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Google App Engine compliant variation of FreeMarker. FreeMarker is a "template engine"; a generic tool to generate text output based on templates.
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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.
 */

package freemarker.core;

import java.io.IOException;
import java.io.Writer;
import java.text.NumberFormat;
import java.util.Locale;

import freemarker.template.TemplateException;
import freemarker.template.utility.StringUtil;

/**
 * An instruction that outputs the value of a numerical expression.
 */
final class NumericalOutput extends Interpolation {

    private final Expression expression;
    private final boolean hasFormat;
    private final int minFracDigits;
    private final int maxFracDigits;
    /** For OutputFormat-based auto-escaping */
    private final MarkupOutputFormat autoEscapeOutputFormat;
    private volatile FormatHolder formatCache; // creating new NumberFormat is slow operation

    NumericalOutput(Expression expression, MarkupOutputFormat autoEscapeOutputFormat) {
        this.expression = expression;
        hasFormat = false;
        this.minFracDigits = 0;
        this.maxFracDigits = 0;
        this.autoEscapeOutputFormat = autoEscapeOutputFormat;
    }

    NumericalOutput(Expression expression,
            int minFracDigits, int maxFracDigits,
            MarkupOutputFormat autoEscapeOutputFormat) {
        this.expression = expression;
        hasFormat = true;
        this.minFracDigits = minFracDigits;
        this.maxFracDigits = maxFracDigits;
        this.autoEscapeOutputFormat = autoEscapeOutputFormat;
    }

    @Override
    TemplateElement[] accept(Environment env) throws TemplateException, IOException {
        String s = calculateInterpolatedStringOrMarkup(env);
        Writer out = env.getOut();
        if (autoEscapeOutputFormat != null) {
            autoEscapeOutputFormat.output(s, out);
        } else {
            out.write(s);
        }
        return null;
    }

    @Override
    protected String calculateInterpolatedStringOrMarkup(Environment env) throws TemplateException {
        Number num = expression.evalToNumber(env);
        
        FormatHolder fmth = formatCache;  // atomic sampling
        if (fmth == null || !fmth.locale.equals(env.getLocale())) {
            synchronized (this) {
                fmth = formatCache;
                if (fmth == null || !fmth.locale.equals(env.getLocale())) {
                    NumberFormat fmt = NumberFormat.getNumberInstance(env.getLocale());
                    if (hasFormat) {
                        fmt.setMinimumFractionDigits(minFracDigits);
                        fmt.setMaximumFractionDigits(maxFracDigits);
                    } else {
                        fmt.setMinimumFractionDigits(0);
                        fmt.setMaximumFractionDigits(50);
                    }
                    fmt.setGroupingUsed(false);
                    formatCache = new FormatHolder(fmt, env.getLocale());
                    fmth = formatCache;
                }
            }
        }
        // We must use Format even if hasFormat == false.
        // Some locales may use non-Arabic digits, thus replacing the
        // decimal separator in the result of toString() is not enough.
        String s = fmth.format.format(num);
        return s;
    }

    @Override
    protected String dump(boolean canonical, boolean inStringLiteral) {
        StringBuilder buf = new StringBuilder("#{");
        final String exprCF = expression.getCanonicalForm();
        buf.append(inStringLiteral ? StringUtil.FTLStringLiteralEnc(exprCF, '"') : exprCF);
        if (hasFormat) {
            buf.append(" ; ");
            buf.append("m");
            buf.append(minFracDigits);
            buf.append("M");
            buf.append(maxFracDigits);
        }
        buf.append("}");
        return buf.toString();
    }
    
    @Override
    String getNodeTypeSymbol() {
        return "#{...}";
    }

    @Override
    boolean heedsOpeningWhitespace() {
        return true;
    }

    @Override
    boolean heedsTrailingWhitespace() {
        return true;
    }
    
    private static class FormatHolder {
        final NumberFormat format;
        final Locale locale;
        
        FormatHolder(NumberFormat format, Locale locale) {
            this.format = format;
            this.locale = locale;
        }
    }

    @Override
    int getParameterCount() {
        return 3;
    }

    @Override
    Object getParameterValue(int idx) {
        switch (idx) {
        case 0: return expression;
        case 1: return hasFormat ? Integer.valueOf(minFracDigits) : null;
        case 2: return hasFormat ? Integer.valueOf(maxFracDigits) : null;
        default: throw new IndexOutOfBoundsException();
        }
    }

    @Override
    ParameterRole getParameterRole(int idx) {
        switch (idx) {
        case 0: return ParameterRole.CONTENT;
        case 1: return ParameterRole.MINIMUM_DECIMALS;
        case 2: return ParameterRole.MAXIMUM_DECIMALS;
        default: throw new IndexOutOfBoundsException();
        }
    }

    @Override
    boolean isNestedBlockRepeater() {
        return false;
    }
}