Increase performance for partitionBy

snippets/partitionBy.md

@@ -1,34 +1,28 @@
 ---
 title: partitionBy
-tags: array,object,function,intermediate
+tags: array,object,function,advanced
 ---
 
 Applies `fn` to each value in `arr`, splitting it each time `fn` returns a new value.
 
-Use `Array.prototype.reduce()` and `Array.prototype.push()` to create the start index of each partition.
-Use `Array.prototype.map()` to create the slice interval of each partition.
-Use `Array.prototype.slice()` to create each partition.
+Use `Array.prototype.reduce()` with an accumulator object that will hold the resulting array and the last value returned from `fn`.
+Use `Array.prototype.push()` to add each value in `arr` to the appropriate partition in the accumulator array.
 
 ```js
 const partitionBy = (arr, fn) =>
-  arr
-    .reduce((acc, val, i, a) => {
-      acc.push(fn(val, i, a));
-      return acc;
-    }, [])
-    .reduce((acc, val, i, a) => {
-      if (val !== a[i + 1]) {
-        acc.push(i + 1);
-      }
-      return acc;
-    }, [])
-    .map((interval, i, a) =>
-      i === 0 ? arr.slice(...[0, interval]) : arr.slice(...[a[i - 1], interval])
-    );
+  arr.reduce(
+    ({ res, last }, v, i, a) => {
+      const next = fn(v, i, a);
+      if (next !== last) res.push([v]);
+      else res[res.length - 1].push(v);
+      return { res, last: next };
+    },
+    { res: [] }
+  ).res;
 ```
 
 ```js
-const numbers = [1, 1, 1, 1, 1, 3, 3, 3, 3, 3, 4, 5, 5, 5, 5, 5];
-partitionBy(numbers, n => n % 2 === 0); // = > [ [ 1, 1, 1, 1, 1, 3, 3, 3, 3, 3 ], [ 4 ], [ 5, 5, 5, 5, 5 ] ]
-partitionBy(numbers, n => n); // => [ [ 1, 1, 1, 1, 1 ], [ 3, 3, 3, 3, 3 ], [ 4 ], [ 5, 5, 5, 5, 5 ] ]
+const numbers = [1, 1, 3, 3, 4, 5, 5, 5];
+partitionBy(numbers, n => n % 2 === 0); // [[1, 1, 3, 3], [4], [5, 5, 5]]
+partitionBy(numbers, n => n); // [[1, 1], [3, 3], [4], [5, 5, 5]]
 ```