grid symmetry helpers

cruciverb-core/src/grid/symmetry.rs

@@ -0,0 +1,110 @@
+use crate::config::SymmetryType;
+
+pub fn mirror_coords(
+    row: usize,
+    col: usize,
+    width: usize,
+    height: usize,
+    symmetry: SymmetryType,
+) -> Vec<(usize, usize)> {
+    let mut coords = vec![(row, col)];
+
+    let mirror_row = |i: usize| height.checked_sub(1).and_then(|h| h.checked_sub(i));
+    let mirror_col = |i: usize| width.checked_sub(1).and_then(|w| w.checked_sub(i));
+
+    match symmetry {
+        SymmetryType::Rotational180 => {
+            if let (Some(mr), Some(mc)) = (mirror_row(row), mirror_col(col)) {
+                coords.push((mr, mc));
+            }
+        }
+        SymmetryType::Rotational90 => {
+            // each 90-degree image is kept only when it lands inside the grid,
+            // so non-square grids no longer underflow or go out of bounds
+            if let Some(mc) = mirror_col(row) {
+                if col < height {
+                    coords.push((col, mc));
+                }
+            }
+            if let (Some(mr), Some(mc)) = (mirror_row(row), mirror_col(col)) {
+                coords.push((mr, mc));
+            }
+            if let Some(mr) = mirror_row(col) {
+                if row < width {
+                    coords.push((mr, row));
+                }
+            }
+        }
+        SymmetryType::Diagonal => {
+            if width == height {
+                coords.push((col, row));
+            }
+        }
+        SymmetryType::Mirror => {
+            if let Some(mc) = mirror_col(col) {
+                coords.push((row, mc));
+            }
+        }
+        SymmetryType::None => {}
+    }
+    coords.sort();
+    coords.dedup();
+    coords
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn rotational_180() {
+        let coords = mirror_coords(0, 0, 5, 5, SymmetryType::Rotational180);
+        assert_eq!(coords, vec![(0, 0), (4, 4)]);
+    }
+
+    #[test]
+    fn rotational_180_center() {
+        let coords = mirror_coords(2, 2, 5, 5, SymmetryType::Rotational180);
+        assert_eq!(coords, vec![(2, 2)]);
+    }
+
+    #[test]
+    fn rotational_90() {
+        let coords = mirror_coords(0, 1, 5, 5, SymmetryType::Rotational90);
+        assert!(coords.contains(&(0, 1)));
+        assert!(coords.contains(&(1, 4)));
+        assert!(coords.contains(&(4, 3)));
+        assert!(coords.contains(&(3, 0)));
+    }
+
+    #[test]
+    fn mirror_lr() {
+        let coords = mirror_coords(1, 0, 5, 5, SymmetryType::Mirror);
+        assert_eq!(coords, vec![(1, 0), (1, 4)]);
+    }
+
+    #[test]
+    fn diagonal() {
+        let coords = mirror_coords(1, 3, 5, 5, SymmetryType::Diagonal);
+        assert_eq!(coords, vec![(1, 3), (3, 1)]);
+    }
+
+    #[test]
+    fn rotational_90_non_square_stays_in_bounds() {
+        let coords = mirror_coords(0, 3, 5, 7, SymmetryType::Rotational90);
+        for (r, c) in &coords {
+            assert!(*r < 7 && *c < 5, "point ({}, {}) out of bounds", r, c);
+        }
+        assert!(coords.contains(&(0, 3)));
+        let coords2 = mirror_coords(6, 4, 5, 7, SymmetryType::Rotational90);
+        for (r, c) in &coords2 {
+            assert!(*r < 7 && *c < 5, "point ({}, {}) out of bounds", r, c);
+        }
+    }
+
+    #[test]
+    fn no_symmetry() {
+        let coords = mirror_coords(1, 2, 5, 5, SymmetryType::None);
+        assert_eq!(coords, vec![(1, 2)]);
+    }
+}