feat: alphaTex writer and GP round trip

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

tests/score/roundtrip.test.ts

@@ -0,0 +1,82 @@
+import { test, expect } from "bun:test";
+import { songToAlphaTex } from "../../src/score/alphatex.ts";
+import { songToGp, songToMidi, truthFromGp, alphaTexDiagnostics } from "../../src/score/gp.ts";
+import { ticksToSeconds } from "../../src/core/time.ts";
+import { GUITAR_STANDARD, type Song, type TimedNote } from "../../src/core/types.ts";
+
+const Q = 960;
+const S = 240;
+const BPM = 90;
+const gn = (tick: number, dur: number, string: number, fret: number) => ({
+  tick, dur, string, fret, pitch: GUITAR_STANDARD[string - 1]! + fret,
+});
+
+const song: Song = {
+  title: 'Round "Trip"', tier: "t1", seed: 1, bpm: BPM, timeSig: [4, 4], bars: 3, key: "C",
+  tracks: [
+    {
+      name: "Lead", role: "guitar", program: 25, tuning: GUITAR_STANDARD, pan: 0, fx: "none", gainDb: 0,
+      notes: [
+        // C major chord, eighth
+        gn(0, Q / 2, 5, 3), gn(0, Q / 2, 4, 2), gn(0, Q / 2, 3, 0), gn(0, Q / 2, 2, 1), gn(0, Q / 2, 1, 0),
+        // 5 sixteenths
+        gn(Q / 2, 5 * S, 3, 2),
+        // whole note from beat 3 of bar 1 (crosses barline)
+        gn(2 * Q, 4 * Q, 2, 3),
+      ],
+    },
+    {
+      name: "Vox", role: "vocals", program: 53, pan: 0, fx: "none", gainDb: 0,
+      notes: [
+        { tick: 0, dur: Q, pitch: 60 },
+        { tick: 3 * Q, dur: Q, pitch: 61 },
+        { tick: 4 * Q + 2 * Q, dur: 2 * Q, pitch: 64 },
+      ],
+    },
+    {
+      name: "Kit", role: "drums", program: 0, pan: 0, fx: "none", gainDb: 0,
+      notes: [
+        { tick: 0, dur: S, pitch: 36 }, { tick: 0, dur: S, pitch: 42 }, { tick: Q, dur: S, pitch: 38 },
+      ],
+    },
+  ],
+};
+
+const round3 = (x: number) => Math.round(x * 1000) / 1000;
+const sortNotes = (ns: TimedNote[]) => [...ns].sort((a, b) => a.onset - b.onset || a.pitch - b.pitch);
+const t = (tick: number) => ticksToSeconds(tick, BPM);
+
+test("alphaTex has no diagnostics", () => {
+  const tex = songToAlphaTex(song);
+  expect(alphaTexDiagnostics(tex)).toEqual([]);
+});
+
+test("GP round trip preserves notes, ties, strings, roles", () => {
+  const truth = truthFromGp(songToGp(song));
+  expect(truth.bpm).toBe(BPM);
+  expect(truth.timeSig).toEqual([4, 4]);
+  const g = truth.tracks[0]!;
+  const expectedGuitar = song.tracks[0]!.notes
+    .map((n) => [n.pitch, round3(t(n.tick))] as [number, number])
+    .sort((a, b) => a[1] - b[1] || a[0] - b[0]);
+  expect(sortNotes(g.notes).map((n) => [n.pitch, round3(n.onset)])).toEqual(expectedGuitar);
+  const tiedPitch = GUITAR_STANDARD[1]! + 3;
+  const tied = g.notes.find((n) => n.pitch === tiedPitch)!;
+  expect(tied.offset - tied.onset).toBeCloseTo((4 * 60) / 90, 3);
+  const five = g.notes.find((n) => n.pitch === GUITAR_STANDARD[2]! + 2)!;
+  expect(five.offset - five.onset).toBeCloseTo(t(5 * S), 3);
+  expect(g.notes.every((n) => n.string! >= 1 && n.string! <= 6)).toBe(true);
+  expect(g.notes.find((n) => n.pitch === 64)!.string).toBe(1);
+  expect(g.tuning).toEqual(GUITAR_STANDARD);
+  const vox = truth.tracks[1]!;
+  expect(sortNotes(vox.notes).map((n) => [n.pitch, round3(n.onset)])).toEqual([
+    [60, 0], [61, round3(t(3 * Q))], [64, round3(t(6 * Q))],
+  ]);
+  expect(truth.tracks.find((x) => x.role === "drums")!.notes.map((n) => n.pitch).sort()).toEqual([36, 38, 42]);
+  expect(truth.tracks.map((x) => x.role)).toEqual(["guitar", "vocals", "drums"]);
+  expect(truth.tracks.map((x) => x.name)).toEqual(["Lead", "Vox", "Kit"]);
+});
+
+test("songToMidi generates events", () => {
+  expect(songToMidi(song).tracks.length).toBeGreaterThan(0);
+});