asciiToTrytes.js

//
//  Conversion of ascii encoded bytes to trytes.
//  Input is a string (can be stringified JSON object), return value is Trytes
//
//  How the conversion works:
//    2 Trytes === 1 Byte
//    There are a total of 27 different tryte values: 9ABCDEFGHIJKLMNOPQRSTUVWXYZ
//
//    1. We get the decimal value of an individual ASCII character
//    2. From the decimal value, we then derive the two tryte values by basically calculating the tryte equivalent (e.g. 100 === 19 + 3 * 27)
//      a. The first tryte value is the decimal value modulo 27 (27 trytes)
//      b. The second value is the remainder (decimal value - first value), divided by 27
//    3. The two values returned from Step 2. are then input as indices into the available values list ('9ABCDEFGHIJKLMNOPQRSTUVWXYZ') to get the correct tryte value
//
//   EXAMPLES
//      Lets say we want to convert the ASCII character "Z".
//        1. 'Z' has a decimal value of 90.
//        2. 90 can be represented as 9 + 3 * 27. To make it simpler:
//           a. First value: 90 modulo 27 is 9. This is now our first value
//           b. Second value: (90 - 9) / 27 is 3. This is our second value.
//        3. Our two values are now 9 and 3. To get the tryte value now we simply insert it as indices into '9ABCDEFGHIJKLMNOPQRSTUVWXYZ'
//           a. The first tryte value is '9ABCDEFGHIJKLMNOPQRSTUVWXYZ'[9] === "I"
//           b. The second tryte value is '9ABCDEFGHIJKLMNOPQRSTUVWXYZ'[3] === "C"
//        Our tryte pair is "IC"
//
//      RESULT:
//        The ASCII char "Z" is represented as "IC" in trytes.
//
function toTrytes(input) {

    // If input is not a string, return null
    if ( typeof input !== 'string' ) return null

    var TRYTE_VALUES = "9ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    var trytes = "";

    for (var i = 0; i < input.length; i++) {
        var char = input[i];
        var asciiValue = char.charCodeAt(0);

        // If not recognizable ASCII character, return null
        if (asciiValue > 255) {
            //asciiValue = 32
            return null;
        }

        var firstValue = asciiValue % 27;
        var secondValue = (asciiValue - firstValue) / 27;

        var trytesValue = TRYTE_VALUES[firstValue] + TRYTE_VALUES[secondValue];

        trytes += trytesValue;
    }

    return trytes;
}


//
//  Trytes to bytes
//  Reverse operation from the byteToTrytes function in send.js
//  2 Trytes == 1 Byte
//  We assume that the trytes are a JSON encoded object thus for our encoding:
//    First character = {
//    Last character = }
//    Everything after that is 9's padding
//
function fromTrytes(inputTrytes) {

    // If input is not a string, return null
    if ( typeof inputTrytes !== 'string' ) return null

    // If input length is odd, return null
    if ( inputTrytes.length % 2 ) return null

    var TRYTE_VALUES = "9ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    var outputString = "";

    for (var i = 0; i < inputTrytes.length; i += 2) {
        // get a trytes pair
        var trytes = inputTrytes[i] + inputTrytes[i + 1];

        var firstValue = TRYTE_VALUES.indexOf(trytes[0]);
        var secondValue = TRYTE_VALUES.indexOf(trytes[1]);

        var decimalValue = firstValue + secondValue * 27;

        var character = String.fromCharCode(decimalValue);

        outputString += character;
    }

    return outputString;
}

module.exports = {
    toTrytes: toTrytes,
    fromTrytes: fromTrytes
}