MRPGI/mrpgi-core/src/framebuffer.rs

//! AGI's core rendering trick: every room is drawn into TWO buffers at once.
//!
//! * the **visual** buffer is what the player sees (palette indices), and
//! * the **priority** buffer is invisible — it encodes depth bands and
//!   control lines so the ego can walk *behind* a column and be blocked by a
//!   wall, all without any per-object Z bookkeeping.
//!
//! Values 4..=15 in the priority buffer are depth bands (higher = nearer the
//! camera = drawn on top). Values 0..=3 are "control" lines with special
//! meaning to the movement system.

pub const PIC_W: usize = 160;
pub const PIC_H: usize = 168;

/// The "open" color the visual buffer clears to (white, like AGI fills).
pub const BG_VISUAL: u8 = 15;

// --- Control-line values (priority buffer 0..=3) ---------------------------
/// Unconditional barrier — nothing may step here.
pub const CTRL_BLOCK: u8 = 0;
/// Water: sets the on-water state for the ego. (reserved for later)
pub const CTRL_WATER: u8 = 1;
/// Signal line: stepping on it can trigger room logic. (reserved for later)
pub const CTRL_TRIGGER: u8 = 2;
/// Conditional barrier. (reserved for later)
pub const CTRL_COND: u8 = 3;

/// Depth bands run from here up to 15.
pub const FIRST_BAND: u8 = 4;

/// Map a screen row to its default depth priority: 4 at the top of the play
/// area down to 15 at the very bottom. This is what makes a flat floor sort
/// correctly with no extra work from the room author.
#[inline]
pub fn band(y: usize) -> u8 {
    let b = FIRST_BAND as usize + (y * 12) / PIC_H;
    b.min(15) as u8
}

pub struct FrameBuffer {
    pub visual: Vec<u8>,
    pub priority: Vec<u8>,
}

impl FrameBuffer {
    pub fn new() -> Self {
        let mut fb = FrameBuffer {
            visual: vec![BG_VISUAL; PIC_W * PIC_H],
            priority: vec![FIRST_BAND; PIC_W * PIC_H],
        };
        fb.clear();
        fb
    }

    /// Reset to a blank room: white visual, default depth bands on priority.
    pub fn clear(&mut self) {
        for y in 0..PIC_H {
            let p = band(y);
            for x in 0..PIC_W {
                let i = y * PIC_W + x;
                self.visual[i] = BG_VISUAL;
                self.priority[i] = p;
            }
        }
    }

    #[inline]
    pub fn in_bounds(x: i32, y: i32) -> bool {
        x >= 0 && y >= 0 && (x as usize) < PIC_W && (y as usize) < PIC_H
    }

    /// Priority at a point; off-screen reads as a solid wall.
    #[inline]
    pub fn priority_at(&self, x: i32, y: i32) -> u8 {
        if Self::in_bounds(x, y) {
            self.priority[y as usize * PIC_W + x as usize]
        } else {
            CTRL_BLOCK
        }
    }
}

impl Default for FrameBuffer {
    fn default() -> Self {
        Self::new()
    }
}