Fix sprites disappearing too early when sliding left off screen

src/lib.rs

@@ -1,6 +1,7 @@
 pub mod frontend;
 pub mod snes;
 pub mod tickable;
+pub mod util;
 
 #[cfg(test)]
 pub mod test;

src/snes/ppu/bus.rs

@@ -2,10 +2,7 @@ use super::*;
 
 use crate::frontend::Renderer;
 use crate::snes::bus::{Address, BusMember};
-
-use num::traits::{WrappingShl, WrappingShr};
-use num::Integer;
-use std::mem::size_of_val;
+use crate::util::sign_extend;
 
 macro_rules! write_m7x {
     ($self:ident, $reg:ident, $val:expr) => {{
@@ -24,11 +21,6 @@ macro_rules! write_m7x_13b {
     }};
 }
 
-fn sign_extend<T: Integer + WrappingShl + WrappingShr>(val: T, nbits: u32) -> T {
-    let notherbits = size_of_val(&val) as u32 * 8 - nbits;
-    val.wrapping_shl(notherbits).wrapping_shr(notherbits)
-}
-
 impl<TRenderer> BusMember<Address> for PPU<TRenderer>
 where
     TRenderer: Renderer,

src/snes/ppu/render.rs

@@ -198,23 +198,35 @@ where
             }
 
             if (e.y..(e.y + e.height)).contains(&scanline) {
-                for x in e.x..(e.x + e.width) {
-                    if x >= state.idx.len() {
+                for x in e.x..(e.x + e.width as i32) {
+                    if x >= state.idx.len() as i32 || x < 0 {
                         // Outside of visible area.
-                        break;
+                        continue;
                     }
-                    if state.window.sprites[x] && state.windowlayermask & (1 << LAYER_SPRITES) != 0
+
+                    if state.window.sprites[x as usize]
+                        && state.windowlayermask & (1 << LAYER_SPRITES) != 0
                     {
                         // Masked by window.
                         continue;
                     }
 
-                    let t_x = (x - e.x) / TILE_WIDTH;
-                    let t_y = (scanline - e.y) / TILE_HEIGHT;
+                    // Coordinates within the tile
+                    let (in_x, in_y) = (
+                        ((x - e.x) % TILE_WIDTH as i32) as usize,
+                        ((scanline - e.y) % TILE_HEIGHT) as usize,
+                    );
+                    // Sub-tile coordinates
+                    let (t_x, t_y) = (
+                        ((x - e.x) / TILE_WIDTH as i32) as usize,
+                        ((scanline - e.y) / TILE_HEIGHT) as usize,
+                    );
+                    // Should be positive from here on
+                    let x = x as usize;
+
                     let sprite = self.get_sprite_tile(&e, t_x, t_y);
 
-                    let coloridx =
-                        sprite.get_coloridx((x - e.x) % TILE_WIDTH, (scanline - e.y) % TILE_HEIGHT);
+                    let coloridx = sprite.get_coloridx(in_x, in_y);
                     if coloridx == 0 || state.idx[x] != 0 {
                         continue;
                     }

src/snes/ppu/sprites.rs

@@ -1,10 +1,11 @@
 use super::*;
+use crate::util::sign_extend;
 
 pub const OAM_ENTRIES: usize = 128;
 
 #[derive(Debug)]
 pub struct OAMEntry {
-    pub x: usize,
+    pub x: i32,
     pub y: usize,
     pub priority: u8,
     pub attr: u8,
@@ -107,7 +108,7 @@ where
         };
 
         OAMEntry {
-            x: usize::from(e[0]) | (usize::from(ext) & 0x01) << 8,
+            x: sign_extend(i32::from(e[0]) | (i32::from(ext) & 0x01) << 8, 9),
             y: e[1].into(),
             tileidx: e[2] as u16 | (e[3] as u16 & 0x01) << 8,
             attr: e[3],

src/util.rs

@@ -0,0 +1,9 @@
+use num::traits::{WrappingShl, WrappingShr};
+use num::Integer;
+use std::mem::size_of_val;
+
+/// Sign-extend a value
+pub fn sign_extend<T: Integer + WrappingShl + WrappingShr>(val: T, nbits: u32) -> T {
+    let notherbits = size_of_val(&val) as u32 * 8 - nbits;
+    val.wrapping_shl(notherbits).wrapping_shr(notherbits)
+}