add MatBuf::shift_up() operation

mino/src/matrix.rs

@@ -233,6 +233,15 @@ impl<const LEN: usize> MatBuf<LEN> {
         self.rows = dst as i16;
         rng
     }
+
+    pub fn shift_up(&mut self) {
+        if self.rows as usize == LEN {
+            panic!("not enough available buffer space to shift up");
+        }
+        self.buf.copy_within(0..self.rows as usize, 1);
+        self.buf[0] = EMPTY_ROW;
+        self.rows += 1;
+    }
 }
 
 impl<const LEN: usize> Default for MatBuf<LEN> {
@@ -575,4 +584,40 @@ mod test {
         let mut buf: MatBuf<4> = MatBuf::default();
         buf.fill_row(4, 0b1001);
     }
+
+    #[test]
+    fn test_shift_up() {
+        let mat0 = mat! {
+            ".x........";
+            "xxxxx.xxxx";
+            "x.xxxxxxxx";
+            ".xxxxxxxxx";
+        };
+        let mat1 = mat! {
+            ".x........";
+            "xxxxx.xxxx";
+            "x.xxxxxxxx";
+            ".xxxxxxxxx";
+            "..........";
+        };
+        let mat2 = mat! {
+            ".x........";
+            "xxxxx.xxxx";
+            "x.xxxxxxxx";
+            ".xxxxxxxxx";
+            "..........";
+            "..........";
+        };
+        let mut buf = MatBuf::new();
+        buf.copy_from(mat0);
+        buf.shift_up();
+        assert_eq!(buf, mat1);
+        buf.shift_up();
+        assert_eq!(buf, mat2);
+        assert_eq!(buf.clear_lines(), 0..2);
+        assert_eq!(buf, mat0);
+        buf.shift_up();
+        assert_eq!(buf, mat1);
+        assert_eq!(buf.clear_lines(), 0..1);
+    }
 }