feat(buffers): add IReadBuffer.peek()

- update `FIFOBuffer` & `LIFOBuffer` impls
- add tests

packages/buffers/src/api.ts

@@ -17,9 +17,15 @@ export interface IReadBuffer<T> {
 	/**
 	 * Unguarded read operation. Assumes the caller checked
 	 * {@link IReadBuffer.readable} immediately before. Returns next value from
-	 * buffer.
+	 * buffer and removes it (from buffer).
 	 */
 	read(): T;
+	/**
+	 * Unguarded & immutable read operation. Assumes the caller checked
+	 * {@link IReadBuffer.readable} immediately before. Returns what would be
+	 * the next value read from buffer (without removing it).
+	 */
+	peek(): T;
 }
 
 /**

packages/buffers/src/fifo.ts

@@ -59,6 +59,11 @@ export class FIFOBuffer<T> implements IReadWriteBuffer<T> {
 		return val;
 	}
 
+	peek() {
+		const { buf, rpos } = this;
+		return buf[rpos]!;
+	}
+
 	writable() {
 		return (this.wpos + 1) % this.buf.length !== this.rpos;
 	}

packages/buffers/src/lifo.ts

@@ -42,6 +42,10 @@ export class LIFOBuffer<T> implements IReadWriteBuffer<T> {
 		return this.buf.pop()!;
 	}
 
+	peek() {
+		return this.buf[this.buf.length - 1];
+	}
+
 	writable() {
 		return this.buf.length < this.cap;
 	}

packages/buffers/test/main.test.ts

@@ -6,6 +6,7 @@ test("fifo", () => {
 	expect(b.readable()).toBeFalse();
 	expect(b.writable()).toBeTrue();
 	expect(b.write(1)).toBeTrue();
+	expect(b.peek()).toBe(1);
 	expect(b.readable()).toBeTrue();
 	expect(b.writable()).toBeTrue();
 	expect(b.write(2)).toBeTrue();
@@ -28,6 +29,7 @@ test("lifo", () => {
 	expect(b.readable()).toBeFalse();
 	expect(b.writable()).toBeTrue();
 	expect(b.write(1)).toBeTrue();
+	expect(b.peek()).toBe(1);
 	expect(b.readable()).toBeTrue();
 	expect(b.writable()).toBeTrue();
 	expect(b.write(2)).toBeTrue();