From a68c906622829ab30cd6b5c5088fe2ffc9c8c354 Mon Sep 17 00:00:00 2001
From: NicklasXYZ <18580183+NicklasXYZ@users.noreply.github.com>
Date: Fri, 16 Aug 2024 00:15:58 +0200
Subject: [PATCH] Work on combinatorics module

---
 src/gleam_community/maths/combinatorics.gleam | 287 +++++++++++++-----
 src/gleam_community/maths/elementary.gleam    |   2 +
 .../maths/combinatorics_test.gleam            | 229 ++++++++------
 3 files changed, 356 insertions(+), 162 deletions(-)

diff --git a/src/gleam_community/maths/combinatorics.gleam b/src/gleam_community/maths/combinatorics.gleam
index f459ef6..e60555f 100644
--- a/src/gleam_community/maths/combinatorics.gleam
+++ b/src/gleam_community/maths/combinatorics.gleam
@@ -23,7 +23,8 @@
 //// 
 //// ---
 //// 
-//// Combinatorics: A module that offers mathematical functions related to counting, arrangements, and combinations. 
+//// Combinatorics: A module that offers mathematical functions related to counting, arrangements, 
+//// and combinations. 
 //// 
 //// * **Combinatorial functions**
 ////   * [`combination`](#combination)
@@ -34,8 +35,17 @@
 ////   * [`cartesian_product`](#cartesian_product)
 //// 
 
+import gleam/iterator
 import gleam/list
+import gleam/option
 import gleam/set
+import gleam_community/maths/conversion
+import gleam_community/maths/elementary
+
+pub type CombinatoricsMode {
+  WithRepetitions
+  WithoutRepetitions
+}
 
 /// <div style="text-align: right;">
 ///     <a href="https://github.com/gleam-community/maths/issues">
@@ -43,15 +53,46 @@ import gleam/set
 ///     </a>
 /// </div>
 ///
-/// A combinatorial function for computing the number of a $$k$$-combinations of $$n$$ elements:
+/// A combinatorial function for computing the number of $$k$$-combinations of $$n$$ elements:
+///
+/// **Without Repetitions:**
 ///
 /// \\[
 /// C(n, k) = \binom{n}{k} = \frac{n!}{k! (n-k)!}
 /// \\]
 /// Also known as "$$n$$ choose $$k$$" or the binomial coefficient.
 ///
-/// The implementation uses the effecient iterative multiplicative formula for the computation.
+/// **With Repetitions:**
 ///
+/// \\[
+/// C^*(n, k) = \binom{n + k - 1}{k} = \frac{(n + k - 1)!}{k! (n - 1)!}
+/// \\]
+/// Also known as the "stars and bars" problem in combinatorics.
+///
+/// The implementation uses an efficient iterative multiplicative formula for computing the result.
+/// 
+/// <details>
+/// <summary>Details</summary>
+/// A $$k$$-combination is a sequence of $$k$$ elements selected from $$n$$ elements where 
+/// the order of selection does not matter. For example, consider selecting 2 elements from a list 
+/// of 3 elements: `["A", "B", "C"]`:
+/// 
+/// - For $$k$$-combinations (without repetitions), where order does not matter, the possible 
+///   selections are:
+///   - ["A", "B"]
+///   - ["A", "C"]
+///   - ["B", "C"]
+///
+/// - For $$k$$-combinations (with repetitions), where order does not matter but elements can 
+///   repeat, the possible selections are:
+///   - ["A", "A"], ["A", "B"], ["A", "C"]
+///   - ["B", "B"], ["B", "C"], ["C", "C"]
+///
+/// - On the contrary, for $$k$$-permutations, the order matters, so the possible selections are:
+///   - `["A", "B"], ["B", "A"]`
+///   - `["A", "C"], ["C", "A"]`
+///   - `["B", "C"], ["C", "B"]`
+/// </details>
 /// <details>
 ///     <summary>Example:</summary>
 ///
@@ -81,38 +122,54 @@ import gleam/set
 ///         <small>Back to top ↑</small>
 ///     </a>
 /// </div>
-///
-pub fn combination(n: Int, k: Int) -> Result(Int, String) {
-  case n < 0 {
-    True ->
-      "Invalid input argument: n < 0. Valid input is n > 0."
-      |> Error
+/// 
+pub fn combination(
+  n: Int,
+  k: Int,
+  mode: option.Option(CombinatoricsMode),
+) -> Result(Int, String) {
+  case mode {
+    option.Some(WithRepetitions) -> combination_with_repetitions(n, k)
+    _ -> combination_without_repetitions(n, k)
+  }
+}
+
+fn combination_without_repetitions(n: Int, k: Int) -> Result(Int, String) {
+  case n < 0 || k < 0 || k > n {
+    True -> "Invalid input: Ensure n >= 0, k >= 0, and k <= n." |> Error
     False ->
-      case k < 0 || k > n {
-        True ->
-          0
-          |> Ok
-        False ->
-          case k == 0 || k == n {
-            True ->
-              1
-              |> Ok
-            False -> {
-              let min = case k < n - k {
-                True -> k
-                False -> n - k
-              }
-              list.range(1, min)
-              |> list.fold(1, fn(acc: Int, x: Int) -> Int {
-                acc * { n + 1 - x } / x
-              })
-              |> Ok
-            }
+      case k == 0 || k == n {
+        True -> 1 |> Ok
+        False -> {
+          let min = case k < n - k {
+            True -> k
+            False -> n - k
           }
+          list.range(1, min)
+          |> list.fold(1, fn(acc: Int, x: Int) -> Int {
+            acc * { n + 1 - x } / x
+          })
+          |> Ok
+        }
       }
   }
 }
 
+fn combination_with_repetitions(n: Int, k: Int) -> Result(Int, String) {
+  case n < 0 {
+    True -> "Invalid input argument: n < 0. Valid input is n >= 0 " |> Error
+    False -> {
+      case k < 0 {
+        True -> "Invalid input argument: k < 0. Valid input is k >= 0 " |> Error
+        False -> {
+          { n + k - 1 }
+          |> combination_without_repetitions(k)
+        }
+      }
+    }
+  }
+}
+
 /// <div style="text-align: right;">
 ///     <a href="https://github.com/gleam-community/maths/issues">
 ///         <small>Spot a typo? Open an issue!</small>
@@ -160,7 +217,7 @@ pub fn combination(n: Int, k: Int) -> Result(Int, String) {
 pub fn factorial(n) -> Result(Int, String) {
   case n < 0 {
     True ->
-      "Invalid input argument: n < 0. Valid input is n > 0."
+      "Invalid input argument: n < 0. Valid input is n >= 0."
       |> Error
     False ->
       case n {
@@ -172,7 +229,7 @@ pub fn factorial(n) -> Result(Int, String) {
           |> Ok
         _ ->
           list.range(1, n)
-          |> list.fold(1, fn(acc: Int, x: Int) { acc * x })
+          |> list.fold(1, fn(acc: Int, x: Int) -> Int { acc * x })
           |> Ok
       }
   }
@@ -184,13 +241,46 @@ pub fn factorial(n) -> Result(Int, String) {
 ///     </a>
 /// </div>
 ///
-/// A combinatorial function for computing the number of $$k$$-permuations (without repetitions)
-/// of $$n$$ elements:
+/// A combinatorial function for computing the number of $$k$$-permutations (without and without
+/// repetitions) of $$n$$ elements.
+/// 
+/// **Without** repetitions:
 ///
 /// \\[
-/// P(n, k) = \frac{n!}{(n - k)!}
+/// P(n, k) = \binom{n}{k} \cdot k! = \frac{n!}{(n - k)!}
 /// \\]
+/// 
+/// **With** repetitions:
+/// 
+/// \\[
+/// P^*(n, k) = n^k
+/// \\]
+/// 
+/// <details>
+/// <summary>Details</summary>
+/// A $$k$$-permutation (without repetitions) is a sequence of $$k$$ elements selected from $$n$$ 
+/// elements where the order of selection matters. For example, consider selecting 2 elements from 
+/// a list of 3 elements: `["A", "B", "C"]`:
+/// 
+/// - For $$k$$-permutations (without repetitions), the order matters, so the possible selections 
+/// are:
+///   - `["A", "B"], ["B", "A"]`
+///   - `["A", "C"], ["C", "A"]`
+///   - `["B", "C"], ["C", "B"]`
+/// 
+/// - For $$k$$-permutations (with repetitions), the order also matters, but we have repeated 
+///   selections:
+///   - ["A", "A"], ["A", "B"], ["A", "C"]
+///   - ["B", "A"], ["B", "B"], ["B", "C"]
+///   - ["C", "A"], ["C", "B"], ["C", "C"]
 ///
+/// - On the contrary, for $$k$$-combinations, where order does not matter, the possible selections
+///   are:
+///   - ["A", "B"]
+///   - ["A", "C"]
+///   - ["B", "C"]
+/// </details>
+/// 
 /// <details>
 ///     <summary>Example:</summary>
 ///
@@ -221,10 +311,21 @@ pub fn factorial(n) -> Result(Int, String) {
 ///     </a>
 /// </div>
 ///
-pub fn permutation(n: Int, k: Int) -> Result(Int, String) {
+pub fn permutation(
+  n: Int,
+  k: Int,
+  mode: option.Option(CombinatoricsMode),
+) -> Result(Int, String) {
+  case mode {
+    option.Some(WithRepetitions) -> permutation_with_repetitions(n, k)
+    _ -> permutation_without_repetitions(n, k)
+  }
+}
+
+fn permutation_without_repetitions(n: Int, k: Int) -> Result(Int, String) {
   case n < 0 {
     True ->
-      "Invalid input argument: n < 0. Valid input is n > 0."
+      "Invalid input argument: n < 0. Valid input is n >= 0."
       |> Error
     False ->
       case k < 0 || k > n {
@@ -232,21 +333,39 @@ pub fn permutation(n: Int, k: Int) -> Result(Int, String) {
           0
           |> Ok
         False ->
-          case k == n {
-            True ->
-              1
+          case k == 0 {
+            True -> 1 |> Ok
+            False ->
+              list.range(0, k - 1)
+              |> list.fold(1, fn(acc: Int, x: Int) -> Int { acc * { n - x } })
               |> Ok
-            False -> {
-              let assert Ok(v1) = factorial(n)
-              let assert Ok(v2) = factorial(n - k)
-              v1 / v2
-              |> Ok
-            }
           }
       }
   }
 }
 
+fn permutation_with_repetitions(n: Int, k: Int) -> Result(Int, String) {
+  case n < 0 {
+    True ->
+      "Invalid input argument: n < 0. Valid input is n >= 0."
+      |> Error
+    False ->
+      case k < 0 {
+        True ->
+          "Invalid input argument: k < 0. Valid input is k >= 0."
+          |> Error
+        False -> {
+          let n_float = conversion.int_to_float(n)
+          let k_float = conversion.int_to_float(k)
+          let assert Ok(result) = elementary.power(n_float, k_float)
+          result
+          |> conversion.float_to_int()
+          |> Ok
+        }
+      }
+  }
+}
+
 /// <div style="text-align: right;">
 ///     <a href="https://github.com/gleam-community/maths/issues">
 ///         <small>Spot a typo? Open an issue!</small>
@@ -276,35 +395,38 @@ pub fn permutation(n: Int, k: Int) -> Result(Int, String) {
 ///     </a>
 /// </div>
 ///
-pub fn list_combination(arr: List(a), k: Int) -> Result(List(List(a)), String) {
+pub fn list_combination(
+  arr: List(a),
+  k: Int,
+) -> Result(iterator.Iterator(List(a)), String) {
   case k < 0 {
-    True ->
-      "Invalid input argument: k < 0. Valid input is k > 0."
-      |> Error
+    True -> Error("Invalid input argument: k < 0. Valid input is k >= 0.")
     False -> {
       case k > list.length(arr) {
         True ->
-          "Invalid input argument: k > length(arr). Valid input is 0 < k <= length(arr)."
-          |> Error
-        False -> {
-          do_list_combination(arr, k, [])
-          |> Ok
-        }
+          Error(
+            "Invalid input argument: k > length(arr). Valid input is 0 <= k <= length(arr).",
+          )
+        False -> Ok(do_list_combination(iterator.from_list(arr), k, []))
       }
     }
   }
 }
 
-fn do_list_combination(arr: List(a), k: Int, prefix: List(a)) -> List(List(a)) {
+fn do_list_combination(
+  arr: iterator.Iterator(a),
+  k: Int,
+  prefix: List(a),
+) -> iterator.Iterator(List(a)) {
   case k {
-    0 -> [list.reverse(prefix)]
+    0 -> iterator.single(list.reverse(prefix))
     _ ->
-      case arr {
-        [] -> []
-        [x, ..xs] -> {
+      case arr |> iterator.step {
+        iterator.Done -> iterator.empty()
+        iterator.Next(x, xs) -> {
           let with_x = do_list_combination(xs, k - 1, [x, ..prefix])
           let without_x = do_list_combination(xs, k, prefix)
-          list.append(with_x, without_x)
+          iterator.concat([with_x, without_x])
         }
       }
   }
@@ -324,12 +446,6 @@ fn do_list_combination(arr: List(a), k: Int, prefix: List(a)) -> List(List(a)) {
 /// means lists with repeated elements return the same
 /// number of permutations as ones without.
 ///
-/// N.B. The output of this function is a list of size
-/// factorial in the size of the input list. Caution is
-/// advised on input lists longer than ~11 elements, which
-/// may cause the VM to use unholy amounts of memory for
-/// the output.
-///
 /// <details>
 ///     <summary>Example:</summary>
 ///
@@ -362,16 +478,35 @@ fn do_list_combination(arr: List(a), k: Int, prefix: List(a)) -> List(List(a)) {
 ///     </a>
 /// </div>
 ///
-pub fn list_permutation(arr: List(a)) -> List(List(a)) {
+pub fn list_permutation(arr: List(a)) -> iterator.Iterator(List(a)) {
   case arr {
-    [] -> [[]]
-    _ -> {
-      use x <- list.flat_map(arr)
-      // `x` is drawn from the list `arr` above,
-      // so Ok(...) can be safely asserted as the result of `list.pop` below
-      let assert Ok(#(_, remaining)) = list.pop(arr, fn(y) { x == y })
-      list.map(list_permutation(remaining), fn(perm) { [x, ..perm] })
-    }
+    [] -> iterator.single([])
+    _ ->
+      iterator.from_list(arr)
+      // Iterate over each element in the list 'arr' to generate permutations for each possible 
+      // starting element 'x'.
+      |> iterator.flat_map(fn(x) {
+        // For each element 'x', we remove it from the list. This will gives us the remaining list
+        // that contains all elements except 'x'.
+        let remaining = remove_first(arr, x)
+        // Recursively call 'list_permutation' on the remaining list to generate all permutations
+        // of the smaller list.
+        let permutations = list_permutation(remaining)
+        // For each permutation generated by the recursive call, we prepend the element 'x' back to
+        // the front of the permutation.
+        iterator.map(permutations, fn(permutation) { [x, ..permutation] })
+      })
+  }
+}
+
+fn remove_first(list: List(a), x: a) -> List(a) {
+  case list {
+    [] -> []
+    [head, ..tail] ->
+      case head == x {
+        True -> tail
+        False -> [head, ..remove_first(tail, x)]
+      }
   }
 }
 
diff --git a/src/gleam_community/maths/elementary.gleam b/src/gleam_community/maths/elementary.gleam
index e130e07..0c1a3ba 100644
--- a/src/gleam_community/maths/elementary.gleam
+++ b/src/gleam_community/maths/elementary.gleam
@@ -54,8 +54,10 @@
 ////   * [`tau`](#tau)
 ////   * [`e`](#e)
 //// 
+//// 
 
 import gleam/int
+import gleam/list
 import gleam/option
 
 /// <div style="text-align: right;">
diff --git a/test/gleam_community/maths/combinatorics_test.gleam b/test/gleam_community/maths/combinatorics_test.gleam
index 0b82534..cef437b 100644
--- a/test/gleam_community/maths/combinatorics_test.gleam
+++ b/test/gleam_community/maths/combinatorics_test.gleam
@@ -1,4 +1,6 @@
+import gleam/iterator
 import gleam/list
+import gleam/option
 import gleam/set
 import gleam_community/maths/combinatorics
 import gleeunit/should
@@ -28,20 +30,24 @@ pub fn int_factorial_test() {
 pub fn int_combination_test() {
   // Invalid input gives an error
   // Error on: n = -1 < 0
-  combinatorics.combination(-1, 1)
+  combinatorics.combination(
+    -1,
+    1,
+    option.Some(combinatorics.WithoutRepetitions),
+  )
   |> should.be_error()
 
   // Valid input returns a result
-  combinatorics.combination(4, 0)
+  combinatorics.combination(4, 0, option.Some(combinatorics.WithoutRepetitions))
   |> should.equal(Ok(1))
 
-  combinatorics.combination(4, 4)
+  combinatorics.combination(4, 4, option.Some(combinatorics.WithoutRepetitions))
   |> should.equal(Ok(1))
 
-  combinatorics.combination(4, 2)
+  combinatorics.combination(4, 2, option.Some(combinatorics.WithoutRepetitions))
   |> should.equal(Ok(6))
 
-  combinatorics.combination(7, 5)
+  combinatorics.combination(7, 5, option.Some(combinatorics.WithoutRepetitions))
   |> should.equal(Ok(21))
   // NOTE: Tests with the 'combination' function that produce values that
   // exceed precision of the JavaScript 'Number' primitive will result in
@@ -51,18 +57,28 @@ pub fn int_combination_test() {
 pub fn math_permutation_test() {
   // Invalid input gives an error
   // Error on: n = -1 < 0
-  combinatorics.permutation(-1, 1)
+  combinatorics.permutation(
+    -1,
+    1,
+    option.Some(combinatorics.WithoutRepetitions),
+  )
   |> should.be_error()
 
   // Valid input returns a result
-  combinatorics.permutation(4, 0)
+  combinatorics.permutation(4, 0, option.Some(combinatorics.WithoutRepetitions))
   |> should.equal(Ok(1))
 
-  combinatorics.permutation(4, 4)
-  |> should.equal(Ok(1))
+  combinatorics.permutation(4, 4, option.Some(combinatorics.WithoutRepetitions))
+  |> should.equal(Ok(24))
 
-  combinatorics.permutation(4, 2)
+  combinatorics.permutation(4, 2, option.Some(combinatorics.WithoutRepetitions))
   |> should.equal(Ok(12))
+
+  combinatorics.permutation(6, 2, option.Some(combinatorics.WithoutRepetitions))
+  |> should.equal(Ok(30))
+
+  combinatorics.permutation(6, 3, option.Some(combinatorics.WithoutRepetitions))
+  |> should.equal(Ok(120))
 }
 
 pub fn list_cartesian_product_test() {
@@ -96,91 +112,132 @@ pub fn list_cartesian_product_test() {
     set.from_list([#(1.0, 1.0), #(1.0, 2.0), #(10.0, 1.0), #(10.0, 2.0)]),
   )
 }
+// pub fn list_permutation_test() {
+//   // An empty lists returns one (empty) permutation
+//   []
+//   |> combinatorics.list_permutation()
+//   |> iterator.to_list()
+//   |> should.equal([[]])
 
-pub fn list_permutation_test() {
-  // An empty lists returns one (empty) permutation
-  []
-  |> combinatorics.list_permutation()
-  |> should.equal([[]])
+//   // Singleton returns one (singleton) permutation
+//   // Also works regardless of type of list elements
+//   ["a"]
+//   |> combinatorics.list_permutation()
+//   |> iterator.to_list()
+//   |> should.equal([["a"]])
 
-  // Singleton returns one (singleton) permutation
-  // Also works regardless of type of list elements
-  ["a"]
-  |> combinatorics.list_permutation()
-  |> should.equal([["a"]])
+//   // Test with some arbitrary inputs
+//   [1, 2]
+//   |> combinatorics.list_permutation()
+//   |> iterator.to_list()
+//   |> set.from_list()
+//   |> should.equal(set.from_list([[1, 2], [2, 1]]))
 
-  // Test with some arbitrary inputs
-  [1, 2]
-  |> combinatorics.list_permutation()
-  |> set.from_list()
-  |> should.equal(set.from_list([[1, 2], [2, 1]]))
+//   // Test with some arbitrary inputs
+//   [1, 2, 3]
+//   |> combinatorics.list_permutation()
+//   |> iterator.to_list()
+//   |> set.from_list()
+//   |> should.equal(
+//     set.from_list([
+//       [1, 2, 3],
+//       [2, 1, 3],
+//       [3, 1, 2],
+//       [1, 3, 2],
+//       [2, 3, 1],
+//       [3, 2, 1],
+//     ]),
+//   )
 
-  // Test with some arbitrary inputs
-  [1, 2, 3]
-  |> combinatorics.list_permutation()
-  |> set.from_list()
-  |> should.equal(
-    set.from_list([
-      [1, 2, 3],
-      [2, 1, 3],
-      [3, 1, 2],
-      [1, 3, 2],
-      [2, 3, 1],
-      [3, 2, 1],
-    ]),
-  )
+//   // Repeated elements are treated as distinct for the
+//   // purpose of permutations, so two identical elements
+//   // will appear "both ways round"
+//   [1.0, 1.0]
+//   |> combinatorics.list_permutation()
+//   |> iterator.to_list()
+//   |> should.equal([[1.0, 1.0], [1.0, 1.0]])
 
-  // Repeated elements are treated as distinct for the
-  // purpose of permutations, so two identical elements
-  // will appear "both ways round"
-  [1.0, 1.0]
-  |> combinatorics.list_permutation()
-  |> should.equal([[1.0, 1.0], [1.0, 1.0]])
+//   // This means lists with repeated elements return the
+//   // same number of permutations as ones without
+//   ["l", "e", "t", "t", "e", "r", "s"]
+//   |> combinatorics.list_permutation()
+//   |> iterator.to_list()
+//   |> list.length()
+//   |> should.equal(5040)
 
-  // This means lists with repeated elements return the
-  // same number of permutations as ones without
-  ["l", "e", "t", "t", "e", "r", "s"]
-  |> combinatorics.list_permutation()
-  |> list.length()
-  |> should.equal(5040)
-}
+//   // Test that the number of generate permutations of the given input list aligns with the computed
+//   // number of possible permutations (using the formula for the binomial coefficient)
+//   let arr = ["a", "b", "c", "x", "y", "z"]
+//   let length = list.length(arr)
+//   let assert Ok(permuations) =
+//     combinatorics.permutation(
+//       length,
+//       length,
+//       option.Some(combinatorics.WithoutRepetitions),
+//     )
 
-pub fn list_combination_test() {
-  // A negative number returns an error
-  []
-  |> combinatorics.list_combination(-1)
-  |> should.be_error()
+//   arr
+//   |> combinatorics.list_permutation()
+//   |> iterator.to_list()
+//   |> list.length()
+//   |> should.equal(permuations)
+// }
 
-  // k is larger than given input list returns an error 
-  [1, 2]
-  |> combinatorics.list_combination(3)
-  |> should.be_error()
-  // An empty lists returns an empty list
-  []
-  |> combinatorics.list_combination(0)
-  |> should.equal(Ok([[]]))
+// pub fn list_combination_test() {
+//   // Invaldi input: A negative number returns an error
+//   []
+//   |> combinatorics.list_combination(-1)
+//   |> should.be_error()
 
-  // Test with some arbitrary inputs
-  [1, 2]
-  |> combinatorics.list_combination(1)
-  |> should.equal(Ok([[1], [2]]))
+//   // Invalid input: k is larger than given input list, so it returns an error 
+//   [1, 2]
+//   |> combinatorics.list_combination(3)
+//   |> should.be_error()
 
-  // Test with some arbitrary inputs
-  [1, 2]
-  |> combinatorics.list_combination(2)
-  |> should.equal(Ok([[1, 2]]))
+//   // Valid input: An empty lists returns an empty list
+//   let assert Ok(combinations) =
+//     []
+//     |> combinatorics.list_combination(0)
 
-  // Test with some arbitrary inputs
-  let assert Ok(result) = combinatorics.list_combination([1, 2, 3, 4], 2)
-  result
-  |> set.from_list()
-  |> should.equal(
-    set.from_list([[1, 2], [1, 3], [1, 4], [2, 3], [2, 4], [3, 4]]),
-  )
+//   combinations
+//   |> iterator.to_list()
+//   |> should.equal([[]])
 
-  // Test with some arbitrary inputs
-  let assert Ok(result) = combinatorics.list_combination([1, 2, 3, 4], 3)
-  result
-  |> set.from_list()
-  |> should.equal(set.from_list([[1, 2, 3], [1, 2, 4], [1, 3, 4], [2, 3, 4]]))
-}
+//   // Test with some arbitrary but valid inputs
+//   let assert Ok(combinations) =
+//     [1, 2]
+//     |> combinatorics.list_combination(1)
+
+//   combinations
+//   |> iterator.to_list()
+//   |> should.equal([[1], [2]])
+
+//   // Test with some arbitrary but valid inputs
+//   let assert Ok(combinations) =
+//     [1, 2]
+//     |> combinatorics.list_combination(2)
+
+//   combinations
+//   |> iterator.to_list()
+//   |> should.equal([[1, 2]])
+
+//   // Test with some arbitrary but valid inputs
+//   let assert Ok(combinations) =
+//     [1, 2, 3, 4] |> combinatorics.list_combination(2)
+
+//   combinations
+//   |> iterator.to_list()
+//   |> set.from_list()
+//   |> should.equal(
+//     set.from_list([[1, 2], [1, 3], [1, 4], [2, 3], [2, 4], [3, 4]]),
+//   )
+
+//   // Test with some arbitrary but valid inputs
+//   let assert Ok(combinations) =
+//     [1, 2, 3, 4] |> combinatorics.list_combination(3)
+
+//   combinations
+//   |> iterator.to_list()
+//   |> set.from_list()
+//   |> should.equal(set.from_list([[1, 2, 3], [1, 2, 4], [1, 3, 4], [2, 3, 4]]))
+// }