cyclicity

cyclicity.js

var lastDigit = function(str1, str2){
  var x = Number(str1.substr(str1.length - 1));
  
  if(Number(str2) === 0) {
    return 1;
  }
  if([0, 1, 5, 6].indexOf(x) >= 0) {
    return x;
  }
  if([4,9].indexOf(x) >= 0) {
    return cycle(2, x, str2);
  }
  if([2, 3, 8, 7].indexOf(x) >= 0) {
    return cycle(4, x, str2);
  }
}

function cycle(cyclicity, x, y) {
  var t = Number(modulo(y, cyclicity));
  if(t === 0) {
    return Math.pow(x, cyclicity) % 10;
  }
  return Math.pow(x, t) % 10;
}

function modulo (divident, divisor) {
    var cDivident = '';
    var cRest = '';

    for (var i in divident ) {
        var cChar = divident[i];
        var cOperator = cRest + '' + cDivident + '' + cChar;

        if ( cOperator < parseInt(divisor) ) {
                cDivident += '' + cChar;
        } else {
                cRest = cOperator % divisor;
                if ( cRest == 0 ) {
                    cRest = '';
                }
                cDivident = '';
        }

    }
    cRest += '' + cDivident;
    if (cRest == '') {
        cRest = 0;
    }
    return cRest;
}