1 files changed, 48 insertions, 0 deletions
diff --git a/src/Enmarcha.Backend/Helpers/ContrastHelper.cs b/src/Enmarcha.Backend/Helpers/ContrastHelper.cs
new file mode 100644
index 0000000..abc3dba
--- /dev/null
+++ b/src/Enmarcha.Backend/Helpers/ContrastHelper.cs
@@ -0,0 +1,48 @@
+namespace Enmarcha.Backend.Helpers;
+
+using System;
+using System.Globalization;
+
+public static class ContrastHelper
+{
+    public static string GetBestTextColour(string backgroundHex)
+    {
+        // Strip #
+        backgroundHex = backgroundHex.TrimStart('#');
+
+        if (backgroundHex.Length != 6)
+            throw new ArgumentException("Hex colour must be 6 characters (RRGGBB)");
+
+        // Parse RGB
+        int r = int.Parse(backgroundHex.Substring(0, 2), NumberStyles.HexNumber);
+        int g = int.Parse(backgroundHex.Substring(2, 2), NumberStyles.HexNumber);
+        int b = int.Parse(backgroundHex.Substring(4, 2), NumberStyles.HexNumber);
+
+        // Convert to relative luminance
+        double luminance = GetRelativeLuminance(r, g, b);
+
+        // Contrast ratios
+        double contrastWithWhite = (1.0 + 0.05) / (luminance + 0.05);
+        double contrastWithBlack = (luminance + 0.05) / 0.05;
+
+        if (contrastWithWhite >= 2.5)
+        {
+            return "#FFFFFF";
+        }
+
+        return "#000000";
+    }
+
+    private static double GetRelativeLuminance(int r, int g, int b)
+    {
+        double rs = r / 255.0;
+        double gs = g / 255.0;
+        double bs = b / 255.0;
+
+        rs = rs <= 0.03928 ? rs / 12.92 : Math.Pow((rs + 0.055) / 1.055, 2.4);
+        gs = gs <= 0.03928 ? gs / 12.92 : Math.Pow((gs + 0.055) / 1.055, 2.4);
+        bs = bs <= 0.03928 ? bs / 12.92 : Math.Pow((bs + 0.055) / 1.055, 2.4);
+
+        return 0.2126 * rs + 0.7152 * gs + 0.0722 * bs;
+    }
+}