//! Cubin compilation for the LOB simulator CUDA kernels.
//! Mirrors crates/ml-alpha/build.rs.
//! Per feedback_no_nvrtc.md: cubins only, no runtime nvrtc.

use std::path::PathBuf;
use std::process::Command;

fn main() -> Result<(), String> {
    let cuda_dir = PathBuf::from(env!("CARGO_MANIFEST_DIR")).join("cuda");
    let out_dir = PathBuf::from(std::env::var("OUT_DIR").map_err(|e| e.to_string())?);

    // Per pearl_build_rs_rerun_if_env_changed.md: pair every env::var
    // with rerun-if-env-changed.
    // Default sm_86 covers RTX 3050 (local dev). Production Argo workflows
    // override via FOXHUNT_CUDA_ARCH=sm_89 (L40S Ada) or sm_90 (H100).
    let arch = std::env::var("FOXHUNT_CUDA_ARCH").unwrap_or_else(|_| "sm_86".into());
    println!("cargo:rerun-if-env-changed=FOXHUNT_CUDA_ARCH");
    println!("cargo:rerun-if-env-changed=CUDA_PATH");
    println!("cargo:rerun-if-env-changed=NVCC");

    let nvcc = std::env::var("NVCC")
        .map(PathBuf::from)
        .unwrap_or_else(|_| PathBuf::from("nvcc"));

    if !cuda_dir.exists() {
        // No kernels yet (e.g. before C4 lands them) — quietly noop.
        return Ok(());
    }

    println!("cargo:rerun-if-changed={}", cuda_dir.display());
    let entries = std::fs::read_dir(&cuda_dir).map_err(|e| format!("read cuda dir: {e}"))?;
    for entry in entries {
        let entry = entry.map_err(|e| e.to_string())?;
        let path = entry.path();
        if path.extension().and_then(|s| s.to_str()) != Some("cu") {
            continue;
        }
        let stem = path
            .file_stem()
            .ok_or("cu file has no stem")?
            .to_string_lossy()
            .to_string();
        let cubin = out_dir.join(format!("{stem}.cubin"));
        println!("cargo:rerun-if-changed={}", path.display());

        let mut cmd = Command::new(&nvcc);
        cmd.args(["--cubin", "-O3", "-std=c++17"])
            .arg(format!("-arch={arch}"))
            .arg("-I")
            .arg(&cuda_dir)
            .arg("-o")
            .arg(&cubin)
            .arg(&path);
        let status = cmd
            .status()
            .map_err(|e| format!("spawn nvcc for {}: {e}", path.display()))?;
        if !status.success() {
            return Err(format!("nvcc failed for {}", path.display()));
        }
    }
    Ok(())
}