DavisDevelopment · October 30, 2015 18:20 · Oct 30, 2015
diff --git a/Luhn.java b/Luhn.java
@@ -0,0 +1,42 @@
+
+public class Luhn {
+  /**
+	 * validate the given card-number with the luhn mod10 algorithm
+	 */
+	public static boolean luhn(String card) {
+		int total = 0; // sum of all digits
+		int v; // variable to hold the numeric value of the "current" char
+		int i = card.length(); // current position in the String
+		/*
+		 [action] represents what we're currently doing with the chars we read
+		------
+		 action=true: odd character
+		 action=false: even character
+		*/
+		boolean action = false;
+
+		while(0 < i--) {
+		// get the numeric value of the char
+			v = (int)(card.charAt(i) - 48);
+			// if we're an even number of steps into this loop
+			if ( action ) {
+				v *= 2; // double the value of [v]
+				// and if [v] is now a double-digit number
+				if (v > 9) {
+				  // reduce it by 9, effectively adding the two digits
+					v -= 9;
+				}
+			}
+
+			// increase [total] by
+			total += v;
+			// toggle the value of [action]
+			action = !action;
+		}
+
+		/*
+		 return whether [total] is divisible by 10
+		*/
+		return ((total % 10) == 0);
+	}
+}