// rainbow rings
//
#include <stdio.h>
#include <math.h>
 
#include "ppm.h"
 
#define N 401	// N-by-N pixels total
#define R 25	// ring radius
#define C 200	// center x,y coordinates

// RGB values for Roy G. Biv
// from https://simple.wikipedia.org/wiki/Rainbow
//
struct { int r, g, b; } rgb[] =
{
  {   0,   0,   0 },  // black
  { 255,   0,   0 },  // red
  { 255, 127,   0 },  // orange
  { 255, 255,   0 },  // yellow
  {   0, 255,   0 },  // green
  {   0,   0, 255 },  // blue
  {  75,   0, 130 },  // indigo
  { 139,   0, 255 },  // violet
  { 255, 255, 255 }   // white
};

// interpolate, a + alpha*(b-a), 0.0 <= alpha <= 1.0
//
int interp( double alpha, int a, int b)
{
  return 0.5 + a + alpha*(b-a); // round to int
}

// square
//
double sqr( double x)
{
  return x*x;
}

int main( void)
{
  ppm a = ppm_new( N, N);
 
  for( int i = 0; i < N; ++i)
    for( int j = 0; j < N; ++j)
    {
      double r = sqrt( sqr(i-C) + sqr(j-C)); // distance from center

      r /= R; // RBG array index

      if( r >= 8) // too big, make it white
      {
	a.r[i][j] = a.g[i][j] = a.b[i][j] = 255;
	continue;
      }

      int k = r; // RBG lower index

      double alpha = r - k; // interpolation factor

      a.r[i][j] = interp( alpha, rgb[k].r, rgb[k+1].r);
      a.g[i][j] = interp( alpha, rgb[k].g, rgb[k+1].g);
      a.b[i][j] = interp( alpha, rgb[k].b, rgb[k+1].b);
    }
 
  ppm_write( a);
 
  return 0;
}