From 22546de9db05def062da6dbf6ad73332617a5cd7 Mon Sep 17 00:00:00 2001
From: Fabien Freling <public@ffreling.com>
Date: Sun, 22 Jun 2014 16:08:40 +0200
Subject: [PATCH] Fix draw_lines for x1 = x2 and y1 = y2.

Some lines are dotted though.
---
 rotation.cpp | 35 ++++++++++++++++++++++++++---------
 1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/rotation.cpp b/rotation.cpp
index cf0a220..65461b7 100644
--- a/rotation.cpp
+++ b/rotation.cpp
@@ -355,20 +355,33 @@ Image rotate(Image const& src, double angle)
 
 void draw_line(Image& img, unsigned int x1, unsigned int y1, unsigned int x2, unsigned int y2)
 {
-  if (x1 > x2)
-  {
-    swap(x1, x2);
-    swap(y1, y2);
-  }
+  int x_inc = x1 <= x2 ? 1 : -1;
+
   unsigned int const y_min = min(y1, y2);
   unsigned int const y_max = max(y1, y2);
-  double const slope = ((double) y2 - y1) / ((double) x2 - x1);
+  double slope = (double) y2 - y1;
+  if (x1 != x2)
+    slope = ((double) y2 - y1) / abs((double) x2 - x1);
   int y_inc = slope > 0 ? 1 : -1;
 
-  unsigned int previous_y = y1;
-  for (unsigned int i = x1; i <= x2; ++i)
+  if (x1 == x2)
   {
-    unsigned int y = slope * (i - x1) + y1;
+    for (unsigned int runner = y1; runner != y2; runner+= y_inc)
+      img.set_pixel(x1, runner, 255, 0, 0); // set line to red
+    return;
+  }
+
+  if (y1 == y2)
+  {
+    for (unsigned int runner = x1; runner != x2; runner+= x_inc)
+      img.set_pixel(runner, y1, 255, 0, 0); // set line to red
+    return;
+  }
+
+  unsigned int previous_y = y1;
+  for (unsigned int i = x1, steps = 0; i != x2; i += x_inc, ++steps)
+  {
+    unsigned int y = slope * steps + y1;
     y = min(y, y_max);
     y = max(y, y_min);
     for (unsigned int runner = previous_y; runner != y; runner+= y_inc)
@@ -505,9 +518,13 @@ bool check_trigo()
 void check_lines()
 {
   Image const square(500, 500);
+  draw_outline(square, 5, "square");
+  draw_outline(square, 12, "square");
   draw_outline(square, 22, "square");
   draw_outline(square, 33, "square");
   draw_outline(square, 45, "square");
+  draw_outline(square, 60, "square");
+  draw_outline(square, 75, "square");
   draw_outline(square, 90, "square");
 
   Image const rect1(640, 480);