From 0cd630609ea14432751cf015467af181340c61ca Mon Sep 17 00:00:00 2001
From: Bad Diode <bd@badd10de.dev>
Date: Fri, 16 Apr 2021 17:09:54 +0200
Subject: Update draw_line to fix offset error

---
 src/main.c | 42 ++++++++++++++++--------------------------
 1 file changed, 16 insertions(+), 26 deletions(-)

diff --git a/src/main.c b/src/main.c
index b86a830..6dcbf64 100644
--- a/src/main.c
+++ b/src/main.c
@@ -102,17 +102,15 @@ put_text(int x, int y, Color clr, char *msg) {
 // Bresenham's line drawing algorithm using exclusively integer arithmetic.
 static void
 draw_line(int x0, int y0, int x1, int y1, Color clr) {
-    // The line length in color units.
-    int pitch = SCREEN_WIDTH;
-
     // Pointer to the initial position of the screen buffer where we will start
-    // writing our data. We need to multiply by 2 because in mode 3 we have
-    // 2 bytes per pixel.
-    vu16 *destination = (u16*)(SCREEN_BUFFER + y0 * pitch * 2 + x0 * 2);
+    // writing our data.
+    vu16 *destination = (u16*)(SCREEN_BUFFER + y0 * SCREEN_WIDTH + x0);
 
     // Adjust the step direction and calculate deltas.
     int x_step;
+    int y_step;
     int dx;
+    int dy;
     if (x0 > x1) {
         x_step = -1;
         dx = x0 - x1;
@@ -120,20 +118,14 @@ draw_line(int x0, int y0, int x1, int y1, Color clr) {
         x_step = 1;
         dx = x1 - x0;
     }
-    int y_step;
-    int dy;
     if (y0 > y1) {
-        y_step = -pitch;
+        y_step = -SCREEN_WIDTH;
         dy = y0 - y1;
     } else {
-        y_step = +pitch;
+        y_step = +SCREEN_WIDTH;
         dy = y1 - y0;
     }
 
-    // Precalculate 2 * deltas for x and y.
-    int ddx = dx + dx;
-    int ddy = dy + dy;
-
     if(dy == 0) {
         // Horizontal line.
         for(int i = 0; i <= dx; i++) {
@@ -146,27 +138,27 @@ draw_line(int x0, int y0, int x1, int y1, Color clr) {
         }
     } else if (dx >= dy){
         // Positive slope.
-        int diff = ddy - dx;
+        int diff = 2 * dy - dx;
         for (int i = 0; i <= dx; ++i) {
             *destination = clr;
             if (diff >= 0) {
                 destination += y_step;
-                diff -= ddx;
+                diff -= 2 * dx;
             }
             destination += x_step;
-            diff += ddy;
+            diff += 2 * dy;
         }
     } else {
         // Negative slope.
-        int diff = ddx - dy;
+        int diff = 2 * dx - dy;
         for (int i = 0; i <= dy; ++i) {
             *destination = clr;
             if (diff >= 0) {
                 destination += x_step;
-                diff -= ddy;
+                diff -= 2 * dy;
             }
             destination += y_step;
-            diff += ddx;
+            diff += 2 * dx;
         }
     }
 }
@@ -316,10 +308,10 @@ int main(void) {
     draw_fill_rect(0, 0, SCREEN_WIDTH, SCREEN_HEIGHT, COLOR_GREY);
 
     int side = 60;
-    int x = 88;
-    int y = 70;
     int line = 35;
-    int height = sqrt(side * side - (side / 2) * (side / 2));
+    int height = side * 0.5;
+    int x = SCREEN_WIDTH / 2 - height / 2;
+    int y = SCREEN_HEIGHT / 2;
 
     // Draw red triangle.
     draw_line(x + height - 1, y - side / 2, x, y - 1, COLOR_RED);
@@ -335,8 +327,6 @@ int main(void) {
     // Draw white line at triangle tip.
     draw_line(x + height, y - side / 2, x + height, y + side / 2, COLOR_WHITE);
     draw_line(x + height + 1, y - side / 2, x + height + 1, y + side / 2, COLOR_WHITE);
-    draw_line(x + height + 2, y - side / 2, x + height + 2, y + side / 2, COLOR_WHITE);
-    // draw_line(x + height + 3, y - side / 2, x + height + 3, y + side / 2, COLOR_WHITE);
 
     // Double triangle line.
     draw_line(x - 1, y - side / 2, x - 1, y + side / 2, COLOR_WHITE);
@@ -349,7 +339,7 @@ int main(void) {
     draw_line(x - line, y + 1, x, y + 1, COLOR_WHITE);
     draw_line(x + height, y + 1, x + height + line, y + 1, COLOR_WHITE);
 
-    put_text(77, 125, COLOR_RED, "0xbadd10de");
+    put_text(SCREEN_WIDTH / 2 - 8 * 10 / 2, 125, COLOR_RED, "0xbadd10de");
 
     int frame_counter = 0;
     while(true) {
-- 
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