Songreader.js

// Given a song reader state, move the reader to the new time.
//
// - prev_state: the previous song reader state
// - song: the song being read
//
function f_next_song_reader_state(prev_state, song) {
    let next_state = {...prev_state}

    // Pattern commands don't transfer from state to state, so we must clear these out
    next_state.pattern_commands = []

    let prev_pattern = song.patterns[ prev_state.pattern_index ]
    let tick_period = 60.0 / (prev_pattern.bpm * prev_pattern.ticks_per_beat)

    // At end of pattern? Start next one, or repeat entire song if option provided. Set done
    // if not repeating.
    let next_pattern_command_index = prev_state.next_pattern_command_index
    if (!prev_state.started_playing) {
        // Special case: We've just started playing, so we're already on the right tick and pattern.
        next_state.started_playing = true
    } else if (prev_state.tick_index == prev_pattern.ticks_per_pattern - 1) {
        next_state.tick_index = 0
        next_state.pattern_index++
        next_state.pattern_start_time = prev_state.pattern_start_time + tick_period * prev_pattern.ticks_per_pattern
        next_pattern_command_index = 0

        if (next_state.pattern_index == song.patterns.length) {
            if (prev_state.song_reader_options.repeats) {
                next_state.pattern_index = 0
            } else {
                next_state.done = true
            }
        }
    } else {
        // Not done pattern, so move onto next tick.
        next_state.tick_index++
    }

    if (!next_state.done) {
        let curr_pattern = song.patterns[ next_state.pattern_index ]

        next_state.pattern_commands = []

        // Find the patterns that apply to this current tick
        while (next_pattern_command_index < curr_pattern.pattern_commands.length
                && curr_pattern.pattern_commands[ next_pattern_command_index].tick_index < next_state.tick_index ) {
            next_pattern_command_index++
        }

        if (next_pattern_command_index < curr_pattern.pattern_commands.length
                && curr_pattern.pattern_commands[ next_pattern_command_index ].tick_index == next_state.tick_index) {
            // We found a pattern command that applies to this tick.
            next_state.pattern_commands.push(curr_pattern.pattern_commands[ next_pattern_command_index ])
            next_pattern_command_index++
            
            // Keep going until we go past the commands with the same index
            while (next_pattern_command_index < curr_pattern.pattern_commands.length
                    && curr_pattern.pattern_commands[next_pattern_command_index].tick_index == next_state.tick_index ) {
                next_state.pattern_commands.push(curr_pattern.pattern_commands[ next_pattern_command_index ])
                next_pattern_command_index++
            }
        }
        
        next_state.next_pattern_command_index = next_pattern_command_index
    }

    return next_state
}