feat: add find_index interface to array (#558)

* feat: add find_index interface to array

* update

* less return

---------

Co-authored-by: Hongbo Zhang <bobzhang1988@gmail.com>

array/array_test.mbt

@@ -441,3 +441,9 @@ test "shuffle" {
   let shuffled = Array::shuffle(arr, ~rand)
   inspect(shuffled, content="[2, 7, 6, 1, 5, 3, 4]")?
 }
+
+test "find_index" {
+  let arr : Array[Int] = [1, 2, 3, 4, 5, 6, 7]
+  let index = arr.find_index(fn(x) { x == 3 })
+  @assertion.assert_eq(index, Some(2))?
+}

builtin/array.mbt

@@ -955,3 +955,24 @@ pub fn to_string[T : Show](self : Array[T]) -> String {
     continue i + 1, "\(init), \(cur)"
   }
 }
+
+/// Find the index of the first element that satisfies the predicate.
+//
+/// # Example
+///
+/// ```
+/// let v = [1, 2, 3, 4, 5]
+/// match v.find_index(fn(x) { x == 3 }) {
+///   Some(index) => println(index) // 2
+///   None => println("Not found")
+/// }
+/// ```
+pub fn find_index[T](self : Array[T], f : (T) -> Bool) -> Int? {
+  for i = 0; i < self.length(); i = i + 1 {
+    if f(self[i]) {
+      break Some(i)
+    }
+  } else {
+    None
+  }
+}

builtin/builtin.mbti

@@ -50,6 +50,7 @@ impl Array {
   ends_with[T : Eq](Self[T], Self[T]) -> Bool
   extract_if[T](Self[T], (T) -> Bool) -> Self[T]
   filter[T](Self[T], (T) -> Bool) -> Self[T]
+  find_index[T](Self[T], (T) -> Bool) -> Int?
   flatten[T](Self[Self[T]]) -> Self[T]
   fold_left[T, U](Self[T], (U, T) -> U, ~init : U) -> U
   fold_lefti[T, U](Self[T], (Int, U, T) -> U, ~init : U) -> U