improved sql parser.

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

@@ -0,0 +1,59 @@
+package com.alibaba.druid.sql.parser;
+
+/**
+ * @author wenshao[szujobs@hotmail.com]
+ */
+public class SymbolTable {
+    public static SymbolTable global = new SymbolTable(8192);
+
+    private final Entry[] entries;
+    private final int      indexMask;
+
+    public SymbolTable(int tableSize){
+        this.indexMask = tableSize - 1;
+        this.entries = new Entry[tableSize];
+
+//        this.addSymbol("id", 0, 2, "id".hashCode());
+    }
+
+
+
+    public String addSymbol(String buffer, int offset, int len, long hash) {
+        final int bucket = ((int) hash) & indexMask;
+
+        Entry entry = entries[bucket];
+        if (entry != null) {
+            if (hash == entry.hash //
+                    && len == entry.len) {
+                return entry.value;
+            }
+
+            String str = subString(buffer, offset, len);
+
+            return str;
+        }
+
+        String str = subString(buffer, offset, len);
+        entry = new Entry(hash, len, str);
+        entries[bucket] = entry;
+        return str;
+    }
+
+    private static String subString(String src, int offset, int len) {
+        char[] chars = new char[len];
+        src.getChars(offset, offset + len, chars, 0);
+        return new String(chars);
+    }
+
+    private static class Entry {
+        public final long hash;
+        public final int len;
+        public final String value;
+
+        public Entry(long hash, int len, String value) {
+            this.hash = hash;
+            this.len = len;
+            this.value = value;
+        }
+    }
+}