exprevalc.nc

nesC写的heed算法

// $Id: ExprEvalC.nc,v 1.2.12.3 2003/08/26 09:08:14 cssharp Exp $

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/** Expression evaluator that allows for more complicated select and aggregate expressions.
 *  For example 	"select avg(s.temp * 2) from sensors as s
 *		 where s.temp / 5 > 25
 *		 group by s.light >> 2"
 *
 *  Author:  Kyle Stanek 
 *           Designed by Kyle Stanek and Sam Madden
 *  Created on July 15, 2002 at 3:52 PM
 * @author Kyle Stanek
 * @author Designed by Kyle Stanek and Sam Madden
 */ 

module ExprEvalC {
  provides {
    interface ExprEval;
  }
}

implementation
{
  // Prototype for local utility function
  short evaluateSimpleExpr(short a, short op, short b);
  
  command short ExprEval.evaluate(Expr *e, short fieldValue) {
    short fieldOp = e->fieldOp;
    short fieldConst = e->fieldConst;

    return evaluateSimpleExpr(fieldValue, fieldOp, fieldConst);
  }
  
  command short ExprEval.evaluateGroupBy(AggregateExpression a, short grpByFieldValue) {
    short grpByFieldOp = a.groupFieldOp;
    short grpByFieldConst = a.groupFieldConst;

    return evaluateSimpleExpr(grpByFieldValue, grpByFieldOp, grpByFieldConst);
  }

  command short ExprEval.evaluateSimple(short value, short op, short constVal) {
	return evaluateSimpleExpr(value,op,constVal);
  }
  
  //returns c, where c = a op b
  short evaluateSimpleExpr(short a, short op, short b) {
    
    //  Op codes are defined in TinyDB.h
    switch(op) { 
    case FOP_NOOP: //no op
      break;
    case FOP_TIMES:
      return (a * b); 
    case FOP_DIVIDE: 
      return (a / b); 
    case FOP_ADD: 
      return (a + b); 
    case FOP_SUBTRACT: 
      return (a - b); 
    case FOP_MOD: 
      return (a % b); 
    case FOP_RSHIFT: 
      return (a >> b); 
    }
    
    return a;
  }
}