add exslices.Slice function

Author: ImSingee

exslices/exslices.go

@@ -0,0 +1,46 @@
+package exslices
+
+// Slice returns a slice view of arr from start up to, but not including, end.
+// Negative indices are resolved from the end, and an omitted end means len(arr).
+func Slice[T any](arr []T, start int, end_ ...int) []T {
+	length := len(arr)
+
+	if length == 0 {
+		return []T{}
+	}
+
+	begin := start
+	if begin < 0 {
+		begin = length + begin
+		if begin < 0 {
+			begin = 0
+		}
+	}
+	if begin > length {
+		begin = length
+	}
+
+	end := length
+	if len(end_) > 0 {
+		end = end_[0]
+		if end < 0 {
+			end = length + end
+		}
+		if end < 0 {
+			end = 0
+		}
+		if end > length {
+			end = length
+		}
+	}
+
+	if end < begin {
+		end = begin
+	}
+
+	if end == begin {
+		return []T{}
+	}
+
+	return arr[begin:end]
+}

exslices/exslices_test.go

@@ -0,0 +1,137 @@
+package exslices
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestSlice(t *testing.T) {
+	nums := []int{0, 1, 2, 3, 4, 5}
+
+	t.Run("only provide start", func(t *testing.T) {
+		assert.Equal(t,
+			[]int{0, 1, 2, 3, 4, 5},
+			Slice(nums, 0),
+		)
+
+		assert.Equal(t,
+			[]int{2, 3, 4, 5},
+			Slice(nums, 2),
+		)
+
+		assert.Equal(t,
+			[]int{5},
+			Slice(nums, 5),
+		)
+
+		assert.Equal(t,
+			[]int{},
+			Slice(nums, 6),
+		)
+
+		assert.Equal(t,
+			[]int{},
+			Slice(nums, 999),
+		)
+	})
+
+	t.Run("provide start and end", func(t *testing.T) {
+		assert.Equal(t,
+			[]int{0, 1, 2, 3, 4, 5},
+			Slice(nums, 0, 6),
+		)
+
+		assert.Equal(t,
+			[]int{0, 1, 2, 3},
+			Slice(nums, 0, 4),
+		)
+
+		assert.Equal(t,
+			[]int{2, 3},
+			Slice(nums, 2, 4),
+		)
+
+		assert.Equal(t,
+			[]int{1, 2, 3, 4},
+			Slice(nums, 1, 5),
+		)
+
+		assert.Equal(t,
+			[]int{1, 2, 3, 4, 5},
+			Slice(nums, 1, 999),
+		)
+
+		assert.Equal(t,
+			[]int{},
+			Slice(nums, 1, 1),
+		)
+
+		assert.Equal(t,
+			[]int{},
+			Slice(nums, 999, 1),
+		)
+	})
+
+	t.Run("negative start", func(t *testing.T) {
+		assert.Equal(t,
+			[]int{4, 5},
+			Slice(nums, -2),
+		)
+
+		assert.Equal(t,
+			[]int{0, 1, 2, 3, 4, 5},
+			Slice(nums, -999),
+		)
+	})
+
+	t.Run("negative end", func(t *testing.T) {
+		assert.Equal(t,
+			[]int{0, 1, 2, 3, 4},
+			Slice(nums, 0, -1),
+		)
+
+		assert.Equal(t,
+			[]int{0, 1, 2, 3},
+			Slice(nums, 0, -2),
+		)
+
+		assert.Equal(t,
+			[]int{},
+			Slice(nums, 0, -999),
+		)
+	})
+
+	t.Run("negative start and end", func(t *testing.T) {
+		assert.Equal(t,
+			[]int{1, 2, 3, 4},
+			Slice(nums, -5, -1),
+		)
+
+		assert.Equal(t,
+			[]int{3},
+			Slice(nums, -3, -2),
+		)
+
+		assert.Equal(t,
+			[]int{},
+			Slice(nums, -1, -999),
+		)
+
+		assert.Equal(t,
+			[]int{0, 1, 2, 3, 4},
+			Slice(nums, -999, -1),
+		)
+	})
+
+	t.Run("returning result must be a reference", func(t *testing.T) {
+		nums := []int{0, 1, 2, 3, 4, 5}
+
+		view := Slice(nums, 1, -1)
+		assert.Equal(t, []int{1, 2, 3, 4}, view)
+
+		view[1] = 999
+		assert.Equal(t, []int{1, 999, 3, 4}, view)
+		assert.Equal(t, []int{0, 1, 999, 3, 4, 5}, nums)
+	})
+}