Add diagonal line drawing and tests

2 files changed, 34 insertions, 3 deletions

diff --git a/src/main.c b/src/main.c
index 522ef70..96682f9 100644
--- a/src/main.c
+++ b/src/main.c
@@ -36,6 +36,7 @@ WITH REGARD TO THIS SOFTWARE.
         txt_printf("FLIP:  %.9lu\n", flip_cycles);\
         txt_printf("CHR:   %.9lu\n", test_chr_cycles);\
         txt_printf("ICN:   %.9lu\n", test_icn_cycles);\
+        txt_printf("LINE:  %.9lu\n", test_lines_cycles);\
         txt_render();\
         txt_clear();\
         frame_counter++;\
@@ -47,6 +48,7 @@ WITH REGARD TO THIS SOFTWARE.
     u32 test_clear_cycles = 0;\
     u32 test_chr_cycles = 0;\
     u32 test_icn_cycles = 0;\
+    u32 test_lines_cycles = 0;\
     u32 flip_cycles = 0;
 
 void
@@ -97,6 +99,16 @@ test_icn(void) {
     }
 }
 
+void
+test_lines(void) {
+    for (size_t i = 0; i < 20; i++) {
+        draw_line(0, i * 8, (30 * 8 - 1), ((20 - i) * 8 - 1), 5);
+    }
+    for (size_t i = 0; i < 30; i++) {
+        draw_line(i * 8, (20 * 8 - 1), ((30 - i) * 8 - 1), 0, 5);
+    }
+}
+
 int main(void) {
     // Adjust system wait times.
     SYSTEM_WAIT = SYSTEM_WAIT_CARTRIDGE;
@@ -116,6 +128,7 @@ int main(void) {
         bios_vblank_wait();
 
         PROF(test_clear(), test_clear_cycles);
+        PROF(test_lines(), test_lines_cycles);
         PROF(test_rect(), test_rect_cycles);
         PROF(test_fill_rect(), test_fill_rect_cycles);
         PROF(test_chr(), test_chr_cycles);
diff --git a/src/renderer_m4.c b/src/renderer_m4.c
index 21dc192..cbf3c3f 100644
--- a/src/renderer_m4.c
+++ b/src/renderer_m4.c
@@ -136,15 +136,33 @@ void
 draw_line(size_t x0, size_t y0, size_t x1, size_t y1, u8 clr) {
     BOUNDCHECK_SCREEN(x0, y0);
     BOUNDCHECK_SCREEN(x1, y1);
-    MAYBE_SWAP(x0, x1);
-    MAYBE_SWAP(y0, y1);
 
     if (y0 == y1) {
+        MAYBE_SWAP(x0, x1);
         draw_hline(x0, x1, y0, clr);
     } else if (x0 == x1) {
+        MAYBE_SWAP(y0, y1);
         draw_vline(x0, y0, y1, clr);
+    } else {
+        // Diagonal line.
+        int dx = x0 > x1 ? x0 - x1 : x1 - x0;
+        int dy = y0 > y1 ? y1 - y0 : y0 - y1;
+        int x_step = x0 < x1 ? 1 : -1;
+        int y_step = y0 < y1 ? 1 : -1;
+        int err = dx + dy;
+        while (!(x0 == x1 && y0 == y1)) {
+            draw_pixel(x0, y0, clr);
+            int diff = 2 * err;
+            if (diff >= dy) {
+                err += dy;
+                x0 += x_step;
+            }
+            if (diff <= dx) {
+                err += dx;
+                y0 += y_step;
+            }
+        }
     }
-    // TODO: diagonal (bresengham)
     screen_updated = true;
 }