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Apache POI - Java API To Access Microsoft Format Files
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/* ====================================================================
   Licensed to the Apache Software Foundation (ASF) under one or more
   contributor license agreements.  See the NOTICE file distributed with
   this work for additional information regarding copyright ownership.
   The ASF licenses this file to You 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.

   2012 - Alfresco Software, Ltd.
   Alfresco Software has modified source of this file
   The details of changes as svn diff can be found in svn at location root/projects/3rd-party/src 
==================================================================== */
package org.apache.poi.ss.usermodel;

import java.math.BigDecimal;
import java.math.MathContext;
import java.math.RoundingMode;
import java.text.DecimalFormat;
import java.text.DecimalFormatSymbols;
import java.text.FieldPosition;
import java.text.Format;
import java.text.ParsePosition;
import java.util.Locale;

/**
 * A format that formats a double as Excel would, ignoring FieldPosition.
 * All other operations are unsupported.
 **/
public class ExcelGeneralNumberFormat extends Format {

    private static final long serialVersionUID = 1L;

    private static final MathContext TO_10_SF = new MathContext(10, RoundingMode.HALF_UP);

    private final DecimalFormatSymbols decimalSymbols;
    private final DecimalFormat integerFormat;
    private final DecimalFormat decimalFormat;
    private final DecimalFormat scientificFormat;

    public ExcelGeneralNumberFormat(final Locale locale) {
        decimalSymbols = DecimalFormatSymbols.getInstance(locale);
        scientificFormat = new DecimalFormat("0.#####E0", decimalSymbols);
        DataFormatter.setExcelStyleRoundingMode(scientificFormat);
        integerFormat = new DecimalFormat("#", decimalSymbols);
        DataFormatter.setExcelStyleRoundingMode(integerFormat);
        decimalFormat = new DecimalFormat("#.##########", decimalSymbols);
        DataFormatter.setExcelStyleRoundingMode(decimalFormat);
    }

    public StringBuffer format(Object number, StringBuffer toAppendTo, FieldPosition pos) {
        final double value;
        if (number instanceof Number) {
            value = ((Number)number).doubleValue();
            if (Double.isInfinite(value) || Double.isNaN(value)) {
                return integerFormat.format(number, toAppendTo, pos);
            }
        } else {
            // testBug54786 gets here with a date, so retain previous behaviour
            return integerFormat.format(number, toAppendTo, pos);
        }

        final double abs = Math.abs(value);
        if (abs >= 1E11 || (abs <= 1E-10 && abs > 0)) {
            return scientificFormat.format(number, toAppendTo, pos);
        } else if (Math.floor(value) == value || abs >= 1E10) {
            // integer, or integer portion uses all 11 allowed digits
            return integerFormat.format(number, toAppendTo, pos);
        }
        // Non-integers of non-scientific magnitude are formatted as "up to 11
        // numeric characters, with the decimal point counting as a numeric
        // character". We know there is a decimal point, so limit to 10 digits.
        // https://support.microsoft.com/en-us/kb/65903
        final double rounded = new BigDecimal(value).round(TO_10_SF).doubleValue();
        return decimalFormat.format(rounded, toAppendTo, pos);
    }

    public Object parseObject(String source, ParsePosition pos) {
        throw new UnsupportedOperationException();
    }

}