// In-class work on string hashCode() implementations 
// and on using polynomial codes to combine sequences 
// of hash code values
public class String{
  private char [] s;

  
  // O(N^2) where N is the length of the string
  public int hashCode(){
    int ret = 0;
    for(int i=0; i<s.length; i++){
      ret += s[i]*mypow(31,s.length-i+1);
    }
    return ret;    
  }
  public int mypow(int base, int pow){
    int ans = 1;
    for(int i=0; i<pow; i++){
      ans = ans*base;
    }
    return ans;
  }

  // O(N) version which manually computes powers
  public int hashCode(){
    int exp = s.length-1;
    int nx = mypow(31,exp);
    int ret = 0;
    for(int i=0; i<s.length; i++){
      ret += s[i] * nx;
      nx = nx/31;
      exp--;
    }
    return ret;
  }
  
  // The actual version of hashCode() used by java
  // is O(N) and looks somewhat different than the above.
  // It appears in the lecture slides for Hash Codes
}

// Using polynomial code for an array of integers
public static int hashCodeArray(int [] a){
  int hc = 0;
  for(int i=0; i<a.length; i++){
    hc = hc*31 + a[i];
  }
  return hc;
}


// Example of applying a polynomial code to an array of doubles
// for which hashCode() methods can be invoked.
public static int hashCodeArray(Double [] a){
  int hc = 0;
  for(int i=0; i<a.length; i++){
    hc = hc*31 + a[i].hashCode();
  }
  return hc;
}


// Use a polynomial code to compute a hashCode() for a class
// by treating its fields as a sequence of hashable values
public class Flurb{
  int a;
  boolean b;
  char [] str;
  double x;
  public int hashCode(){
    int hc = 0;
    hc = hc*31 + a;
    hc = hc*31 + (new Boolean(b)).hashCode();
    for(int i=0; i<str.length; i++){
      hc = hc*31 + str[i];
    }
    hc = hc*31 + (new Double(x)).hashCode();
    return hc;
  }
}