func directAt(b *testing.B, o *offsetSlice, l int) {
	for i := 0; i < b.N; i++ {
		for j := 0; j < 1e3; j++ {
			_ = o.slice[l-o.offset]
		}
	}
}

func methodAt(b *testing.B, o *offsetSlice, l int) {
	for i := 0; i < b.N; i++ {
		for j := 0; j < 1e3; j++ {
			_ = o.at(l)
		}
	}
}

func main() {
	o := &offsetSlice{
		slice:  make([]int, 5000),
		offset: 500,
	}
	for i, f := range []func(*testing.B, *offsetSlice, int){directAt, methodAt, directSet, methodSet} {
		r := testing.Benchmark(func(b *testing.B) {
			f := f
			f(b, o, 1000)
		})
		fmt.Printf("%s: %v ns / index\n", []string{"directAt", "methodAt", "directSet", "methodSet"}[i], float64(r.NsPerOp())/1e3)
	}
}