fixed resuce [the third element is not skipped now]

pkg/pipe/pipe.go

@@ -159,8 +159,8 @@ func (p *Pipe[T]) Reduce(fn func(T, T) T) *T {
 		return &data[0]
 	default:
 		res := data[0]
-		for i := range data[1:] {
-			res = fn(res, data[i+1])
+		for _, val := range data[1:] {
+			res = fn(res, val)
 		}
 		return &res
 	}

pkg/pipe/pipe_test.go

@@ -293,3 +293,31 @@ func TestSort_ok_parallel_large(t *testing.T) {
 		require.GreaterOrEqual(t, item, prevItem)
 	}
 }
+
+func TestReduce(t *testing.T) {
+	res := pipe.Func(func(i int) (int, bool) {
+		return i, true
+	}).
+		Gen(6000).
+		Reduce(func(a, b int) int { return a + b })
+
+	expected := 0
+	for i := 1; i < 6000; i++ {
+		expected += i
+	}
+	require.Equal(t, expected, *res)
+}
+
+func TestSum(t *testing.T) {
+	res := pipe.Func(func(i int) (int, bool) {
+		return i, true
+	}).
+		Gen(6000).
+		Sum(func(a, b int) int { return a + b })
+
+	expected := 0
+	for i := 1; i < 6000; i++ {
+		expected += i
+	}
+	require.Equal(t, expected, *res)
+}

pkg/pipe/prefixpipe.go

@@ -30,7 +30,7 @@ func Reduce[SrcT any, DstT any](p *Pipe[SrcT], fn func(DstT, SrcT) DstT, initVal
 	default:
 		res := fn(initVal, data[0])
 		for i := range data[1:] {
-			res = fn(res, data[i+1])
+			res = fn(res, data[i])
 		}
 		return res
 	}