feat: color a site from its address, and offer a failed web page to the system browser

app/lib/presentation/reader/oklch.dart

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+import 'dart:math' as math;
+
+import 'package:flutter/material.dart';
+
+/// OkLCH color math. Lightness and chroma are perceptual here, so a palette
+/// can be moved to another hue without the per-hue lightness corrections that
+/// rotating an HSL hue would need.
+
+/// Builds a color from lightness (0..1), chroma and hue (degrees). Chroma is
+/// reduced until the color fits in sRGB, so a hue that can't hold the asked
+/// chroma comes back muted rather than clipped to a wrong hue.
+Color oklch(double lightness, double chroma, double hue) {
+  var low = 0.0;
+  var high = chroma;
+  if (_fitsSrgb(_linearSrgb(lightness, chroma, hue))) {
+    return _color(_linearSrgb(lightness, chroma, hue));
+  }
+  for (var i = 0; i < 16; i++) {
+    final mid = (low + high) / 2;
+    if (_fitsSrgb(_linearSrgb(lightness, mid, hue))) {
+      low = mid;
+    } else {
+      high = mid;
+    }
+  }
+  return _color(_linearSrgb(lightness, low, hue));
+}
+
+/// The WCAG contrast ratio, from 1 (identical) to 21 (black on white).
+double contrastRatio(Color a, Color b) {
+  final la = _relativeLuminance(a);
+  final lb = _relativeLuminance(b);
+  final lighter = math.max(la, lb);
+  final darker = math.min(la, lb);
+  return (lighter + 0.05) / (darker + 0.05);
+}
+
+/// How far a color sits from gray, in the same units [oklch] takes: the
+/// bigger it is, the more vivid the color looks.
+double chromaOf(Color color) {
+  double linear(double value) =>
+      value <= 0.04045 ? value / 12.92 : math.pow((value + 0.055) / 1.055, 2.4) as double;
+  final r = linear(color.r);
+  final g = linear(color.g);
+  final b = linear(color.b);
+  final l = math.pow(0.4122214708 * r + 0.5363325363 * g + 0.0514459929 * b, 1 / 3) as double;
+  final m = math.pow(0.2119034982 * r + 0.6806995451 * g + 0.1073969566 * b, 1 / 3) as double;
+  final s = math.pow(0.0883024619 * r + 0.2817188376 * g + 0.6299787005 * b, 1 / 3) as double;
+  final a = 1.9779984951 * l - 2.4285922050 * m + 0.4505937099 * s;
+  final bb = 0.0259040371 * l + 0.7827717662 * m - 0.8086757660 * s;
+  return math.sqrt(a * a + bb * bb);
+}
+
+List<double> _linearSrgb(double lightness, double chroma, double hue) {
+  final radians = hue * math.pi / 180;
+  final a = chroma * math.cos(radians);
+  final b = chroma * math.sin(radians);
+  final l = math.pow(lightness + 0.3963377774 * a + 0.2158037573 * b, 3) as double;
+  final m = math.pow(lightness - 0.1055613458 * a - 0.0638541728 * b, 3) as double;
+  final s = math.pow(lightness - 0.0894841775 * a - 1.2914855480 * b, 3) as double;
+  return [
+    4.0767416621 * l - 3.3077115913 * m + 0.2309699292 * s,
+    -1.2684380046 * l + 2.6097574011 * m - 0.3413193965 * s,
+    -0.0041960863 * l - 0.7034186147 * m + 1.7076147010 * s,
+  ];
+}
+
+bool _fitsSrgb(List<double> rgb) =>
+    rgb.every((channel) => channel >= -0.0001 && channel <= 1.0001);
+
+Color _color(List<double> rgb) => Color.fromARGB(
+      255,
+      _encode(rgb[0]),
+      _encode(rgb[1]),
+      _encode(rgb[2]),
+    );
+
+int _encode(double channel) {
+  final clamped = channel.clamp(0.0, 1.0);
+  final encoded = clamped <= 0.0031308
+      ? 12.92 * clamped
+      : 1.055 * math.pow(clamped, 1 / 2.4) - 0.055;
+  return (encoded * 255).round().clamp(0, 255);
+}
+
+double _relativeLuminance(Color color) {
+  double channel(double value) =>
+      value <= 0.04045 ? value / 12.92 : math.pow((value + 0.055) / 1.055, 2.4) as double;
+  return 0.2126 * channel(color.r) + 0.7152 * channel(color.g) + 0.0722 * channel(color.b);
+}