From 96c4179094b229c67701f0f36abd5e118a4331c9 Mon Sep 17 00:00:00 2001
From: Bad Diode <bd@badd10de.dev>
Date: Sun, 6 Jun 2021 10:58:09 +0200
Subject: Add diagonal line implementation

---
 src/renderer.c | 103 +++++++++++++++++++++------------------------------------
 1 file changed, 37 insertions(+), 66 deletions(-)

(limited to 'src/renderer.c')

diff --git a/src/renderer.c b/src/renderer.c
index d247d06..2b1f0d2 100644
--- a/src/renderer.c
+++ b/src/renderer.c
@@ -47,65 +47,6 @@ draw_pixel(size_t x, size_t y, u8 color) {
 IWRAM_CODE
 void
 draw_line(size_t x0, size_t y0, size_t x1, size_t y1, u8 clr) {
-    // Pointer to the initial position of the screen buffer where we will start
-    // writing our data.
-    // vu16 *destination = (u16*)(SCREEN_BUFFER + y0 * SCREEN_WIDTH + x0);
-
-//     // Adjust the step direction and calculate deltas.
-//     size_t x_step;
-//     size_t y_step;
-//     size_t dx;
-//     size_t dy;
-//     if (x0 > x1) {
-//         x_step = -1;
-//         dx = x0 - x1;
-//     } else {
-//         x_step = 1;
-//         dx = x1 - x0;
-//     }
-//     if (y0 > y1) {
-//         y_step = -SCREEN_WIDTH;
-//         dy = y0 - y1;
-//     } else {
-//         y_step = +SCREEN_WIDTH;
-//         dy = y1 - y0;
-//     }
-
-//     if(dy == 0) {
-//         // Horizontal line.
-//         for(int i = 0; i <= dx; i++) {
-//             draw_pixel(x0 + i, y0, clr);
-//         }
-//     } else if(dx == 0) {
-//         // Vertical line.
-//         for(int i = 0; i <= dy; i++) {
-//             draw_pixel(x0, y0 + i, clr);
-//         }
-//     } else if (dx >= dy){
-//         // Positive slope.
-//         int diff = 2 * dy - dx;
-//         for (int i = 0; i <= dx; ++i) {
-//             // *destination = clr;
-//             if (diff >= 0) {
-//                 // destination += y_step;
-//                 diff -= 2 * dx;
-//             }
-//             // destination += x_step;
-//             diff += 2 * dy;
-//         }
-//     } else {
-//         // Negative slope.
-//         int diff = 2 * dx - dy;
-//         for (int i = 0; i <= dy; ++i) {
-//             // *destination = clr;
-//             if (diff >= 0) {
-//                 // destination += x_step;
-//                 diff -= 2 * dy;
-//             }
-//             // destination += y_step;
-//             diff += 2 * dx;
-//         }
-//     }
     BOUNDCHECK_SCREEN(x0, y0);
     BOUNDCHECK_SCREEN(x1, y1);
 
@@ -122,15 +63,13 @@ draw_line(size_t x0, size_t y0, size_t x1, size_t y1, u8 clr) {
     // Get a pointer to the backbuffer and the tile row.
     u32 *backbuffer = &BACKBUF[start_row0 + (tile_x0 + tile_y0 * 32) * 8];
 
-    size_t dx = tile_x1 - tile_x0;
-    size_t dy = tile_y1 - tile_y0;
-
     if (y0 == y1) {
-        // There are 3 cases:
+        // Horizontal line. There are 3 cases:
         //     1. Lines fit on a single tile.
         //     2. Lines go through 2 tiles, both require partial row updates.
         //     3. Lines go through 3 or more tiles, first and last tiles use
         //        partial row updates, rows in the middle can write the.
+        size_t dx = tile_x1 - tile_x0;
         if (dx < 1) {
             u32 row_mask = 0xFFFFFFFF;
             row_mask >>= (7 - start_col1 - dx) * 4;
@@ -155,7 +94,8 @@ draw_line(size_t x0, size_t y0, size_t x1, size_t y1, u8 clr) {
             dirty_tiles[tile_y0] |= 1 << (tile_x0 + dx);
         }
     } else if (x0 == x1) {
-        //The vertical line cases are analogous to the horizontal ones.
+        // Vertical line. The cases are analogous to the horizontal ones.
+        size_t dy = tile_y1 - tile_y0;
         u32 row_mask = 0xF << start_col0 * 4;
         u32 row_left = (0x11111111 * clr) & row_mask;
         if (dy < 1) {
@@ -178,9 +118,40 @@ draw_line(size_t x0, size_t y0, size_t x1, size_t y1, u8 clr) {
                 backbuffer[0] = (backbuffer[0] & ~row_mask) | row_left;
             }
         }
+    } else {
+        // Diagonal line.
+        int x_step = x0 > x1 ? -1 : 1;
+        int y_step = y0 > y1 ? -1 : 1;
+        size_t dx = x0 > x1 ? x0 - x1 : x1 - x0;
+        size_t dy = y0 > y1 ? y0 - y1 : y1 - y0;
+        size_t x = x0;
+        size_t y = y0;
+        if (dx >= dy) {
+            // Positive slope.
+            int diff = 2 * dy - dx;
+            for (int i = 0; i <= dx; ++i) {
+                draw_pixel(x, y, clr);
+                if (diff >= 0) {
+                    y += y_step;
+                    diff -= 2 * dx;
+                }
+                x += x_step;
+                diff += 2 * dy;
+            }
+        } else {
+            // Negative slope.
+            int diff = 2 * dx - dy;
+            for (int i = 0; i <= dy; ++i) {
+                draw_pixel(x, y, clr);
+                if (diff >= 0) {
+                    x += x_step;
+                    diff -= 2 * dy;
+                }
+                y -= y_step;
+                diff += 2 * dx;
+            }
+        }
     }
-
-    // TODO: Add slope line drawing.
 }
 
 IWRAM_CODE
-- 
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