Rolling Hash(Rabin-Karp 算法)

// 闲言少许，直奔主题，我是宋鹏举。

import java.io.PrintStream;

public class RollingHash
{
  private static final int R = 31;
  private int _hash;
  private int _rK;
  private CharSequence _c;
  private int _k;
  private int _offset;
  
  public RollingHash(int k)
  {
    if (k <= 0) {
      throw new IllegalArgumentException("k must be positive");
    }
    this._k = k;
    

    this._rK = 1;
    for (int i = 1; i < k; i++) {
      this._rK *= 31;
    }
  }
  
  public void init(CharSequence c)
  {
    if (this._k > c.length()) {
      throw new IllegalArgumentException("k is greater then lenght of content");
    }
    this._c = c;
    this._offset = 0;
    

    this._hash = 0;
    for (int i = 0; i < this._k; i++) {
      this._hash = (this._hash * 31 + this._c.charAt(i));
    }
  }
  
  public boolean hasNext()
  {
    if (this._offset + this._k >= this._c.length()) {
      return false;
    }
    return true;
  }
  
  public int next()
  {
    if (this._offset + this._k >= this._c.length()) {
      throw new IllegalStateException("Past the end of content.");
    }
    int currentHash = this._hash;
    
    this._hash = ((this._hash - this._rK * this._c.charAt(this._offset)) * 31 + this._c.charAt(this._offset + this._k));
    
    this._offset += 1;
    
    return currentHash;
  }
  
  public static void main(String[] args)
  {
    String text = "01234567890123456789012345678901234567890123456789testesteskdjfgdkfjghdkfjghdkfjghdkjgh";
    
    int k = 20;
    
    RollingHash rh = new RollingHash(k);
    rh.init(text);
    for (int i = 0; rh.hasNext(); i++) {
      System.out.println("Hash " + i + ": " + rh.next());
    }
  }
}