From d35bd9ef2e77ae5b552f6f0ea1808593144ca7b4 Mon Sep 17 00:00:00 2001
From: Kirill Zhuravlev <kazhuravlev@fastmail.com>
Date: Sun, 19 May 2024 12:59:57 +0200
Subject: [PATCH] add new slice helper

---
 slice.go      | 28 ++++++++++++++++++++++++++++
 slice_test.go | 46 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 74 insertions(+)

diff --git a/slice.go b/slice.go
index fc9e11d..b2907aa 100644
--- a/slice.go
+++ b/slice.go
@@ -742,3 +742,31 @@ func SliceShuffleCopy[T any](in []T) []T {
 
 	return res
 }
+
+// SliceLastN return up to last n elements from input slice in original order.
+func SliceLastN[T any](in []T, n int) []T {
+	if n < 0 {
+		panic("n should be greater than 0")
+	}
+
+	inLen := len(in)
+
+	if inLen == 0 || n == 0 {
+		return make([]T, 0)
+	}
+
+	if inLen == n {
+		res := make([]T, n)
+		copy(res, in)
+
+		return res
+	}
+
+	n = Min(inLen, n)
+	res := make([]T, n)
+	for i := n - 1; i >= 0; i-- {
+		res[i] = in[inLen-n+i]
+	}
+
+	return res
+}
diff --git a/slice_test.go b/slice_test.go
index 50ac4bf..eb61bef 100644
--- a/slice_test.go
+++ b/slice_test.go
@@ -1816,3 +1816,49 @@ func TestSliceShuffleCopy(t *testing.T) {
 		})
 	}
 }
+
+func TestSliceLastN(t *testing.T) {
+	table := []struct {
+		in  []int
+		n   int
+		exp []int
+	}{
+		{
+			in:  nil,
+			n:   10,
+			exp: []int{},
+		},
+		{
+			in:  nil,
+			n:   0,
+			exp: []int{},
+		},
+		{
+			in:  []int{10},
+			n:   0,
+			exp: []int{},
+		},
+		{
+			in:  []int{10},
+			n:   1,
+			exp: []int{10},
+		},
+		{
+			in:  []int{10},
+			n:   6,
+			exp: []int{10},
+		},
+		{
+			in:  []int{10, 20, 30, 40, 50},
+			n:   2,
+			exp: []int{40, 50},
+		},
+	}
+
+	for _, row := range table {
+		t.Run("", func(t *testing.T) {
+			res := just.SliceLastN(row.in, row.n)
+			require.Equal(t, row.exp, res)
+		})
+	}
+}