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0003 The blueprint is the song

Status: accepted

Context

A claudius song is edited from many directions: a script changes the tempo, the studio paints drum steps, mutation swaps a bass style, an assistant rewrites the form, analysis of an audio file produces a new song. It has to be saved, undone, printed as a script and evolved, and two versions of it have to be comparable.

A DAW stores the arranged session: tracks with their notes and clips. That is what it plays, and it is the obvious thing to store. But a session records results, not decisions. A bass line stored as 144 notes no longer knows that it was an off-beat house bass over a progression; changing the key means moving every note; trying another style means writing it again by hand; and a mutation operating on notes can only nudge notes.

Decision

A song is its Blueprint (crates/claudius/src/model.rs): the decisions a producer makes, stored as data. Tempo, key and swing; progressions as Roman numerals; the form; drum lanes as step patterns; each melodic part as a recipe (a role, a style, a register, a density and a seed of its own); pattern tracks as the code that makes them; every sound; the mix. The arranged Project with its notes and clips is derived from it by compose::arrange, deterministically, every time it is needed, and thrown away after rendering.

The blueprint is what is saved (blueprint.json), what undo restores, what scripts print and edit, and what mutation and crossover change. Notes are stored in a blueprint only where no recipe produced them: hand-written or analysed notes in a part’s notes, and the drum steps.

Rejected: storing the project as the truth, for the reasons above, and storing both, which would leave two copies of the song that drift apart the first time one is edited.

Consequences

  • Edits happen at the level a producer thinks at. A new seed is a new bass line in the same style; a new key moves every part, because progressions are numerals and parts are written from them; a style change rewrites a part in a sentence. Mutation operators are a few lines each (Evolution).
  • A blueprint is small: about 30 KB of JSON for a full house song, against about 300 KB for its project. Undo keeps 200 whole blueprints, and evolution hashes them to skip duplicates.
  • A song can print as a script that runs back into itself (Patterns and the script engine), because every field is a decision a script function can state.
  • The notes of a generated part depend on the writer’s code. When theory code changes, an existing blueprint renders differently: a saved song is not frozen across versions of claudius. The generator snapshot tests make such changes visible in review, but they cannot keep old songs as they were.
  • Single notes of a generated part cannot be edited in place. To change one, the part has to become hand-written notes or a pattern track, and then it no longer follows the key and style.
  • A track plays the same way in every section it is in; there is no per-bar arrangement edit. A variation takes a second track.
  • Everything that derives the project must be deterministic, which shapes the random number generator, the seeds stored in parts, and the hash-based randomness of patterns (Architecture).