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 (50,009 lines → 304 modules)
SYNSPEC 54: Synthetic spectrum evaluator (23,917 lines → 168 modules)

Environment Variables

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

# 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)
python3 extract_fortran.py tlusty/tlusty208.f tlusty/extracted/
cp tlusty/*.FOR tlusty/extracted/
cd tlusty/extracted && make    # Output: build/tlusty_extracted

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

Running Tests

# TLUSTY: H-He model test
cd tests/tlusty/hhe
$TLUSTY/tlusty/tlusty.exe < hhe35lt.5 > hhe35lt.6
cp fort.7 hhe35lt.7
diff hhe35lt.7 hhe35lt.7.bak  # Verify against expected

# SYNSPEC: spectrum test
cd tests/synspec/hhe
ln -sf $TLUSTY/data data       # MUST be symlink, not file
cp hhe35nl.7 fort.8
ln -sf fort.55.con fort.55
$TLUSTY/synspec/synspec.exe < hhe35nl.5
# Output: fort.7 (spectrum), fort.17 (continuum)

Module Extraction

extract_fortran.py splits monolithic Fortran into individual .f files:

Generates Makefile with correct flags
Analyzes COMMON block dependencies
Identifies pure functions (no COMMON) for independent testing
Handles unnamed BLOCK DATA units

Key Architecture

TLUSTY include files define COMMON blocks shared across subroutines:

BASICS.FOR: Array dimensions (MDEPTH=100, MFREQ=135000, MLEVEL=1134)
ATOMIC.FOR: Atomic masses, abundances, energy levels
MODELQ.FOR: Temperature, density, populations
ARRAY1.FOR: Main linear equation arrays

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

Refactoring Notes

Pure functions (no COMMON dependency) can be rewritten independently
TLUSTY has 195 pure units, SYNSPEC has 93
See memory/MEMORY.md for detailed extraction results and test procedures

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