Address code review feedback on refactoring constraints and fix plot_line overflow

crates/rustille/src/canvas.rs

@@ -24,10 +24,12 @@ use crate::error::{Error, Result};
 
 /// Largest canvas Rustille will allocate, in character cells.
 ///
-/// One cell is one byte, so this caps a canvas at 256 MiB — roughly a
-/// 32,768 × 65,536 dot surface. Anything larger is rejected with
+/// One cell is one byte, so this caps a canvas at 16 MiB — roughly a
+/// 8,192 × 16,384 dot surface. Anything larger is rejected with
 /// [`Error::InvalidDimensions`] rather than aborting on allocation failure.
-pub const MAX_CANVAS_CELLS: u64 = 1 << 28;
+/// This acts as a safeguard against accidental or malicious memory exhaustion
+/// (e.g. over web APIs or FFI boundaries).
+pub const MAX_CANVAS_CELLS: u64 = 1 << 24;
 
 /// A fixed-size grid of Braille dots.
 #[derive(Clone, PartialEq, Eq)]
@@ -247,8 +249,8 @@ impl Canvas {
     }
 
     fn plot_line(&mut self, x0: i32, y0: i32, x1: i32, y1: i32, value: bool) {
-        let dx = (x1 - x0).abs();
-        let dy = -(y1 - y0).abs();
+        let dx = (i64::from(x1) - i64::from(x0)).abs();
+        let dy = -(i64::from(y1) - i64::from(y0)).abs();
         let step_x = if x0 < x1 { 1 } else { -1 };
         let step_y = if y0 < y1 { 1 } else { -1 };
         let mut error = dx + dy;

crates/rustille/src/options.rs

@@ -401,9 +401,11 @@ impl RenderOptions {
         if self.height == Some(0) {
             return Err(Error::invalid_options("height must be greater than zero"));
         }
-        if !self.cell_aspect_ratio.is_finite() || self.cell_aspect_ratio <= 0.0 {
+        // Prevent extremely small values that could cause division by zero or infinities
+        // during rendering size calculations.
+        if !self.cell_aspect_ratio.is_finite() || self.cell_aspect_ratio <= 1e-6 {
             return Err(Error::invalid_options(format!(
-                "cell_aspect_ratio must be a positive finite number, got {}",
+                "cell_aspect_ratio must be a positive finite number >= 1e-6, got {}",
                 self.cell_aspect_ratio
             )));
         }

crates/rustille/src/render.rs

@@ -280,7 +280,8 @@ fn pack_cells(
 fn cover_crop(surface: &Surface, cells_width: u32, cells_height: u32, cell_aspect: f32) -> Surface {
     let box_aspect = f64::from(cells_width) / (f64::from(cells_height) * f64::from(cell_aspect));
     let source_aspect = f64::from(surface.width()) / f64::from(surface.height());
-    if (source_aspect - box_aspect).abs() < 1e-9 {
+    // Use a small epsilon to handle precision issues
+    if (source_aspect - box_aspect).abs() < 1e-6 {
         return surface.clone();
     }
     if source_aspect > box_aspect {