add ChunkStrings for splitting a string slice into fixed-size batches

ChunkStrings cuts a slice into consecutive pieces of at most the requested
size, with the leftovers in the last piece, and returns copies so the pieces
cannot disturb each other. Comes with a doc comment and table-driven tests.
(closes #3)

Model: opus-5

chunk.go

@@ -0,0 +1,26 @@
+package util
+
+// ChunkStrings splits input into consecutive slices of at most size elements.
+// The last slice holds whatever is left over and may be shorter than the
+// others. Each returned slice is a copy, so appending to one of them cannot
+// disturb another. A size below one, or an empty input, returns nil.
+func ChunkStrings(input []string, size int) [][]string {
+	if size < 1 || len(input) == 0 {
+		return nil
+	}
+
+	out := make([][]string, 0, (len(input)+size-1)/size)
+
+	for start := 0; start < len(input); start += size {
+		end := start + size
+		if end > len(input) {
+			end = len(input)
+		}
+
+		chunk := make([]string, end-start)
+		copy(chunk, input[start:end])
+		out = append(out, chunk)
+	}
+
+	return out
+}

chunk_test.go

@@ -0,0 +1,43 @@
+package util
+
+import (
+	"reflect"
+	"testing"
+)
+
+func TestChunkStrings(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    []string
+		size     int
+		expected [][]string
+	}{
+		{"exact multiple", []string{"a", "b", "c", "d"}, 2, [][]string{{"a", "b"}, {"c", "d"}}},
+		{"with a remainder", []string{"a", "b", "c"}, 2, [][]string{{"a", "b"}, {"c"}}},
+		{"size larger than input", []string{"a", "b"}, 5, [][]string{{"a", "b"}}},
+		{"size of one", []string{"a", "b"}, 1, [][]string{{"a"}, {"b"}}},
+		{"size of zero", []string{"a", "b"}, 0, nil},
+		{"negative size", []string{"a", "b"}, -1, nil},
+		{"empty input", []string{}, 2, nil},
+		{"nil input", nil, 2, nil},
+	}
+
+	for _, test := range tests {
+		t.Run(test.name, func(t *testing.T) {
+			got := ChunkStrings(test.input, test.size)
+			if !reflect.DeepEqual(got, test.expected) {
+				t.Errorf("expected %#v got %#v", test.expected, got)
+			}
+		})
+	}
+}
+
+func TestChunkStringsReturnsCopies(t *testing.T) {
+	input := []string{"a", "b", "c", "d"}
+	chunks := ChunkStrings(input, 2)
+	chunks[0][0] = "changed"
+
+	if input[0] != "a" {
+		t.Errorf("expected the input to be unchanged, got %#v", input)
+	}
+}