import 'dart:math';
import 'dart:ui';

import 'package:flutter/material.dart' show Colors;
import 'package:flutter_test/flutter_test.dart';
import 'package:tane/ui/theme.dart';

/// WCAG 2.x relative luminance of a color (sRGB).
double _luminance(Color c) {
  double channel(double v) =>
      v <= 0.03928 ? v / 12.92 : pow((v + 0.055) / 1.055, 2.4).toDouble();
  return 0.2126 * channel(c.r) + 0.7152 * channel(c.g) + 0.0722 * channel(c.b);
}

/// WCAG contrast ratio between two colors (order-independent, 1..21).
double _contrast(Color a, Color b) {
  final la = _luminance(a);
  final lb = _luminance(b);
  final lighter = max(la, lb);
  final darker = min(la, lb);
  return (lighter + 0.05) / (darker + 0.05);
}

void main() {
  void expectAa(Color foreground, Color background, String label) {
    expect(
      _contrast(foreground, background),
      greaterThanOrEqualTo(4.5),
      reason: '$label must meet WCAG AA (4.5:1) for normal text',
    );
  }

  group('palette meets WCAG AA (4.5:1) for text', () {
    test('seedMuted (hints, subtitles) on every background it sits on', () {
      // White: search-field fill and home cards. Canvas/field: M3 surfaces.
      expectAa(seedMuted, Colors.white, 'seedMuted on white');
      expectAa(seedMuted, seedCanvas, 'seedMuted on canvas');
      expectAa(seedMuted, seedField, 'seedMuted on field fill');
    });

    test('body and secondary text on the canvas', () {
      expectAa(seedOnSurface, seedCanvas, 'onSurface on canvas');
      expectAa(seedTitle, seedCanvas, 'titles on canvas');
      expectAa(seedOnSurfaceVariant, seedCanvas, 'onSurfaceVariant on canvas');
    });

    test('white text on the green action colours', () {
      expectAa(Colors.white, seedGreen, 'white on primary green');
      expectAa(Colors.white, seedAppBar, 'white on app-bar green');
    });
  });
}