From fa9e170cc81d9fddb346972712dc55f26e0ee654 Mon Sep 17 00:00:00 2001
From: Dan Schafer <dan.schafer@gmail.com>
Date: Wed, 29 Jul 2026 10:28:32 -0700
Subject: [PATCH] Harden text and image puzzle parsing

Return errors for gridless images, calculate mark ratios over the inspected image area, and preserve edge spaces as blank partial-state squares.
---
 CHANGELOG.md |  3 +++
 src/file.rs  | 22 ++++++++++++++++++++--
 src/image.rs | 28 +++++++++++++++++++++++++++-
 3 files changed, 50 insertions(+), 3 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index f0273bd..5e634ff 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Fixed
 
 - Give out-of-range values passed to bitset-backed sets consistent, descriptive panics.
+- Return an error instead of panicking when an image contains no detectable grid.
+- Calculate queen and X pixel ratios against the inspected image area.
+- Preserve spaces used as blank markers at the edges of partial-state grids.
 
 ## [1.0.0] - 2025-04-23
 
diff --git a/src/file.rs b/src/file.rs
index 4cdce9e..c9ad39b 100644
--- a/src/file.rs
+++ b/src/file.rs
@@ -56,7 +56,7 @@ impl FromStr for InputSquares {
     type Err = anyhow::Error;
 
     fn from_str(s: &str) -> Result<Self> {
-        let lines = s.trim().lines().collect::<Vec<_>>();
+        let lines = s.trim_matches(['\n', '\r']).lines().collect::<Vec<_>>();
         let size = lines.len();
         for (line_num, &line) in lines.iter().enumerate() {
             ensure!(
@@ -115,7 +115,7 @@ impl FromStr for QueensFile {
     type Err = anyhow::Error;
 
     fn from_str(s: &str) -> Result<Self> {
-        let lines = s.trim().lines().collect::<Vec<_>>();
+        let lines = s.trim_matches(['\n', '\r']).lines().collect::<Vec<_>>();
         let lines_len = lines.len();
         ensure!(lines_len != 0, "Invalid solve state: no lines found.");
 
@@ -151,6 +151,15 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn input_squares_preserve_edge_spaces() -> Result<()> {
+        let input_squares = InputSquares::from_str(" Qxx\nxx..\nx...\n    \n")?;
+
+        assert_eq!(input_squares.0[0], None);
+        assert!(input_squares.0[12..].iter().all(Option::is_none));
+        Ok(())
+    }
+
     #[test]
     fn input_squares_display() -> Result<()> {
         let input_str = "Qxxx\nxx..\nx...\nx...";
@@ -239,4 +248,13 @@ mod tests {
         );
         Ok(())
     }
+    #[test]
+    fn queens_file_preserves_edge_spaces() -> Result<()> {
+        let file = QueensFile::from_str("wwww\nkkkk\nrrrr\nbbbb\n\n    \n....\n____\n    \n")?;
+        let squares = &file.squares.as_ref().unwrap().0;
+
+        assert!(squares[..4].iter().all(Option::is_none));
+        assert!(squares[12..].iter().all(Option::is_none));
+        Ok(())
+    }
 }
diff --git a/src/image.rs b/src/image.rs
index 9bbf9d1..b3ade55 100644
--- a/src/image.rs
+++ b/src/image.rs
@@ -191,7 +191,9 @@ fn get_other_ratio(view: &SubImage<&RgbImage>, rgb_color: &Rgb<u8>) -> f32 {
         .pixels()
         .filter(|(_, _, p)| color_distance(*p, *rgb_color) > COLOR_DISTANCE_THRESHOLD)
         .count();
-    (other_count as f32) / ((width * height) as f32)
+    let center_width = width - (2 * width / BORDER_DENOM);
+    let center_height = height - (2 * height / BORDER_DENOM);
+    (other_count as f32) / ((center_width * center_height) as f32)
 }
 
 fn find_grid_ranges(img: &RgbImage, range: Range<u32>, is_vertical: bool) -> Vec<Range<u32>> {
@@ -208,6 +210,9 @@ fn find_grid_ranges(img: &RgbImage, range: Range<u32>, is_vertical: bool) -> Vec
         });
     grid_ranges.extend(grid_ranges_iter);
     let grid_ranges_len = grid_ranges.len();
+    if grid_ranges_len == 0 {
+        return grid_ranges;
+    }
     let median_grid_length = grid_ranges
         .clone()
         .select_nth_unstable_by(grid_ranges_len / 2, |a, b| a.len().cmp(&b.len()))
@@ -318,3 +323,24 @@ fn map_image_to_square_colors(image_colors: &[Rgb<u8>]) -> [SquareColor; MAX_UNI
 
     image_to_square_color
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn analyze_grid_image_rejects_blank_image() {
+        let img = RgbImage::from_pixel(64, 64, Rgb([255, 255, 255]));
+        assert!(analyze_grid_image(&img).is_err());
+    }
+
+    #[test]
+    fn other_ratio_uses_only_the_inspected_area() {
+        let white = Rgb([255, 255, 255]);
+        let mut img = RgbImage::from_pixel(10, 10, white);
+        img.put_pixel(0, 0, Rgb([0, 0, 0]));
+        img.put_pixel(5, 5, Rgb([0, 0, 0]));
+
+        assert_eq!(get_other_ratio(&img.view(0, 0, 10, 10), &white), 1.0 / 64.0);
+    }
+}