Annotated Luhn Algorithm implementation, in Java

Luhn.java

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);
	}
}