fix(thrift): use standard XOR zigzag decode to fix numeric message offset

packages/linejs/base/thrift/readwrite/tmc.test.ts

@@ -0,0 +1,30 @@
+import { assertEquals } from "@std/assert";
+import { TMoreCompactProtocol } from "./tmc.ts";
+
+Deno.test("decodeZigZag matches standard thrift zigzag for positive values", () => {
+	const tmc = new TMoreCompactProtocol();
+	assertEquals(tmc.decodeZigZag(0), 0);
+	assertEquals(tmc.decodeZigZag(2), 1);
+	assertEquals(tmc.decodeZigZag(4), 2);
+	assertEquals(tmc.decodeZigZag(100), 50);
+	assertEquals(tmc.decodeZigZag(200), 100);
+});
+
+Deno.test("decodeZigZag matches standard thrift zigzag for negative values", () => {
+	const tmc = new TMoreCompactProtocol();
+	assertEquals(tmc.decodeZigZag(1), -1);
+	assertEquals(tmc.decodeZigZag(3), -2);
+	assertEquals(tmc.decodeZigZag(5), -3);
+	assertEquals(tmc.decodeZigZag(7), -4);
+	assertEquals(tmc.decodeZigZag(199), -100);
+});
+
+Deno.test("decodeZigZag round-trips with standard zigzag encode", () => {
+	const tmc = new TMoreCompactProtocol();
+	function zigzagEncode(n: number): number {
+		return (n << 1) ^ (n >> 31);
+	}
+	for (const v of [-1000, -100, -3, -2, -1, 0, 1, 2, 3, 100, 1000]) {
+		assertEquals(tmc.decodeZigZag(zigzagEncode(v)), v);
+	}
+});

packages/linejs/base/thrift/readwrite/tmc.ts

@@ -122,7 +122,8 @@ export class TMoreCompactProtocol {
 
 	// ZigZag decode
 	decodeZigZag(encoded: number): number {
-		return Number((BigInt(encoded) >> 1n) * ((encoded & 1) ? -1n : 1n));
+		const n = BigInt(encoded);
+		return Number((n >> 1n) ^ -(n & 1n));
 	}
 
 	// Read data by type
@@ -216,8 +217,8 @@ export class TMoreCompactProtocol {
 			}
 		} else if (typeId === 16) {
 			// STRING (string ID delta)
-			const temp = BigInt(this.readVarint());
-			const delta = (temp % 2n ? -1n : 1n) * (temp / 2n);
+			const raw = BigInt(this.readVarint());
+			const delta = (raw >> 1n) ^ -(raw & 1n);
 			const stringId = delta + this.lastStringId;
 			this.lastStringId = stringId;
 			value = String(stringId);