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

use ort::session::builder::GraphOptimizationLevel;
use ort::session::Session;

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

/// Build a session for `bytes` on `rung`.
///
/// `strict` is the probe's flag: with it, a provider that would hand any
/// node to the CPU fails the build instead, so "the session built" means
/// "the provider took the graph" and not "the provider registered" (§4).
pub fn build(
    rung: Rung,
    role: Role,
    bytes: &[u8],
    cfg: &Config,
    strict: bool,
) -> ort::Result<Session> {
    let mut b = Session::builder()?
        .with_optimization_level(GraphOptimizationLevel::Level3)?
        .with_intra_threads(threads(cfg))?;
    if strict {
        b = b.with_config_entry("session.disable_cpu_ep_fallback", "1")?;
    }
    // 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::Hexagon => unreachable!("the Hexagon rung is not on a desktop ladder"),
    }
}

#[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 => unreachable!("no NVIDIA rung on Android"),
    }
}