MusicLanguage.java

package music;

import static music.Pitch.OCTAVE;

import java.util.regex.Matcher;
import java.util.regex.Pattern;

/**
 * MusicLanguage defines static methods for constructing and manipulating Music expressions.
 */
public class MusicLanguage {

    // Prevent instantiation
    protected MusicLanguage() {}

    ////////////////////////////////////////////////////
    // Factory methods
    ////////////////////////////////////////////////////

    /**
     * Make Music from a string using a variant of abc notation
     *    (see http://www.walshaw.plus.com/abc/examples/).
     * 
     * <p> The notation consists of whitespace-delimited symbols representing
     * either notes or rests. The vertical bar | may be used as a delimiter
     * for measures; notes() treats it as a space.
     * Grammar:
     * <pre>
     *     notes ::= symbol*
     *     symbol ::= . duration          // for a rest
     *              | pitch duration      // for a note
     *     pitch ::= accidental letter octave*
     *     accidental ::= empty string    // for natural,
     *                  | _               // for flat,
     *                  | ^               // for sharp
     *     letter ::= [A-G]
     *     octave ::= '                   // to raise one octave
     *              | ,                   // to lower one octave
     *     duration ::= empty string      // for 1-beat duration
     *                | /n                // for 1/n-beat duration
     *                | n                 // for n-beat duration
     *                | n/m               // for n/m-beat duration
     * </pre>
     * <p> Examples (assuming 4/4 common time, i.e. 4 beats per measure):
     *     C     quarter note, middle C
     *     A'2   half note, high A
     *     _D/2  eighth note, middle D flat
     * 
     * @param notes string of notes and rests in simplified abc notation given above
     * @param instr instrument to play the notes with
     * @return the music in notes played by instr
     */
    public static Music notes(String notes, Instrument instr) {
        Music music = rest(0);
        for (String sym : notes.split("[\\s|]+")) {
            if (!sym.isEmpty()) {
                music = concat(music, parseSymbol(sym, instr));
            }
        }
        return music;
    }

    /* Parse a symbol into a Note or a Rest. */
    private static Music parseSymbol(String symbol, Instrument instr) {
        Matcher m = Pattern.compile("([^/0-9]*)([0-9]+)?(/[0-9]+)?").matcher(symbol);
        if (!m.matches()) throw new IllegalArgumentException("couldn't understand " + symbol);

        String pitchSymbol = m.group(1);

        double duration = 1.0;
        if (m.group(2) != null) duration *= Integer.valueOf(m.group(2));
        if (m.group(3) != null) duration /= Integer.valueOf(m.group(3).substring(1));

        if (pitchSymbol.equals(".")) return rest(duration);
        else return note(duration, parsePitch(pitchSymbol), instr);
    }

    /* Parse a symbol into a Pitch. */
    private static Pitch parsePitch(String symbol) {
        if (symbol.endsWith("'"))
            return parsePitch(symbol.substring(0, symbol.length()-1)).transpose(OCTAVE);
        else if (symbol.endsWith(","))
            return parsePitch(symbol.substring(0, symbol.length()-1)).transpose(-OCTAVE);
        else if (symbol.startsWith("^"))
            return parsePitch(symbol.substring(1)).transpose(1);
        else if (symbol.startsWith("_"))
            return parsePitch(symbol.substring(1)).transpose(-1);
        else if (symbol.length() != 1)
            throw new IllegalArgumentException("can't understand " + symbol);
        else
            return new Pitch(symbol.charAt(0));
    }

    /**
     * @param duration duration in beats, must be >= 0
     * @param pitch pitch to play
     * @param instrument instrument to use
     * @return pitch played by instrument for duration beats
     */
    public static Music note(double duration, Pitch pitch, Instrument instrument) {
        return new Note(duration, pitch, instrument);
    }

    /**
     * @param duration duration in beats, must be >= 0
     * @return rest that lasts for duration beats
     */
    public static Music rest(double duration) {
        return new Rest(duration);
    }

    ////////////////////////////////////////////////////
    // Producers
    ////////////////////////////////////////////////////

    /**
     * @param m1 first piece of music
     * @param m2 second piece of music
     * @return m1 followed by m2
     */
    public static Music concat(Music m1, Music m2) {
        return new Concat(m1, m2);
    }
}