feat(xls-codec): recover formula text from BIFF8 Ptg token streams

packages/xls-codec/src/biff/cursor.test.ts

@@ -25,6 +25,13 @@ describe("BlockCursor", () => {
     expect(cursor.i32()).toBe(-1);
   });
 
+  it("reads a signed 16-bit integer", () => {
+    const cursor = new BlockCursor([bytes(0xff, 0xff, 0x02, 0x00)]);
+
+    expect(cursor.i16()).toBe(-1);
+    expect(cursor.i16()).toBe(2);
+  });
+
   it("reads an Xnum as an IEEE 754 double", () => {
     // [MS-XLS] 2.5.342: Xnum is a 64-bit binary floating-point number. 1.5 is 0x3FF8000000000000, little-endian on the wire.
     const cursor = new BlockCursor([

packages/xls-codec/src/biff/cursor.ts

@@ -83,6 +83,11 @@ export class BlockCursor {
     return this.u32() | 0;
   }
 
+  /** A signed 16-bit integer, sign-extended by shifting the raw value out of and back into the low 16 bits -- what XTI's itabFirst/itabLast ([MS-XLS] 2.5.344) and a handful of other structures carry. */
+  i16(): number {
+    return (this.u16() << 16) >> 16;
+  }
+
   /** An Xnum ([MS-XLS] 2.5.342): a little-endian IEEE 754 double. */
   f64(): number {
     const raw = this.take(8);