feat(worker): 7z signature-header parser

worker/src/archive/ranged/sevenz-ranged.test.ts

@@ -0,0 +1,30 @@
+import { describe, it, expect } from "vitest";
+import { parseSevenZSignatureHeader } from "./sevenz-ranged.js";
+
+const MAGIC = Buffer.from([0x37, 0x7a, 0xbc, 0xaf, 0x27, 0x1c]);
+
+function buildSignatureHeader(nextOffset: bigint, nextSize: bigint): Buffer {
+  const buf = Buffer.alloc(32);
+  MAGIC.copy(buf, 0);
+  buf.writeUInt8(0, 6); buf.writeUInt8(4, 7);  // version 0.4
+  buf.writeUInt32LE(0, 8);                      // StartHeaderCRC (unused here)
+  buf.writeBigUInt64LE(nextOffset, 12);
+  buf.writeBigUInt64LE(nextSize, 20);
+  buf.writeUInt32LE(0, 28);                     // NextHeaderCRC (unused here)
+  return buf;
+}
+
+describe("parseSevenZSignatureHeader", () => {
+  it("reads NextHeaderOffset and NextHeaderSize", () => {
+    const buf = buildSignatureHeader(1_000_000n, 4096n);
+    expect(parseSevenZSignatureHeader(buf)).toEqual({ nextHeaderOffset: 1_000_000, nextHeaderSize: 4096 });
+  });
+
+  it("returns null on bad magic", () => {
+    expect(parseSevenZSignatureHeader(Buffer.alloc(32))).toBeNull();
+  });
+
+  it("returns null when shorter than 32 bytes", () => {
+    expect(parseSevenZSignatureHeader(MAGIC)).toBeNull();
+  });
+});

worker/src/archive/ranged/sevenz-ranged.ts

@@ -0,0 +1,12 @@
+const SEVENZ_MAGIC = Buffer.from([0x37, 0x7a, 0xbc, 0xaf, 0x27, 0x1c]);
+
+export function parseSevenZSignatureHeader(
+  buf: Buffer,
+): { nextHeaderOffset: number; nextHeaderSize: number } | null {
+  if (buf.length < 32) return null;
+  if (!buf.subarray(0, 6).equals(SEVENZ_MAGIC)) return null;
+  return {
+    nextHeaderOffset: Number(buf.readBigUInt64LE(12)),
+    nextHeaderSize: Number(buf.readBigUInt64LE(20)),
+  };
+}