Use reusable state accessor for compiler decorators (#4465)

Idea that could be expanded/exposed to other libraries in the future.

Define clearly how to access a certain state key.

Doing that now as for paging there is a lot of decorators that are added
and that will simplify.

.chronus/changes/refactor-state-accessor-2024-8-17-15-11-51.md

@@ -0,0 +1,7 @@
+---
+changeKind: internal
+packages:
+  - "@typespec/http-server-csharp"
+---
+
+Fix potential undefined

.chronus/changes/refactor-state-accessor-2024-8-17-21-56-42.md

@@ -0,0 +1,8 @@
+---
+# Change versionKind to one of: internal, fix, dependencies, feature, deprecation, breaking
+changeKind: internal
+packages:
+  - "@typespec/compiler"
+---
+
+Use reusable state accessor for compiler decorators

packages/compiler/package.json

@@ -40,6 +40,10 @@
     "./emitter-framework": {
       "types": "./dist/src/emitter-framework/index.d.ts",
       "default": "./dist/src/emitter-framework/index.js"
+    },
+    "./experimental": {
+      "types": "./dist/src/experimental/index.d.ts",
+      "default": "./dist/src/experimental/index.js"
     }
   },
   "browser": {

packages/compiler/src/experimental/index.ts

@@ -0,0 +1 @@
+export { unsafe_useStateMap, unsafe_useStateSet } from "./state-accessor.js";

packages/compiler/src/experimental/state-accessor.ts

@@ -0,0 +1,30 @@
+import type { Program } from "../core/program.js";
+import type { Type } from "../core/types.js";
+
+type StateMapGetter<K extends Type, V> = (program: Program, type: K) => V | undefined;
+type StateMapSetter<K extends Type, V> = (program: Program, type: K, value: V) => void;
+type StateMapMapGetter<K extends Type, V> = (program: Program) => Map<K, V>;
+
+/** @experimental */
+export function unsafe_useStateMap<K extends Type, V>(
+  key: symbol,
+): [StateMapGetter<K, V>, StateMapSetter<K, V>, StateMapMapGetter<K, V>] {
+  const getter = (program: Program, target: K) => program.stateMap(key).get(target);
+  const setter = (program: Program, target: K, value: V) =>
+    program.stateMap(key).set(target, value);
+  const mapGetter = (program: Program) => program.stateMap(key);
+  return [getter, setter, mapGetter as any];
+}
+
+type StateSetGetter<K extends Type> = (program: Program, type: K) => boolean;
+type StateSetSetter<K extends Type> = (program: Program, type: K) => void;
+
+/** @experimental */
+export function unsafe_useStateSet<K extends Type>(
+  key: symbol,
+): [StateSetGetter<K>, StateSetSetter<K>] {
+  const getter = (program: Program, target: K) => program.stateSet(key).has(target);
+  const setter = (program: Program, target: K) => program.stateSet(key).add(target);
+
+  return [getter, setter];
+}