Fix Buffer::merge

Coordinates returned by Buffer::pos_of were interpreted as local coordinates
while they were global. This was resulting in panics due to out of bounds
accesses. Interpreting the coordinates as global and using correct offsets
when computing the new index within the buffer for each cell fix the issue.

src/buffer.rs

@@ -360,28 +360,24 @@ impl Buffer {
         self.content.resize(area.area() as usize, cell.clone());
 
         // Move original content to the appropriate space
-        let offset_x = self.area.x - area.x;
-        let offset_y = self.area.y - area.y;
         let size = self.area.area() as usize;
         for i in (0..size).rev() {
             let (x, y) = self.pos_of(i);
             // New index in content
-            let k = ((y + offset_y) * area.width + (x + offset_x)) as usize;
-            self.content[k] = self.content[i].clone();
+            let k = ((y - area.y) * area.width + x - area.x) as usize;
             if i != k {
+                self.content[k] = self.content[i].clone();
                 self.content[i] = cell.clone();
             }
         }
 
         // Push content of the other buffer into this one (may erase previous
         // data)
-        let offset_x = other.area.x - area.x;
-        let offset_y = other.area.y - area.y;
         let size = other.area.area() as usize;
         for i in 0..size {
             let (x, y) = other.pos_of(i);
             // New index in content
-            let k = ((y + offset_y) * area.width + (x + offset_x)) as usize;
+            let k = ((y - area.y) * area.width + x - area.x) as usize;
             self.content[k] = other.content[i].clone();
         }
         self.area = area;
@@ -630,4 +626,86 @@ mod tests {
             ]
         );
     }
+
+    #[test]
+    fn buffer_merge() {
+        let mut one = Buffer::filled(
+            Rect {
+                x: 0,
+                y: 0,
+                width: 2,
+                height: 2,
+            },
+            Cell::default().set_symbol("1"),
+        );
+        let two = Buffer::filled(
+            Rect {
+                x: 0,
+                y: 2,
+                width: 2,
+                height: 2,
+            },
+            Cell::default().set_symbol("2"),
+        );
+        one.merge(&two);
+        assert_eq!(one, Buffer::with_lines(vec!["11", "11", "22", "22"]));
+    }
+
+    #[test]
+    fn buffer_merge2() {
+        let mut one = Buffer::filled(
+            Rect {
+                x: 2,
+                y: 2,
+                width: 2,
+                height: 2,
+            },
+            Cell::default().set_symbol("1"),
+        );
+        let two = Buffer::filled(
+            Rect {
+                x: 0,
+                y: 0,
+                width: 2,
+                height: 2,
+            },
+            Cell::default().set_symbol("2"),
+        );
+        one.merge(&two);
+        assert_eq!(
+            one,
+            Buffer::with_lines(vec!["22  ", "22  ", "  11", "  11"])
+        );
+    }
+
+    #[test]
+    fn buffer_merge3() {
+        let mut one = Buffer::filled(
+            Rect {
+                x: 3,
+                y: 3,
+                width: 2,
+                height: 2,
+            },
+            Cell::default().set_symbol("1"),
+        );
+        let two = Buffer::filled(
+            Rect {
+                x: 1,
+                y: 1,
+                width: 3,
+                height: 4,
+            },
+            Cell::default().set_symbol("2"),
+        );
+        one.merge(&two);
+        let mut merged = Buffer::with_lines(vec!["222 ", "222 ", "2221", "2221"]);
+        merged.area = Rect {
+            x: 1,
+            y: 1,
+            width: 4,
+            height: 4,
+        };
+        assert_eq!(one, merged);
+    }
 }