quadratic.java

openmap java写的开源数字地图程序. 用applet实现,可以像google map 那样放大缩小地图.

/**
 * Copyright (C) BBNT Solutions LLC.  All Rights Reserved
 * $Id: Quadratic.java,v 1.1.2.1 2005/12/09 20:58:48 dietrick Exp $
 */

package com.bbn.openmap.geo;

/**
   Computes 0 to 2 real roots of the quadratic:
   <pre>
   a*x^2 + b*x + c = 0
   </pre>
   See http://web.cs.mun.ca/courses/cs2710-w02/lab5/assignment/assign5.html.
   http://www.1728.com/quadratc.htm
   <p>
   Access the roots as a double iterator with methods hasNext() and next().
 */

public class Quadratic {
  double a, b, c;
  double[] solution;
  int index = 0;
  
  public Quadratic (double a, double b, double c) {
    this.a = a;
    this.b = b;
    this.c = c;
    solve();
  }

  /** Returns the solution, x, to the equation a*x+b = 0 **/
  public static double linear (double a, double b) { return -b/a; }

  /** Solve a*x^2 + b*x + c = 0 for x for real values of x.
   * <p>
   *  @see Winston and Horn, LISP p. 167-169. 
   */
  public void solve () {
    if (a == 0.0 && b == 0.0) values();
    else if (a == 0.0 && b != 0.0) values(linear(b, c));
    else if (a != 0.0 && c == 0.0) values(0.0, linear(a, b));
    else {
      double d = b*b - 4.0*a*c;
      if (d < 0.0) values();	// Complex roots.
      else if (d == 0.0)values(-b/(2.0*a));
      else values((-b + Math.sqrt(d))/(2.0*a), (-b - Math.sqrt(d))/(2.0*a));
    }
  }

  /** Evaluate the Quadratic at x. **/
  public double eval (double x) { return (a*x + b)*x + c; }

  public boolean hasNext () { 
    return index < solution.length; 
  }

  public double next () {
    if (this.hasNext()) return solution[index++]; 
    else throw new java.util.NoSuchElementException();
  }
  
  public String toString () {
    return "{" + a + "*x^2 + " + b + "*x + " + c + " = 0 [" + solutionToString() + "]}";
  }

  private String solutionToString () {
    switch (solution.length) {
    case 0: return "";
    case 1: return String.valueOf(solution[0]);
    case 2: return String.valueOf(solution[0]) + " " + String.valueOf(solution[1]);
    default: return "";		// Can't happen.
    }
  }
      
  private void values () { solution = new double[0]; }
  private void values (double s1) { solution = new double[] {s1}; }
  private void values (double s1, double s2) {
    solution = new double[] { s1, s2 };
  }
  
  public String test () {
    String s = this + " test: ";
    switch (solution.length) {
    case 0: return "";
    case 1: return s + this.eval(solution[0]);
    case 2: return s + this.eval(solution[0]) + " " + this.eval(solution[1]);
    default: return "";   // Can't happen.
    }
  }

  static private void p(double a, double b, double c) {
    System.out.println((new Quadratic(a, b, c)).test());
  }
  
  public static void main (String[] args) {
    p(0.0, 0.0, -30.0);
    p(0.0, 4.0, 0.0);
    p(0.0, 4.0, -30.0);
    p(2.0, 4.0, 0.0);
    p(2.0, 4.0, -30.0);
    p(1.0, 4.0, 1.0);
  }
}