VariantSelector.java

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package com.sun.jersey.server.impl;

import com.sun.jersey.core.header.AcceptableLanguageTag;
import com.sun.jersey.core.header.AcceptableMediaType;
import com.sun.jersey.core.header.AcceptableToken;
import com.sun.jersey.core.header.QualityFactor;
import com.sun.jersey.core.header.QualitySourceMediaType;
import com.sun.jersey.server.impl.model.HttpHelper;
import com.sun.jersey.spi.container.ContainerRequest;
import java.util.HashSet;
import java.util.Iterator;
import java.util.LinkedList;
import java.util.List;
import java.util.Locale;
import java.util.Set;
import javax.ws.rs.core.HttpHeaders;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Variant;

/**
 *
 * @author Paul.Sandoz@Sun.Com
 */
public final class VariantSelector {
    
    private VariantSelector() { }

    /**
     * Interface to get a dimension value from a variant and check if an
     * acceptable dimension value is compatible with a dimension value.
     * 
     * @param T the acceptable dimension value type
     * @param U the dimension value type
     */
    private static interface DimensionChecker<T, U> {
        /**
         * Get the dimension value from the variant
         * 
         * @param v the variant
         * @return the dimension value
         */
        U getDimension(VariantHolder v);
        
        /**
         * Get the quality source of the dimension
         * 
         * @return
         */
        int getQualitySource(VariantHolder v, U u);

        /**
         * Ascertain if the acceptable dimension value is compatible with
         * the dimension value
         * 
         * @param t the acceptable dimension value
         * @param u the dimension value
         * @return true if the acceptable dimension value is compatible with
         *         the dimension value
         */
        boolean isCompatible(T t, U u);

        String getVaryHeaderValue();
    }
    
    private static final DimensionChecker<AcceptableMediaType, MediaType> MEDIA_TYPE_DC = 
            new DimensionChecker<AcceptableMediaType, MediaType>() {
        public MediaType getDimension(VariantHolder v) {
            return v.v.getMediaType();
        }

        public boolean isCompatible(AcceptableMediaType t, MediaType u) {
            return t.isCompatible(u);
        }

        public int getQualitySource(VariantHolder v, MediaType u) {
            return v.mediaTypeQs;
        }

        public String getVaryHeaderValue() {
            return HttpHeaders.ACCEPT;
        }
    };
    
    private static final DimensionChecker<AcceptableLanguageTag, Locale> LANGUAGE_TAG_DC = 
            new DimensionChecker<AcceptableLanguageTag, Locale>() {
        public Locale getDimension(VariantHolder v) {
            return v.v.getLanguage();
        }

        public boolean isCompatible(AcceptableLanguageTag t, Locale u) {
            return t.isCompatible(u);
        }

        public int getQualitySource(VariantHolder qsv, Locale u) {
            return QualityFactor.MINUMUM_QUALITY;
        }

        public String getVaryHeaderValue() {
            return HttpHeaders.ACCEPT_LANGUAGE;
        }
    };
    
    private static final DimensionChecker<AcceptableToken, String> CHARSET_DC = 
            new DimensionChecker<AcceptableToken, String>() {
        public String getDimension(VariantHolder v) {
            MediaType m = v.v.getMediaType();
            return (m != null) ? m.getParameters().get("charset") : null;
        }

        public boolean isCompatible(AcceptableToken t, String u) {
            return t.isCompatible(u);
        }        

        public int getQualitySource(VariantHolder qsv, String u) {
            return QualityFactor.MINUMUM_QUALITY;
        }

        public String getVaryHeaderValue() {
            return HttpHeaders.ACCEPT_CHARSET;
        }
    };
    
    private static final DimensionChecker<AcceptableToken, String> ENCODING_DC = 
            new DimensionChecker<AcceptableToken, String>() {
        public String getDimension(VariantHolder v) {
            return v.v.getEncoding();
        }

        public boolean isCompatible(AcceptableToken t, String u) {
            return t.isCompatible(u);
        }        

        public int getQualitySource(VariantHolder qsv, String u) {
            return QualityFactor.MINUMUM_QUALITY;
        }

        public String getVaryHeaderValue() {
            return HttpHeaders.ACCEPT_ENCODING;
        }
    };
    
    /**
     * Select variants for a given dimension. 
     * 
     * @param the collection of variants.
     * 
     * @param as the list of acceptable dimension values, ordered by the quality
     *        parameter, with the highest quality dimension value occurring
     *        first.
     * @param dc the dimension checker
     */
    private static <T extends QualityFactor, U> LinkedList<VariantHolder> selectVariants(
            LinkedList<VariantHolder> vs,
            List<T> as,
            DimensionChecker<T, U> dc,
            Set<String> vary) {
        int cq = QualityFactor.MINUMUM_QUALITY;
        int cqs = QualityFactor.MINUMUM_QUALITY;

        final LinkedList<VariantHolder> selected = new LinkedList<VariantHolder>();

        // Iterate over the acceptable entries
        // This assumes the entries are ordered by the quality
        for (final T a : as) {
            final int q = a.getQuality();

            final Iterator<VariantHolder> iv = vs.iterator();
            while (iv.hasNext()) {
                final VariantHolder v = iv.next();

                // Get the dimension  value of the variant to check
                final U d = dc.getDimension(v);

                if (d != null) {
                    vary.add(dc.getVaryHeaderValue());
                    // Check if the acceptable entry is compatable with
                    // the dimension value
                    final int qs = dc.getQualitySource(v, d);
                    if (qs >= cqs && dc.isCompatible(a, d)) {
                        if (qs > cqs) {
                            cqs = qs;
                            cq = q;
                            // Remove all entries that were added for qs < cqs
                            selected.clear();
                            selected.add(v);
                        } else if (q > cq) {
                            cq = q;
                            // Add variant with higher accept quality at the front
                            selected.addFirst(v);
                        } else if (q == cq) {
                            // Ensure selection is stable with order of variants
                            // with same quality of source and accept quality
                            selected.add(v);
                        }
                        iv.remove();
                    }
                }
            }
        }

        // Add all variants that are not compatible with this dimension
        // to the end
        for (VariantHolder v : vs) {
            if (dc.getDimension(v) == null)
                selected.add(v);
        }
        return selected;
    }

    private static class VariantHolder {
        private final Variant v;
        
        private final int mediaTypeQs;

        public VariantHolder(Variant v) {
            this(v, QualitySourceMediaType.DEFAULT_QUALITY_SOURCE_FACTOR);
        }

        public VariantHolder(Variant v, int mediaTypeQs) {
            this.v = v;
            this.mediaTypeQs = mediaTypeQs;
        }
    }

    private static LinkedList<VariantHolder> getVariantHolderList(final List<Variant> variants) {
        final LinkedList<VariantHolder> l = new LinkedList<VariantHolder>();
        for (Variant v : variants) {
            final MediaType mt = v.getMediaType();
            if (mt != null) {
                if (mt instanceof QualitySourceMediaType || mt.getParameters().
                        containsKey(QualitySourceMediaType.QUALITY_SOURCE_FACTOR)) {
                    int qs = QualitySourceMediaType.getQualitySource(mt);
                    l.add(new VariantHolder(v, qs));
                } else {
                    l.add(new VariantHolder(v));
                }
            } else {
                l.add(new VariantHolder(v));
            }
        }

        return l;
    }

    public static Variant selectVariant(ContainerRequest r, List<Variant> variants) {
        LinkedList<VariantHolder> vhs = getVariantHolderList(variants);

        Set<String> vary = new HashSet<String>();
        vhs = selectVariants(vhs, HttpHelper.getAccept(r), MEDIA_TYPE_DC, vary);
        vhs = selectVariants(vhs, HttpHelper.getAcceptLanguage(r), LANGUAGE_TAG_DC, vary);
        vhs = selectVariants(vhs, HttpHelper.getAcceptCharset(r), CHARSET_DC, vary);
        vhs = selectVariants(vhs, HttpHelper.getAcceptEncoding(r), ENCODING_DC, vary);


        if (vhs.isEmpty()) {
            return null;
        } else {
            StringBuilder varyHeader = new StringBuilder();
            for (String v : vary) {
                if (varyHeader.length() > 0) {
                    varyHeader.append(',');
                }
                varyHeader.append(v);
            }
            r.getProperties().put(ContainerRequest.VARY_HEADER, varyHeader.toString());
            return vhs.iterator().next().v;
        }
    }
}