From 8a5cb6c47367b19745b84a74a8a5ba1705767222 Mon Sep 17 00:00:00 2001
From: Minsang Kim <ben399399@gmail.com>
Date: Sat, 10 Feb 2024 01:07:56 +0900
Subject: [PATCH] fix: A.dropWhile behaves as what it should be

---
 __tests__/Array/dropWhile.test.ts |  2 ++
 src/Array/Array.res               | 26 +++++++++++++++++++-------
 2 files changed, 21 insertions(+), 7 deletions(-)

diff --git a/__tests__/Array/dropWhile.test.ts b/__tests__/Array/dropWhile.test.ts
index 8548131..fa7baab 100644
--- a/__tests__/Array/dropWhile.test.ts
+++ b/__tests__/Array/dropWhile.test.ts
@@ -3,6 +3,7 @@ import { expectType } from 'ts-expect'
 import { A, S, pipe } from '../..'
 
 const xs = [1, 2, 3, 4, 5, 6, 7]
+const ys = [1, 2, 3, 4, 3, 2, 1]
 
 // TODO: expectType
 describe('dropWhile', () => {
@@ -30,5 +31,6 @@ describe('dropWhile (pipe)', () => {
       A.dropWhile(x => x < 4),
     )
     expect(result).toEqual([4, 5, 6, 7])
+    expect(A.dropWhile(ys,x=>x<=3)).toEqual([4, 3, 2, 1])
   })
 })
diff --git a/src/Array/Array.res b/src/Array/Array.res
index 670e05f..7830c2d 100644
--- a/src/Array/Array.res
+++ b/src/Array/Array.res
@@ -211,14 +211,26 @@ let dropExactly = (xs, n) => n < 0 || n > length(xs) ? None : Some(Belt.Array.sl
   "Drops elements from the beginning of the array until an element is reached which does not satisfy the given predicate."
 )
 @gentype
-let dropWhile = (xs, predicateFn) =>
-  Belt.Array.reduceU(xs, [], (. acc, element) => {
-    if !predicateFn(element) {
-      Js.Array2.push(acc, element)->ignore
-    }
-    acc
-  })
+let dropWhile = (xs, predicateFn) => {
+  let index = ref(0)
+  let break = ref(false)
+  let arr = []
 
+  while index.contents < length(xs) && !break.contents {
+    let value = Belt.Array.getUnsafe(xs, index.contents)
+
+    if predicateFn(value) {
+      index := succ(index.contents)
+    } else {
+      break := true
+    }
+  }
+  while index.contents < length(xs) {
+    Js.Array2.push(arr, Belt.Array.getUnsafe(xs, index.contents))->ignore
+    index := succ(index.contents)
+  }
+  arr
+}
 %comment(
   "Splits the provided array into head and tail parts (as a tuple), but only if the array is not empty."
 )