// lib/editor/input/input_arbiter.dart
//
// Pure draw-vs-pan/zoom arbitration for the pen-first canvas (P0 step 4 —
// extracted verbatim from `pen_canvas.dart` so the make-or-break gesture rules
// are decided by ONE testable place rather than inline in a StatefulWidget).
//
// The model (clean-room from Saber, proven live):
//  - A DRAW gesture is exactly ONE active pointer that is a stylus / inverted
//    stylus / mouse, OR (when the finger-drawing toggle is on) a single finger.
//  - >= 2 active pointers ALWAYS means pan/zoom (pinch); never draw.
//  - Palm rejection: a finger never draws unless the user explicitly enabled
//    finger-drawing — so a resting palm pans (or is ignored) instead of marking.
//  - A hardware pen button mapped to `pan` suppresses drawing so the shared
//    InteractiveViewer pans instead.
//
// These are PURE functions (no widget/IO state) so the whole truth table is
// unit-tested; `pen_canvas.dart` owns the live pointer map and delegates the
// decisions here.

import 'package:flutter/gestures.dart' show PointerDeviceKind;

/// Whether [kind] is a pen (tip or flipped eraser end).
bool isStylusKind(PointerDeviceKind kind) =>
    kind == PointerDeviceKind.stylus ||
    kind == PointerDeviceKind.invertedStylus;

/// Decide whether the gesture currently forming should DRAW.
///
/// True iff there is exactly one active pointer, drawing is not suppressed by a
/// hardware pan button, and the pointer is a draw device:
///  - stylus / inverted stylus → always draws,
///  - mouse → always draws (desktop authoring),
///  - touch → draws only when [fingerDrawingEnabled] (else it pans / is palm).
bool shouldDraw({
  required int activePointerCount,
  required PointerDeviceKind kind,
  required bool fingerDrawingEnabled,
  required bool hwPanActive,
}) {
  if (activePointerCount != 1) return false;
  if (hwPanActive) return false;
  if (isStylusKind(kind)) return true;
  if (kind == PointerDeviceKind.mouse) return true;
  if (kind == PointerDeviceKind.touch) return fingerDrawingEnabled;
  return false;
}