//! Criterion benchmark: `composite_scratch` buffer pooling guard.
//!
//! ## Purpose
//! Verifies that the ~33 MB `composite_scratch` buffer inside
//! `render_composite_region()` is allocated once and reused across calls.
//! If a regression replaces the pooled `Option<Vec<u8>>` with a fresh
//! `Vec::new()` on every call, this benchmark detects the allocation storm.
//!
//! ## Logic & Workflow
//! 1. Creates a 3840×2160, 10-layer document with all layers filled.
//! 2. Performs a stroke + composite to warm the scratch buffer.
//! 3. **cold_composite**: Measures a `render_composite_region()` call on a
//!    fresh engine where `composite_scratch` is `None`.
//! 4. **warm_composite**: Measures the same call after the buffer has been
//!    allocated (should be faster — no allocation, just memset + composite).
//!
//! ## Arguments & Returns
//! Standard criterion benchmark — run with:
//! ```bash
//! cargo bench -p hcie-engine-api --bench composite_scratch_pooling
//! ```
//!
//! ## Side Effects / Dependencies
//! Allocates ~400 MB (10 layers × 4K RGBA + scratch + tile caches).

use criterion::{criterion_group, criterion_main, Criterion};
use hcie_engine_api::{BrushStyle, BrushTip, Engine};

/// Canvas dimensions — 4K UHD.
const W: u32 = 3840;
const H: u32 = 2160;
/// Number of raster layers to create (worst-case workload).
const LAYERS: usize = 10;

/// Builds a multi-layer 4K document ready for compositing.
///
/// **Purpose:** Creates the worst-case composite scenario with every layer
/// filled so tile and pixel compositing operate on real data.
/// **Returns:** `(Engine, top_layer_id)`.
fn setup_multilayer_engine() -> (Engine, u64) {
    let mut engine = Engine::new(W, H);

    let bg_id = engine.active_layer_id();
    engine.set_active_layer(bg_id);
    engine.draw_filled_rect_rgba(0, 0, W, H, [240, 240, 240, 255]);

    let mut layer_ids = vec![bg_id];
    for i in 1..LAYERS {
        let id = engine.add_layer(&format!("Layer {}", i));
        engine.set_active_layer(id);
        let color = match i % 4 {
            0 => [180, 80, 80, 255],
            1 => [80, 180, 80, 255],
            2 => [80, 80, 180, 255],
            _ => [160, 140, 100, 255],
        };
        engine.draw_filled_rect_rgba(0, 0, W, H, color);
        layer_ids.push(id);
    }

    let top_id = *layer_ids.last().unwrap();
    engine.set_active_layer(top_id);

    let mut tip = BrushTip::default();
    tip.style = BrushStyle::Round;
    tip.size = 24.0;
    tip.opacity = 1.0;
    tip.hardness = 0.85;
    tip.spacing = 0.1;
    engine.set_brush_tip(tip);
    engine.set_color([220, 60, 60, 255]);
    engine.set_eraser(false);

    (engine, top_id)
}

/// Forces a dirty region and composites it.
///
/// **Purpose:** Exercises the `render_composite_region()` path including
/// scratch buffer allocation/reuse, tile compositing, and dirty rect handling.
/// **Arguments:** `engine` — mutable engine, `layer_id` — target layer.
/// **Side Effects:** Paints a short stroke segment to create a dirty region,
/// then calls `render_composite_region()`.
fn stroke_and_composite(engine: &mut Engine, layer_id: u64) {
    let cx = W as f32 / 2.0;
    let cy = H as f32 / 2.0;
    engine.begin_stroke(layer_id, cx, cy);
    engine.stroke_to(layer_id, cx + 50.0, cy + 50.0, 0.8);
    let (region, ptr, _size) = engine.render_composite_region();
    assert!(!ptr.is_null());
    let _ = region;
    engine.end_stroke(layer_id);
    std::thread::sleep(std::time::Duration::from_millis(10));
    engine.commit_pending_history();
}

/// Benchmark group: cold vs warm `composite_scratch` allocation.
///
/// **Purpose:** Detects regressions where `composite_scratch` is dropped or
/// reallocated on every `render_composite_region()` call. When pooling works,
/// the warm benchmark avoids a ~33 MB allocation and should be measurably faster.
///
/// **Logic & Workflow:**
/// - `cold_composite`: Fresh engine, `composite_scratch = None`. First call
///   allocates the buffer.
/// - `warm_composite`: Engine with pre-allocated scratch buffer. Measures
///   pure composite cost without allocation overhead.
fn bench_composite_scratch_pooling(c: &mut Criterion) {
    let mut group = c.benchmark_group("composite_scratch_pooling");

    // ── Cold: composite on a fresh engine (scratch allocation included) ──
    group.bench_function("cold_composite", |b| {
        b.iter_with_setup(
            || setup_multilayer_engine(),
            |(mut engine, layer_id)| {
                stroke_and_composite(&mut engine, layer_id);
            },
        );
    });

    // ── Warm: composite on a pre-warmed engine (scratch already pooled) ──
    group.bench_function("warm_composite", |b| {
        let (mut engine, layer_id) = setup_multilayer_engine();
        // Warm up: allocate the scratch buffer
        stroke_and_composite(&mut engine, layer_id);

        b.iter(|| {
            // Paint a new segment and composite — scratch should be reused
            let cx = W as f32 / 2.0;
            let cy = H as f32 / 2.0;
            engine.begin_stroke(layer_id, cx, cy);
            engine.stroke_to(layer_id, cx + 30.0, cy - 30.0, 0.7);
            let (region, ptr, _size) = engine.render_composite_region();
            assert!(!ptr.is_null());
            let _ = region;
            engine.end_stroke(layer_id);
            std::thread::sleep(std::time::Duration::from_millis(5));
            engine.commit_pending_history();
        });
    });

    group.finish();
}

criterion_group!(benches, bench_composite_scratch_pooling);
criterion_main!(benches);