add web app & add concurrency

web/src/getContrastYIQ.ts

@@ -0,0 +1,25 @@
+export function getContrastYIQ(hexcolor: string) {
+  // Remove leading # if present
+  if (hexcolor.startsWith("#")) {
+    hexcolor = hexcolor.slice(1);
+  }
+
+  // Handle 3-digit hex codes (e.g., #abc → #aabbcc)
+  if (hexcolor.length === 3) {
+    hexcolor = hexcolor
+      .split("")
+      .map((c) => c + c)
+      .join("");
+  }
+
+  // Parse RGB values
+  const r = parseInt(hexcolor.substr(0, 2), 16);
+  const g = parseInt(hexcolor.substr(2, 2), 16);
+  const b = parseInt(hexcolor.substr(4, 2), 16);
+
+  // Calculate YIQ brightness
+  const yiq = (r * 299 + g * 587 + b * 114) / 1000;
+
+  // Return black or white based on contrast
+  return yiq >= 128 ? "black" : "white";
+}