add list.shuffle (#360)

3 files changed, 66 insertions, 0 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5e60f82..215e54b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -11,6 +11,7 @@
 - Fixed a bug where `regex.scan` would not work correctly on utf8.
 - The performance of `list.flatten` has been greatly improved.
 - The `string_builder` module gains the `join` function.
+- The `list` module gains the  `shuffle` function.
 
 ## v0.24.0 - 2022-10-15
 
diff --git a/src/gleam/list.gleam b/src/gleam/list.gleam
index 04d311e..268642d 100644
--- a/src/gleam/list.gleam
+++ b/src/gleam/list.gleam
@@ -23,6 +23,7 @@
 ////
 
 import gleam/int
+import gleam/float
 import gleam/order.{Order}
 import gleam/pair
 
@@ -1822,3 +1823,44 @@ pub fn transpose(list_of_list: List(List(a))) -> List(List(a)) {
     }
   }
 }
+
+fn do_shuffle_pair_unwrap(list: List(#(Float, a)), acc: List(a)) -> List(a) {
+  case list {
+    [] -> acc
+    _ -> {
+      let [elem_pair, ..enumerable] = list
+      do_shuffle_pair_unwrap(enumerable, [elem_pair.1, ..acc])
+    }
+  }
+}
+
+fn do_shuffle_by_pair_indexes(
+  list_of_pairs: List(#(Float, a)),
+) -> List(#(Float, a)) {
+  sort(
+    list_of_pairs,
+    fn(a_pair: #(Float, a), b_pair: #(Float, a)) -> Order {
+      float.compare(a_pair.0, b_pair.0)
+    },
+  )
+}
+
+/// Takes a list, randomly sorts all items and returns the shuffled list.
+///
+/// ## Example
+///
+/// ```gleam
+/// list.range(1, 10)
+/// |> list.shuffle()
+/// > [1, 6, 9, 10, 3, 8, 4, 2, 7, 5]
+/// ```
+///
+/// Notice: This function uses Erlang's `:rand` module or Javascript's
+/// `Math.random()` to calcuate the index shuffling.
+///
+pub fn shuffle(list: List(a)) -> List(a) {
+  list
+  |> fold(from: [], with: fn(acc, a) { [#(float.random(0.0, 1.0), a), ..acc] })
+  |> do_shuffle_by_pair_indexes()
+  |> do_shuffle_pair_unwrap([])
+}
diff --git a/test/gleam/list_test.gleam b/test/gleam/list_test.gleam
index 13dbbcb..ec95af4 100644
--- a/test/gleam/list_test.gleam
+++ b/test/gleam/list_test.gleam
@@ -1081,3 +1081,26 @@ pub fn transpose_test() {
   list.transpose([[1, 2, 3], [101, 102], [201, 202, 203]])
   |> should.equal([[1, 101, 201], [2, 102, 202], [3, 203]])
 }
+
+pub fn shuffle_test() {
+  []
+  |> list.shuffle
+  |> should.equal([])
+
+  [1, 1]
+  |> list.shuffle
+  |> should.equal([1, 1])
+
+  [1, 1, 1]
+  |> list.shuffle
+  |> should.equal([1, 1, 1])
+
+  list.range(1, 100)
+  |> list.shuffle
+  |> list.sort(int.compare)
+  |> should.equal(list.range(1, 100))
+
+  // TCO test
+  list.range(0, recursion_test_cycles)
+  |> list.shuffle()
+}