# Learnings — tofu-core

Conventions, patterns, and successful approaches discovered during work on this plan.

_Auto-scaffolded by /start-work. Append new entries below - never overwrite._

---

## Task 5 — tofu.vercmp (RPM-style version comparison)

### Module naming
- `version` is a D reserved keyword (conditional compilation). Module is named
  `tofu.vercmp`, file `src/tofu/vercmp.d`.  The `DepConstraint` struct in
  `types.d` uses field `ver` (not `version`) for the same reason.

### Algorithm (exact port from ZETA/lib/vercmp.lua)
- `nextSegment`: skips any non-digit/non-letter chars (separators); then reads
  a contiguous run of same-type chars.  Returns `(segment, nextIndex)`.
- `cmpNumeric`: strips leading zeros from both segments (`"007"` → `"7"`,
  `"000"` → `"0"`).  Longer stripped string wins; then lexicographic.
- `compare`: strips all whitespace, then loops over alternating segments.
  If both segments are digit-typed → `cmpNumeric`, else lexical.  When one
  version exhausts segments: the exhausted one is *older* (shorter = older).
  This matches RPM semantics: `compare("1.0.0", "1.0") > 0`.
- `satisfies`: uses `final switch` on `DepOp` enum.  Maps to `compare` result
  as expected (ge→c>=0, le→c<=0, eq→c==0, ne→c!=0, gt→c>0, lt→c<0).
- `parseDep`: delegates to `DepConstraint.parse` (already ported in types.d by
  task 4).  Both implementations are kept consistent.

### whitespace stripping
- Could not use `filter`+`array` because Phobos auto-decodes `string` to
  `dchar` range elements, returning `dchar[]` not `string`.  Implemented a
  manual `removeWhitespace` helper using raw code-unit indexing, with a
  `@trusted` cast since the freshly-allocated `char[]` has no aliasing.

### Test porting
- All 10 ZETA test categories ported as separate `@safe unittest` blocks.
- 4 modules pass unittests (types, vercmp, plus pre-existing ones).
- `dub build` passes with warnings-as-errors enabled.

### Future considerations
- When task 12 (constraint resolution) imports `tofu.vercmp`, the `compare`
  and `satisfies` functions are ready.  The `parseDep` integration with
  `DepConstraint.parse` ensures consistency between ZETA and tofu's dep
  constraint semantics.

---

## Task 2: Config Loading (src/tofu/config.d)

### toml package API (v1.0.0, Kripth/toml)
- Import: `import toml;` → provides `TOMLDocument`, `TOMLValue`, `parseTOML`, `TOMLException`, `TOMLParserException`
- `TOMLDocument` has `alias table this` — acts like `TOMLValue[string]` AA
- Access sections: `doc["core"]` returns `TOMLValue` with `type == TOML_TYPE.TABLE`
- Access string values: `doc["core"]["key"].str` (throws if type mismatch)
- Check existence: `"key" in doc` returns `TOMLValue*` (null if absent)
- `parseTOML()` is `@system` — must wrap in `@trusted` block when calling from `@safe` code
- Keys without `.str` / `.integer` access will throw `TOMLException`

### D language gotchas
- **Nested function declarations**: D does NOT allow `void foo() { ... }` inside a function body. Use lambdas: `auto foo = () { ... }; foo();` or anonymous `() @trusted { ... }();`
- **`version` is a keyword**: Cannot use `string version;` as field name — use `string version_;` or initialize with `= ""`
- **`parseTOML` is `@system`**: The toml package's parse function isn't marked `@safe` — must wrap in `@trusted` lambda
- **`std.file.readText` is `@system`**: Same — wrap in `@trusted`
- **`std.process.environment.get()`**: Returns empty string when env var is unset; `"VAR" in environment` checks existence but `environment.get("VAR", "default")` is cleaner
- **`stderr.writefln` is `@safe`** in recent DMD (2.106+) — usable directly in `@safe` code
- **AA `in` operator**: Returns pointer (`V*`) or `null` — must check for null before dereferencing

### Pattern: env-overrideable config loading
- Priority: env vars > TOML file > hardcoded defaults
- TOML read happens BEFORE env override — so TOML values serve as base, env vars overwrite
- Missing config file → no error, just skip to defaults
- Malformed TOML → `TOMLParserException` caught, warning to stderr, fall back to defaults
- Testability: `load()` accepts optional `envOverrides` map and explicit `configFile` path to avoid real env/filesystem

### Config design decisions
- `TOFU_CONFIG` env var overrides default config file path (`~/.config/tofu/config.toml`)
- `~` in default paths expanded with `std.path.expandTilde`
- All path fields default to empty string when "find on PATH" — no hardcoded binary paths
- `defaultJobs` typed as `int` (not `size_t`/`ulong`) since it's a user-facing count

### buildOptions warningsAsErrors deprecation
- DUB warns about `warningsAsErrors` in `buildOptions` — recommends `buildRequirements` instead
- Not blocking, informational only

## Task 3 — `tofu.log` (colored, NO_COLOR-aware logging)

- **dmd 2.112 gotcha**: the `std.stdio` globals `stdout`/`stderr` are `@system` to *access* — `makeGlobal` uses `__gshared File result`, which fails `@safe` inference. An `@safe` function cannot call `stdout.writeln(...)` directly. Fix: route every write through a tiny `@trusted` helper (`writeStdout(string)`/`writeStderr(string)`), keep the public log functions `@safe`. Compiler hint: *"using `__gshared` instead of `shared` makes it fail to infer `@safe`"*.
- **`std.process.environment` (dmd 2.112)**: it is an `abstract final class` of *static* methods — `environment.get(name)` is `@safe` and returns `null` when the var is unset (so "NO_COLOR set" == `get !is null`, even for an empty string). `environment["K"] = v` and `environment.remove(name)` are `@trusted`. NOT deprecated in 2.112.
- **Testable color decision**: make `colorEnabled()` re-read the environment on every call instead of caching in `static this()`. Enables NO_COLOR/TERM tests without restarting the process. Costs one `getenv` per log line — negligible.
- **Env save/restore in unittests**: a `private struct ColorEnv` (in `version(unittest)`) that snapshots NO_COLOR+TERM in its ctor and restores them in `~this()` (destructor) — `auto env = ColorEnv("1", "xterm")` scopes the restore. `null` value = remove the var. Double-destruction from a copied temporary is harmless (restore is idempotent).
- **Capturing stdout/stderr in unittests**: swap the global `stdout`/`stderr` File to a `File(name, "w+")`, write, `flush()`, swap back, then `rewind()` + `readln()`. Restore the global before the temp File's destructor runs.
- **zeta log format details**: `warn ` paints the *whole prefix* yellow (`paint("yellow", "warn ") ~ msg`), but `error` paints *only the word* then `" " ~ msg` (`paint("red","error") ~ " " ~ msg`). Assertion must expect `\x1b[31merror\x1b[0m failed deploy` — the reset code sits between "error" and the message.
- **`version` is a D keyword** — cannot be used as a struct field / identifier (trap for the parallel `types.d` work).
- **`std.stdio.writeln` flushes per call** — already satisfies "no buffering"; no explicit `flush()` needed in production paths.
- **Verifying a single module without the full build**: `dmd -main -unittest -i src/tofu/log.d` runs just that module's unittests when sibling modules (`config.d`, `types.d`) are mid-edit by parallel agents. `-i` pulls imports in automatically.

## Task 4 — `tofu.types` (core data structures)

- **`version` is a D keyword** — all struct fields named `version` must be renamed to `ver`. The D compiler treats `version` as a conditional-compilation directive and rejects it as a field name (see task 3 learnings above).
- **Struct field defaults in D**: `string` fields initialise to `null` by default. To enforce empty-string semantics (`""`), every string field must be explicitly `= ""`.
- **`final switch` for exhaustive enum handling**: using `final switch` on enum types forces the compiler to verify all cases are covered — catches missing branches at compile time. Used for `poolToString()`.
- **Manual character scanning preferred over `std.regex`**: the dep-spec parser uses simple char-by-char scanning with `isNameChar()` / `isWhite()` helpers instead of regex. This avoids the `std.regex` dependency, keeps the parser `@safe`, and matches the Lua reference semantics exactly.
- **Dep spec parsing order matters**: 2-character operators (`>=`, `<=`, `==`, `~=`) MUST be checked before 1-character operators (`>`, `<`, `=`) to avoid false matches. The Lua reference uses an ordered table `OPS = { ">=", "<=", "==", "~=", ">", "<", "=" }` — we replicate this with if/else chains.
- **`=` normalises to `==`**: the Lua code has `if op == "=" then op = "==" end`; we map both to `DepOp.eq`.
- **Trailing garbage detection**: after extracting version chars, any non-whitespace remaining in the spec string causes a `TypesException`. The Lua pattern uses `$` anchor for this.
- **`dub test` includes all `.d` files in source paths** for executable targets, not just imported modules. Pre-existing compile errors in sibling modules (config.d) block test runs. Fixed config.d by removing `private` from a nested function (access specifiers not allowed on local functions in D).
- **`BuildResult.failed` as `BuildFailure[]`**: the plan describes it as `struct {string name; string reason}[]` — in D this is a named struct `BuildFailure` with array field `BuildFailure[] failed`. The `failedNames()` helper extracts just the names for caller convenience.
- **`DepConstraint.parse` is a static factory method**: returns a new `DepConstraint` by value, never allocates on the heap. `@safe` throughout.
- **All structs are `@safe`**: no methods do I/O, no `@system` calls. The module stands alone — no imports from other tofu modules.