From dd5bf6cfc680451f7fc69f3251d103739ae6446b Mon Sep 17 00:00:00 2001
From: Bad Diode <bd@badd10de.dev>
Date: Thu, 15 Apr 2021 13:38:20 +0200
Subject: Implement Bresenham's line drawing algorithm

---
 src/main.c | 92 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 88 insertions(+), 4 deletions(-)

diff --git a/src/main.c b/src/main.c
index ea43821..06fe7d9 100644
--- a/src/main.c
+++ b/src/main.c
@@ -84,7 +84,7 @@ put_char(int x, int y, Color clr, u8 chr) {
 }
 
 static inline void
-put_line(int x, int y, Color clr, char *msg) {
+put_text(int x, int y, Color clr, char *msg) {
     int count = 0;
     while (*msg) {
         put_char(x + count, y, clr, *msg++);
@@ -92,6 +92,75 @@ put_line(int x, int y, Color clr, char *msg) {
     }
 }
 
+// Draws a line with the given color between (x0,y0) and (x1,y1) using the
+// Bresenham's line drawing algorithm using exclusively integer arithmetic.
+static inline void
+draw_line(int x0, int y0, int x1, int y1, Color clr) {
+    // Keep track of the coordinate for writing to the memory buffer.
+    int x = x0;
+    int y = y0;
+
+    // Normalize the drawing direction to always draw from (x0, y0)->(x1, y1)
+    int x_step = 1;
+    int y_step = 1;
+    int dx = x1 - x0;
+    int dy = y1 - y0;
+    if (x0 > x1) {
+        x_step = -1;
+        dx = x0 - x1;
+    }
+    if (y0 > y1) {
+        y_step = -1;
+        dy = y0 - y1;
+    }
+
+    // Precalculate line deltas.
+    int ddx = dx + dx;
+    int ddy = dy + dy;
+
+    // These variables are dependant on the slope. We can avoid considering
+    // separate cases for positive and negative slopes by using pointers to
+    // update the step in x or y.
+    int diff;
+    int diff_inc_a;
+    int diff_inc_b;
+    int n_steps;
+    int *a;
+    int *b;
+    int a_step;
+    int b_step;
+    if (dx >= dy) {
+        diff = ddy - dx;
+        diff_inc_a = ddy;
+        diff_inc_b = ddx;
+        n_steps = dx;
+        a = &x;
+        b = &y;
+        a_step = x_step;
+        b_step = y_step;
+    } else {
+        diff = ddx - dy;
+        diff_inc_a = ddx;
+        diff_inc_b = ddy;
+        n_steps = dy;
+        a = &y;
+        b = &x;
+        a_step = y_step;
+        b_step = x_step;
+    }
+
+    // Draw the line with Bresenham's algorithm.
+    for (size_t i = 0; i <= n_steps; ++i) {
+        FRAMEBUFFER[y][x] = clr;
+        *a += a_step;
+        diff += diff_inc_a;
+        if (diff > 0) {
+            *b += b_step;
+            diff -= diff_inc_b;
+        }
+    }
+}
+
 static inline void
 wait_vsync() {
     while(DISP_VCOUNT >= 160);
@@ -105,11 +174,26 @@ wait_vsync() {
 int main(void) {
     set_display_mode(DISP_MODE_3 | DISP_BG2);
 
+
+    put_text(0, 0, rgb15(18, 0, 0), "\n\n\n\n");
+
+    draw_line(0, 0, 3, 8,rgb15(0, 30, 0));
+    draw_line(8, 0, 16, 8,rgb15(0, 0, 30));
+
+    draw_line(0, 0, 8, 3,rgb15(0, 0, 30));
+    draw_line(8, 0, 16, 8,rgb15(0, 0, 30));
+
+    // Testing a "rectangle"
+    draw_line(0, 0, 0, 7,rgb15(0, 0, 30));
+    draw_line(0, 0, 7, 0,rgb15(0, 30, 30));
+    draw_line(0, 7, 7, 7,rgb15(30, 0, 0));
+    draw_line(7, 0, 7, 7,rgb15(0, 30, 0));
+
+    draw_line(8, 7, 15, 0,rgb15(0, 0, 30));
+
+    char scanline_counter[16];
     while(true) {
         wait_vsync();
-        put_line(16,  20 + 16, rgb15(28, 0, 0), "Hello world from the GBA!");
-        put_line(16,  20 + 32, rgb15(0, 28, 28), "Using my little 8x8 bd-font");
-        put_line(16,  20 + 64, rgb15(16, 0, 28), "Isn't that neat? :D");
     };
 
     return 0;
-- 
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