// Timekeeping info for the timer
timer_start = 0;
timer_sec = 0;

// Runs once a second to maintain the timer
// Tablutes how long it's been, subtracts that from timer length, then
// updates the HTML element representing the timer
function timer_tick() {
    // Don't do anything if a timer isn't running
    if (!timer_start) {
        return false
    }

    // Take how long it's been since we started the timer and subtract that
    // from the timer duration to get how long is remaining
    remain = timer_sec - ((Date.now() - timer_start) / 1000);

    // Stop at 0
    if (remain < 0) {remain = 0;}

    // Update the element
    document.getElementById('timer').innerHTML = seconds_to_string(remain);
}

// Utility function to convert seconds to H:MM:SS
function seconds_to_string(seconds) {
    hours = Math.floor(seconds / 3600);
    minutes = Math.floor((seconds % 3600) / 60);
    seconds = Math.floor(seconds % 60);

    return String(hours) + ":" +
           String(minutes).padStart(2, '0') + ":" + 
           String(seconds).padStart(2, '0')
}

// Tabluates how many seconds we want on the timer, stores that info, and
// stops a currently running timer
function reset_timer() {
    hours = parseInt(document.getElementById('timer_h').value);
    minutes = parseInt(document.getElementById('timer_m').value);
    seconds = parseInt(document.getElementById('timer_s').value);

    total = (minutes * 60) + (hours * 3600) + seconds;

    document.getElementById('timer').innerHTML = seconds_to_string(total);

    timer_start = 0;
    timer_sec = total;
}

// Starts the timer by setting 'now' as the timer start time
function start_timer() {
    timer_start = Date.now()
}

// Destroys the timer start time to halt the timer
function stop_timer() {
    timer_start = 0;
}

// Start timer interval
setInterval(timer_tick, 1000)