package com.meidusa.amoeba.sqljep.function;

import com.meidusa.amoeba.sqljep.ASTFunNode;
import com.meidusa.amoeba.sqljep.JepRuntime;
import com.meidusa.amoeba.sqljep.ParseException;

/**
 * 针对String的hash算法
 * 
 * @author hexianmao
 * @version 2008-10-27 下午05:49:02
 */
public class StringHash extends PostfixCommand {

    private static final int  _hash_len = 8;
    private static final int  _bit_len  = 5;

    private static final long _unknown  = -1L;

    public int getNumberOfParameters() {
        return 1;
    }

    public Comparable<?>[] evaluate(ASTFunNode node, JepRuntime runtime) throws ParseException {
        node.childrenAccept(runtime.ev, null);
        Comparable<?> param = runtime.stack.pop();
        return new Comparable<?>[] { param };
    }

    public Comparable<?> getResult(Comparable<?>... comparables) throws ParseException {
        if (comparables[0] != null && comparables[0] instanceof String) {
            return hash((String) comparables[0]);
        }
        return _unknown;
    }

    /**
     * <pre>
     * 字符串hash算法：s[0]*31ˆ(n-1) + s[1]*31ˆ(n-2) + ... + s[n-1]
     * 其中s[]为字符串的字符数组，换算成程序的表达式为：
     * h = 31*h + s.charAt(i); => h = (h << 5) - h + s.charAt(i);
     * 注：对hash的字符串做了长度限定，保证其结果不会超出数据类型的范围。
     * </pre>
     */
    private static long hash(String s) {
        long h = 0;
        for (int i = 0; (i < _hash_len && i < s.length()); i++) {
            h = (h << _bit_len) - h + s.charAt(i);
        }
        return h;
    }
}