gistfile1.py

#!/usr/bin/env python3
# algorithmic tone row composer
# Python 3 only
# Copyright 2014 Chris Muggli-Miller
# rontasu@gmail.com

# Free for personal use
# Do not redistribute without my permission

# Composes a 12-tone row using every tone in the Western chromatic scale
# exactly once. Instead of choosing each note at random, the program is given
# an awareness of the preceeding melodic interval, and it decides the next note
# based on that interval. The result is that the row sounds unusually melodic
# (tone rows normally sound dissonant), but it still adheres to the rule of
# using every note once and only once.

# I think of this as computer-assisted composing, not pure algorithmic
# composing, because the program doesn't have any say over the rules. All it
# does is give us a list of notes based on a predetermined set of rules.
# Another way to think of it is that the program is really just displacing some
# tedious calculations that a human would otherwise need to do.

from random import randint  # very important for decision-making...

# First, create a list of the display names for all of the notes.
ALL_TONES = ['C', 'C#', 'D', 'D#', 'E', 'F', 'F#', 'G', 'Ab', 'A', 'Bb', 'B']

# Here, we have to keep track of each note by marking it '1' so that the
# same note never gets used twice. Each position in the USED_TONES list
# corresponds to the respective note name in the ALL_TONES list.
# 0 = not yet used, 1 = already used
USED_TONES = [0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]


# This function checks whether the desired note is available. If it is, it
# returns the desired note. If not, it randomly chooses a new note. It will
# keep trying until it finds an available note.
def get_new_note(desired_pitch):
    while USED_TONES[desired_pitch] == 1:
        # can't re-use a pitch, so pick a new one
        desired_pitch = randint(0, 11)
    return desired_pitch


# This function moves from one pitch to another based on the chosen interval.
# An interval (for our purposes) is just a number specifying by how many
# semitones the pitch should be moved. I.e., an interval of '2' means return a
# new pitch that is two semitones higher than the given pitch.
def move_by_interval(pitch_class, interval):
    pitch_class += interval  # move the pitch by 'interval' amount of semitones

    # wrap the new pitch into range, if necessary
    # (move it up or down by one octave so that it always falls between 0-11)
    if pitch_class < 0:
        pitch_class += 12
    elif pitch_class > 11:
        pitch_class -= 12

    return pitch_class


def main():
    # Next, these two variables keep track of the last two notes that were
    # chosen so that we can determine the interval that was between them.
    # -1 means the note is yet undefined; valid range is 0-11
    previous_note = -1
    previous_note2 = -1

    # start composing a tone row!
    # pick a first pitch at random (musically, it doesn't matter what note
    # comes first)
    current_note = randint(0, 11)
    USED_TONES[current_note] = 1

    # print each note as we choose it - this is how we display the row to
    # the user
    print(ALL_TONES[current_note], end=' ')

    previous_note = current_note

    # pick a 2nd pitch
    # Now we start to see some logic. Choose from the following list of
    # pleasant-sounding intervals to determine the 2nd note. This way, the
    # first interval in the row will always sound pleasing and never dissonant.
    # (This rule is based on my musical opinion, and it doesn't have to be
    # this way necessarily.)
    intervaltype = randint(1, 4)
    if intervaltype == 1:  # use a major 3rd
        interval = 4
    elif intervaltype == 2:  # minor 3rd
        interval = 3
    elif intervaltype == 3:  # perfect 4th
        interval = 5
    elif intervaltype == 4:  # perfect 5th
        interval = 7

    # store the last two chosen notes
    previous_note2 = previous_note
    previous_note = current_note

    current_note = move_by_interval(previous_note, interval)
    USED_TONES[current_note] = 1

    # display the 2nd note we chose above
    print(ALL_TONES[current_note], end=' ')

    # now pick the next 10 pitches
    # Here the decision-making logic is minimal. We could use more than this,
    # but it turns out this works surprisingly well. We actually don't want TOO
    # much logic or else the rules become too rigid and the program will
    # compose the same thing every time.
    for i in range(10):

        # keep record of the last two chosen notes...
        previous_note2 = previous_note
        previous_note = current_note

        # get the interval of the last two notes
        previous_interval = previous_note2 - previous_note

        # intelligently choose a new pitch based on the interval between the
        # previous two
        if previous_interval == 4:
            chance = randint(1, 4)  # roll a 4-sided die, basically
            if chance == 1:
                # a 1-in-4 chance of skipping the logic and choosing randomly
                current_note = get_new_note(randint(0, 11))
                USED_TONES[current_note] = 1
            else:
                # if the previous interval was a minor 3rd, attempt to follow
                # it with a major 2nd
                current_note = get_new_note(move_by_interval(previous_note, 3))
                USED_TONES[current_note] = 1
        elif previous_interval == 5:
            chance = randint(1, 4)  # roll the die
            if chance == 1:
                current_note = get_new_note(randint(0, 11))
                USED_TONES[current_note] = 1
            else:
                # if the previous interval was a major 3rd, attempt to follow
                # by going down a minor 3rd
                current_note = get_new_note(move_by_interval(previous_note,
                                                             -4))
                USED_TONES[current_note] = 1
        else:
            # if all else fails, just pick randomly
            current_note = get_new_note(randint(0, 11))
            USED_TONES[current_note] = 1

        print(ALL_TONES[current_note], end=' ')

    print('\nEnd of row!')

if __name__ == "__main__":
    main()