use std::env;
use std::path::Path;
use std::process::Command;

fn main() {
    println!("cargo:rustc-check-cfg=cfg(no_cuda)");
    println!("cargo:rerun-if-changed=../../csrc/");

    let cuda_path = env::var("CUDA_HOME")
        .or_else(|_| env::var("CUDA_PATH"))
        .unwrap_or_else(|_| "/usr/local/cuda".to_string());

    // Locally there is no nvcc / GPU. Detect that and skip the CUDA build so
    // `cargo check`/`cargo build` of host-side Rust still works. The `no_cuda`
    // cfg makes the FFI bindings + smoke test compile (but not run) without nvcc.
    if !nvcc_available(&cuda_path) {
        println!("cargo:warning=nvcc not found — skipping CUDA compilation (host-only build).");
        println!("cargo:rustc-cfg=no_cuda");
        return;
    }

    println!("cargo:rustc-link-search=native={cuda_path}/lib64");
    println!("cargo:rustc-link-lib=dylib=cudart");
    println!("cargo:rustc-link-lib=dylib=cuda");
    // cuBLAS is used only as a correctness reference for the hand-written GEMM.
    println!("cargo:rustc-link-lib=dylib=cublas");

    cc::Build::new()
        .cuda(true)
        .cudart("shared")
        .flag("-gencode=arch=compute_120,code=sm_120")
        .file("../../csrc/test/vecadd.cu")
        .file("../../csrc/ops/elementwise.cu")
        .file("../../csrc/ops/gemm.cu")
        .file("../../csrc/ops/nn.cu")
        .file("../../csrc/ops/model.cu")
        .file("../../csrc/ops/optim.cu")
        .file("../../csrc/ops/attention.cu")
        .file("../../csrc/ops/flash_attention.cu")
        .file("../../csrc/ops/repeat_kv.cu")
        .file("../../csrc/ops/cast.cu")
        .file("../../csrc/ops/dropout.cu")
        .compile("xtrain_cuda_kernels");
}

fn nvcc_available(cuda_path: &str) -> bool {
    if Command::new("nvcc").arg("--version").output().is_ok() {
        return true;
    }
    Path::new(&format!("{cuda_path}/bin/nvcc")).exists()
}