Exercises: the band

These exercises practise this part on the real code: the library, the chord sheet, the band and its players, the orchestra, the song’s form, the stops, the soloist, your own styles, the practice tools and the lead sheet. Each one says what is already there, so you extend it rather than build it again. Some are things to try in the app and listen to; some are pieces of Rust with a clear end; the ones called “Think about it” are questions, with no code to write.

Try the Rust ones on a branch of your own. Run cargo test after each change, and cargo fmt --all --check and cargo clippy --workspace --all-targets -- -D warnings before you call one finished: CI runs all three. An exercise that adds something to the project file needs the next free format number (see The version number).

The library

1. A style of your own (lineup-core, easy)

STYLES in library/style.rs is a table of style(...) rows (see Styles: a table in a static). Add one: a progression from a song you like, written in numerals. For example the whole of Pachelbel’s Canon, I | V | vi | iii | IV | I | IV | V (the Piano ballad style has only its first six chords), with Comping::ArpUpDown and Bass::Held. Run cargo test -p lineup-core before you open the app: if a numeral is misspelled, a bar cannot hold its chords or the name is taken, every_style_parses_and_fits_in_a_pattern or names_are_unique_and_rhythms_exist names the style.

Then try it in the app over a song in another key. Which chord names change, and which numerals stay the same?

Practises: const fn tables, struct literals, reading test failures.

2. A rhythm with a cowbell and a fill (lineup-core, easy)

Add a drum pattern to RHYTHMS in library/rhythm.rs, in its genre’s block, with a line for Kick, one for Snare and one for Cowbell, and a fill that exists, such as "Tom fill". In 4/4 every line is sixteen characters, from X, x, o, h, l and .; spaces are only for reading. Put the cowbell on an offbeat or a clave, not on every step, so it is heard as a colour of its own.

Done when every_line_is_whole_bars_of_known_characters and names_are_unique_and_fills_exist pass, the Library lists the rhythm with its sentence, and, with the band playing it, the fill comes in the last bar of every eight.

Practises: data as text, tests that read the whole table.

3. A third chacarera (lineup-core, easy)

The library has a Chacarera rhythm in 6/8 and a Chacarera trunca in 3/4. Read both first: a bar of each is twelve steps. Which steps start the beats of a 6/8 bar, and which those of a 3/4 bar? Now write a third bombo pattern for the same dance, with the head (LowTom) on the 6/8 beats and the rim (Rim) on the 3/4 ones, the two against three of the genre, and a style that uses it. Check in the rack that the beat colours come in two groups of six.

Practises: reading existing data before adding to it, meters with the same number of steps.

4. Near the song’s tempo (the app, easy)

In the Library, on a song at 60 BPM, tick Near 60 BPM. Which tempos does it keep? Then on a song at 180. NEAR_SPEED is 1.15: work the ranges out before you look, and say why a fixed ±15 BPM would have been worse (see The Library tab).

Practises: ratios.

5. The same genre (lineup-app, medium)

Add a fourth box, Same genre as the song’s style, that keeps the presets of the genre the band plays. What does Narrowing need to know, where do you get it from, and what should the box do in a song without a band?

Practises: adding a filter without touching the others.

6. Think about it: closures that run once

write_pattern_style takes write: impl FnOnce(&mut Project, usize). The two closures passed to it today only read what they capture. Would impl Fn work for them? What could a later closure do that Fn would refuse, and why is asking for FnOnce the kindest choice for whoever writes the next one?

The chord sheet

7. How many bars fit (pencil, easy)

Columns::count (see Why 1, 2, 4, 8, 12 or 16) gets the width of the sheet and the width the widest bar needs. Work out by hand how many bars a line holds in a sheet 900 pixels wide when the widest bar needs 100, 120 and 250 pixels. Then add those three cases to the test next to count, and run it.

Practises: reading a short function closely.

8. Insert several bars (lineup-app, easy)

Insert a bar before and Insert a bar after call project.insert_bars(bar, after, 1). Add Insert bars…, which asks how many. Which line of open_bars already makes room for more than one, and what stops a song from growing past its longest?

Practises: using what the core already offers.

9. A bar inside a repeat (lineup-core, medium)

Write a test in bars.rs: a clip repeated three times, and a bar inserted in the middle of its second time round. Write down first which clips, at which steps, you expect afterwards; then run it. The tests a_clip_of_a_pattern_played_elsewhere_too_is_cut_in_two and a_repeated_clip_loses_the_repeats_in_the_deleted_bars show how to set one up.

Practises: writing the expectation before the code tells you.

10. Think about it: before or after

close_bars reads the time signature, the tempo and the key at the end of the deleted bars before it moves anything (see What a deleted bar takes with it). What would it read if it asked afterwards, and what would the song sound like?

11. Two bars again (lineup-core, medium)

% plays the bar before again (repeat_bar). Charts have a sign for the two bars before as well, a % with two slashes, drawn across the bar line of the two bars it fills. Let %% typed in a bar repeat the two bars before it, in that bar and the next, which must be empty. read_typed_bar_after reads one bar and is given one bar before: what must it be given now, and how does the sheet window learn that typing in one bar wrote two? What should text::write write for two bars that repeat the two before?

Practises: a notation that spans more than the place it is typed in.

12. A title in the song (lineup-core and lineup-app, medium)

text::read reads a Title: line into TextSheet::title, but the song does not keep it: Export chord sheet… writes the song’s file name as the title. Keep the title in the project and write it back. It is a new field of the file: what does an older file read as?

Practises: a new field from the text format to the project file.

13. Paste in the song’s key (lineup-app, medium)

A chart pasted from a message keeps its chord names, even when its Key: line says another key than the song’s. Add Paste in the song’s key, which moves the pasted chords from the chart’s key to the song’s. HarmonyName::moved(from, to) already does it for one harmony, keeping letters right. What does the paste need to know that it does not keep today?

Practises: reusing a theory function in an editing action.

14. Bars per line in text (lineup-core, easy)

text::write puts at most four bars on a line, BARS_PER_LINE. Make it a parameter. What in the column layout assumes four, and how does widths have to change?

Practises: turning a constant into a parameter, arrays and vectors.

15. Think about it: other chart formats

ChordPro, a common format for songbooks, writes chords inside the lyrics: [C]Twinkle twinkle [F]little [C]star. It has no bar lines. What would a reader need to guess to make a TextSheet from it?
Some players want the numeral large and the chord name small under it. HarmonyName::caption gives the numeral of a chord. What should the large line say for a set, which has no numeral?

How the band plays

16. Bass lines by eye (the app, easy)

Type C | Am | F | G7 | C and pick Jazz waltz. Look at the bass in the piano roll: which note does it play on beat 3 of each bar, and how far, by name, is it from the next bar’s root? Then pick Swing, in 4/4: what does the walk put on beats 2 and 3 now? Check your answers against walk in The bass.

Practises: reading a bass line.

17. Where the chord changes (the app, easy)

Type a bar Dm7 G7 and a bar C / / G7 in Pop beat. On which beat does G7 start in each? Look at the piano part: does its rhythm change in those bars, or only its notes?

18. Alberti bass (the app, easy)

Pick Alberti bass over C | Am | F | G7. The pattern is “low, high, middle, high”. Which note is low in the Am bar, and why is it not A? (Hint: look at all four bars, and at The harmonies, as notes.)

19. A part you changed (the app, easy)

In a new song, change one note of the bass, then type a new chord in bar 2. Which parts follow the new chord, and which doesn’t? What does the notice say? Click Play it again: what happens to your note?

20. A band over a demo (the app, easy)

Open Canon in D and click Pop ballad in the Library. Look at the channel rack: which channels are muted, and which play on? Then pick No band in the band’s menu. Is the demo back exactly as it was?

21. Think about it: what a fingerprint leaves out

part_print leaves out the track a clip is on and its pattern’s number. What would the user see if it did not?
band_styles() returns &'static [BandStyle]. Why can Band hold a &'static BandStyle without a lifetime parameter, when it could not hold a &BandStyle borrowed from a local Vec?

A bigger band, horns, and following your chords

22. Four guitarists (the app, easy)

Make a chord sheet with C Am F G and try Pop beat, Folk, Rock beat and Swing. Click Piano, Bass and Drums in the You play row, so that only the guitar is left. How does each guitarist play the same four chords?

23. Chord shapes (pencil and the piano roll, easy)

In Pop beat, read the guitar’s notes for G and for F in the piano roll and write their shapes as chord books do (G is 320003; see The music: how a guitarist plays a chord). Why does the F shape have no open strings? Which shape would you play for E7, and which notes of the chord does it double? Check your answers with open_chords_are_the_ones_in_chord_books and other_keys_take_barre_chords.

24. Full chords through distortion (lineup-core, easy)

In band/extras.rs, change Rock beat’s guitar from GuitarPart::Power(Comping::Pulse) to GuitarPart::Strum(Comping::Pulse): full shapes through the overdriven guitar. Listen with a SoundFont. Why do rock guitarists avoid this? Put it back.

25. A guitar for Lo-fi (lineup-core, easy)

Lo-fi has no extra players. Give it a nylon-string guitar picking the chords, GuitarPart::Strum(Comping::ArpUp), in BY_STYLE of extras.rs. Then misspell the style’s name: which test fails, and what does it print?

Practises: tables of constants, tests that guard names.

26. Think about it: the guitar

Over a major chord the boogie shuffle rocks between the fifth and the sixth; over a minor chord it stays on the fifth (see the_boogie_alternates_fifth_and_sixth). What chord do A, C, E and F make, and why would a minor sixth over A minor be risky? What would a major sixth, F♯, turn the chord into?
try_frets takes found: &mut impl FnMut(&[u8]). Try found: impl FnMut(&[u8]). What does the compiler say about the recursive call, and why?

27. A second keyboard (lineup-core, harder)

Add an electric piano (program 4) playing the style’s comping an octave up in the R&B and Neo soul styles. Which parts of band.rs and file.rs must change (look at written and written_too in In the file)? Check that a song whose band has only three players saves with the same format number as before.

28. Horns held and struck (the app, easy)

Tick Horn section in Swing and in Pop ballad, over the same chords. Compare the horns’ notes in the piano roll: what is the same, and what differs? Why does smooth voice leading matter even more for long notes than for short ones?

29. Common tones (the app, easy)

Type C Am F G in Pop beat with the horns, and write down each chord’s horn voicing from the piano roll. Which horn keeps its note from one chord to the next, and for how many chords? A note two chords share is a common tone: why does the horn on it not move?

30. Cheap fifths (lineup-core, easy)

cost charges PARALLEL_FIFTHS, 24 semitones, for each pair of horns moving in parallel fifths. Make it 2 and run cargo test -p lineup-core horns. Which test fails, between which two chords, and which horns move in fifths? Put it back.

31. Think about it: names in two tables

HELD and BY_STYLE in horns.rs name styles by their names. What happens if a style is renamed in the library? Find the test that would catch it.

32. A split point (lineup-app, medium)

Arranger keyboards often have a split point: only the keys below it choose the chord, so the right hand can play a tune. Add one, say below C4, as a second option next to Accompaniment follows chords. Which keys reach chord_held now, and what still happens to the keys above?

33. Two keys a major second apart (lineup-core, easy)

chord_held reads two keys by the interval between them (see Following the chords you hold). Make two keys a major second apart a sus2 chord on the lower one. Which test needs a new case? Before your change, holding F below G asked for no chord; what does it ask for now, and is that what a player holding them would mean?

34. Think about it: chords at once

Notes already sounding keep their old pitch until they end, so a chord held for a whole bar changes only at the next bar. Arranger keyboards play the new chord at once. What would the audio thread need to know to do that, and what would it cost?

The orchestra

35. Clap with the bass drum (the app, easy)

Apply the Orchestra to The Rite of Spring, The Augurs of Spring and mute the Song track. Clap along with the bass drum. Can you predict where the next stroke comes? Unmute the song and listen to the horns’ accents falling on the same eighths.

36. Felt beats by hand (pencil, easy)

TimeSignature::felt_groups gives 13/16 as 2 + 2 + 2 + 2 + 2 + 3. On which sixteenths do the horns strike in a bar of 13/16 where they keep to the felt beats? Open Petrushka, The Moor’s Room, apply the Orchestra, find a bar of 13/16 on the ruler and check. If they play something else there, which rule of Playing along with the score explains it?

37. `in_meter` for the tresillo bar (lineup-core, easy)

Work out by hand the line in_meter writes for a bar of 3+3+2/8 from the Orchestra’s X... .... o... ..... Then add it to a_rhythm_that_follows_the_meter_plays_each_bar_in_its_own in rhythm.rs, and run it.

38. A style in 4/4 over changing meters (the app, easy)

With the Orchestra playing the Ritual of Abduction, click Pop beat in the Library. What happens to the ruler, and why does a style in 4/4 have to do that, when the Orchestra did not?

39. Only the voices (the app, easy)

Open Shchedryk (Carol of the Bells), apply the Orchestra with only Voices / instruments ticked, and open the Mixer. Which instruments do the Song track’s channels play? Which do the new Piano and Bass tracks play?

40. Accents nobody wrote (lineup-core, easy)

In band/following.rs, change TUTTI from (9, 5) to (3, 2) and run cargo test -p lineup-core accents_of_the_augurs. Do accents appear that Stravinsky did not write? Put it back.

Form, variations, sections and locks

41. The intro leads somewhere (the app, easy)

42. The last chord (the app, easy)

Set the key to A minor (the Key menu beside the harmony keys), write a song that starts on Am, and turn the ending on. Which chord does it end on? Now change the first chord to F: which chord does it end on now, and why is that still right?

43. How long is the song (the app, easy)

Export a song with a 4-bar intro, three choruses and an ending as a WAV file. How many bars long is it? Check your answer against the file’s length and the tempo.

44. The form moves (the app, easy)

Give a song an 8-bar intro, change a chord in it, then change the intro to 2 bars. What happens to your chord? Undo it.
Type a chord in the first bar of the ending. Where is the ending now, and where is the end-repeat sign?

45. Think about it: the form in Rust

Form is Copy; Band is not. Look at their fields: why could Band not be?
In played_pieces, the chorus repeats only if from <= start && end <= to. What would a loop region from the middle of the intro to the middle of the chorus play without that test?

46. In two, then walking (the app, easy)

Choose the Jazz ballad style, write | Cmaj7 | Am7 | Dm7 | G7 | twice, and set bars 5 to 8 to variation b. Open the bass’s patterns in the piano roll. How many notes does a bar have in a, and in b? Which notes does the bass play in the Dm7 bar of each?

47. Fills and crashes (the app, easy)

In an eight-bar Pop beat, set bar 5 to variation b and bar 6 back to a. Which bars get a drum fill, and where do the crashes fall? Why could that sound restless in a real song?
Choose Swing and play a few bars in each variation. What changes and what stays? Why does the time stay on the same cymbal?

48. A style for some bars (the app, easy)

Write | C | Am | F | G | twice in Pop beat, and make bars 5 to 8 Bossa nova. Where does the bossa nova’s bass line start, and how does it differ from the pop one?
Play a section in Chorale, which has no drums. What do the drums play there, and in the bar before it?

49. Locks (the app, easy)

Open Ode to Joy, lock Keys in the rack, and apply Doo-wop. Which tracks changed, and what did Keys keep?
In a new song, lock the kick and apply Bossa nova. What does the kick play now?

50. Think about it: markers and borrows

tidy for variation markers sorts, then calls dedup_by, then retain. What would go wrong if it called retain before sorting?
played_in_runs takes play: impl Fn(...). Could it take an FnMut? What would change where it is called?
where_it_plays returns impl Iterator. What would change if it returned a Vec, and why does notes.extend(...) accept both?
StylePattern holds a &'a mut Pattern. What does the borrow checker say if you call another Project method while one is alive, and why is that a good thing?

Stops and the soloist

51. Stop-time (the app, easy)

Type a twelve-bar blues in F with shots on the first four bars: | F7. | F7. | F7. | F7. | Bb7 | Bb7 | F7 | F7 | C7 | Bb7 | F7 | C7 |, and pick Shuffle. Play along in the gaps with the F blues scale (F A♭ B♭ B C E♭). What changes in the feel when the band comes back in bar 5?

52. Pushes (the app, easy)

Type | C | Am | F | G | in Pop beat and listen. Then push every chord after the first: | C | ^Am | ^F | ^G |. Clap the piano’s rhythm both ways. Which beat is no longer struck? Then compare the piano and the bass in the piano roll: how long does each hold a pushed chord?

53. A hold over two bars (lineup-core, medium)

Type C.. in bar 1, nothing in bar 2 and G7 in bar 3, so the hold lasts two bars, and look at the bass: one note. Each player strikes its own way says why the bass uses bass_struck and not Bass::Held. Write a test in band.rs that would fail if it did use Bass::Held.

54. A shot a quarter long (lineup-core, easy)

A shot is an eighth note: SHOT_STEPS in band/stops.rs is 2. Make it a quarter note and run cargo test -p lineup-core. Tests fail not only in stops.rs (a_shot_strikes_once_and_the_band_stops) but in the library’s charts and demos too: why do those care how long a shot is? Fix the expectations, or decide the change was wrong.

55. A push of a quarter (lineup-core, medium)

Add ^^^C, a push of a quarter note: a variant of Push. Let the compiler show you every match that must learn it (steps, marks, describe). Which other places need it? (Hint: read_marks refuses three carets today, and breaks_menu in the app lists the pushes.)

56. Think about it: a fill before a stop

The drums play a fill in the bar before a change. Should a shot or a rest in that bar keep it? Find where drum_part adds the fill and where apply_to_drums takes the groove out, decide, and write a test for what you chose.

57. Guide tones and chord scales (the app, easy)

With the soloist on, open its part in the piano roll and find the notes marked 3 and 7. Where does a 7th move to a 3rd a half step away at a chord change? Sing only the 3rds and 7ths, one a bar, over the band.
Type | Am7 | D7 | Gmaj7 | E7 | Am7 | D7 | G6 | G6 | in G major. Work out the scale of each chord with the rules of Chord scales before you look. One chord plays an F natural, although the key has F♯: which one, and which other note of its scale is not in G major?
In A minor, type | Bm7b5 | E7 | Am | Am |. Which note of E7’s scale is not in A natural minor? Listen for it in the solo.

58. Leaps and enclosures (lineup-core, medium)

The soloist leaps to the second chord tone one time in five (dice.chance(20) in target). Make it 60 and play a few solos with Another solo. How does the line change? Which tests still pass, and why?
Add an enclosure, a favourite bebop figure: before a target, a scale note above it and then the note a half step below it, as D B C into C. Give it a new NoteRole, let the compiler show you every match that must learn it, and write a test in the style of roles_name_what_each_note_does.

59. A new solo every chorus (lineup-core, harder)

With Choruses set to 3, the soloist plays the same solo three times. Make each chorus a new solo. Where is the chorus repeated (look for repeated_chorus), and how could each time round get its own seed while the song still plays the same every time it is opened?

Your own styles and the practice tools

60. Read the files (the app and a text editor, easy)

Save a style from the style editor (see Trying it) and open the .lineup-style file in a text editor. Find its format, bars, chart and the lanes with each channel and its notes. Then apply it to a song, save the song, and search its file for custom_rhythm: the whole style is inside. Why must it be, and what is the song’s format number at least?

61. Out-of-key dominants (lineup-core, medium)

Over a chord outside the key, adapt_pitch in user_style.rs keeps the distances written over C, so the E7 that leads to Am plays E F♯ G♯ A B C♯ D (see Chords from outside the key). The soloist’s chord scales, in harmony/chord_scale.rs, would give it the F and C of A minor. Use them for dominant chords outside the key. Which written notes of a style over E7 in C major now play differently? Write the test first, in the style of template_notes_stay_in_the_key, and listen: is a pattern that sounds the same over every chord worth more than the right notes?

62. How many bars (pencil, easy)

A user style holds at most 256 steps and at most sixteen bars. How many bars can a style in 7/8 have, and one in 6/4?

63. The metronome counts (the app, easy)

Turn on the metronome over Silent Night, in 6/8. How many clicks are there in a bar, and where does the high one fall? Now Greensleeves, in 3/4: the same six eighth notes to a bar, counted differently.
With the count-in on, pause a song halfway through a bar, then press Play again. What do you hear before the music, and why is that better than counting a whole bar and then starting halfway through the next?
Set the speed to 50% and export a WAV file. How long is it, compared with the song at full speed? Why?

64. Tapping (the app, easy)

Tap 4 times at an even pace, then 4 more times faster without stopping. Which tempo do you get, and why not the average of the eight gaps?
In a song in 6/8, tap 80 times a minute. Which tempo does the song get, and why?
Take over the bass with You play, then change a chord, and give the bass back: is the new chord in its part?

65. Think about it: the click and the count-in

ClickVoice::strike(Click::None) returns at once. What would you hear if it set level to 0 instead?
In begin_play, what goes wrong if self.stop() is not called when Play arrives while the song is already playing?

66. The speed trainer by hand (the app, easy)

Set from 50% to 100% by 10% every 2 passes. Write down the speed of each of the first eight times round, then play it with the trace on and check.
Set from 100% to 60%. What does the trainer do now? Think of a use for it (hint: a passage you can play fast, but not yet evenly).
Start a run, Pause in the middle of it, then press Play. Then Stop and press Play. What is the speed each time, and why is it useful that the two buttons differ?
With the trainer on and nothing looped, play a whole song. When does the speed go up?

67. Think about it: the trainer’s messages

In came_round, self.laps_seen.replace(laps) runs before the check for self.training. What would go wrong when you turn the trainer on in the middle of a song if it ran after it?
The audio thread throws an old list of lap tempos away with throw_away when a new request replaces it. What would happen if it simply assigned the new list, and why does that matter on the audio thread in particular?

68. More for the ear (lineup-core, medium)

A tune for the major seventh. Interval::tune has none going up. Find a song whose melody starts with one, check it on a piano, and add it. Does going_up_names_a_tune need to change?
Tunes going down. Add a tune_down (the start of Beethoven’s Fifth Symphony is a major third down) and use it when the question went down.
What you get wrong. Keep, for each interval, how often it was asked and how often it was right, and ask the weak ones more often. How would IntervalQuestion::new use the counts without losing the rule “same seed, same question”?

69. Think about it: ear training

Sixth chords are left out because C6 and Am7 have the same notes. Could they be asked fairly if the chord always played over its root an octave below? Would inversions still make sense?
Let the progression exercise play the song’s own chord sheet, with the names hidden. What would it need from chord_sheet.rs?

The lead sheet and parts

70. Notation choices (lineup-core, medium)

Courtesy accidentals. When a bar has an F♯ and the next bar an F natural by the key signature, many lead sheets print a natural in brackets as a reminder. Where would BarAccidentals need to know about the bar before?
Half-diminished. Some lead sheets write Em7♭5 as Eø7. Add a choice for it. Is it a choice of layout.rs, or of the chord’s name?

71. Think about it: the page

Endings. When the last bar of a repeated section differs the second time, lead sheets use first and second endings: a bracket “1.” over one bar, “2.” over the other. What would the playlist need to tell score.rs?
Bars of rest. A tune silent for eight bars could show one bar with a thick line and “8” over it. Where in systems would it go, and how does it change the bar numbers of the next line?

72. More instruments (lineup-core, medium)

The exact octave. Add a choice that writes every part exactly as the instrument plays it: a ninth up for the tenor saxophone, an octave and a sixth for the baritone. Which function changes? How many leger lines does a tune around C5 need for the tenor, and why do books of lead sheets not do this?
D and G instruments. A trumpet in D sounds a whole tone higher than written, an alto flute in G a fourth lower. Add them. What do note() and semitones() return, and what else in the window and the label has to know about them?
The E♭ clarinet sounds a minor third higher than it reads. Is that a new Transposition, or the alto saxophone’s part in another octave? What would shift need to know to tell them apart?

73. Parts as text, and the concert key (lineup-core, easy)

Give Export chord sheet… the same choice of instruments. Which single function does the work for every chord? (See The chords: moved letters and all.)
Some parts print the song’s own key under the instrument, “Concert key: B♭”. Where in layout.rs would it go, and what does lay_out need to know that it doesn’t now?

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