title: How to transform degrees (angle) to facing number
body: <p>I have a image which contains 40 rotated images.</p>
  
  <p>With image index starting at 0. 0-39 actually.</p>
  
  <p>Here is the code which transforms 0-39 to degrees</p>
  
  <pre><code>int image_direction = 0; //Can be 0-39
  int facing_degrees = (int)(360.0 * (-(image_direction- 10.0))/40.0);
  while(facing_degrees &lt; 0)
      facing_degrees += 360;
  while (facing_degrees &gt; 360)
      facing_degrees -= 360;
  </code></pre>
  
  <p>So yeah it can also give out negative degrees as well as degrees over 360. So thats why there is the 2 while loops.</p>
  
  <p>Now I wish to reverse this process say I specify 90 degrees I would like to get back 0..</p>
  
  <p>I was thinking of doing something like</p>
  
  <pre><code> if(degrees == 90 || degrees &gt;= 80 &amp;&amp; degrees &lt;= 99)
      image_direction = 0;
   elseif(degrees == 100 || degrees &gt;= 91 &amp;&amp; degrees &lt;= 109)
      image_direction = 39;
  //etc.......
  </code></pre>
  
  <p>Well I'm not good at mathematics, well I forgot about this kind of stuff.</p>
  
  <p>I'm wondering how can you reverse the function that gives degrees from image_direction to run backwards in a plain equation to avoid the huge case of if statements.</p>
  
  <p>Here is some results
  <img src="https://i.stack.imgur.com/09c7m.png" alt="rotation results"></p>
  
  <pre><code>Image Index = 0 Image Degree = 90
  Image Index = 1 Image Degree = 81
  Image Index = 2 Image Degree = 72
  Image Index = 3 Image Degree = 63
  Image Index = 4 Image Degree = 54
  Image Index = 5 Image Degree = 45
  Image Index = 6 Image Degree = 36
  Image Index = 7 Image Degree = 27
  Image Index = 8 Image Degree = 18
  Image Index = 9 Image Degree = 9
  Image Index = 10 Image Degree = 0
  Image Index = 11 Image Degree = 351
  Image Index = 12 Image Degree = 342
  Image Index = 13 Image Degree = 333
  Image Index = 14 Image Degree = 324
  Image Index = 15 Image Degree = 315
  Image Index = 16 Image Degree = 306
  Image Index = 17 Image Degree = 297
  Image Index = 18 Image Degree = 288
  Image Index = 19 Image Degree = 279
  Image Index = 20 Image Degree = 270
  Image Index = 21 Image Degree = 261
  Image Index = 22 Image Degree = 252
  Image Index = 23 Image Degree = 243
  Image Index = 24 Image Degree = 234
  Image Index = 25 Image Degree = 225
  Image Index = 26 Image Degree = 216
  Image Index = 27 Image Degree = 207
  Image Index = 28 Image Degree = 198
  Image Index = 29 Image Degree = 189
  Image Index = 30 Image Degree = 180
  Image Index = 31 Image Degree = 171
  Image Index = 32 Image Degree = 162
  Image Index = 33 Image Degree = 153
  Image Index = 34 Image Degree = 144
  Image Index = 35 Image Degree = 135
  Image Index = 36 Image Degree = 126
  Image Index = 37 Image Degree = 117
  Image Index = 38 Image Degree = 108
  Image Index = 39 Image Degree = 99
  Image Index = 0 Image Degree = 90
  Image Index = 1 Image Degree = 81
  Image Index = 2 Image Degree = 72
  Image Index = 3 Image Degree = 63
  Image Index = 4 Image Degree = 54
  Image Index = 5 Image Degree = 45
  Image Index = 6 Image Degree = 36
  Image Index = 7 Image Degree = 27
  Image Index = 8 Image Degree = 18
  </code></pre>