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Java utilities (core)
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package com.dua3.utility.xml;

import com.dua3.cabe.annotations.Nullable;

import javax.xml.namespace.NamespaceContext;
import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Optional;

/**
 * A simple {@link NamespaceContext} implementation that automatically creates the reverse mapping from URI to
 * namespace. When a default namespace is set via "xmlns=..." it will be associated with the namespace name "ns"
 * if that is not already in use. Otherwise, the next free namespace name of the form "ns#number" is used.
 * Whether a default namespace has been set and what name it uses can be queried via {@link #getDefaultPrefix()}.
 */
public class SimpleNamespaceContext implements NamespaceContext {
    private final HashMap nsToUri;
    private final HashMap> uriToNs;
    private final String defaultPrefix;

    /**
     * Construct new instance with mappings.
     *
     * @param nsToUriMapping mapping from namespace to URI
     */
    public SimpleNamespaceContext(Map nsToUriMapping) {
        this(nsToUriMapping, null);
    }

    /**
     * Construct new instance with mappings and default namespace.
     *
     * @param defaultUri URI for the default namespace
     */
    public SimpleNamespaceContext(@Nullable String defaultUri) {
        this(Collections.emptyMap(), defaultUri);
    }

    /**
     * Construct new instance with mappings and default namespace.
     *
     * @param nsToUriMapping mapping from namespace to URI
     * @param defaultUri     URI for the default namespace
     */
    public SimpleNamespaceContext(Map nsToUriMapping, @Nullable String defaultUri) {
        this.nsToUri = new HashMap<>(nsToUriMapping);

        // determine a namespace name for the default namespace
        if (defaultUri != null) {
            String prefix = "ns";
            for (int i = 1; nsToUri.containsKey(prefix); i++) {
                //noinspection StringConcatenationMissingWhitespace
                prefix = "ns" + i;
            }
            this.defaultPrefix = prefix;
            nsToUri.put(defaultPrefix, defaultUri);
        } else {
            this.defaultPrefix = null;
        }

        // create a map reverse mappings
        this.uriToNs = new HashMap<>();
        nsToUri.forEach((k, v) -> uriToNs.computeIfAbsent(v, k_ -> new ArrayList<>()).add(k));
    }

    @Override
    public String getNamespaceURI(String prefix) {
        return nsToUri.get(prefix);
    }

    @Override
    public String getPrefix(String namespaceURI) {
        List namespaces = uriToNs.get(namespaceURI);
        return namespaces == null || namespaces.isEmpty() ? null : namespaces.get(0);
    }

    @Override
    public Iterator getPrefixes(String namespaceURI) {
        return Collections.unmodifiableList(uriToNs.getOrDefault(namespaceURI, Collections.emptyList())).iterator();
    }

    /**
     * Get prefix for default namespace.
     *
     * @return the default namespace prefix (optional)
     */
    public Optional getDefaultPrefix() {
        return Optional.ofNullable(defaultPrefix);
    }

    /**
     * Returns the list of prefixes currently stored in the mapping.
     *
     * @return a list containing all prefixes in the mapping
     */
    public List getPrefixes() {
        return new ArrayList<>(nsToUri.keySet());
    }

    /**
     * Retrieves a list of all the namespace URIs currently stored in the mapping.
     *
     * @return A list of namespace URIs.
     */
    public List getNamespaceURIs() {
        return new ArrayList<>(uriToNs.keySet());
    }
}