////
/// Return a frequency based on piano key
/// @param {Number} key [49]
/// @return {Number} frequency
////
const getFrequencyFromKey = (key = 49) => {
    return 2 ** ((key - 49) / 12) * 440;
};

////
/// Convert a string-based key to a number-based key
////
const convertKeyStringToNumber = (key = "C4") => {
    let series = key.match(/\d/g)[0];
    let notes = ["A", "A#Bb", "B", "C", "C#Db", "D", "D#Eb", "E", "F", "F#Gb", "G", "G#Ab"];
    let note = key.replace(/\d/g, '');

    return (note.match(/[AB]/) ? series : series - 1) * 12 + notes.indexOf(note) + 1;
};

////
/// Return an array of frequencies from a starting key, array of semitone
/// intervals, and number of keys to convert.
/// @param {Number} keyStart [41] Middle C
/// @param {Array} keyIntervals [[0]]
/// @param {Number} keyCount [1]
/// @return {Array} frequencies
////
const getFrequenciesFromKeys = (keyStart = 41, keyIntervals = [0], keyCount = 1) => {
    let frequencies = [getFrequencyFromKey(keyStart)];
    let keyInterval = 0;

    for (let count = 1; count < keyCount; count++) {
        keyInterval += Number.parseFloat(keyIntervals[count % keyIntervals.length]);
        frequencies.push(getFrequencyFromKey(keyStart + keyInterval));
    }

    return frequencies;
};