From e4e8f5a361ba5a6c9802e26900563fb9655f6789 Mon Sep 17 00:00:00 2001
From: aereal <trasty.loose@gmail.com>
Date: Mon, 17 Nov 2025 20:32:03 +0900
Subject: [PATCH] feat: add seq.Pairwise()

---
 seq/seq.go      | 25 +++++++++++++++++++++++++
 seq/seq_test.go | 44 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 69 insertions(+)

diff --git a/seq/seq.go b/seq/seq.go
index 2f263df..cf2dc7a 100644
--- a/seq/seq.go
+++ b/seq/seq.go
@@ -195,3 +195,28 @@ func FlatMap[T any, R any](s iter.Seq[T], f func(T) iter.Seq[R]) iter.Seq[R] {
 		}
 	}
 }
+
+// Pairwise returns an iterator over consecutive pairs of elements.
+// For example, [a, b, c, d] yields [(a, b), (b, c), (c, d)].
+func Pairwise[T any](s iter.Seq[T]) iter.Seq2[T, T] {
+	return func(yield func(T, T) bool) {
+		next, stop := iter.Pull(s)
+		defer stop()
+
+		prev, ok := next()
+		if !ok {
+			return
+		}
+
+		for {
+			current, ok := next()
+			if !ok {
+				return
+			}
+			if !yield(prev, current) {
+				return
+			}
+			prev = current
+		}
+	}
+}
diff --git a/seq/seq_test.go b/seq/seq_test.go
index abcef0d..2d8a765 100644
--- a/seq/seq_test.go
+++ b/seq/seq_test.go
@@ -333,6 +333,50 @@ func TestFlatMap(t *testing.T) {
 	}
 }
 
+func TestPairwise(t *testing.T) {
+	testCases := []struct {
+		name  string
+		input []string
+		want  []pair[string, string]
+	}{
+		{
+			name:  "empty",
+			input: []string{},
+			want:  []pair[string, string]{},
+		},
+		{
+			name:  "only one element",
+			input: []string{"a"},
+			want:  []pair[string, string]{},
+		},
+		{
+			name:  "ok",
+			input: []string{"a", "b", "c", "d"},
+			want: []pair[string, string]{
+				{"a", "b"},
+				{"b", "c"},
+				{"c", "d"},
+			},
+		},
+		{
+			name:  "3 elements",
+			input: []string{"a", "b", "c"},
+			want: []pair[string, string]{
+				{"a", "b"},
+				{"b", "c"},
+			},
+		},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			got := pairs(seq.Pairwise(slices.Values(tc.input)))
+			if !reflect.DeepEqual(got, tc.want) {
+				t.Errorf("result mismatch:\n\twant: %#v\n\t got: %#v", tc.want, got)
+			}
+		})
+	}
+}
+
 func list[T any](xs ...T) iter.Seq[T] {
 	return func(yield func(T) bool) {
 		for _, x := range xs {