feat: Subject correction

src/Subject.ts

@@ -1,5 +1,6 @@
 import { IObservable } from './Observable';
 import { Observer } from './Observer';
+import { Subscriber } from './Subscriber';
 import { Subscription } from './Subscription';
 
 /**
@@ -16,24 +17,47 @@ export interface SubjectConstructor {
  * Implementation
  */
 export class Subject<T> implements IObservable<T>, Observer<T> {
+  private closed = false;
+  private closedByError = false;
+  private lastError: unknown | undefined;
+
+  private subscribers: Subscriber<T>[] = [];
+
   public constructor() {}
 
+  private isClosed(): boolean {
+    return this.closed || this.closedByError;
+  }
+
   public next(value: T): void {
-    void value;
-    throw new Error('TODO');
+    if (this.isClosed()) return;
+    this.subscribers.forEach(s => s.next(value));
   }
 
   public error(err: unknown): void {
-    void err;
-    throw new Error('TODO');
+    if (this.isClosed()) return;
+    this.closedByError = true;
+    this.lastError = err;
+    this.subscribers.forEach(s => s.error(err));
   }
 
   public complete(): void {
-    throw new Error('TODO');
+    if (this.isClosed()) return;
+    this.closed = true;
+    this.subscribers.forEach(s => s.complete());
   }
 
   public subscribe(obs: Partial<Observer<T>>): Subscription {
-    void obs;
-    throw new Error('TODO');
+    const subscriber = new Subscriber(obs);
+
+    if (this.closed) {
+      subscriber.complete();
+    } else if (this.closedByError) {
+      subscriber.error(this.lastError);
+    } else {
+      this.subscribers.push(subscriber);
+    }
+
+    return subscriber;
   }
 }