ffcharge 0.2.0

A lightweight pure Rust library for fast, residue-based force field charge assignment (AMBER/CHARMM) in molecular modeling pipelines.
Documentation
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use std::collections::HashMap;
use std::env;
use std::fs::File;
use std::io::{BufWriter, Write};
use std::path::Path;

// =============================================================================
// Data Structures
// =============================================================================

#[derive(Debug, serde::Deserialize)]
struct Record {
    scheme: String,
    position: String,
    residue: String,
    atom: String,
    charge: f32,
}

#[derive(Debug, Default)]
struct WaterData {
    o: Option<f32>,
    h1: Option<f32>,
    h2: Option<f32>,
}

/// Parsed charge data organized by scheme → position → residue → atoms.
type AtomData = HashMap<String, HashMap<String, HashMap<String, Vec<(String, f32)>>>>;
/// Ion data organized by scheme → residue → charge.
type IonData = HashMap<String, HashMap<String, f32>>;
/// Water data organized by scheme → WaterData.
type WaterMap = HashMap<String, WaterData>;

// =============================================================================
// Helper Functions
// =============================================================================

/// Sanitizes a string for use as a Rust identifier.
fn sanitize(s: &str) -> String {
    s.replace('-', "_").replace('+', "_plus")
}

/// Generates a PHF map name from scheme and position.
fn map_name(scheme: &str, pos: &str) -> String {
    let scheme_ident = sanitize(scheme).to_uppercase();
    let pos_ident = if pos.is_empty() {
        "EMPTY".to_string()
    } else {
        sanitize(pos).to_uppercase()
    };
    format!("MAP_{}_{}", scheme_ident, pos_ident)
}

// =============================================================================
// Code Generation
// =============================================================================

struct CodeGenerator {
    atom_data: AtomData,
    ion_data: IonData,
    water_data: WaterMap,
}

impl CodeGenerator {
    fn new() -> Self {
        Self {
            atom_data: HashMap::new(),
            ion_data: HashMap::new(),
            water_data: HashMap::new(),
        }
    }

    fn load(&mut self, path: &Path) {
        let mut rdr = csv::ReaderBuilder::new()
            .comment(Some(b'#'))
            .trim(csv::Trim::All)
            .from_path(path)
            .expect("Failed to open data/charges.csv");

        for result in rdr.deserialize() {
            let record: Record = result.expect("Failed to parse CSV record");
            self.process_record(record);
        }
    }

    fn process_record(&mut self, record: Record) {
        match (record.residue.as_str(), record.scheme.as_str()) {
            ("HOH", _) => self.add_water(&record),
            (_, "classic") => self.add_ion(&record),
            _ => self.add_atom(&record),
        }
    }

    fn add_water(&mut self, record: &Record) {
        let entry = self.water_data.entry(record.scheme.clone()).or_default();
        match record.atom.as_str() {
            "O" => entry.o = Some(record.charge),
            "H1" => entry.h1 = Some(record.charge),
            "H2" => entry.h2 = Some(record.charge),
            other => panic!("Unknown water atom: {}", other),
        }
    }

    fn add_ion(&mut self, record: &Record) {
        self.ion_data
            .entry(record.scheme.clone())
            .or_default()
            .insert(record.residue.clone(), record.charge);
    }

    fn add_atom(&mut self, record: &Record) {
        self.atom_data
            .entry(record.scheme.clone())
            .or_default()
            .entry(record.position.clone())
            .or_default()
            .entry(record.residue.clone())
            .or_default()
            .push((record.atom.clone(), record.charge));
    }

    fn generate_lib(&self, path: &Path) {
        let mut f = BufWriter::new(File::create(path).unwrap());

        writeln!(f, "// Auto-generated by build.rs - DO NOT EDIT").unwrap();
        writeln!(f).unwrap();

        self.write_atom_maps(&mut f);
        self.write_ion_map(&mut f);
        self.write_water_map(&mut f);

        writeln!(f).unwrap();
        writeln!(
            f,
            "// ============================================================================="
        )
        .unwrap();
        writeln!(f, "// Lookup Functions").unwrap();
        writeln!(
            f,
            "// ============================================================================="
        )
        .unwrap();

        self.write_lookup_fn(&mut f, "get_protein_charge", &["n", "n-", "c", "c+", "m"]);
        self.write_lookup_fn(&mut f, "get_nucleic_charge", &["5", "3", "m"]);
        self.write_ion_lookup_fn(&mut f);
        self.write_water_lookup_fn(&mut f);
    }

    fn generate_test(&self, path: &Path) {
        let mut f = BufWriter::new(File::create(path).unwrap());

        writeln!(f, "// Auto-generated by build.rs - DO NOT EDIT").unwrap();
        writeln!(
            f,
            "// This file provides test utilities for charge validation."
        )
        .unwrap();
        writeln!(f).unwrap();

        self.write_atom_maps(&mut f);

        writeln!(f).unwrap();
        writeln!(
            f,
            "// ============================================================================="
        )
        .unwrap();
        writeln!(f, "// Test Utility Functions").unwrap();
        writeln!(
            f,
            "// ============================================================================="
        )
        .unwrap();

        self.write_atoms_lookup_fn(&mut f, "get_protein_atoms", &["n", "n-", "c", "c+", "m"]);
        self.write_atoms_lookup_fn(&mut f, "get_nucleic_atoms", &["5", "3", "m"]);
    }

    fn write_atom_maps(&self, f: &mut BufWriter<File>) {
        for (scheme, pos_map) in &self.atom_data {
            for (pos, res_map) in pos_map {
                let name = map_name(scheme, pos);
                self.write_atom_phf_map(f, &name, res_map);
            }
        }
    }

    fn write_atom_phf_map(
        &self,
        f: &mut BufWriter<File>,
        name: &str,
        res_map: &HashMap<String, Vec<(String, f32)>>,
    ) {
        let entries: Vec<_> = res_map
            .iter()
            .map(|(res, atoms)| {
                let atoms_str = atoms
                    .iter()
                    .map(|(a, c)| format!("(\"{}\", {}_f32)", a, c))
                    .collect::<Vec<_>>()
                    .join(", ");
                (res.clone(), format!("&[{}]", atoms_str))
            })
            .collect();

        let mut phf = phf_codegen::Map::new();
        for (res, val) in &entries {
            phf.entry(res.as_str(), val.as_str());
        }

        writeln!(
            f,
            "static {}: phf::Map<&'static str, &'static [(&'static str, f32)]> = {};",
            name,
            phf.build()
        )
        .unwrap();
    }

    fn write_ion_map(&self, f: &mut BufWriter<File>) {
        for (scheme, res_map) in &self.ion_data {
            let name = format!("ION_MAP_{}", sanitize(scheme).to_uppercase());

            let entries: Vec<_> = res_map
                .iter()
                .map(|(res, charge)| (res.clone(), format!("{}_f32", charge)))
                .collect();

            let mut phf = phf_codegen::Map::new();
            for (res, val) in &entries {
                phf.entry(res.as_str(), val.as_str());
            }

            writeln!(
                f,
                "static {}: phf::Map<&'static str, f32> = {};",
                name,
                phf.build()
            )
            .unwrap();
        }
    }

    fn write_water_map(&self, f: &mut BufWriter<File>) {
        let entries: Vec<_> = self
            .water_data
            .iter()
            .map(|(scheme, data)| {
                let val = format!(
                    "crate::WaterCharges {{ o: {}_f32, h1: {}_f32, h2: {}_f32 }}",
                    data.o.expect("Missing O"),
                    data.h1.expect("Missing H1"),
                    data.h2.expect("Missing H2")
                );
                (scheme.clone(), val)
            })
            .collect();

        let mut phf = phf_codegen::Map::new();
        for (scheme, val) in &entries {
            phf.entry(scheme.as_str(), val.as_str());
        }

        writeln!(
            f,
            "static WATER_CHARGES: phf::Map<&'static str, crate::WaterCharges> = {};",
            phf.build()
        )
        .unwrap();
    }

    fn write_lookup_fn(&self, f: &mut BufWriter<File>, fn_name: &str, positions: &[&str]) {
        let arms = self.build_scheme_match_arms(positions, |name| format!("{}.get(res),", name));

        writeln!(f).unwrap();
        writeln!(f, "#[inline(always)]").unwrap();
        writeln!(
            f,
            "pub fn {}(scheme: &str, pos: &str, res: &str, atom: &str) -> Option<f32> {{",
            fn_name
        )
        .unwrap();
        writeln!(f, "    let atoms = match scheme {{").unwrap();
        for arm in &arms {
            writeln!(f, "        {}", arm).unwrap();
        }
        writeln!(f, "        _ => None,").unwrap();
        writeln!(f, "    }}?;").unwrap();
        writeln!(f).unwrap();
        writeln!(
            f,
            "    atoms.iter().find(|(a, _)| *a == atom).map(|(_, c)| *c)"
        )
        .unwrap();
        writeln!(f, "}}").unwrap();
    }

    fn write_atoms_lookup_fn(&self, f: &mut BufWriter<File>, fn_name: &str, positions: &[&str]) {
        let arms =
            self.build_scheme_match_arms(positions, |name| format!("{}.get(res).copied(),", name));

        writeln!(f).unwrap();
        writeln!(f, "#[inline(always)]").unwrap();
        writeln!(
            f,
            "pub fn {}(scheme: &str, pos: &str, res: &str) -> Option<&'static [(&'static str, f32)]> {{",
            fn_name
        )
        .unwrap();
        writeln!(f, "    match scheme {{").unwrap();
        for arm in &arms {
            writeln!(f, "        {}", arm).unwrap();
        }
        writeln!(f, "        _ => None,").unwrap();
        writeln!(f, "    }}").unwrap();
        writeln!(f, "}}").unwrap();
    }

    fn build_scheme_match_arms<F>(&self, positions: &[&str], map_access: F) -> Vec<String>
    where
        F: Fn(&str) -> String,
    {
        let mut arms = Vec::new();

        for (scheme, pos_map) in &self.atom_data {
            let pos_arms: Vec<_> = pos_map
                .keys()
                .filter(|p| positions.contains(&p.as_str()))
                .map(|pos| {
                    let name = map_name(scheme, pos);
                    format!("\"{}\" => {}", pos, map_access(&name))
                })
                .collect();

            if !pos_arms.is_empty() {
                let pos_match = format!(
                    "match pos {{\n            {}\n            _ => None,\n        }}",
                    pos_arms.join("\n            ")
                );
                arms.push(format!(
                    "\"{}\" => {{\n            {}\n        }}",
                    scheme, pos_match
                ));
            }
        }

        arms
    }

    fn write_ion_lookup_fn(&self, f: &mut BufWriter<File>) {
        let arms: Vec<_> = self
            .ion_data
            .keys()
            .map(|scheme| {
                let name = format!("ION_MAP_{}", sanitize(scheme).to_uppercase());
                format!("\"{}\" => {}.get(res).copied(),", scheme, name)
            })
            .collect();

        writeln!(f).unwrap();
        writeln!(f, "#[inline(always)]").unwrap();
        writeln!(
            f,
            "pub fn get_ion_charge(scheme: &str, res: &str) -> Option<f32> {{"
        )
        .unwrap();
        writeln!(f, "    match scheme {{").unwrap();
        for arm in &arms {
            writeln!(f, "        {}", arm).unwrap();
        }
        writeln!(f, "        _ => None,").unwrap();
        writeln!(f, "    }}").unwrap();
        writeln!(f, "}}").unwrap();
    }

    fn write_water_lookup_fn(&self, f: &mut BufWriter<File>) {
        writeln!(f).unwrap();
        writeln!(f, "#[inline(always)]").unwrap();
        writeln!(
            f,
            "pub fn get_water_charges(scheme: &str) -> Option<crate::WaterCharges> {{"
        )
        .unwrap();
        writeln!(f, "    WATER_CHARGES.get(scheme).copied()").unwrap();
        writeln!(f, "}}").unwrap();
    }
}

// =============================================================================
// Main Entry Point
// =============================================================================

fn main() {
    let csv_path = Path::new("data/charges.csv");
    println!("cargo:rerun-if-changed={}", csv_path.display());

    let out_dir = env::var("OUT_DIR").unwrap();
    let out_path = Path::new(&out_dir);

    let mut generator = CodeGenerator::new();
    generator.load(csv_path);
    generator.generate_lib(&out_path.join("codegen.rs"));
    generator.generate_test(&out_path.join("codegen_test.rs"));
}