BUG: Buffer::pos_of panics on inside-bounds index

- Add tests for this behavior.
- Extend documentation of Buffer::pos_of and Buffer::index_of
  - Clarify that the coordinates should be in global coordinate-space
  (rather than local).
  - Document panics.
  - Add examples.

src/buffer.rs

@@ -153,7 +153,34 @@ impl Buffer {
         &mut self.content[i]
     }
 
-    /// Returns the index in the Vec<Cell> for the given (x, y)
+    /// Returns the index in the Vec<Cell> for the given global (x, y) coordinates.
+    ///
+    /// Global coordinates are offset by the Buffer's area offset (`x`/`y`).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tui::buffer::Buffer;
+    /// # use tui::layout::Rect;
+    /// let rect = Rect::new(200, 100, 10, 10);
+    /// let buffer = Buffer::empty(rect);
+    /// // Global coordinates to the top corner of this buffer's area
+    /// assert_eq!(buffer.index_of(200, 100), 0);
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// Panics when given an coordinate that is outside of this Buffer's area.
+    ///
+    /// ```should_panic
+    /// # use tui::buffer::Buffer;
+    /// # use tui::layout::Rect;
+    /// let rect = Rect::new(200, 100, 10, 10);
+    /// let buffer = Buffer::empty(rect);
+    /// // Top coordinate is outside of the buffer in global coordinate space, as the Buffer's area
+    /// // starts at (200, 100).
+    /// buffer.index_of(0, 0); // Panics
+    /// ```
     pub fn index_of(&self, x: u16, y: u16) -> usize {
         debug_assert!(
             x >= self.area.left() && x < self.area.right() && y >= self.area.top()
@@ -166,10 +193,36 @@ impl Buffer {
         ((y - self.area.y) * self.area.width + (x - self.area.x)) as usize
     }
 
-    /// Returns the coordinates of a cell given its index
+    /// Returns the (global) coordinates of a cell given its index
+    ///
+    /// Global coordinates are offset by the Buffer's area offset (`x`/`y`).
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use tui::buffer::Buffer;
+    /// # use tui::layout::Rect;
+    /// let rect = Rect::new(200, 100, 10, 10);
+    /// let buffer = Buffer::empty(rect);
+    /// assert_eq!(buffer.pos_of(0), (200, 100));
+    /// assert_eq!(buffer.pos_of(14), (204, 101));
+    /// ```
+    ///
+    /// # Panics
+    ///
+    /// Panics when given an index that is outside the Buffer's content.
+    ///
+    /// ```should_panic
+    /// # use tui::buffer::Buffer;
+    /// # use tui::layout::Rect;
+    /// let rect = Rect::new(0, 0, 10, 10); // 100 cells in total
+    /// let buffer = Buffer::empty(rect);
+    /// // Index 100 is the 101th cell, which lies outside of the area of this Buffer.
+    /// buffer.pos_of(100); // Panics
+    /// ```
     pub fn pos_of(&self, i: usize) -> (u16, u16) {
         debug_assert!(
-            i >= self.content.len(),
+            i < self.content.len(),
             "Trying to get the coords of a cell outside the buffer: i={} len={}",
             i,
             self.content.len()
@@ -252,3 +305,42 @@ impl Buffer {
         self.area = area;
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn it_translates_to_and_from_coordinates() {
+        let rect = Rect::new(200, 100, 50, 80);
+        let buf = Buffer::empty(rect);
+
+        // First cell is at the upper left corner.
+        assert_eq!(buf.pos_of(0), (200, 100));
+        assert_eq!(buf.index_of(200, 100), 0);
+
+        // Last cell is in the lower right.
+        assert_eq!(buf.pos_of(buf.content.len() - 1), (249, 179));
+        assert_eq!(buf.index_of(249, 179), buf.content.len() - 1);
+    }
+
+    #[test]
+    #[should_panic(expected = "outside the buffer")]
+    fn pos_of_panics_on_out_of_bounds() {
+        let rect = Rect::new(0, 0, 10, 10);
+        let buf = Buffer::empty(rect);
+
+        // There are a total of 100 cells; zero-indexed means that 100 would be the 101st cell.
+        buf.pos_of(100);
+    }
+
+    #[test]
+    #[should_panic(expected = "outside the buffer")]
+    fn index_of_panics_on_out_of_bounds() {
+        let rect = Rect::new(0, 0, 10, 10);
+        let buf = Buffer::empty(rect);
+
+        // width is 10; zero-indexed means that 10 would be the 11th cell.
+        buf.index_of(10, 0);
+    }
+}