feat: Add "intersect" for joining two arrays using predicate and unio…

…n functions

src/index.js

@@ -32,6 +32,7 @@ export { hist } from "./hist/hist"
 export { i } from "./i/i"
 export { inc } from "./inc/inc"
 export { indexBy } from "./index-by/index-by"
+export { intersect } from "./intersect/intersect"
 export { is, isNothing, isTrue, isFalse, isObject } from "./is/is"
 export { isEmpty, isNotEmpty } from "./is-empty/is-empty"
 export { isMatch } from "./is-match/is-match"

src/intersect/intersect.js

@@ -0,0 +1,26 @@
+import { findIndex } from "../find-index/find-index"
+
+export const intersect = (predicateFn, unionFn) => (aList, bList) => {
+  if (aList.length === 0) {
+    return bList
+  }
+
+  if (bList.length === 0) {
+    return aList
+  }
+
+  const result = [].concat(aList)
+
+  for (let index = 0; index < bList.length; ++index) {
+    const b = bList[index]
+    const aIndex = findIndex(item => predicateFn(item, b))(result)
+
+    if (aIndex === -1) {
+      result.push(b)
+    } else {
+      result[aIndex] = unionFn(result[aIndex], b)
+    }
+  }
+
+  return result
+}

src/intersect/intersect.test.js

@@ -0,0 +1,55 @@
+import test from "tape"
+
+import { intersect } from ".."
+
+test("array::intersect", t => {
+  t.deepEqual(
+    intersect(
+      (a, b) => a === b,
+      a => a
+    )([], [1, 2, 3]),
+    [1, 2, 3],
+    "First array empty"
+  )
+
+  t.deepEqual(
+    intersect(
+      (a, b) => a === b,
+      a => a
+    )([1, 2, 3], []),
+    [1, 2, 3],
+    "Second array empty"
+  )
+
+  t.deepEqual(
+    intersect(
+      (a, b) => a === b,
+      a => a
+    )([1, 2, 3], [3, 4, 5]),
+    [1, 2, 3, 4, 5],
+    "Join with common"
+  )
+
+  t.deepEqual(
+    intersect(
+      (a, b) => a === b,
+      a => a
+    )([1, 2], [3, 4, 5]),
+    [1, 2, 3, 4, 5],
+    "Join without common"
+  )
+
+  t.deepEqual(
+    intersect(
+      (a, b) => a.id === b.id,
+      (a, b) => ({ ...a, ...b })
+    )(
+      [{ id: 1, overwrite: 1 }, { id: 2 }],
+      [{ id: 1, overwrite: 2 }, { id: 3 }]
+    ),
+    [{ id: 1, overwrite: 2 }, { id: 2 }, { id: 3 }],
+    "Join objects - same order as input arrays"
+  )
+
+  t.end()
+})