diff options
| -rw-r--r-- | CHANGELOG.md | 1 | ||||
| -rw-r--r-- | src/gleam/list.gleam | 24 | ||||
| -rw-r--r-- | test/gleam/list_test.gleam | 23 |
3 files changed, 48 insertions, 0 deletions
diff --git a/CHANGELOG.md b/CHANGELOG.md index cd5516a..36ef33d 100644 --- a/CHANGELOG.md +++ b/CHANGELOG.md @@ -7,6 +7,7 @@ - The `result` module gains the `lazy_or` and `lazy_unwrap` functions. - The `bool` module gains the `nand`, `nor`, `exclusive_nor`, and `exclusive_or` functions. - The `bit_builder` module gains the `from_string_builder` function. +- The `list` modules gains the `permutations` function. ## v0.12.0 - 2020-11-04 diff --git a/src/gleam/list.gleam b/src/gleam/list.gleam index 1af34e8..44d1510 100644 --- a/src/gleam/list.gleam +++ b/src/gleam/list.gleam @@ -1052,3 +1052,27 @@ pub fn partition( ) -> tuple(List(a), List(a)) { do_partition(list, categorise, [], []) } + +/// Return all the permutations of a list +/// All values must be unique +/// +/// ## Examples +/// +/// > permutations([1, 2]) +/// [[1, 2], [2, 1]] +/// +pub fn permutations(l: List(a)) -> List(List(a)) { + case l { + [] -> [[]] + _ -> + map( + l, + fn(x) { + filter(l, fn(y) { y != x }) + |> permutations + |> map(append([x], _)) + }, + ) + |> flatten + } +} diff --git a/test/gleam/list_test.gleam b/test/gleam/list_test.gleam index ff94c3b..d9a4285 100644 --- a/test/gleam/list_test.gleam +++ b/test/gleam/list_test.gleam @@ -481,3 +481,26 @@ pub fn partition_test() { |> list.partition(int.is_odd) |> should.equal(tuple([1, 3, 5, 7], [2, 4, 6])) } + +pub fn permutations_test() { + [1, 2] + |> list.permutations + |> should.equal([[1, 2], [2, 1]]) + + let expected = [ + [1, 2, 3], + [1, 3, 2], + [2, 1, 3], + [2, 3, 1], + [3, 1, 2], + [3, 2, 1], + ] + + [1, 2, 3] + |> list.permutations + |> should.equal(expected) + + ["a", "b"] + |> list.permutations + |> should.equal([["a", "b"], ["b", "a"]]) +} |