From 08e3ddc83dbff47f236db139dada5fa4d501916b Mon Sep 17 00:00:00 2001
From: Josep M Sobrepere <jm.sobrepere@gmail.com>
Date: Wed, 5 Oct 2022 23:23:05 +0200
Subject: [PATCH] context-state: `combineStates`

---
 packages/context-state/src/combineStates.ts | 47 +++++++++++++++++++++
 1 file changed, 47 insertions(+)
 create mode 100644 packages/context-state/src/combineStates.ts

diff --git a/packages/context-state/src/combineStates.ts b/packages/context-state/src/combineStates.ts
new file mode 100644
index 0000000..4a8e402
--- /dev/null
+++ b/packages/context-state/src/combineStates.ts
@@ -0,0 +1,47 @@
+import { of } from "rxjs"
+import {
+  mapRecord,
+  detachedNode,
+  EMPTY_VALUE,
+  recordEntries,
+  children,
+} from "./internal"
+import { StateNode, StringRecord } from "./types"
+
+type StringRecordNodeToNodeStringRecord<
+  States extends StringRecord<StateNode<any>>,
+> = StateNode<{
+  [K in keyof States]: States[K] extends StateNode<infer V> ? V : never
+}>
+
+export const combineStates = <States extends StringRecord<StateNode<any>>>(
+  states: States,
+): StringRecordNodeToNodeStringRecord<States> => {
+  let inactiveStates = Object.keys(states).length
+  let emptyStates = 0
+  const activeStates = mapRecord(states, () => false)
+  const latestStates = mapRecord(states, () => null)
+
+  const [result, run] = detachedNode(() => of({ ...latestStates }))
+
+  let latestValue: boolean | EMPTY_VALUE = false
+  recordEntries(states).forEach(([key, node]) => {
+    children.get(node)!.add((isActive, value) => {
+      if (isActive !== activeStates[key]) {
+        inactiveStates += isActive ? -1 : +1
+        activeStates[key] = isActive
+      }
+
+      if (value !== latestStates[key]) {
+        emptyStates +=
+          latestStates[key] === EMPTY_VALUE ? -1 : value === EMPTY_VALUE ? 1 : 0
+        latestStates[key] = value
+      }
+
+      latestValue = emptyStates === 0 ? !latestValue : EMPTY_VALUE
+      run(inactiveStates === 0, latestValue)
+    })
+  })
+
+  return result as StringRecordNodeToNodeStringRecord<States>
+}