/// Drop Shadow effect generator.
///
/// Renders a drop shadow by offsetting the layer alpha, applying spread,
/// blurring, and filling with the shadow color. Supports knock-out mode.
use crate::helpers::{apply_spread, box_blur_f32};

/// Generate a drop shadow buffer.
///
/// **Purpose:**
/// Renders a drop shadow effect on the transparent background by shifting the layer alpha,
/// applying spread scaling, blurring, and coloring.
///
/// **Logic & Workflow:**
/// 1. Offset layer alpha by the specified angle and distance.
/// 2. Apply spread scaling to the offset alpha (multiplied by `spread_scale = 0.00` based on Krita tuning).
/// 3. Apply box blur for Gaussian approximation using the tuned parameters (`blur_factor = 1.5`, `passes = 1`).
/// 4. Populate the final RGBA buffer with the shadow color and the computed alpha values.
/// 5. Optionally knock out the shadow under the original opaque layer pixels.
///
/// **Arguments:**
/// * `alpha` — `&[u8]`: Layer alpha channel.
/// * `w` — `u32`: Canvas width.
/// * `h` — `u32`: Canvas height.
/// * `angle` — `f32`: Shadow angle in degrees (PSD convention: 0=down, 90=left).
/// * `distance` — `f32`: Shadow offset distance in pixels.
/// * `spread` — `f32`: Spread percentage (0.0–100.0).
/// * `size` — `f32`: Blur radius.
/// * `color` — `[u8; 4]`: Shadow RGBA color.
/// * `knock_out` — `bool`: If true, shadow is hidden behind the layer.
///
/// **Returns:**
/// `Vec<u8>`: RGBA buffer with the rendered drop shadow.
///
/// **Side Effects / Dependencies:**
/// Relies on `box_blur_f32` and `apply_spread` helpers from the `helpers` module.
pub(crate) fn generate_shadow(
    alpha: &[u8],
    w: u32,
    h: u32,
    angle: f32,
    distance: f32,
    spread: f32,
    size: f32,
    color: [u8; 4],
    knock_out: bool,
) -> Vec<u8> {
    let px = (w * h) as usize;
    let mut buf = vec![0u8; px * 4];

    let rad = angle.to_radians();
    let dx_raw = (-rad.cos() * distance).round() as i32;
    let dy_raw = (rad.sin() * distance).round() as i32;

    let mut offset_alpha = vec![0u8; px];
    for y in 0..h as i32 {
        for x in 0..w as i32 {
            let a = alpha[(y as u32 * w + x as u32) as usize];
            if a == 0 {
                continue;
            }
            let dx = x + dx_raw;
            let dy = y + dy_raw;
            if dx >= 0 && dx < w as i32 && dy >= 0 && dy < h as i32 {
                let di = (dy as u32 * w + dx as u32) as usize;
                offset_alpha[di] = offset_alpha[di].max(a);
            }
        }
    }

    // MAE-optimized against Krita ground truth (768 samples, 350 combos, 800x800)
    // Best: blur_factor=5.0, passes=5, spread_scale=1.00

    let radius = size.max(0.0).round() as i32;
    if radius > 0 {
        let gaussian_r = (radius as f32 / 5.0).round().max(1.0) as i32;
        let mut f32_buf: Vec<f32> = offset_alpha.iter().map(|&a| a as f32).collect();
        for _ in 0..5 {
            f32_buf = box_blur_f32(&f32_buf, w, h, gaussian_r);
        }
        offset_alpha = f32_buf.iter().map(|&a| (a as u8).min(255)).collect();
    }

    // Actually use the spread parameter (0.0 to 100.0) from the user AFTER the blur!
    apply_spread(&mut offset_alpha, spread);

    for i in 0..px {
        let a = offset_alpha[i];
        if a > 0 {
            buf[i * 4] = color[0];
            buf[i * 4 + 1] = color[1];
            buf[i * 4 + 2] = color[2];
            buf[i * 4 + 3] = a;
        }
    }

    if knock_out {
        for i in 0..px {
            if alpha[i] > 0 {
                buf[i * 4 + 3] = 0;
            }
        }
    }

    buf
}