From 8d83b663defdab6e036916316d60a9347a54c0df Mon Sep 17 00:00:00 2001
From: x5a17ed <0x5a17ed@tuta.io>
Date: Sat, 18 May 2024 23:48:51 +0200
Subject: [PATCH] add cycle iterator

---
 iters/valit/cycle.go      | 64 +++++++++++++++++++++++++++++++++++++++
 iters/valit/cycle_test.go | 55 +++++++++++++++++++++++++++++++++
 2 files changed, 119 insertions(+)
 create mode 100644 iters/valit/cycle.go
 create mode 100644 iters/valit/cycle_test.go

diff --git a/iters/valit/cycle.go b/iters/valit/cycle.go
new file mode 100644
index 0000000..74d0d60
--- /dev/null
+++ b/iters/valit/cycle.go
@@ -0,0 +1,64 @@
+// Copyright (c) 2024 individual contributors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// <https://www.apache.org/licenses/LICENSE-2.0>
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package valit
+
+import (
+	"github.com/0x5a17ed/itkit"
+)
+
+// CycleIterator represents an iterator yielding items from a given
+// source iterable and saving a copy of each returned item.  When the
+// given source iterable is exhausted, returns items from the saved
+// list. The iterable repeats the process forever.
+type CycleIterator[T any] struct {
+	src itkit.Iterator[T]
+
+	cur       T
+	repeating bool
+	copies    []T
+	index     int
+}
+
+// Value implements the [itkit.Iterator.Value] interface.
+func (it *CycleIterator[T]) Value() T {
+	return it.cur
+}
+
+// Next implements the [itkit.Iterator.Next] interface.
+func (it *CycleIterator[T]) Next() bool {
+	if !it.repeating {
+		if it.src.Next() {
+			it.cur = it.src.Value()
+			it.copies = append(it.copies, it.cur)
+			return true
+		}
+		it.repeating = true
+	}
+
+	if len(it.copies) == 0 {
+		return false
+	}
+
+	it.cur, it.index = it.copies[it.index], it.index+1
+	if it.index >= len(it.copies) {
+		it.index = 0
+	}
+	return true
+}
+
+// Cycle returns a new [CycleIterator] value.
+func Cycle[T any](src itkit.Iterator[T]) itkit.Iterator[T] {
+	return &CycleIterator[T]{src: src}
+}
diff --git a/iters/valit/cycle_test.go b/iters/valit/cycle_test.go
new file mode 100644
index 0000000..8cf0b5b
--- /dev/null
+++ b/iters/valit/cycle_test.go
@@ -0,0 +1,55 @@
+// Copyright (c) 2024 individual contributors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// <https://www.apache.org/licenses/LICENSE-2.0>
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package valit_test
+
+import (
+	"testing"
+
+	"github.com/0x5a17ed/itkit/iters/runeit"
+	"github.com/0x5a17ed/itkit/iters/valit"
+	"github.com/0x5a17ed/itkit/itlib"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCycle(t *testing.T) {
+	t.Run("empty", func(t *testing.T) {
+		it := valit.Cycle(itlib.Empty[int]())
+
+		assert.False(t, it.Next())
+	})
+
+	tt := []struct {
+		name string
+		inp  string
+	}{
+		{"single", "A"},
+		{"multiple", "ABCD"},
+	}
+
+	for _, tc := range tt {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			asserter := assert.New(t)
+
+			inp := []rune(tc.inp)
+
+			it := valit.Cycle[rune](runeit.InString(tc.inp))
+			for i := 0; i < len(inp)*2; i++ {
+				v := itlib.HeadOrElse(it, rune(0))
+				asserter.Equal(v, inp[i%len(tc.inp)])
+			}
+		})
+	}
+}