Add HashMap.countBy (#4935)

Author: vinassefranche

.changeset/plain-horses-train.md

@@ -0,0 +1,13 @@
+---
+"effect": minor
+---
+
+Add HashMap.countBy
+
+```ts
+import { HashMap } from "effect"
+
+const map = HashMap.make([1, "a"], [2, "b"], [3, "c"])
+const result = HashMap.countBy(map, (_v, key) => key % 2 === 1)
+console.log(result) // 2
+```

packages/effect/src/HashMap.ts

@@ -282,6 +282,27 @@ export const toEntries = <K, V>(self: HashMap<K, V>): Array<[K, V]> => Array.fro
  */
 export const size: <K, V>(self: HashMap<K, V>) => number = HM.size
 
+/**
+ * Counts all the element of the given HashMap that pass the given predicate
+ *
+ * **Example**
+ *
+ * ```ts
+ * import { HashMap } from "effect"
+ *
+ * const map = HashMap.make([1, "a"], [2, "b"], [3, "c"])
+ * const result = HashMap.countBy(map, (_v, key) => key % 2 === 1)
+ * console.log(result) // 2
+ * ```
+ *
+ * @since 3.17.0
+ * @category folding
+ */
+export const countBy: {
+  <K, V>(predicate: (value: NoInfer<V>, key: NoInfer<K>) => boolean): (self: HashMap<K, V>) => number
+  <K, V>(self: HashMap<K, V>, predicate: (value: NoInfer<V>, key: NoInfer<K>) => boolean): number
+} = HM.countBy
+
 /**
  * Marks the `HashMap` as mutable.
  *

packages/effect/src/internal/hashMap.ts

@@ -334,6 +334,20 @@ export const entries = <K, V>(self: HM.HashMap<K, V>): IterableIterator<[K, V]>
 /** @internal */
 export const size = <K, V>(self: HM.HashMap<K, V>): number => (self as HashMapImpl<K, V>)._size
 
+/** @internal */
+export const countBy = Dual.dual<
+  <K, V>(predicate: (value: NoInfer<V>, key: NoInfer<K>) => boolean) => (self: HM.HashMap<K, V>) => number,
+  <K, V>(self: HM.HashMap<K, V>, predicate: (value: NoInfer<V>, key: NoInfer<K>) => boolean) => number
+>(2, (self, f) => {
+  let count = 0
+  for (const [k, a] of self) {
+    if (f(a, k)) {
+      count++
+    }
+  }
+  return count
+})
+
 /** @internal */
 export const beginMutation = <K, V>(self: HM.HashMap<K, V>): HM.HashMap<K, V> =>
   makeImpl(

packages/effect/test/HashMap.test.ts

@@ -458,9 +458,20 @@ describe("HashMap", () => {
   })
 
   it("findFirst", () => {
-    const map1 = HM.make([key(0), value("a")], [key(1), value("bb")])
-    assertSome(HM.findFirst(map1, (_v, k) => k.n === 0), [key(0), value("a")])
-    assertSome(HM.findFirst(map1, (v, _k) => v.s === "bb"), [key(1), value("bb")])
-    assertNone(HM.findFirst(map1, (v, k) => k.n === 0 && v.s === "bb"))
+    const map = HM.make([key(0), value("a")], [key(1), value("bb")])
+    assertSome(HM.findFirst(map, (_v, k) => k.n === 0), [key(0), value("a")])
+    assertSome(HM.findFirst(map, (v, _k) => v.s === "bb"), [key(1), value("bb")])
+    assertNone(HM.findFirst(map, (v, k) => k.n === 0 && v.s === "bb"))
+  })
+
+  it("countBy", () => {
+    const map = HM.make([key(1), value("a")], [key(2), value("b")], [key(3), value("c")])
+    strictEqual(HM.countBy(map, (_v, k) => k.n % 2 === 1), 2)
+    strictEqual(HM.countBy(map, (v, k) => k.n % 2 === 1 && v.s === "a"), 1)
+    strictEqual(HM.countBy(map, (v, k) => k.n % 2 === 1 && v.s === "b"), 0)
+
+    strictEqual(pipe(map, HM.countBy((_v, k) => k.n % 2 === 1)), 2)
+    strictEqual(pipe(map, HM.countBy((v, k) => k.n % 2 === 1 && v.s === "a")), 1)
+    strictEqual(pipe(map, HM.countBy((v, k) => k.n % 2 === 1 && v.s === "b")), 0)
   })
 })