utils: add 'Reduce()'

Add a 'Reduce()' function to collapse a given slice into a single
accumulator. 'Reduce()' loops over the slice in order and applies the
specified reduce function to the slice element and accumulator. Add unit
tests applying 'Reduce()' to various slice and accumulator types.

Signed-off-by: Victoria Dye <[email protected]>

Author: vdye

internal/utils/slices.go

@@ -9,3 +9,10 @@ func Map[T any, S any](in []T, fn func(t T) S) []S {
 	}
 	return out
 }
+
+func Reduce[T any, S any](in []T, acc S, fn func(t T, acc S) S) S {
+	for _, t := range in {
+		acc = fn(t, acc)
+	}
+	return acc
+}

internal/utils/slices_test.go

@@ -79,3 +79,58 @@ func TestMap(t *testing.T) {
 		tt.runTest(t)
 	}
 }
+
+type reduceTest[T any, S any] struct {
+	title string
+
+	// Inputs
+	in  []T
+	acc S
+	fn  func(T, S) S
+
+	// Outputs
+	expectedOut S
+}
+
+var reduceTests = []testable{
+	reduceTest[int, int]{
+		title: "int -> int",
+
+		in:  []int{1, 2, 3},
+		acc: 0,
+		fn:  func(t int, acc int) int { return acc + t },
+
+		expectedOut: 6,
+	},
+	reduceTest[string, []string]{
+		title: "string -> []string",
+
+		in:  []string{"A", "B", "C"},
+		acc: []string{"test"},
+		fn:  func(t string, acc []string) []string { return append(acc, t) },
+
+		expectedOut: []string{"test", "A", "B", "C"},
+	},
+	reduceTest[twoInts, int]{
+		title: "named struct -> int",
+
+		in:  []twoInts{{int1: 12, int2: 34}, {int1: 56, int2: 78}},
+		acc: 0,
+		fn:  func(t twoInts, acc int) int { return acc + t.int1 + t.int2 },
+
+		expectedOut: 180,
+	},
+}
+
+func (tt reduceTest[T, S]) runTest(t *testing.T) {
+	t.Run(tt.title, func(t *testing.T) {
+		out := utils.Reduce(tt.in, tt.acc, tt.fn)
+		assert.Equal(t, tt.expectedOut, out)
+	})
+}
+
+func TestReduce(t *testing.T) {
+	for _, tt := range reduceTests {
+		tt.runTest(t)
+	}
+}