# Architecture

See [`CLAUDE.md`](../CLAUDE.md) for build commands and the sharp edges to
know before changing the engine. This document is the visual overview.

## Components

![Component architecture](images/architecture.png)

- **`res/`** holds the source assets: the 22 Rider-Waite-Smith Major
  Arcana card images and A. E. Waite's *Pictorial Key to the Tarot* PDF
  (public domain), which `tarot_data.c`'s card and position text is
  condensed from.
- **`engine/third_party/astronomy/`** is a vendored, unmodified copy of
  [cosinekitty/astronomy](https://github.com/cosinekitty/astronomy)
  (MIT), pinned to a specific commit — see its `VENDORED.md`.
- **`engine/src/`** is the core engine. Everything except `main.c`,
  `reading.c`'s `reading_print_text`/`reading_print_html` functions, and
  `i18n.c`'s file-loading half is free of `stdio`/CLI assumptions, so it
  links straight into a Pebble watchapp without rework.
  - `rng.c` — deterministic string-seeded PRNG (FNV-1a + splitmix64).
  - `tarot.c` / `tarot_data.c` — the Celtic Cross draw and its content.
  - `astro.c` — natal chart + daily transits, built on the vendored
    Astronomy Engine.
  - `i18n.c` — loads a `key=value` translation file into a small
    fixed-size catalog; every display-text lookup elsewhere falls back to
    the built-in English string if no catalog is loaded or a key is
    missing from it.
  - `reading.c` — orchestrates the above into one `DailyReading` and
    (desktop-only) renders it as text or HTML.
  - `main.c` — CLI argument parsing, the only consumer of `reading_print_*`.
- **`engine/i18n/`** holds the translation source files (`en.lang`,
  `de.lang`, ...), one `key=value` file per language — see
  [`CLAUDE.md`](../CLAUDE.md) for the key-naming convention.
- **`dist/`** is generated by the single root `Makefile`: the
  `deck-engine` and `interpreter-cli` binaries, a copy of the card art
  under `img/`, a copy of the translation files under `i18n/`,
  `run-engine.sh`/`run-interpreter.sh`, and `user.properties`. Since
  `dist/` itself is disposable, `user.properties` also gets backed up to
  `engine/scripts/user.properties.local` (gitignored) once it's filled
  in, and restored from there if `dist/` is ever wiped — see
  `CLAUDE.md`'s "Build & run".

## Dataflow

![Dataflow for one reading](images/dataflow.png)

A single reading is produced by one call: `reading_generate(seed, birth,
utc_moment, &out)` in `reading.c`. It fans out to three independent
computations and combines their results:

1. **`astro_compute_natal_chart(birth)`** → `NatalChart` — geocentric
   ecliptic sign/degree for the Sun through Pluto, the Ascendant, and
   whole-sign houses, all from the birth date/time/location.
2. **`astro_compute_daily_transits(utc_moment, natal_chart)`** →
   `DailyTransits` — today's planetary positions, the Moon phase, and
   every transiting-to-natal aspect within orb. Depends on the natal
   chart (read-only) to compute aspects against it.
3. **`tarot_draw_celtic_cross(seed)`** → `CelticCrossSpread` — ten cards
   with position and upright/reversed orientation, fully determined by
   the seed string alone.

These three results are combined into one `DailyReading` struct — the
real API surface. `reading_print_text`/`reading_print_html` (used by the
CLI) are just one way of consuming it; any caller, including watch UI
code, can walk the struct's fields directly. See
[`input-output-format.md`](input-output-format.md) for the exact fields
and output formats.

Every display string produced along the way (`tarot_data.c`'s card and
position text, `astro.c`'s body/sign/moon-phase/aspect names, and
`reading.c`'s section headers) is looked up in `i18n.c`'s translation
catalog, with the built-in English text as the fallback. `main.c` loads
the catalog from `--lang`/`--i18n-dir` (or `run-engine.sh`'s `lang=`
property) before calling `reading_generate`, so this is orthogonal to
the three computations above — it only affects how their results are
rendered as text, not the underlying `TarotCard`/`ZodiacSign`/etc. enum
values.

Both entry points (direct CLI flags, or `run-engine.sh` pulling the
seed/date from the OS clock and birth data from `user.properties`)
converge on the same `main.c` argument parsing before calling
`reading_generate` — there is exactly one code path from parsed input to
a reading. `run-interpreter.sh` reuses those same OS-clock/`user.properties`
inputs, but calls `deck-engine --format json` and pipes the result into
`interpreter-cli` instead of printing a reading directly.

## Regenerating the diagrams

The diagrams are Graphviz sources in `docs/diagrams/`, rendered to PNG:

```bash
cd docs
dot -Tpng -Gdpi=150 diagrams/architecture.dot -o images/architecture.png
dot -Tpng -Gdpi=150 diagrams/dataflow.dot -o images/dataflow.png
```
