//! Board-change wipe animation.
//!
//! When the player steps through a portal both the old and new boards are rendered
//! into off-screen ratatui [`Buffer`]s at init time. Each call to
//! [`Animation::update`] advances the LFSR-driven reveal bitmap, and
//! [`Animation::draw`] blends the two buffers — old cells for unrevealed positions,
//! new cells for revealed ones.

use kiln_core::Board;
use ratatui::{
    buffer::Buffer,
    layout::Rect,
    widgets::Widget,
};
use crate::animation::{Animation, AnimationLayer};
use crate::input::InputMode;
use crate::render::BoardWidget;

/// How long the wipe lasts in seconds.
pub const TRANSITION_DURATION: f32 = 0.25;

/// Advance a 16-bit Galois LFSR one step.
///
/// Polynomial 0xB400 gives a maximal period of 65 535, covering every realistic
/// terminal viewport (e.g. 256 × 200 = 51 200 cells).
fn lfsr_step(state: u16) -> u16 {
    let lsb = state & 1;
    let s = state >> 1;
    if lsb != 0 { s ^ 0xB400 } else { s }
}

/// Build a permutation of `[0, total)` using the 16-bit LFSR.
///
/// The LFSR visits every value in `1..=65535` exactly once per cycle.
/// Mapping `state → state - 1` gives `0..=65534`; values in `[0, total)` are
/// kept in encounter order. `total` must not exceed 65 535.
fn lfsr_sequence(total: usize) -> Vec<u32> {
    let mut seq = Vec::with_capacity(total);
    let mut state: u16 = 1; // non-zero start; LFSR never visits 0
    loop {
        let idx = state as u32 - 1;
        if (idx as usize) < total {
            seq.push(idx);
            if seq.len() == total { break; }
        }
        state = lfsr_step(state);
        if state == 1 { break; } // full cycle — shouldn't happen for total <= 65535
    }
    seq
}

/// A board-to-board wipe animation driven by an LFSR.
///
/// Both boards are rendered into off-screen [`Buffer`]s once at construction.
/// Implements [`Animation`]: [`update`](Animation::update) marks newly-due cells
/// as revealed each frame; [`draw`](Animation::draw) blends old and new buffers.
pub struct TransitionAnimation {
    /// Pre-rendered old board.
    old_buf: Buffer,
    /// Pre-rendered new board.
    new_buf: Buffer,
    /// LFSR-ordered permutation of screen-cell indices `[0, width×height)`.
    reveal_order: Vec<u32>,
    /// Which cells have been revealed (show `new_buf`).
    revealed: Vec<bool>,
    /// How many entries of `reveal_order` are currently revealed.
    revealed_count: usize,
    /// Elapsed seconds since the animation started.
    elapsed: f32,
}

impl TransitionAnimation {
    /// Render both boards into off-screen buffers and build the LFSR sequence.
    ///
    /// `area` must match the area that will be passed to [`Animation::draw`].
    pub fn new(old: &Board, new: &Board, area: Rect) -> Self {
        let total = area.width as usize * area.height as usize;

        let mut old_buf = Buffer::empty(area);
        BoardWidget::new(old).render(area, &mut old_buf);

        let mut new_buf = Buffer::empty(area);
        BoardWidget::new(new).render(area, &mut new_buf);

        let reveal_order = lfsr_sequence(total);

        Self {
            old_buf,
            new_buf,
            reveal_order,
            revealed: vec![false; total],
            revealed_count: 0,
            elapsed: 0.0,
        }
    }
}

impl Animation for TransitionAnimation {
    fn update(&mut self, dt: f32) -> bool {
        self.elapsed += dt;
        let total = self.revealed.len();
        let target =
            ((self.elapsed / TRANSITION_DURATION) * total as f32) as usize;
        let target = target.min(total);

        // Mark each newly revealed cell in the bitmap.
        for &idx in &self.reveal_order[self.revealed_count..target] {
            self.revealed[idx as usize] = true;
        }
        self.revealed_count = target;

        self.elapsed >= TRANSITION_DURATION
    }

    fn draw(&self, area: Rect, buf: &mut Buffer) {
        let w = area.width as usize;
        for row in 0..area.height {
            for col in 0..area.width {
                let idx = row as usize * w + col as usize;
                // Out-of-range cells (e.g. after a terminal resize) default to new board.
                let src = if self.revealed.get(idx).copied().unwrap_or(true) {
                    &self.new_buf
                } else {
                    &self.old_buf
                };
                let pos = (area.x + col, area.y + row);
                if let (Some(s), Some(d)) = (src.cell(pos), buf.cell_mut(pos)) {
                    *d = s.clone();
                }
            }
        }
    }

    fn layer(&self) -> AnimationLayer { AnimationLayer::Board }

    fn input_mode_after(&self) -> InputMode { InputMode::Board }
}