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kiln-tui/src/render.rs

@@ -50,6 +50,68 @@ impl<'a> BoardWidget<'a> {
     }
 }
 
+/// Maps a board cell `(bx, by)` to its screen position within `area`, using the
+/// same centering/scrolling layout as [`BoardWidget`]. Returns `None` when the
+/// cell is scrolled out of view.
+///
+/// The inverse of [`screen_to_board`]; both reuse [`BoardWidget::axis`] so they
+/// stay consistent with how the board is actually drawn. (Unlike
+/// `speech::board_screen_pos`, which clamps off-screen cells to the edge, this
+/// reports off-screen cells as `None` — needed for exact cursor placement.)
+pub fn board_to_screen(area: Rect, board: &Board, bx: usize, by: usize) -> Option<(u16, u16)> {
+    let (off_x, pad_x, cols) = BoardWidget::axis(board.width, area.width as usize, board.player.x);
+    let (off_y, pad_y, rows) = BoardWidget::axis(board.height, area.height as usize, board.player.y);
+    // Cell must fall within the visible window on both axes.
+    if bx < off_x || bx >= off_x + cols || by < off_y || by >= off_y + rows {
+        return None;
+    }
+    let sx = area.x + pad_x + (bx - off_x) as u16;
+    let sy = area.y + pad_y + (by - off_y) as u16;
+    Some((sx, sy))
+}
+
+/// Maps a screen position `(sx, sy)` to the board cell drawn there within `area`,
+/// the inverse of [`board_to_screen`]. Returns `None` when the point lies outside
+/// the drawn board (in the centering pad or past the visible cells).
+pub fn screen_to_board(area: Rect, board: &Board, sx: u16, sy: u16) -> Option<(usize, usize)> {
+    let (off_x, pad_x, cols) = BoardWidget::axis(board.width, area.width as usize, board.player.x);
+    let (off_y, pad_y, rows) = BoardWidget::axis(board.height, area.height as usize, board.player.y);
+    // Screen-space offset from the first drawn cell; reject anything before it.
+    let col = (sx as i32) - (area.x as i32 + pad_x as i32);
+    let row = (sy as i32) - (area.y as i32 + pad_y as i32);
+    if col < 0 || row < 0 || col as usize >= cols || row as usize >= rows {
+        return None;
+    }
+    Some((off_x + col as usize, off_y + row as usize))
+}
+
+#[cfg(test)]
+mod tests {
+    use super::BoardWidget;
+
+    #[test]
+    fn axis_centers_a_small_board() {
+        // 10-cell board in a 20-cell view: no scroll, 5 cells of pad each side.
+        let (off, pad, count) = BoardWidget::axis(10, 20, 3);
+        assert_eq!((off, pad, count), (0, 5, 10));
+    }
+
+    #[test]
+    fn axis_scrolls_a_large_board_to_follow_the_player() {
+        // 100-cell board in a 20-cell view, player at 50: centered, no pad.
+        let (off, pad, count) = BoardWidget::axis(100, 20, 50);
+        assert_eq!((off, pad, count), (40, 0, 20));
+    }
+
+    #[test]
+    fn axis_clamps_scroll_at_the_edges() {
+        // Player near the left edge can't scroll past 0.
+        assert_eq!(BoardWidget::axis(100, 20, 2), (0, 0, 20));
+        // Player near the right edge clamps to board_len - view.
+        assert_eq!(BoardWidget::axis(100, 20, 99), (80, 0, 20));
+    }
+}
+
 impl Widget for BoardWidget<'_> {
     fn render(self, area: Rect, buf: &mut Buffer) {
         let board = self.board;