/**

 * An implementation of the REPLACE function:

 * Replaces part of a text string based on the number of characters 

 * you specify, with another text string.

 * @author Manda Wilson < wilson at c bio dot msk cc dot org >

 */

public class Replace extends TextFunction {

	/**

	 * Replaces part of a text string based on the number of characters 

	 * you specify, with another text string.

	 * 

	 * @see org.apache.poi.hssf.record.formula.eval.Eval

	 */

    public Eval evaluate(Eval[] operands, int srcCellRow, short srcCellCol) {		

    	Eval retval = null;

        String oldStr = null;

        String newStr = null;

        int startNum = 0;

        int numChars = 0;

        switch (operands.length) {

	        default:

	            retval = ErrorEval.VALUE_INVALID;

	        case 4:	        

	        	// first operand is text string containing characters to replace

	            // second operand is position of first character to replace

	            // third operand is the number of characters in the old string

	            // you want to replace with new string

	            // fourth operand is the new string

	            ValueEval firstveval = singleOperandEvaluate(operands[0], srcCellRow, srcCellCol);

	            ValueEval secondveval = singleOperandEvaluate(operands[1], srcCellRow, srcCellCol);

	            ValueEval thirdveval = singleOperandEvaluate(operands[2], srcCellRow, srcCellCol);

	            ValueEval fourthveval = singleOperandEvaluate(operands[3], srcCellRow, srcCellCol);

	            if (firstveval instanceof StringValueEval

	            	&& secondveval instanceof NumericValueEval

	            	&& thirdveval instanceof NumericValueEval

	            	&& fourthveval instanceof StringValueEval) {

	                StringValueEval oldStrEval = (StringValueEval) firstveval;

	                oldStr = oldStrEval.getStringValue();

	                NumericValueEval startNumEval = (NumericValueEval) secondveval;

	                // NOTE: it is safe to cast to int here

	                // because in Excel =REPLACE("task", 2.7, 3, "est") 

	                // returns test 

	                // so 2.7 must be truncated to 2

	                // and =REPLACE("task", 1, 1.9, "") returns ask 

	                // so 1.9 must be truncated to 1

	                startNum = (int) startNumEval.getNumberValue();

	                NumericValueEval numCharsEval = (NumericValueEval) thirdveval;

	                numChars = (int) numCharsEval.getNumberValue();

	                StringValueEval newStrEval = (StringValueEval) fourthveval;

	                newStr = newStrEval.getStringValue();

	            } else {

	            	retval = ErrorEval.VALUE_INVALID;

	            }

	    }

        if (retval == null) {

			if (startNum < 1 || numChars < 0) {

				retval = ErrorEval.VALUE_INVALID;

			} else {

				StringBuffer strBuff = new StringBuffer(oldStr);

				// remove any characters that should be replaced

				if (startNum <= oldStr.length() && numChars != 0) {

					strBuff.delete(startNum - 1, startNum - 1 + numChars);

				} 

				// now insert (or append) newStr

				if (startNum > strBuff.length()) {

					strBuff.append(newStr);

				} else {

					strBuff.insert(startNum - 1, newStr);

				}

				retval = new StringEval(strBuff.toString());

			}

        } 

		return retval;

    }