//! The Whimsy look (see the vision's K-UI note): a wallpaper glowing with
//! the theme's orbs, and glass panels over it, the wallpaper blurred and
//! tinted through them. Both are made once per theme and window size, at
//! half the size each way (they are soft, and an app's memory is small),
//! by plain integer code; a frame only copies from them, each pixel
//! doubled, so it costs about what clearing the window does.

use alloc::vec;
use alloc::vec::Vec;

use crate::layout::Rect;
use crate::pixels::{between_corners, blend, corner_row, cover, height, span};
use crate::theme::{Rgba, Theme};

/// The backdrop is made at 1 / `SCALE` of the window, each way.
const SCALE: usize = 2;
/// Where the orbs sit, as fractions of the window, and how big they are
/// (of its larger side).
const ORBS: [(f32, f32, f32); 3] = [(0.18, 0.22, 0.38), (0.82, 0.38, 0.32), (0.45, 0.92, 0.42)];
/// An orb is at full strength out to this fraction of its radius, then
/// fades: a shape with an edge, which the blur softens behind the glass.
const ORB_CORE: f32 = 0.6;

/// An image in pixels of the bytes R, G, B, A (opaque).
struct Image {
    w: usize,
    h: usize,
    px: Vec<[u8; 4]>,
}

/// The wallpaper, and the glass over it (blurred, tinted), for one theme
/// and window size.
pub struct Backdrop {
    key: (Rgba, Rgba, [Rgba; 3], u8, u8, usize, usize),
    wall: Image,
    glass: Image,
}

/// `a` moved toward `b` by `k` 256ths.
fn mix(a: [u8; 4], b: Rgba, k: u32) -> [u8; 4] {
    let m = |x: u8, y: u8| ((x as u32 * (256 - k) + y as u32 * k) >> 8) as u8;
    [m(a[0], b.0), m(a[1], b.1), m(a[2], b.2), 255]
}

/// How strongly an orb shows (256ths) at a squared distance of `i`/`FALL_STEPS`
/// of its radius squared.
const FALL_STEPS: usize = 256;

fn falloff() -> [u32; FALL_STEPS + 1] {
    let mut t = [0u32; FALL_STEPS + 1];
    for (i, k) in t.iter_mut().enumerate() {
        let d = libm::sqrtf(i as f32 / FALL_STEPS as f32);
        *k = (((1.0 - d) / (1.0 - ORB_CORE)).clamp(0.0, 1.0) * theme::ORB_STRENGTH * 256.0) as u32;
    }
    t
}

impl Backdrop {
    /// The backdrop for `t` in a window `w` by `h`: `cached` if it still is.
    pub fn for_window(cached: Option<Backdrop>, t: &Theme, w: u32, h: u32) -> Option<Backdrop> {
        let (w, h) = (w as usize, h as usize);
        let key = (t.background, t.surface, t.orbs, t.blur_radius, t.panel_opacity, w, h);
        if let Some(b) = cached.filter(|b| b.key == key) {
            return Some(b);
        }
        let (sw, sh) = (w.div_ceil(SCALE).max(1), h.div_ceil(SCALE).max(1));
        let bg = [t.background.0, t.background.1, t.background.2, 255];
        let mut wall = Image { w: sw, h: sh, px: vec![bg; sw * sh] };
        let big = sw.max(sh) as f32;
        let fall = falloff();
        for (orb, (fx, fy, fr)) in t.orbs.iter().zip(ORBS) {
            let (cx, cy, r) = ((fx * sw as f32) as i64, (fy * sh as f32) as i64, ((fr * big) as i64).max(1));
            let r2 = r * r;
            let rows = (cy - r).max(0) as usize..((cy + r + 1) as usize).min(sh);
            let cols = (cx - r).max(0) as usize..((cx + r + 1) as usize).min(sw);
            for y in rows {
                let dy2 = (y as i64 - cy) * (y as i64 - cy);
                for x in cols.clone() {
                    let d2 = (x as i64 - cx) * (x as i64 - cx) + dy2;
                    if d2 < r2 {
                        let p = &mut wall.px[y * sw + x];
                        *p = mix(*p, *orb, fall[(d2 * FALL_STEPS as i64 / r2) as usize]);
                    }
                }
            }
        }
        // Blurred at half this size again (it is soft), then brought back.
        let mut small = halve(&wall);
        blur(&mut small, (t.blur_radius as usize).div_ceil(2 * SCALE));
        let mut glass = double(&small, sw, sh);
        let opacity = t.panel_opacity as u32 * 256 / 100;
        for p in &mut glass.px {
            *p = mix(*p, t.surface, opacity);
        }
        Some(Backdrop { key, wall, glass })
    }

    /// The wallpaper over `part` of the window's pixels `pm`
    /// (premultiplied RGBA, `width` pixels a row).
    pub fn wall(&self, pm: &mut [u8], width: usize, part: Rect) {
        copy(&self.wall, pm, width, part, 0.0);
    }

    /// `part` of `pm` moved back toward the wallpaper by `k` 256ths: what
    /// is drawn there fading in.
    pub fn fade(&self, pm: &mut [u8], width: usize, part: Rect, k: u32) {
        let (cols, rows) = span(part, width, height(pm, width));
        for y in rows {
            let src = &self.wall.px[(y / SCALE).min(self.wall.h - 1) * self.wall.w..][..self.wall.w];
            let row = pm[(y * width + cols.start) * 4..(y * width + cols.end) * 4].as_chunks_mut::<4>().0;
            for (i, d) in row.iter_mut().enumerate() {
                blend(d, src[((cols.start + i) / SCALE).min(self.wall.w - 1)], k);
            }
        }
    }

    /// A glass panel over `r`, its corners rounded by `radius`.
    pub fn glass(&self, pm: &mut [u8], width: usize, r: Rect, radius: f32) {
        copy(&self.glass, pm, width, r, radius);
    }
}

/// Copies `img` (scaled up) into `r` of `pm`, the corners rounded by
/// `radius` and anti-aliased there.
fn copy(img: &Image, pm: &mut [u8], width: usize, r: Rect, radius: f32) {
    let radius = radius.min(r.w / 2.0).min(r.h / 2.0).max(0.0);
    let (cols, rows) = span(r, width, height(pm, width));
    let mid = between_corners(r, radius, &cols);
    let (x0, x1) = (cols.start, cols.end);
    let mut copied: Option<usize> = None;
    for y in rows {
        let corner = corner_row(r, radius, y);
        let at = y * width * 4;
        // A row made from the same source row as the one above: the same.
        if let Some(above) = copied.filter(|a| *a + 1 == y && a / SCALE == y / SCALE && !corner) {
            let from = above * width * 4;
            pm.copy_within(from + x0 * 4..from + x1 * 4, at + x0 * 4);
            copied = Some(y);
            continue;
        }
        let src = &img.px[(y / SCALE).min(img.h - 1) * img.w..][..img.w];
        let straight = if corner { mid.clone() } else { cols.clone() };
        let row = &mut pm[at + straight.start * 4..at + straight.end * 4];
        for (i, d) in row.as_chunks_mut::<4>().0.iter_mut().enumerate() {
            *d = src[((straight.start + i) / SCALE).min(img.w - 1)];
        }
        if corner {
            let row = pm[at..at + width * 4].as_chunks_mut::<4>().0;
            for x in (x0..mid.start).chain(mid.end..x1) {
                let s = src[(x / SCALE).min(img.w - 1)];
                match cover(r, radius, x, y) {
                    256 => row[x] = s,
                    0 => {}
                    k => blend(&mut row[x], s, k),
                }
            }
        }
        copied = (!corner).then_some(y);
    }
}

/// `img` at half the size each way, each pixel the average of four.
fn halve(img: &Image) -> Image {
    let (w, h) = (img.w.div_ceil(2), img.h.div_ceil(2));
    let mut px = vec![[0u8; 4]; w * h];
    for y in 0..h {
        let (r0, r1) = (2 * y, (2 * y + 1).min(img.h - 1));
        for x in 0..w {
            let (c0, c1) = (2 * x, (2 * x + 1).min(img.w - 1));
            let mut sum = [0u32; 4];
            for p in [img.px[r0 * img.w + c0], img.px[r0 * img.w + c1], img.px[r1 * img.w + c0], img.px[r1 * img.w + c1]] {
                add(&mut sum, p);
            }
            px[y * w + x] = sum.map(|s| (s / 4) as u8);
        }
    }
    Image { w, h, px }
}

/// The average of two pixels, all four bytes at once.
fn mid(a: [u8; 4], b: [u8; 4]) -> [u8; 4] {
    let (a, b) = (u32::from_le_bytes(a), u32::from_le_bytes(b));
    ((a & b) + (((a ^ b) & 0xfefe_fefe) >> 1)).to_le_bytes()
}

/// `img` at twice the size (cut to `w` by `h`), the new pixels between
/// the old ones the average of their neighbours: the even rows from the
/// source rows, the odd ones from the rows either side.
fn double(img: &Image, w: usize, h: usize) -> Image {
    let mut px = vec![[0u8; 4]; w * h];
    for y in (0..h).step_by(2) {
        let src = &img.px[(y / 2).min(img.h - 1) * img.w..][..img.w];
        let out = &mut px[y * w..(y + 1) * w];
        for (x, o) in out.iter_mut().enumerate() {
            let s = src[(x / 2).min(img.w - 1)];
            *o = if x % 2 == 0 { s } else { mid(s, src[(x / 2 + 1).min(img.w - 1)]) };
        }
    }
    for y in (1..h).step_by(2) {
        let below = if y + 1 < h { y + 1 } else { y - 1 };
        for x in 0..w {
            px[y * w + x] = mid(px[(y - 1) * w + x], px[below * w + x]);
        }
    }
    Image { w, h, px }
}

/// A box blur run three times over (close to a Gaussian), `r` pixels each
/// way: along each row, then down every column at once, so memory is
/// walked in order.
fn blur(img: &mut Image, r: usize) {
    if r == 0 {
        return;
    }
    let mut line = vec![[0u8; 4]; img.w];
    let mut sums = vec![[0u32; 4]; img.w];
    for _ in 0..3 {
        for y in 0..img.h {
            across(&mut img.px[y * img.w..(y + 1) * img.w], r, &mut line);
        }
        down(img, r, &mut sums);
    }
}

/// Dividing a sum of `2r + 1` bytes by that, as a multiply and a shift:
/// exact for such sums.
fn reciprocal(r: usize) -> u32 {
    (65536 + 2 * r as u32) / (2 * r as u32 + 1)
}

fn add(sum: &mut [u32; 4], p: [u8; 4]) {
    sum[0] += p[0] as u32;
    sum[1] += p[1] as u32;
    sum[2] += p[2] as u32;
    sum[3] += p[3] as u32;
}

fn sub(sum: &mut [u32; 4], p: [u8; 4]) {
    sum[0] -= p[0] as u32;
    sum[1] -= p[1] as u32;
    sum[2] -= p[2] as u32;
    sum[3] -= p[3] as u32;
}

fn average(sum: &[u32; 4], inv: u32) -> [u8; 4] {
    [((sum[0] * inv) >> 16) as u8, ((sum[1] * inv) >> 16) as u8, ((sum[2] * inv) >> 16) as u8, ((sum[3] * inv) >> 16) as u8]
}

/// One pass along `row`, its ends held.
fn across(row: &mut [[u8; 4]], r: usize, line: &mut [[u8; 4]]) {
    let n = row.len();
    let line = &mut line[..n];
    line.copy_from_slice(row);
    let inv = reciprocal(r);
    let mut sum = [0u32; 4];
    for i in 0..=2 * r {
        add(&mut sum, line[i.saturating_sub(r).min(n - 1)]);
    }
    for (i, out) in row.iter_mut().enumerate() {
        *out = average(&sum, inv);
        add(&mut sum, line[(i + r + 1).min(n - 1)]);
        sub(&mut sum, line[i.saturating_sub(r)]);
    }
}

/// One pass down every column, a row at a time; the top and bottom held.
fn down(img: &mut Image, r: usize, sums: &mut [[u32; 4]]) {
    let (w, h) = (img.w, img.h);
    let src = img.px.clone();
    let row = |i: usize| &src[i.min(h - 1) * w..][..w];
    let inv = reciprocal(r);
    sums.fill([0; 4]);
    for i in 0..=2 * r {
        for (s, p) in sums.iter_mut().zip(row(i.saturating_sub(r))) {
            add(s, *p);
        }
    }
    for y in 0..h {
        for (out, s) in img.px[y * w..(y + 1) * w].iter_mut().zip(sums.iter()) {
            *out = average(s, inv);
        }
        for ((s, a), b) in sums.iter_mut().zip(row(y + r + 1)).zip(row(y.saturating_sub(r))) {
            add(s, *a);
            sub(s, *b);
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    fn image(w: usize, h: usize, c: [u8; 4]) -> Image {
        Image { w, h, px: vec![c; w * h] }
    }

    #[test]
    fn blur_spreads_a_shape_and_keeps_flat_colour_flat() {
        let mut img = image(40, 40, [10, 20, 30, 255]);
        blur(&mut img, 3);
        assert!(img.px.iter().all(|p| *p == [10, 20, 30, 255]), "nothing to blur, nothing changes");
        for y in 15..25 {
            for x in 15..25 {
                img.px[y * 40 + x] = [250, 250, 250, 255];
            }
        }
        blur(&mut img, 3);
        let red = |x: usize, y: usize| img.px[y * 40 + x][0];
        assert!(red(20, 20) > 150, "the middle, {}", red(20, 20));
        assert!((40..200).contains(&red(13, 20)), "just outside the edge: between, {}", red(13, 20));
        assert_eq!(red(2, 2), 10, "far away: untouched");
    }

    #[test]
    fn the_backdrop_is_made_once_per_theme_and_size_and_glass_is_tinted() {
        let t = Theme::WHIMSY_DARK;
        let b = Backdrop::for_window(None, &t, 100, 60).unwrap();
        assert_eq!((b.wall.w, b.wall.h), (50, 30));
        let flat = Theme { orbs: [t.background; 3], ..t };
        let f = Backdrop::for_window(Some(b), &flat, 100, 60).unwrap();
        let bg = [t.background.0, t.background.1, t.background.2, 255];
        assert!(f.wall.px.iter().all(|p| *p == bg), "no orbs, no glow");
        assert_eq!(f.glass.px[0], mix(bg, t.surface, t.panel_opacity as u32 * 256 / 100));
    }

    #[test]
    fn a_panel_is_copied_with_round_soft_corners() {
        let t = Theme { orbs: [Rgba::rgb(0xffffff); 3], ..Theme::WHIMSY_DARK };
        let b = Backdrop::for_window(None, &t, 40, 40).unwrap();
        let mut pm = vec![0u8; 40 * 40 * 4];
        b.glass(&mut pm, 40, Rect { x: 10.0, y: 10.0, w: 20.0, h: 20.0 }, 8.0);
        let at = |x: usize, y: usize| pm[(y * 40 + x) * 4 + 3];
        assert_eq!(at(20, 20), 255, "inside");
        assert_eq!(at(10, 10), 0, "the very corner is cut off");
        assert!((1..255).contains(&at(12, 12)), "the corner's edge is soft: {}", at(12, 12));
        assert_eq!(at(5, 20), 0, "outside");
    }
}