//! Selection pixel-clearing regression tests.
//!
//! **Purpose:** Verifies that destructive selection operations use the active
//! document selection mask rather than the brush stroke's temporary mask cache.
//!
//! **Logic & Workflow:** Each test creates an opaque raster layer through the
//! public engine API, installs a deterministic selection mask, clears selected
//! pixels, and compares alpha values inside and outside that mask.
//!
//! **Side Effects / Dependencies:** Uses only in-memory `hcie-engine-api` state;
//! no files, network services, GUI runtime, or engine internals are accessed.

use hcie_engine_api::Engine;

/// Confirms that selection clearing removes internal pixels and preserves all
/// pixels external to the selection.
///
/// **Purpose:** Prevents a regression where `clear_selection_pixels` consulted
/// an empty stroke cache and consequently cleared the complete active layer.
///
/// **Logic & Workflow:** Creates a 3×3 opaque layer, selects only its center
/// pixel, invokes selection clearing, then checks every resulting alpha value.
///
/// **Arguments & Returns:** Takes no arguments and returns nothing; assertion
/// failures report whether an internal or external pixel was handled wrongly.
///
/// **Side Effects / Dependencies:** Mutates only the local in-memory engine and
/// records the normal undo snapshot generated by the clear operation.
#[test]
fn clear_selection_pixels_clears_inside_and_preserves_outside() {
    let mut engine = Engine::new(3, 3);
    engine.set_active_layer_pixels(vec![255; 3 * 3 * 4]);

    let mut mask = vec![0; 3 * 3];
    mask[4] = 255;
    engine.set_selection_mask(mask);

    engine.clear_selection_pixels();

    let pixels = engine
        .get_active_layer_pixels()
        .expect("a new engine must have an active raster layer");
    for pixel_index in 0..9 {
        let alpha = pixels[pixel_index * 4 + 3];
        if pixel_index == 4 {
            assert_eq!(alpha, 0, "selected center pixel must be cleared");
        } else {
            assert_eq!(
                alpha, 255,
                "unselected pixel {pixel_index} must be preserved"
            );
        }
    }
}