# Repository Guidelines

## Project Structure & Module Organization
- `src/main.rs`: Entry point; wires CLI to parser.
- `src/cli.rs`: Clap-based argument parsing (flags like `--file`, `--input-format`).
- `src/color.rs`: Minimal ANSI color helpers.
- `src/parser/`: Parser submodule containing all DER decoding and pretty-printing logic.
  - `mod.rs`: Public API surface for the parser (re-exports types and `pretty_print_der`, `parse_and_print`).
  - `asn1_types.rs`: TLV parsing, OID decode, helpers.
  - `encoding_utils.rs`: Hex/bits formatting utilities and truncation logic.
  - `pretty_printer.rs`: Recursive DER walker and structure rendering.
  - `value_printer.rs`: Leaf value formatting (strings, times, OIDs, bit/octet strings).
  - `certificate_utils.rs`: X.509-specific helpers and detection.
  - `time_utils.rs`: UTCTime/GeneralizedTime humanization.
  - `oid_registry.rs`: Friendly OID names.
- `tests/data/`: Sample keys/certs used by integration tests or manual runs.
- `Cargo.toml`: Rust 2021 package config and dependencies.

## Build, Test, and Development Commands
- Build: `cargo build` (use `--release` for optimized binary).
- Run help: `cargo run -- --help`.
- Parse DER file: `cargo run -- --file tests/data/test.cert.der --input-format der`.
- Parse hex string: `cargo run -- 3082...` (whitespace allowed; auto-detected).
- Lint: `cargo clippy --all-targets -- -D warnings`.
- Format: `cargo fmt`.
- Tests: `cargo test`.

## Linting & Defaults
- Clippy: configured via `clippy.toml`; we keep warnings at deny to avoid regressions.
- BIT STRINGs: truncated by default for readability (bits: 128, hex bytes: 16). Override with `--bits-truncate` or `--bits-truncate-bytes`.

## Coding Style & Naming Conventions
- Rust 2021; 4-space indent; `rustfmt` defaults (`cargo fmt`).
- Naming: modules `snake_case`; types/enums `CamelCase`; functions `snake_case`; constants `SCREAMING_SNAKE_CASE`.
- Avoid `unsafe`; prefer clear, small functions. Keep CLI parsing in `cli.rs`, decoding/formatting in `src/parser/`.
- Run `cargo clippy` locally and fix lints before PRs.

## Testing Guidelines
- Framework: built-in Rust tests (`cargo test`).
- Unit tests: add `#[cfg(test)] mod tests { ... }` near code in the relevant file under `src/parser/`.
- Integration tests: create Rust files under `tests/` (e.g., `tests/parse_certificate.rs`) and use fixtures in `tests/data/`.
- Naming: prefer descriptive test names (e.g., `it_parses_bit_string_flags`).
- If output formats change, update or add tests that assert on rendered lines.

## Commit & Pull Request Guidelines
- Commits: write imperative, concise subjects (e.g., "parse: handle BIT STRING hex mode"). Conventional Commit prefixes are welcome.
- PRs: include a clear description, reproduction or sample command, and before/after output when changing formatting.
- Link related issues; add tests or fixtures when fixing bugs or adding options.
- CI expectations: code compiles, `cargo fmt` has no diff, and `cargo clippy` shows no warnings.

## Notes & Tips
- Binary DER from stdin isn’t supported in text mode; use `--file` with `--input-format der`.
- Colors auto-detect TTY; force with `--color always`.