BadNote/test/pinch_scale_solver_test.dart

// Proves the absolute pinch-zoom math, focused on the re-baseline case that
// produced the on-device flicker: when a finger blips (2→1→2) mid-pinch the
// viewer captures a fresh baseline, and the OLD code multiplied that fresh
// scaleStart by the recognizer's still-cumulative details.scale — popping the
// zoom to a wrong value and snapping back. absolutePinchScale() normalizes
// against the baseline so the pop cannot happen.

import 'package:flutter_test/flutter_test.dart';

import 'package:badnote/editor/canvas/pinch_scale_solver.dart';

void main() {
  const min = 0.5, max = 8.0;

  double solve(double scaleStart, double baseline, double raw) =>
      absolutePinchScale(
        scaleStart: scaleStart,
        rawScaleAtBaseline: baseline,
        rawScale: raw,
        minScale: min,
        maxScale: max,
      );

  test('at gesture start (baseline 1.0) target tracks raw directly', () {
    // scaleStart 1.0, baseline 1.0: pinch out to raw 2.0 → scale 2.0.
    expect(solve(1.0, 1.0, 2.0), closeTo(2.0, 1e-9));
    // pinch in to raw 0.5 → scale 0.5.
    expect(solve(1.0, 1.0, 0.5), closeTo(0.5, 1e-9));
  });

  test('a re-baseline at the SAME instant does not change the scale', () {
    // Pinch in: start(2.0,1.0) → raw 0.6 gives scale 1.2 (well inside clamp).
    final before = solve(2.0, 1.0, 0.6);
    expect(before, closeTo(1.2, 1e-9));

    // A finger blips: the viewer re-baselines RIGHT HERE — scaleStart becomes
    // the current scale (1.2) and rawScaleAtBaseline becomes the current raw
    // (0.6). Re-evaluating the same instant must yield the SAME scale (no pop).
    final after = solve(1.2, 0.6, 0.6);
    expect(after, closeTo(before, 1e-9),
        reason: 're-baseline must be continuous, not a jump');
  });

  test('after a re-baseline the pinch stays smooth (no pop)', () {
    // Re-baselined at scale 1.2 / raw 0.6. Continue pinching in: raw 0.54.
    // Correct: 1.2 * (0.54 / 0.6) = 1.08 — a gentle 10% step, monotonic.
    final next = solve(1.2, 0.6, 0.54);
    expect(next, closeTo(1.08, 1e-9));

    // The OLD bug multiplied the fresh scaleStart by the un-normalized raw:
    // 1.2 * 0.54 = 0.648 — a sudden ~46% drop (the flicker). Guard against it.
    expect(next, greaterThan(1.0),
        reason: 'must NOT collapse to scaleStart*raw (the old 0.648 pop)');
  });

  test('result is clamped to [min, max]', () {
    expect(solve(4.0, 1.0, 4.0), max); // 16 → 8
    expect(solve(1.0, 1.0, 0.1), min); // 0.1 → 0.5
  });

  test('degenerate baseline (0) is treated as no cumulative change', () {
    expect(solve(2.0, 0.0, 5.0), closeTo(2.0, 1e-9));
  });
}