/**
 * A class to represent Gold Sequence Generators for GPS
 */

/**
 * @author Kees
 *
 */
public class GoldSequence
{
  /**
   * note: in the literature the shiftregisters
   * are numbered from left to right 1 to 10
   * we number them 0 to 9,
   * so tap n in literature, here is tap n-1
   */
  
  // the two ten-bit shiftregisters
  private int[] shreg1, shreg2;
  private int tap1, tap2;
  private int count4, temp4;
  private int[][] magnum = 
    { {2,6}, {3,7}, {4,8}, {5,9}, {1,9}, {2,10}, {1,8},
      {2,9}, {3,10},{2,3}, {3,4}, {5,6}, {6,7},  {7,8},
      {8,9}, {9,10},{1,4}, {2,5}, {3,6}, {4,7},  {5,8},
      {6,9}, {1,3}, {4,6}, {5,7}, {6,8}, {7,9},  {8,10},
      {1,6}, {2,7}, {3,8}, {4,9}, {5,10},{4,10}, {1,7},
      {2,8}, {4,10}
    };
  
  /** construct a GoldSequence */
  
  public GoldSequence(int a, int b)
  {
    tap1 = a - 1;
    tap2 = b - 1;
    shreg1 = new int[10];
    shreg2 = new int[10];
    count4 = 0;
    temp4 = 0;
    for (int i=0; i < 10; i++)
    {
      shreg1[i] = 1;
      shreg2[i] = 1;
    }
  }
  
  public GoldSequence(int n)
  { // n = 1..37
    this(0, 0);
    tap1 = magnum[n - 1][0] - 1;
    tap2 = magnum[n - 1][1] - 1;
  }
  
  public int next()
  {
    int shback1, shback2, temp1;
    int result = 0;
    // save rightmost bits
    // calculate the shiftback-values
    shback1 = shreg1[9] ^ shreg1[2];
    shback2 = shreg2[9] ^ shreg2[8] ^ shreg2[7] ^ shreg2[5] ^ shreg2[2] ^ shreg2[1];
    temp1 = shreg1[9];
    result = shreg2[tap1] ^ shreg2[tap2] ^ temp1;
    for (int i = 9; i > 0; i--)
    {
      shreg1[i] = shreg1[i - 1];
      shreg2[i] = shreg2[i - 1];
    }
    shreg1[0] = shback1;
    shreg2[0] = shback2;

    return result;
  }
  
  public void reset()
  {
    count4 = 0;
    temp4 = 0;
    for (int i=0; i < 10; i++)
    {
      shreg1[i] = 1;
      shreg2[i] = 1;      
    }
  }
  
  public int nextan()
  {
    // return analogue result +1 or -1
    int result = 0;
    result = 1 - 2 * this.next();
    return result;
  }
  
  public int nextan4()
  {
    int result = temp4;
    if (count4 == 0)
    {
      temp4 = this.nextan();
    }
    count4++;
    if (count4 == 4)
    {
      count4 = 0;
    } 
    return result;
  }
  
  public String getRegs()
  {
    String result1 = "";
    String result2 = "";
    for (int i=0; i<10; i++)
    {
      result1 = result1 + shreg1[i];
      result2 = result2 + shreg2[i];
    }
    return result1 + " " + result2;
  }
}

public void fillArray9()
    {
      GoldSequence gold26 = new GoldSequence(2,6);
      System.out.print("First 10 chips: ");
      for (int i = 0; i < 10; i++)
      {
        System.out.print(gold26.next());
      }
      System.out.println();
 
    }

public void fillArray10()
    {
      // show the sequence is repeating after 1023 chips
      
      GoldSequence gold26 = new GoldSequence(2,6);
      System.out.print("First 20 chips:  ");
      for (int i = 0; i < 20; i++)
      {
        System.out.print(gold26.next());
      }
      System.out.println();
      for (int i = 20; i < 1023; i++)
      {
        gold26.next();
      }
      System.out.print("After 1023 chips:");
      for (int i = 0; i < 20; i++)
      {
        System.out.print(gold26.next());
      }
    }

public void fillArray11()
    {
      // correlation experiments
      // construct a test-input with 4000 samples of Gold26
      // find the match-point
      
      int inplen = 4000;
      int[] testinput = new int[inplen];
      int[] bins = new int[inplen];
      
      System.out.println("experiment 11. Correlation");
      
      GoldSequence gold26 = new GoldSequence(2,6);

      // throw away first 35 chips
      for (int i = 0; i < 35; i++)
      {
        gold26.nextan();
      }
      
      for (int i = 0; i < testinput.length; i++)
      {
        testinput[i] = gold26.nextan();
      }
      
      gold26.reset();
      
      for (int i = 0; i < inplen; i++)
      {
        gold26.reset();
        int acc = 0;
        for (int j = 0; j < 1023; j++)
        {
          if ((i + j) < inplen)
          {
            acc = acc + testinput[i + j] * gold26.nextan();
          }
        }
        bins[i] = acc;
      }
      
      for (int i = 0; i < inplen; i++)
      {
        System.out.println(i + " " + bins[i]);
      }
    } 

public void fillArray12()
    {
      System.out.println("Experiment 12. Print internal regs");
      GoldSequence gold26 = new GoldSequence(2,6);      
      for (int i=0; i<1040; i++)
      {
        System.out.println(i + " " + gold26.getRegs());
        gold26.next();
      }
    }