/*
    * Copyright 2002-2014 the original author or authors.
    *
    * 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 org.springframework.expression.spel.ast;
  
  import org.springframework.asm.Label;
  import org.springframework.asm.MethodVisitor;
  import org.springframework.expression.EvaluationException;
  import org.springframework.expression.TypedValue;
  import org.springframework.expression.spel.CodeFlow;
  import org.springframework.expression.spel.ExpressionState;
  import org.springframework.util.StringUtils;
  
  /**
   * Represents the elvis operator ?:. For an expression "a?:b" if a is not null, the value
   * of the expression is "a", if a is null then the value of the expression is "b".
   *
   * @author Andy Clement
   * @author Juergen Hoeller
   * @since 3.0
   */
  public class Elvis extends SpelNodeImpl {
  
  	public Elvis(int pos, SpelNodeImpl... args) {
  		super(pos,args);
  	}
  
  
  	/**
  	 * Evaluate the condition and if not null, return it.
  	 * If it is null, return the other value.
  	 * @param state the expression state
  	 * @throws EvaluationException if the condition does not evaluate correctly
  	 * to a boolean or there is a problem executing the chosen alternative
  	 */
  	@Override
  	public TypedValue getValueInternal(ExpressionState state) throws EvaluationException {
  		TypedValue value = this.children[0].getValueInternal(state);
  		if (!StringUtils.isEmpty(value.getValue())) {
  			return value;
  		}
  		else {
  			TypedValue result = this.children[1].getValueInternal(state);
  			computeExitTypeDescriptor();
  			return result;
  		}
  	}
  
  	@Override
  	public String toStringAST() {
  		return getChild(0).toStringAST() + " ?: " + getChild(1).toStringAST();
  	}
  
  	@Override
  	public boolean isCompilable() {
  		SpelNodeImpl condition = this.children[0];
  		SpelNodeImpl ifNullValue = this.children[1];
  		return (condition.isCompilable() && ifNullValue.isCompilable() &&
  				condition.exitTypeDescriptor != null && ifNullValue.exitTypeDescriptor != null);
  	}
  
  	@Override
  	public void generateCode(MethodVisitor mv, CodeFlow cf) {
  		// exit type descriptor can be null if both components are literal expressions
  		computeExitTypeDescriptor();
  		this.children[0].generateCode(mv, cf);
  		Label elseTarget = new Label();
  		Label endOfIf = new Label();
  		mv.visitInsn(DUP);
  		mv.visitJumpInsn(IFNULL, elseTarget);
  		mv.visitJumpInsn(GOTO, endOfIf);
  		mv.visitLabel(elseTarget);
  		mv.visitInsn(POP);
  		this.children[1].generateCode(mv, cf);
  		if (!CodeFlow.isPrimitive(this.exitTypeDescriptor)) {
  			CodeFlow.insertBoxIfNecessary(mv, cf.lastDescriptor().charAt(0));
  		}
  		mv.visitLabel(endOfIf);
  		cf.pushDescriptor(this.exitTypeDescriptor);
  	}
  
  	private void computeExitTypeDescriptor() {
  		if (this.exitTypeDescriptor == null && this.children[0].exitTypeDescriptor != null &&
  				this.children[1].exitTypeDescriptor != null) {
  			String conditionDescriptor = this.children[0].exitTypeDescriptor;
  			String ifNullValueDescriptor = this.children[1].exitTypeDescriptor;
 			if (conditionDescriptor.equals(ifNullValueDescriptor)) {
 				this.exitTypeDescriptor = conditionDescriptor;
 			}
 			else if (conditionDescriptor.equals("Ljava/lang/Object") && !CodeFlow.isPrimitive(ifNullValueDescriptor)) {
 				this.exitTypeDescriptor = ifNullValueDescriptor;
 			}
 			else if (ifNullValueDescriptor.equals("Ljava/lang/Object") && !CodeFlow.isPrimitive(conditionDescriptor)) {
 				this.exitTypeDescriptor = conditionDescriptor;
 			}
 			else {
 				// Use the easiest to compute common super type
 				this.exitTypeDescriptor = "Ljava/lang/Object";
 			}
 		}
 	}
 
 }