Druid/src/main/java/com/alibaba/druid/sql/parser/SymbolTable.java

/*
 * Copyright 1999-2011 Alibaba Group Holding Ltd.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.druid.sql.parser;

/**
 * @author wenshao<szujobs@hotmail.com>
 */
public class SymbolTable {

    //
    // Constants
    //

    /** Default table size. */
    public static final int DEFAULT_TABLE_SIZE = 128;

    /** Buckets. */
    private final Entry[]   buckets;
    private final String[]  symbols;
    private final char[][]  symbols_char;

    // actual table size
    private final int       indexMask;

    //
    // Constructors
    //

    /** Constructs a symbol table with a default number of buckets. */
    public SymbolTable(){
        this(DEFAULT_TABLE_SIZE);
    }

    /** Constructs a symbol table with a specified number of buckets. */
    public SymbolTable(int tableSize){
        this.indexMask = tableSize - 1;
        this.buckets = new Entry[tableSize];
        this.symbols = new String[tableSize];
        this.symbols_char = new char[tableSize][];
    }

    //
    // Public methods
    //

    /**
     * Adds the specified symbol to the symbol table and returns a reference to the unique symbol. If the symbol already
     * exists, the previous symbol reference is returned instead, in order guarantee that symbol references remain
     * unique.
     * 
     * @param symbol The new symbol.
     */
    public String addSymbol(String symbol) {
        return addSymbol(symbol.toCharArray(), 0, symbol.length(), symbol.hashCode());
    }

    public int getIndexMask() {
        return indexMask;
    }

    public String addSymbol(char[] buffer, int offset, int len) {
        // search for identical symbol
        int hash = hash(buffer, offset, len);
        return addSymbol(buffer, offset, len, hash);
    }

    /**
     * Adds the specified symbol to the symbol table and returns a reference to the unique symbol. If the symbol already
     * exists, the previous symbol reference is returned instead, in order guarantee that symbol references remain
     * unique.
     * 
     * @param buffer The buffer containing the new symbol.
     * @param offset The offset into the buffer of the new symbol.
     * @param len The length of the new symbol in the buffer.
     */
    public String addSymbol(char[] buffer, int offset, int len, int hash) {

        // int bucket = indexFor(hash, tableSize);
        final int bucket = hash & indexMask;

        String sym = symbols[bucket];

        boolean match = true;

        if (sym != null) {
            if (sym.length() == len) {
                char[] characters = symbols_char[bucket];

                for (int i = 0; i < len; i++) {
                    if (buffer[offset + i] != characters[i]) {
                        match = false;
                        break;
                    }
                }

                if (match) {
                    return sym;
                }
            } else {
                match = false;
            }
        }

        OUTER: for (Entry entry = buckets[bucket]; entry != null; entry = entry.next) {
            char[] characters = entry.characters;
            if (len == characters.length && hash == entry.hashCode) {
                for (int i = 0; i < len; i++) {
                    if (buffer[offset + i] != characters[i]) {
                        continue OUTER;
                    }
                }
                return entry.symbol;
            }
        }

        // add new entry
        Entry entry = new Entry(buffer, offset, len, hash, buckets[bucket]);
        buckets[bucket] = entry;
        if (match) {
            symbols[bucket] = entry.symbol;
            symbols_char[bucket] = entry.characters;
        }
        return entry.symbol;

    } // addSymbol(char[],int,int):String

    public static final int hash(String symbol) {
        return hash(symbol.toCharArray(), 0, symbol.length());
    }

    // same as String.hashCode
    public static final int hash(char[] buffer, int offset, int len) {
        int h = 0;
        int off = offset;

        for (int i = 0; i < len; i++) {
            h = 31 * h + buffer[off++];
        }
        return h;
    }

    //
    // Classes
    //

    /**
     * This class is a symbol table entry. Each entry acts as a node in a linked list.
     */
    protected static final class Entry {

        public final String symbol;
        public final int    hashCode;

        public final char[] characters;

        /** The next entry. */
        public Entry        next;

        //
        // Constructors
        //

        /**
         * Constructs a new entry from the specified symbol information and next entry reference.
         */
        public Entry(char[] ch, int offset, int length, int hash, Entry next){
            characters = new char[length];
            System.arraycopy(ch, offset, characters, 0, length);
            symbol = new String(characters).intern();
            this.next = next;
            this.hashCode = hash;
        }

    } // class Entry

} // class SymbolTable