// lib/editor/canvas/pinch_scale_solver.dart
//
// Pure math for the pen canvas's absolute pinch-zoom. Extracted so the
// re-baseline behavior (the subtle part) can be unit-tested without simulating
// a flaky multi-pointer gesture.
//
// The pinch is driven ABSOLUTELY: the scale shown is always
//   scaleStart * (rawScale / rawScaleAtBaseline)
// where `scaleStart` is the matrix scale captured at the current baseline and
// `rawScaleAtBaseline` is the recognizer's cumulative `details.scale` at that
// same baseline. Dividing by `rawScaleAtBaseline` re-normalizes the cumulative
// scale so it reads 1.0 at the baseline instant.
//
// Why this matters: a baseline is captured at gesture start AND on every
// pointer-count change (a finger blips 2→1→2, routine on Windows touch). At
// gesture start `details.scale` is 1.0, so a naive `scaleStart * rawScale` is
// correct. But at a MID-GESTURE re-baseline `details.scale` is whatever the
// pinch has accumulated (e.g. 0.40) — multiplying the fresh `scaleStart` by
// that stale 0.40 popped the zoom to a wrong scale and snapped back (the
// reported flicker). Normalizing against `rawScaleAtBaseline` removes the pop.

import 'package:flutter/foundation.dart' show clampDouble;

/// Returns the absolute target scale for a pinch frame.
///
/// [scaleStart] — matrix scale captured at the current baseline.
/// [rawScaleAtBaseline] — recognizer cumulative `details.scale` at that
///   baseline (1.0 at gesture start; the live value at a re-baseline).
/// [rawScale] — the recognizer's current cumulative `details.scale`.
/// Result is clamped to [minScale, maxScale].
double absolutePinchScale({
  required double scaleStart,
  required double rawScaleAtBaseline,
  required double rawScale,
  required double minScale,
  required double maxScale,
}) {
  final double cumulative =
      rawScaleAtBaseline > 0 ? rawScale / rawScaleAtBaseline : 1.0;
  return clampDouble(scaleStart * cumulative, minScale, maxScale);
}