//! One session builder per rung (docs/dev/inference.md §2, §7, §9).

use ort::session::Session;

use crate::{Config, Role, Rung};

/// Build a session for `bytes` on `rung`.
///
/// Not strict about the CPU: `session.disable_cpu_ep_fallback` was tried as
/// the probe's proof that a provider took the graph, and it refuses the
/// Hexagon over the ten quantise/dequantise nodes at the graph's edges that
/// QNN declines by policy and that cost microseconds. The probe's proof is
/// its clock instead (§4): a provider that hands real work to the CPU is
/// slower than the CPU floor and rejected by the same measurement.
pub fn build(rung: Rung, role: Role, bytes: &[u8], cfg: &Config) -> ort::Result<Session> {
    // No optimisation level named. ONNX Runtime's default is already its
    // fullest, and on tract any level but "disabled" means `into_optimized`,
    // whose optimiser divides by zero inside yolo26n-seg (tract-data
    // `stack_tensors`) — a panic across the C API, which is an abort. The
    // app never asked tract for that and does not start now.
    let mut b = Session::builder()?.with_intra_threads(threads(cfg))?;
    // A Hexagon session loads the compiled context when there is one and
    // compiles it from the model when there is not; the engine thread is
    // what makes the second case rare (§6).
    let context = (rung == Rung::Hexagon).then(|| crate::engines::context_path(cfg, bytes));
    let ready = context.as_ref().is_some_and(|p| p.is_file());
    b = providers(
        b,
        rung,
        role,
        cfg,
        if ready { None } else { context.as_deref() },
    )?;
    match (ready, context) {
        (true, Some(path)) => b.commit_from_file(path),
        _ => b.commit_from_memory(bytes),
    }
}

/// The intra-op pool: what the config says, else the cores less two for
/// the compositor and the decoder (§9). tract ignores it.
fn threads(cfg: &Config) -> usize {
    if cfg.threads > 0 {
        return cfg.threads;
    }
    std::thread::available_parallelism()
        .map(|n| n.get().saturating_sub(2).max(1))
        .unwrap_or(1)
}

#[cfg(not(target_os = "android"))]
fn providers(
    b: ort::session::builder::SessionBuilder,
    rung: Rung,
    role: Role,
    cfg: &Config,
    _generate_context: Option<&std::path::Path>,
) -> ort::Result<ort::session::builder::SessionBuilder> {
    use ort::ep;
    match rung {
        Rung::Cpu => Ok(b),
        Rung::Cuda => {
            Ok(b.with_execution_providers([ep::CUDA::default().build().error_on_failure()])?)
        }
        Rung::TensorRt => {
            let cache = cfg.cache_dir.join("tensorrt");
            let _ = std::fs::create_dir_all(&cache);
            let cache = cache.to_string_lossy().into_owned();
            // fp16 for everything but the embedder, whose comparability
            // across devices is worth more than its 0.2 ms (§7). The
            // workspace cap keeps the develop view's tiles on the card
            // (NFR-RES-2). CUDA behind it takes any node TensorRT declines.
            Ok(b.with_execution_providers([
                ep::TensorRT::default()
                    .with_fp16(role != Role::Embedder)
                    .with_engine_cache(true)
                    .with_engine_cache_path(&cache)
                    .with_timing_cache(true)
                    .with_timing_cache_path(&cache)
                    .with_max_workspace_size(512 << 20)
                    .build()
                    .error_on_failure(),
                ep::CUDA::default().build(),
            ])?)
        }
        Rung::MiGraphX => {
            // fp16 on the same terms as TensorRT (§7). MIGraphX compiles a
            // program per graph — 20–60 s here — and keeps it in the cache
            // directory, keyed on the graph, the GPU and its own version
            // but not the precision: hence one directory per precision.
            // The CPU takes any node it declines.
            let fp16 = role != Role::Embedder;
            let cache = cfg
                .cache_dir
                .join("migraphx")
                .join(if fp16 { "fp16" } else { "f32" });
            let _ = std::fs::create_dir_all(&cache);
            let mut b = b;
            migraphx(&mut b, fp16, &cache)?;
            Ok(b)
        }
        Rung::Hexagon => unreachable!("the Hexagon rung is not on a desktop ladder"),
    }
}

/// Register MIGraphX through ONNX Runtime's generic key/value entry point.
///
/// `ort`'s own builder (`ep::MIGraphX`) fills the legacy
/// `OrtMIGraphXProviderOptions`, and 1.29 reads that struct for its
/// precision flags and nothing else — the compiled-program cache directory
/// is only a key in the generic map (`migraphx_model_cache_dir`), and
/// without it every session is a full compile. Registration through the
/// generic entry point needs no `ort` feature: it is one call on the API
/// table, which is why the crate's `ort` dependency names no AMD feature.
#[cfg(not(target_os = "android"))]
fn migraphx(
    b: &mut ort::session::builder::SessionBuilder,
    fp16: bool,
    cache: &std::path::Path,
) -> ort::Result<()> {
    use ort::AsPointer;
    use std::ffi::CString;
    let keys = [c"migraphx_fp16_enable", c"migraphx_model_cache_dir"];
    let values = [
        CString::new(if fp16 { "1" } else { "0" }).unwrap(),
        CString::new(cache.to_string_lossy().as_bytes())
            .map_err(|e| ort::Error::new(e.to_string()))?,
    ];
    let key_ptrs: Vec<_> = keys.iter().map(|k| k.as_ptr()).collect();
    let value_ptrs: Vec<_> = values.iter().map(|v| v.as_ptr()).collect();
    // SAFETY: the documented C call over arrays that outlive it; the
    // runtime copies the strings into its own options map before returning.
    unsafe {
        let status = (ort::api().SessionOptionsAppendExecutionProvider)(
            b.ptr_mut(),
            c"MIGraphX".as_ptr(),
            key_ptrs.as_ptr(),
            value_ptrs.as_ptr(),
            keys.len(),
        );
        ort::Error::result_from_status(status)
    }
}

#[cfg(target_os = "android")]
fn providers(
    b: ort::session::builder::SessionBuilder,
    rung: Rung,
    _role: Role,
    _cfg: &Config,
    generate_context: Option<&std::path::Path>,
) -> ort::Result<ort::session::builder::SessionBuilder> {
    use ort::ep;
    match rung {
        Rung::Cpu => Ok(b),
        Rung::Hexagon => {
            // The HTP compiles the graph once per device (0.8–1.7 s here).
            // With `ep.context_enable` ONNX Runtime writes the compiled
            // context beside the probe cache; the next session loads that
            // file as its model and skips the compile (§5).
            let mut b = b;
            if let Some(ctx) = generate_context {
                let _ = std::fs::create_dir_all(ctx.parent().unwrap());
                b = b
                    .with_config_entry("ep.context_enable", "1")?
                    .with_config_entry("ep.context_file_path", ctx.to_string_lossy())?
                    .with_config_entry("ep.context_embed_mode", "0")?;
            }
            // Quantise/dequantise at the graph's edges stay on the NPU too,
            // so a strict build is a whole-graph build.
            Ok(b.with_execution_providers([ep::QNN::default()
                .with_backend_path("libQnnHtp.so")
                .with_performance_mode(ep::qnn::PerformanceMode::Burst)
                .with_offload_graph_io_quantization(false)
                .build()
                .error_on_failure()])?)
        }
        Rung::Cuda | Rung::TensorRt | Rung::MiGraphX => {
            unreachable!("no desktop GPU rung on Android")
        }
    }
}