/**
 * flag.c
 *   flag simple implementation of the Dutch National Flag algorithm
 *   and some related things
 */

// +---------+-------------------------------------------------------
// | Headers |
// +---------+

#include "flag.h"
#include <stdio.h>

// +------------------+----------------------------------------------
// | Local Procedures |
// +------------------+

void 
swap (color flag[], int a, int b)
{
  color tmp = flag[a];
  flag[a] = flag[b];
  flag[b] = tmp;
}

// +---------------------+-------------------------------------------
// | Exported Procedures |
// +---------------------+


void 
dnf (color flag[], int len)
{
  int p3 = len;
  int p1 = 0;
  int p2 = 0;
  // R, W, B, p1 >= 0, p2 >= 0
  while (p3 > p2)
    {
      // R, W, B, p1 >= 0, p2 >= 0, p3 > p2
      // R, W, B, p1 >= 0, p2 >= 0, p3 > 0
      if (WHITE == flag[p2])
        {
          // R, B, flag[p1..p2) is white
          // R, B, flag[p1..p2+1) is white
          p2 += 1;
          // We've shrunk p3-p2
        } 
      else if (RED == flag[p2])
        {
          // flag[0..p1) is red, flag[p1..p2) is white, B
          // Note: flag[p1] is white (or maybe not???)
          swap(flag,p1,p2);
          // flag[0..p1+1) is red, flag[p1+1..p2+1) is white
          p1 += 1;
          p2 += 1;
          // We've shrunk p3-p2;
        }
      else if (BLUE == flag[p2])
        {
          swap(flag, p2, p3);
          // R, W, B
          // We haven't changed the size.  Danger!  
        }
      // Option 1: flag[p3-1] is blue.  This is safe.
      if (BLUE == flag[p3-1])
        {
          // R, W, flag[p3..n) is blue
          // R, W, flag[p3-1..n) is blue
          p3 -= 1;
          // R, W, B
          // We've shrunk p3-p2
        } 
    } //
} // dnf

void 
printflag (color flag[], int len)
{
  for (int i = 0; i < len; i++)
    {
      switch (flag[i])
        {
          case RED: printf ("R"); break;
          case WHITE: printf ("W"); break;
          case BLUE: printf ("B"); break;
          default: printf ("?"); break;
        } // switch
    } // for
  printf ("\n");
} // printflag