From 0912bacf8c5ac474a83bffc0408656b37d68ab32 Mon Sep 17 00:00:00 2001
From: NicklasXYZ <nsa200293@live.dk>
Date: Sun, 5 Feb 2023 12:19:37 +0100
Subject: [PATCH] Add cartesian_product func

---
 src/gleam_community/maths/list.gleam          | 28 +++++++++++++++++--
 .../gleam_community_maths_list_test.gleam     | 26 +++++++++++++++++
 2 files changed, 51 insertions(+), 3 deletions(-)

diff --git a/src/gleam_community/maths/list.gleam b/src/gleam_community/maths/list.gleam
index 40e7e75..c555b45 100644
--- a/src/gleam_community/maths/list.gleam
+++ b/src/gleam_community/maths/list.gleam
@@ -29,6 +29,8 @@
 import gleam/list
 import gleam/int
 import gleam/float
+import gleam/set
+import gleam/io
 
 /// <div style="text-align: right;">
 ///     <a href="https://github.com/gleam-community/maths/issues">
@@ -146,7 +148,7 @@ pub fn permutation(arr: List(a)) -> List(a) {
 ///     </a>
 /// </div>
 ///
-/// Generate a list containing all combinations of one element from each of two given lists.
+/// Generate a list containing all combinations of pairs of elements coming from two given lists.
 ///
 /// <details>
 ///     <summary>Example:</summary>
@@ -165,6 +167,26 @@ pub fn permutation(arr: List(a)) -> List(a) {
 ///     </a>
 /// </div>
 ///
-pub fn cartesian_product(xarr: List(a), yarr: List(a)) -> List(a) {
-  todo
+pub fn cartesian_product(xarr: List(a), yarr: List(a)) -> List(#(a, a)) {
+  let xset: set.Set(a) =
+    xarr
+    |> set.from_list()
+  let yset: set.Set(a) =
+    yarr
+    |> set.from_list()
+  xset
+  |> set.fold(
+    set.new(),
+    fn(accumulator0: set.Set(#(a, a)), member0: a) -> set.Set(#(a, a)) {
+      set.fold(
+        yset,
+        accumulator0,
+        fn(accumulator1: set.Set(#(a, a)), member1: a) -> set.Set(#(a, a)) {
+          io.debug(#(member0, member1))
+          set.insert(accumulator1, #(member0, member1))
+        },
+      )
+    },
+  )
+  |> set.to_list()
 }
diff --git a/test/gleam/gleam_community_maths_list_test.gleam b/test/gleam/gleam_community_maths_list_test.gleam
index 6ac7e9d..abd0b81 100644
--- a/test/gleam/gleam_community_maths_list_test.gleam
+++ b/test/gleam/gleam_community_maths_list_test.gleam
@@ -20,3 +20,29 @@ pub fn list_trim_test() {
   |> listx.trim(1, 4)
   |> should.equal(Ok([2.0, 3.0, 4.0, 5.0]))
 }
+
+pub fn list_cartesian_product_test() {
+  // An empty lists returns an empty list
+  []
+  |> listx.cartesian_product([])
+  |> should.equal([])
+
+  // Test with some arbitrary inputs
+  [1, 2, 3]
+  |> listx.cartesian_product([1, 2, 3])
+  |> should.equal([
+    #(1, 1),
+    #(1, 2),
+    #(1, 3),
+    #(2, 1),
+    #(2, 2),
+    #(2, 3),
+    #(3, 1),
+    #(3, 2),
+    #(3, 3),
+  ])
+
+  [1.0, 10.0]
+  |> listx.cartesian_product([1.0, 2.0])
+  |> should.equal([#(1.0, 1.0), #(1.0, 2.0), #(10.0, 1.0), #(10.0, 2.0)])
+}