fix: slices.Fold and slices.FoldReverse (#40)

* Fix slices.Fold and slices.FoldReverse

* Add tests for slices.Fold and slices.FoldReverse

* Update CHANGELOG.md

* 'nil' -> 'nil slice'

* update .github/workflows/go-format.yml?

---------

Co-authored-by: Alexander Fougner <alexander.fougner@iver.se>

Author: Alexamakans

.github/workflows/go-format.yml

@@ -17,7 +17,7 @@ jobs:
     steps:
       - uses: actions/setup-go@v2
         with:
-          go-version: '1.18'
+          go-version: '1.23'
 
       - name: Install goimports
         run: go install golang.org/x/tools/cmd/goimports@latest

CHANGELOG.md

@@ -10,6 +10,10 @@ SPDX-License-Identifier: CC-BY-4.0
 
 This project tries to follow [SemVer 2.0.0](https://semver.org/).
 
+## v4.3.1 (WIP)
+
+- Fixed `slices.Fold` and `slices.FoldReverse` bugs. (#39)
+
 ## v4.3.0 (2023-05-04)
 
 - Added `sync2.Map.Len()` method. (#38)

slices/slices.go

@@ -272,7 +272,7 @@ func Filter[S ~[]E, E any](slice S, match func(value E) bool) S {
 func Fold[S ~[]E, State, E any](slice S, seed State, acc func(state State, value E) State) State {
 	state := seed
 	for _, v := range slice {
-		seed = acc(state, v)
+		state = acc(state, v)
 	}
 	return state
 }
@@ -282,8 +282,8 @@ func Fold[S ~[]E, State, E any](slice S, seed State, acc func(state State, value
 // seed value as-is if the slice is empty.
 func FoldReverse[S ~[]E, State, E any](slice S, seed State, acc func(state State, value E) State) State {
 	state := seed
-	for i := len(slice) - 1; i >= 0; i++ {
-		seed = acc(state, slice[i])
+	for i := len(slice) - 1; i >= 0; i-- {
+		state = acc(state, slice[i])
 	}
 	return state
 }

slices/slices_test.go

@@ -188,6 +188,70 @@ func TestRemoveSlice(t *testing.T) {
 	}
 }
 
+func TestFold(t *testing.T) {
+	testCases := []struct {
+		name  string
+		slice []string
+		seed  string
+		want  string
+	}{
+		{
+			name:  "values",
+			slice: []string{"a", "b", "c"},
+			seed:  "",
+			want:  "abc",
+		},
+		{
+			name:  "nil slice",
+			slice: nil,
+			seed:  "seed",
+			want:  "seed",
+		},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			gotStr := Fold(tc.slice, tc.seed, func(state string, seed string) string {
+				return state + seed
+			})
+			if gotStr != tc.want {
+				t.Errorf("want %q, got %q", tc.want, gotStr)
+			}
+		})
+	}
+}
+
+func TestFoldReverse(t *testing.T) {
+	testCases := []struct {
+		name  string
+		slice []string
+		seed  string
+		want  string
+	}{
+		{
+			name:  "values",
+			slice: []string{"a", "b", "c"},
+			seed:  "",
+			want:  "cba",
+		},
+		{
+			name:  "nil slice",
+			slice: nil,
+			seed:  "seed",
+			want:  "seed",
+		},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			gotStr := FoldReverse(tc.slice, tc.seed, func(state string, seed string) string {
+				return state + seed
+			})
+			if gotStr != tc.want {
+				t.Errorf("want %q, got %q", tc.want, gotStr)
+			}
+		})
+	}
+}
+
 func TestConcat(t *testing.T) {
 	testCases := []struct {
 		a    string