Simple generic slice filtering function for Golang 1.18+

slicefilter.go

package slicefilter

// FilterSliceFunc filters the input slice based on a pass function
// and returns the filtered slice
// it filters inplace and modifies the input slice, so be careful!
func FilterSliceFunc[S any](in []S, pass func(S) bool) []S {
	outIdx := 0
	for _, v := range in {
		if !pass(v) {
			continue
		}

		in[outIdx] = v
		outIdx++
	}

	return in[:outIdx]
}

slicefilter_test.go

package slicefilter

import (
	"testing"

	"github.com/stretchr/testify/assert"
)

func TestFilter(t *testing.T) {
	a := []int{1, 2, 5, 4, 1, 2, 3, 6, 7, 0, 1, 4, 0, 1, 4, 1}
	b := []int{2, 5, 4, 2, 3, 6, 7, 0, 4, 0, 4}

	res := FilterSliceFunc(a, func(v int) bool {
		return v != 1
	})

	assert.Equal(t, b, res)
}

func TestEmpty(t *testing.T) {
	a := []int{2, 5, 4, 1, 2, 3, 6, 1, 7, 0, 1, 4, 0, 1, 4, 1}
	b := []int{}

	res := FilterSliceFunc(a, func(int) bool {
		return false
	})

	assert.Equal(t, b, res)
}

func BenchmarkNop(b *testing.B) {
	a := []int{2, 5, 4, 1, 2, 3, 6, 1, 7, 0, 1, 4, 0, 1, 4, 1}

	for i := 0; i < b.N; i++ {
		FilterSliceFunc(a, func(int) bool {
			return true
		})
	}
}

func BenchmarkFilter(b *testing.B) {
	for i := 0; i < b.N; i++ {
		a := []int{75, 13, 46, 88, 74, 58, 28, 19, 48, 51,
			73, 10, 94, 52, 35, 9, 71, 1, 41, 50,
			37, 60, 98, 86, 17, 30, 27, 55, 82, 100,
			23, 90, 33, 38, 8, 24, 7, 57, 56, 54,
			77, 39, 83, 70, 2, 44, 3, 18, 78, 32,
			95, 65, 34, 76, 84, 14, 97, 85, 80, 5,
			42, 89, 79, 68, 25, 49, 93, 99, 59, 11,
			69, 4, 66, 96, 87, 61, 92, 43, 40, 20,
			67, 53, 47, 72, 63, 91, 16, 12, 29, 6,
			21, 81, 64, 15, 22, 62, 36, 31, 26, 45}

		FilterSliceFunc(a, func(v int) bool {
			return v < 50
		})
	}
}