Added HtmlHelper.IsDarkColor and enhanced UnitTests

AMWD.Common.AspNetCore/Utilities/HtmlHelper.cs

@@ -0,0 +1,55 @@
+using System;
+using System.Linq;
+using System.Text.RegularExpressions;
+
+namespace AMWD.Common.AspNetCore.Utilities
+{
+	/// <summary>
+	/// Provides helpers for webpages.
+	/// </summary>
+	public static class HtmlHelper
+	{
+		/// <summary>
+		/// Checks whether a color is considered as dark.
+		/// </summary>
+		/// <param name="color">The color (hex or rgb - as defined in CSS)</param>
+		/// <returns><c>true</c> when the color is dark otherwise <c>false</c>.</returns>
+		public static bool IsDarkColor(string color)
+		{
+			if (string.IsNullOrWhiteSpace(color))
+				return false;
+
+			int r, g, b;
+
+			var rgbMatch = Regex.Match(color, @"^rgba?\(([0-9]+), ?([0-9]+), ?([0-9]+)");
+			var hexMatchFull = Regex.Match(color, @"^#?([0-9a-fA-F]{2})([0-9a-fA-F]{2})([0-9a-fA-F]{2})$");
+			var hexMatchLite = Regex.Match(color, @"^#?([0-9a-fA-F]{1})([0-9a-fA-F]{1})([0-9a-fA-F]{1})$");
+
+			if (rgbMatch.Success)
+			{
+				r = Convert.ToInt32(rgbMatch.Groups[1].Value);
+				g = Convert.ToInt32(rgbMatch.Groups[2].Value);
+				b = Convert.ToInt32(rgbMatch.Groups[3].Value);
+			}
+			else if (hexMatchFull.Success)
+			{
+				r = Convert.ToInt32(hexMatchFull.Groups[1].Value, 16);
+				g = Convert.ToInt32(hexMatchFull.Groups[2].Value, 16);
+				b = Convert.ToInt32(hexMatchFull.Groups[3].Value, 16);
+			}
+			else if (hexMatchLite.Success)
+			{
+				r = Convert.ToInt32(new string(hexMatchLite.Groups[1].Value.First(), 2), 16);
+				g = Convert.ToInt32(new string(hexMatchLite.Groups[2].Value.First(), 2), 16);
+				b = Convert.ToInt32(new string(hexMatchLite.Groups[3].Value.First(), 2), 16);
+			}
+			else
+			{
+				return false;
+			}
+
+			double luminance = (r * 0.299 + g * 0.587 + b * 0.114) / 255;
+			return luminance <= 0.5;
+		}
+	}
+}