# CLAUDE.md

This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.

## Project Overview

Fortran stellar atmosphere modeling suite being refactored to Rust. Strategy: **split Fortran into modules first, then incrementally rewrite in Rust**.

- **TLUSTY 208**: Non-LTE stellar atmosphere calculator (~50,000 lines → 304 modules)
- **SYNSPEC 54**: Synthetic spectrum evaluator (~24,000 lines → 168 modules)
- **Progress**: 120/~472 Fortran units translated to Rust

## Environment Variables

```bash
export TL208=/home/fmq/program/tlusty
export TLUSTY=$TL208/tl208-s54
export LINELIST=$TL208/linelist
export IRON=$TL208/irondata
export OPTABLES=$TL208/optables
```

## Build Commands

### Rust
```bash
cargo build              # Debug build
cargo build --release    # Release build
cargo test               # Run all tests (includes Fortran comparison tests)
cargo test test_expo     # Run specific test
```

### Fortran
```bash
# Production (single file)
gfortran -O3 -fno-automatic -mcmodel=large -o tlusty/tlusty.exe tlusty/tlusty208.f
gfortran -O3 -fno-automatic -mcmodel=large -o synspec/synspec.exe synspec/synspec54.f

# Development (modular)
cd rust/tlusty/extracted && make    # Output: build/tlusty_extracted
```

**Fortran compile flags:**
- `-mcmodel=large`: Required for large COMMON blocks (>2GB address space)
- `-fno-automatic`: Static storage (old Fortran compatibility)
- **Never use** `-ffixed-line-length-none`: Breaks columns 73-80 handling

## Rust Architecture

```
src/
├── lib.rs              # Module exports
├── data.rs             # Static data arrays (translated from BLOCK DATA)
├── math/               # Pure math functions (no COMMON dependency) - 120 modules
│   ├── expint.rs       # Exponential integrals
│   ├── voigt.rs        # Voigt profile
│   ├── tridag.rs       # Tridiagonal solver
│   └── ...
├── state/              # COMMON block translations as structs
│   ├── constants.rs    # Physical/math constants, array dimensions
│   ├── config.rs       # Runtime config
│   ├── atomic.rs       # Atomic/ion/level data
│   ├── model.rs        # Atmosphere model state (largest struct)
│   ├── arrays.rs       # Main linear equation arrays
│   ├── iterat.rs       # Iteration control
│   ├── alipar.rs       # ALI (Accelerated Lambda Iteration) arrays
│   └── odfpar.rs       # ODF (Opacity Distribution Function) data
└── physics/            # Physics calculations (placeholder)
```

## Running Tests

### Rust Tests
```bash
cargo test                                    # All unit tests
cargo test --test fortran_comparison          # Fortran comparison tests only
```

### Fortran Integration Tests
```bash
# TLUSTY: H-He model test
cd $TLUSTY/tests/tlusty/hhe
$TLUSTY/tlusty/tlusty.exe < hhe35lt.5 > hhe35lt.6
cp fort.7 hhe35lt.7
diff hhe35lt.7 hhe35lt.7.bak

# SYNSPEC: spectrum test
cd $TLUSTY/tests/synspec/hhe
ln -sf $TLUSTY/data data
cp hhe35nl.7 fort.8
ln -sf fort.55.con fort.55
$TLUSTY/synspec/synspec.exe < hhe35nl.5
```

## Refactoring Workflow

1. **Find pure functions**: Check `rust/tlusty/extracted/_PURE_UNITS.txt` for units without COMMON dependencies
2. **Translate**: Create `src/math/<name>.rs`, add to `src/math/mod.rs`
3. **Verify**: Add test case in `tests/fortran_comparison.rs` with Fortran reference values

## Key Architecture

**TLUSTY COMMON blocks** (mapped to `src/state/` structs):
- `BASICS.FOR` → `constants.rs`: Array dimensions (`MDEPTH`=100, `MFREQ`=135000, `MLEVEL`=1134)
- `ATOMIC.FOR` → `atomic.rs`: Atomic masses, abundances, energy levels
- `MODELQ.FOR` → `model.rs`: Temperature, density, populations
- `ARRAY1.FOR` → `arrays.rs`: Main linear equation arrays

**SYNSPEC** reads model atmosphere from `fort.8`, outputs spectrum to `fort.7`

## Fortran → Rust Translation Notes

Critical patterns to avoid mistakes:

- **Index conversion**: Fortran 1-indexed → Rust 0-indexed. `arr(i)` → `arr[i-1]`, `DO I=1,N` → `for i in 0..n`
- **Expression precedence**: `-LOG(X)` is `-(LOG(X))`, not `LOG(-X)`
- **Decrementing loops**: Use `isize`/`i32` when loop variable may become negative
- **COMMON dependency**: Check INCLUDE statements before assuming a function is pure
- **Large structs**: Functions with many COMMON dependencies use lifetime structs:
  ```rust
  pub struct Params<'a> {
      pub input: &'a [f64],
      pub output: &'a mut [f64],
  }
  ```
- **Type inference**: `(z1 - z2).powi(2)` may fail; use `(z1 - z2) * (z1 - z2)`
- **Test tolerance**: Polynomial approximations (Abramowitz-Stegun) need relaxed tolerance (~1e-7)