set.intersection

Author: lpil

CHANGELOG.md

@@ -6,7 +6,8 @@
   `from_list`, `fold`, `run`, `to_list`, `take`, `drop`, `map`, `filter`,
   `cycle`, and `range` functions.
 - Created the `set` module with the `new`, `insert`, `delete`, `to_list`,
-  `from_list`, `fold`, `take`, and `contains` functions.
+  `from_list`, `fold`, `take`, `union`, `intersection`, and `contains`
+  functions.
 - Created the `io` module with the `print`, `println`, and `debug` functions.
 - Created the `queue` module with the `new`, `from_list`, `to_list`,
   `is_empty`, `length`, `push_back`, `push_front`, `pop_back`, `pop_front`,

src/gleam/set.gleam

@@ -169,3 +169,44 @@ pub fn take(
 ) -> Set(member) {
   Set(map.take(from: set.map, keeping: desired))
 }
+
+fn order(
+  first: Set(member),
+  second: Set(member),
+) -> tuple(Set(member), Set(member)) {
+  case map.size(first.map) > map.size(second.map) {
+    True -> tuple(first, second)
+    False -> tuple(second, first)
+  }
+}
+
+/// Create a new set that contains all members of both given sets.
+///
+/// This function runs in loglinear time.
+///
+/// ## Examples
+///
+///     > union(from_list([1, 2]), from_list([2, 3])) |> to_list
+///     [1, 2, 3]
+///
+pub fn union(of first: Set(member), and second: Set(member)) -> Set(member) {
+  let tuple(larger, smaller) = order(first, second)
+  fold(over: smaller, from: larger, with: fn(m, a) { insert(a, m) })
+}
+
+/// Create a new set that contains members that are present in both given sets.
+///
+/// This function runs in loglinear time.
+///
+/// ## Examples
+///
+///     > intersection(from_list([1, 2]), from_list([2, 3])) |> to_list
+///     [2]
+///
+pub fn intersection(
+  of first: Set(member),
+  and second: Set(member),
+) -> Set(member) {
+  let tuple(larger, smaller) = order(first, second)
+  take(from: larger, keeping: to_list(smaller))
+}

test/gleam/set_test.gleam

@@ -79,3 +79,15 @@ pub fn take_test() {
   |> set.take([1, 3, 5])
   |> should.equal(set.from_list([1, 3]))
 }
+
+pub fn union_test() {
+  set.union(set.from_list([1, 2]), set.from_list([2, 3]))
+  |> set.to_list
+  |> should.equal([1, 2, 3])
+}
+
+pub fn intersection_test() {
+  set.intersection(set.from_list([1, 2]), set.from_list([2, 3]))
+  |> set.to_list
+  |> should.equal([2])
+}