Add lines

sys/sys_data.c

@@ -6,4 +6,15 @@ sys_i32 sys_max_i32(sys_i32 x, sys_i32 y) {
 
 sys_i32 sys_min_i32(sys_i32 x, sys_i32 y) {
     return x < y ? x : y;
+}
+
+sys_i32 sys_abs_i32(sys_i32 x) {
+    if (x < 0) { return -x; }
+    return x;
+}
+
+sys_i32 sys_sgn_i32(sys_i32 x) {
+    if (x < 0) { return -1; }
+    if (x > 0) { return 1; }
+    return 0;
 }

sys/sys_data.h

@@ -11,6 +11,8 @@ sys_screen_color sys_make_screen_color(uint8_t r, uint8_t g, uint8_t b);
 
 sys_i32 sys_max_i32(sys_i32 x, sys_i32 y); 
 sys_i32 sys_min_i32(sys_i32 x, sys_i32 y); 
+sys_i32 sys_abs_i32(sys_i32 x); 
+sys_i32 sys_sgn_i32(sys_i32 x); 
 
 #define SYS_COLOR_TRANSPARENT 255
 #define SYS_COLOR_N 256

sys/sys_graphics.c

@@ -121,9 +121,12 @@ void sys_oval_fill(
 void sys_oval_draw_ext(
     sys_i32 x0, sys_i32 y0, sys_i32 x1, sys_i32 y1, sys_color c, bool fill
 ) {
+    assert(sys_get_initialized());
+
     if (x0 == x1 || y0 == y1) { return; }
     if (x0 > x1) { sys_i32 tmp = x0; x0 = x1; x1 = tmp; }
     if (y0 > y1) { sys_i32 tmp = y0; y0 = y1; y1 = tmp; }
+
     // alois' algorithm for this implies the bounds are inclusive 
     x1 -= 1; y1 -= 1;
 
@@ -162,6 +165,28 @@ void sys_oval_draw_ext(
     }
 }
 
+void sys_line_draw(
+    sys_i32 x0, sys_i32 y0, sys_i32 x1, sys_i32 y1, sys_color c
+) {
+    assert(sys_get_initialized());
+
+    if (x0 == x1 || y0 == y1) { return; }
+
+    sys_i32 dx = sys_abs_i32(x1 - x0); 
+    sys_i32 sx = sys_sgn_i32(x1 - x0);
+    sys_i32 dy = -sys_abs_i32(y1 - y0); 
+    sys_i32 sy = sys_sgn_i32(y1 - y0);
+    sys_i32 err = dx + dy;
+
+    while (true) {
+        if (x0 == x1 || y0 == y1) { return; }
+        sys_pixel_internal_set(x0, y0, c);
+        sys_i32 e2 = 2 * err;
+        if (e2 >= dy) { err += dy; x0 += sx; }
+        if (e2 <= dx) { err += dx; y0 += sy; }
+    }
+}
+
 void sys_spal_set(sys_color c0, sys_screen_color rc1) {
     assert(sys_get_initialized());
 

sys/sys_graphics.h

@@ -97,6 +97,8 @@ void sys_oval_draw_ext(
 
 /**
  * Draw a line from x0 to y0.
+ * 
+ * The point `(x1, y1)` is not drawn. (because the line is a half-open interval)
  */
 void sys_line_draw(sys_i32 x0, sys_i32 y0, sys_i32 x1, sys_i32 y1, sys_color c);