perf(at): Improve performance and compatibility with lodash (#798)

* perf(at): Improve performance and compatibility with lodash

* Change the implementation to match `Array.prototype.at` implementation

* Avoid truncating numbers if they are already integers

benchmarks/performance/at.bench.ts

@@ -6,24 +6,35 @@ const atToolkit = atToolkit_;
 const atLodash = atLodash_;
 
 describe('at', () => {
+  const data = ['a', 'b', 'c', 'd', 'e'];
+  const indices = [0, 2, -2, 1.5, -1.5];
+
   bench('es-toolkit/at', () => {
-    atToolkit(['a', 'b', 'c', 'd', 'e'], [0, 2, 4]);
+    atToolkit(data, indices);
   });
 
   bench('lodash/at', () => {
-    atLodash(['a', 'b', 'c', 'd', 'e'], [0, 2, 4]);
+    atLodash(data, indices);
+  });
+
+  bench('Array.prototype.at', () => {
+    indices.map(i => data.at(i));
   });
 });
 
 describe('at/largeArray', () => {
-  const largeArray = Array.from({ length: 10000 }, (_, i) => i);
+  const largeData = Array.from({ length: 10000 }, (_, i) => i);
-  const largeIndex = Array.from({ length: 1000 }, (_, i) => i * 2);
+  const largeIndices = Array.from({ length: 1000 }, (_, i) => i * 2);
 
   bench('es-toolkit/at', () => {
-    atToolkit(largeArray, largeIndex);
+    atToolkit(largeData, largeIndices);
   });
 
   bench('lodash/at', () => {
-    atLodash(largeArray, largeIndex);
+    atLodash(largeData, largeIndices);
+  });
+
+  bench('Array.prototype.at', () => {
+    largeIndices.map(i => largeData.at(i));
   });
 });

src/array/at.spec.ts

@@ -13,10 +13,17 @@ describe('at', () => {
   });
 
   it('should return `undefined` for nonexistent keys', () => {
-    expect(at(['a', 'b', 'c'], [2, 4, 0])).toEqual(['c', undefined, 'a']);
+    expect(at(['a', 'b', 'c'], [2, 4, 0, -4])).toEqual(['c', undefined, 'a', undefined]);
   });
 
   it('should return an empty array when no keys are given', () => {
     expect(at(['a', 'b', 'c'], [])).toEqual([]);
   });
+
+  it('should return undefined for non-integer indices', () => {
+    const data = ['a', 'b', 'c'];
+    const indices = [1.5, -1.5, NaN, Infinity, -Infinity];
+
+    expect(at(data, indices)).toEqual(indices.map(i => data.at(i)));
+  });
 });

src/array/at.ts

@@ -14,12 +14,19 @@
  * console.log(result); // [20, 40, 50]
  */
 export function at<T>(arr: readonly T[], indices: number[]): Array<T | undefined> {
-  const result: Array<T | undefined> = new Array(indices.length);
+  const result = new Array<T | undefined>(indices.length);
+  const length = arr.length;
 
   for (let i = 0; i < indices.length; i++) {
-    const index = indices[i];
+    let index = indices[i];
 
-    result[i] = arr.at(index);
+    index = Number.isInteger(index) ? index : Math.trunc(index) || 0;
+
+    if (index < 0) {
+      index += length;
+    }
+
+    result[i] = arr[index];
   }
 
   return result;