From 20904ceb0f19d4d3fb6d8cf9a5a324c0938dc3fb Mon Sep 17 00:00:00 2001
From: Louis Pilfold <louis@lpil.uk>
Date: Thu, 12 Dec 2019 10:47:23 +0000
Subject: cycle

---
 src/gleam/iterator.gleam       | 17 +++++++++++++++++
 test/gleam/iterator_test.gleam |  8 ++++++++
 2 files changed, 25 insertions(+)

diff --git a/src/gleam/iterator.gleam b/src/gleam/iterator.gleam
index a2ca7d8..a4c9361 100644
--- a/src/gleam/iterator.gleam
+++ b/src/gleam/iterator.gleam
@@ -160,3 +160,20 @@ pub fn filter(iterator: Iterator(a), for predicate: fn(a) -> Bool) -> Iterator(a
   |> do_filter(_, predicate)
   |> opaque
 }
+
+fn do_cycle(next: fn() -> Action(a), reset: fn() -> Action(a)) {
+  fn() {
+    case next() {
+      Continue(e, iterator) -> Continue(e, do_cycle(iterator, reset))
+      Stop -> do_cycle(reset, reset)()
+    }
+  }
+}
+
+// TODO: document
+pub fn cycle(iterator: Iterator(a)) -> Iterator(a) {
+  let iterator = iterator |> unopaque
+  iterator
+  |> do_cycle(_, iterator)
+  |> opaque
+}
diff --git a/test/gleam/iterator_test.gleam b/test/gleam/iterator_test.gleam
index b587d12..447e912 100644
--- a/test/gleam/iterator_test.gleam
+++ b/test/gleam/iterator_test.gleam
@@ -99,3 +99,11 @@ pub fn repeat_test() {
   |> iterator.take(_, 5)
   |> expect.equal(_, [1, 1, 1, 1, 1])
 }
+
+pub fn cycle_test() {
+  [1, 2, 3]
+  |> iterator.from_list
+  |> iterator.cycle
+  |> iterator.take(_, 9)
+  |> expect.equal(_, [1, 2, 3, 1, 2, 3, 1, 2, 3])
+}
-- 
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