//! Partition function for H2O from EXOMOL data.
//!
//! Translated from SYNSPEC `h2opf` subroutine.

use std::sync::OnceLock;

const TABLE_SIZE: usize = 10000;
const DATA_FILE: &str = "./data/h2o_exomol.pf";

static TABLE: OnceLock<Option<(Vec<f64>, Vec<f64>)>> = OnceLock::new();

fn load_table() -> Option<(Vec<f64>, Vec<f64>)> {
    let content = std::fs::read_to_string(DATA_FILE).ok()?;
    let mut ttab = Vec::with_capacity(TABLE_SIZE);
    let mut pftab = Vec::with_capacity(TABLE_SIZE);
    for line in content.lines().take(TABLE_SIZE) {
        let parts: Vec<&str> = line.split_whitespace().collect();
        if parts.len() >= 2
            && let (Ok(t), Ok(pf)) = (parts[0].parse::<f64>(), parts[1].parse::<f64>()) {
                ttab.push(t);
                pftab.push(pf);
            }
    }
    Some((ttab, pftab))
}

/// Evaluate H2O partition function at temperature `t` by linear interpolation.
///
/// Returns `None` if the data file cannot be loaded.
pub fn h2opf(t: f64) -> Option<f64> {
    let table = TABLE.get_or_init(load_table).as_ref()?;
    let (ref ttab, ref pftab) = *table;
    let n = ttab.len();
    if n < 2 || t < ttab[0] || t > ttab[n - 1] {
        return None;
    }
    let itab = t.floor() as usize;
    if itab >= n - 1 {
        return None;
    }
    let idx = itab.min(n - 2);
    let pf = pftab[idx] + (t - ttab[idx]) * (pftab[idx + 1] - pftab[idx]);
    Some(pf)
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_h2opf_basic() {
        // Without the data file, should return None
        // This test just verifies the function compiles and runs
        let _ = h2opf(5000.0);
    }
}