Added delete routine in screen and adjusted the code

kernel/screen/include/cursor.h

@@ -15,14 +15,14 @@
 
 /*
  * _screen_move_cursor:
- *  Moves the cursor of the framebuffer to a give position calculated using
- *  @x and @y coordinates.
+ *  Moves the cursor of the framebuffer to a given position calculated using
+ *  the arguments.
  *
- * @param x - horizontal position
- * @param y - vertical position
+ * @param column - Vertical position
+ * @param row    - Horizontal position
  *
  */
 
-void _screen_move_cursor(uint8_t x, uint8_t y);
+void _screen_move_cursor(uint8_t column, uint8_t row);
 
 #endif

kernel/screen/include/screen_internals.h

@@ -17,14 +17,14 @@ size_t screen_color;
  *  Put character at a specified position or put character at the next position
  *  available in framebuffer acording to state variables.
  *
- *  @param c     - Char to be displayed
- *  @param color - Color attributes of @c
- *  @param x     - Vertical position
- *  @param y     - Horizontal position
+ *  @param c          - Char to be displayed
+ *  @param color      - Color attributes of @c
+ *  @param column     - Vertical position
+ *  @param row        - Horizontal position
  *
  */
 
-void _screen_putchar_at(char c, uint16_t color, size_t x, size_t y);
+void _screen_putchar_at(char c, uint16_t color, size_t column, size_t row);
 void _screen_putchar(char c);
 
 /*
@@ -35,4 +35,37 @@ void _screen_putchar(char c);
 
 void _screen_scroll_up(void);
 
+/*
+ * _screen_delete_at:
+ *  Deletes a character at a given position.
+ *
+ * @param column - Vertical position
+ * @param row    - Horizontal position
+ *
+ */
+
+void _screen_delete_at(size_t column, size_t row);
+
+/*
+ * _find_last_nonwhite_column:
+ *
+ *
+ * @return - the column with the last nonwhite character on the previous
+ * line.
+ */
+
+size_t _find_last_nonwhite_column(void);
+
+/*
+ * _is_whitespace:
+ *  
+ * @param column - Vertical position
+ * @param row    - Horizontal poistion
+ * @return       - 1 if it is whitespace
+ *                 0 else
+ *  
+ */
+
+int _is_whitespace(size_t column, size_t row);
+
 #endif

kernel/screen/src/cursor.c

@@ -16,9 +16,9 @@
  *
  */
 
-void _screen_move_cursor(uint8_t x, uint8_t y) {
+void _screen_move_cursor(uint8_t column, uint8_t row) {
 
-    uint16_t sindex = y * VGA_WIDTH + x;
+    uint16_t sindex = row * VGA_WIDTH + column;
 
     outb(CURSOR_COMMAND_PORT, CURSOR_HIGH_BYTE_COMMAND);
     outb(CURSOR_DATA_PORT, ((sindex >> 8)) & 0x00FF);

kernel/screen/src/screen.c

@@ -2,6 +2,7 @@
 
 #include "framebuffer.h"
 #include "screen_internals.h"
+#include "cursor.h"
 
 #include "lib/vga.h"
 
@@ -25,14 +26,9 @@ void screen_init(void) {
 
     fbuffer = (uint16_t *) FBUFFER_START_ADDR;
 
-    for (size_t y = 0; y < VGA_HEIGHT; y++) {
-        for (size_t x = 0; x < VGA_WIDTH; x++) {
-
-            const size_t index = y * VGA_WIDTH + x;
-            fbuffer[index] = vga_entry(' ', screen_color);
-
-        }
-    }
+    for (size_t row = 0; row < VGA_HEIGHT; row++)
+        for (size_t column = 0; column < VGA_WIDTH; column++) 
+            _screen_delete_at(column, row);
 
 }
 
@@ -54,3 +50,35 @@ size_t screen_write(char *buf, size_t len) {
     return i;
 }
 
+/*
+ * screen_delete:
+ * --------------
+ *
+ *  Deletes the current character and updates the cursor. If current position
+ *  is (0, 0) then do nothing. If current position is at the beginning of a
+ *  line then find the last nonwhite element on the last line and move the
+ *  cursor there. Else just delete the current element.
+ *
+ *  Side effects:
+ *   cursor position will be affected too.
+ *
+ */
+
+void screen_delete(void) {
+
+    if (!screen_column && !screen_row)
+        return;
+
+    if (!screen_column && screen_row) {
+
+        screen_column = _find_last_nonwhite_column();
+        screen_row--;
+
+    } else
+        screen_column--;
+
+    _screen_delete_at(screen_column, screen_row);
+    _screen_move_cursor(screen_column, screen_row);
+
+}
+

kernel/screen/src/screen_internals.c

@@ -6,6 +6,21 @@
 #include <stddef.h>
 #include <stdint.h>
 
+/*
+ * _screen_delete_at:
+ * ------------------
+ *
+ *  Replaces the character at (row, column) with a whitespace.
+ *
+ */
+
+void _screen_delete_at(size_t column, size_t row) {
+
+    const size_t index = row * VGA_WIDTH + column;
+    fbuffer[index] = vga_entry(' ', screen_color);
+
+}
+
 /*
  * _screen_putchar_at:
  * ------------------
@@ -15,9 +30,9 @@
  *
  */
 
-void _screen_putchar_at(char c, uint16_t color, size_t x, size_t y) {
+void _screen_putchar_at(char c, uint16_t color, size_t column, size_t row) {
 
-    const size_t index = y * VGA_WIDTH + x;
+    const size_t index = row * VGA_WIDTH + column;
     fbuffer[index] = vga_entry(c, color);
 
 }
@@ -38,7 +53,7 @@ void _screen_scroll_up(void) {
         fbuffer[i] = fbuffer[i + 80];
 
     for (i = 0; i < VGA_WIDTH; i++)
-        fbuffer[(VGA_HEIGHT - 1) * VGA_WIDTH + i] = vga_entry(' ', screen_color);
+        _screen_delete_at(i, VGA_HEIGHT - 1);
 
 }
 
@@ -75,7 +90,44 @@ void _screen_putchar(char c) {
 
     }
 
-    _screen_putchar_at(c, screen_color, screen_column++, screen_row);
+    _screen_putchar_at(c, screen_color, screen_column, screen_row);
+    screen_column++;
     _screen_move_cursor(screen_column, screen_row);
 
 }
+
+/*
+ * _is_whitespace:
+ * ---------------
+ *
+ */
+
+int _is_whitespace(size_t column, size_t row) {
+
+    int index = row * VGA_WIDTH + column;
+    return fbuffer[index] == vga_entry(' ', screen_color);
+
+}
+
+/*
+ * _find_last_nonwhite_column:
+ * ---------------------------
+ *
+ *  Goes on the last line and returns the first column that has a nonwhite
+ *  char. The traversal is from the end of the line to the beginning.
+ *  It will return 0 if there is no nonwhite character.
+ *
+ */
+
+size_t _find_last_nonwhite_column(void) {
+
+    int row = screen_row - 1;
+    int y = VGA_WIDTH;
+
+    for (; y > 0; y--)
+        if (!_is_whitespace(y, row))
+            break;
+
+    return y;
+
+}