xml_serializer.c

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/*
   +----------------------------------------------------------------------+
   | Copyright (c) The PHP Group                                          |
   +----------------------------------------------------------------------+
   | This source file is subject to version 3.01 of the PHP license,      |
   | that is bundled with this package in the file LICENSE, and is        |
   | available through the world-wide-web at the following url:           |
   | https://www.php.net/license/3_01.txt                                 |
   | If you did not receive a copy of the PHP license and are unable to   |
   | obtain it through the world-wide-web, please send a note to          |
   | license@php.net so we can mail you a copy immediately.               |
   +----------------------------------------------------------------------+
   | Authors: Niels Dossche <nielsdos@php.net>                            |
   +----------------------------------------------------------------------+
*/
 
#ifdef HAVE_CONFIG_H
#include <config.h>
#endif
 
#include "php.h"
#if defined(HAVE_LIBXML) && defined(HAVE_DOM)
#include "xml_serializer.h"
#include "private_data.h"
#include "namespace_compat.h"
#include "serialize_common.h"
#include "internal_helpers.h"
#include <libxml/chvalid.h>
 
// TODO: implement iterative approach instead of recursive?
 
/* This file implements the XML serialization algorithm.
 * https://w3c.github.io/DOM-Parsing/#dom-xmlserializer-serializetostring (Date 2021-05-02)
 *
 * The following are spec issues that were fixed in this implementation, but are not yet fixed
 * in the spec itself:
 * https://github.com/w3c/DOM-Parsing/issues/28
 * https://github.com/w3c/DOM-Parsing/issues/29
 * https://github.com/w3c/DOM-Parsing/issues/38
 * https://github.com/w3c/DOM-Parsing/issues/43
 * https://github.com/w3c/DOM-Parsing/issues/44
 * https://github.com/w3c/DOM-Parsing/issues/45
 * https://github.com/w3c/DOM-Parsing/issues/47
 * https://github.com/w3c/DOM-Parsing/issues/50
 * https://github.com/w3c/DOM-Parsing/issues/52
 * https://github.com/w3c/DOM-Parsing/issues/59
 * https://github.com/w3c/DOM-Parsing/issues/71
 */
 
#define TRY(x) do { if (UNEXPECTED((x) < 0)) { return -1; } } while (0)
#define TRY_OR_CLEANUP(x) do { if (UNEXPECTED((x) < 0)) { goto cleanup; } } while (0)
 
#define xmlOutputBufferWriteLit(out, literal) xmlOutputBufferWrite((out), strlen("" literal), "" literal)
 
/* https://w3c.github.io/DOM-Parsing/#dfn-namespace-prefix-map
 * This associates a namespace uri with a list of possible prefixes. */
typedef struct {
    HashTable *ht;
} dom_xml_ns_prefix_map;
 
/* https://w3c.github.io/DOM-Parsing/#dfn-local-prefixes-map */
typedef struct {
    HashTable ht;
} dom_xml_local_prefix_map;
 
typedef struct {
    const xmlChar *prefix, *name;
} dom_qname_pair;
 
typedef struct dom_xml_serialize_ctx {
    xmlSaveCtxtPtr ctxt;
    xmlOutputBufferPtr out;
    php_dom_private_data *private_data;
} dom_xml_serialize_ctx;
 
static int dom_xml_serialization_algorithm(
    dom_xml_serialize_ctx *ctx,
    dom_xml_ns_prefix_map *namespace_prefix_map,
    xmlNodePtr node,
    const xmlChar *namespace,
    unsigned int *prefix_index,
    int indent,
    bool require_well_formed
);
 
static bool dom_xml_str_equals_treat_nulls_as_empty(const xmlChar *s1, const xmlChar *s2)
{
    if (s1 == s2) {
        return true;
    }
    if (s1 == NULL) {
        return s2 == NULL || *s2 == '\0';
    }
    if (s2 == NULL) {
        /* Note: at this point we know that s1 != NULL. */
        return *s1 == '\0';
    }
    return strcmp((const char *) s1, (const char *) s2) == 0;
}
 
static zend_always_inline bool dom_xml_str_equals_treat_nulls_as_nulls(const xmlChar *s1, const xmlChar *s2)
{
    if (s1 == s2) {
        return true;
    }
    if (s1 == NULL || s2 == NULL) {
        return false;
    }
    return strcmp((const char *) s1, (const char *) s2) == 0;
}
 
static zend_always_inline void dom_xml_ns_prefix_map_ctor(dom_xml_ns_prefix_map *map)
{
    ALLOC_HASHTABLE(map->ht);
    zend_hash_init(map->ht, 8, NULL, NULL, false);
}
 
static void dom_xml_ns_prefix_map_destroy(dom_xml_ns_prefix_map *map)
{
    HashTable *list;
    ZEND_HASH_MAP_FOREACH_PTR(map->ht, list) {
        if (GC_DELREF(list) == 0) {
            zval *tmp;
            ZEND_HASH_PACKED_FOREACH_VAL(list, tmp) {
                if (DOM_Z_IS_OWNED(tmp)) {
                    efree(Z_PTR_P(tmp));
                }
            } ZEND_HASH_FOREACH_END();
 
            zend_hash_destroy(list);
            efree(list);
        }
    } ZEND_HASH_FOREACH_END();
 
    zend_hash_destroy(map->ht);
    efree(map->ht);
    map->ht = NULL;
}
 
static zend_always_inline void dom_xml_ns_prefix_map_dtor(dom_xml_ns_prefix_map *map)
{
    if (GC_DELREF(map->ht) == 0) {
        dom_xml_ns_prefix_map_destroy(map);
    }
}
 
static zend_always_inline void dom_xml_ns_prefix_map_copy(dom_xml_ns_prefix_map *dst, const dom_xml_ns_prefix_map *src)
{
    dst->ht = src->ht;
    GC_ADDREF(dst->ht);
}
 
static zend_always_inline void dom_xml_local_prefix_map_ctor(dom_xml_local_prefix_map *map)
{
    zend_hash_init(&map->ht, 8, NULL, NULL, false);
}
 
static zend_always_inline void dom_xml_local_prefix_map_dtor(dom_xml_local_prefix_map *map)
{
    zend_hash_destroy(&map->ht);
}
 
static zend_always_inline void dom_xml_local_prefix_map_add(
    dom_xml_local_prefix_map *map,
    const xmlChar *prefix,
    size_t prefix_len,
    const xmlChar *ns
)
{
    ZEND_ASSERT(prefix != NULL);
    zend_hash_str_add_ptr(&map->ht, (const char *) prefix, prefix_len, (void *) ns);
}
 
static zend_always_inline const xmlChar *dom_xml_local_prefix_map_find(
    const dom_xml_local_prefix_map *map,
    const xmlChar *prefix,
    size_t prefix_len
)
{
    ZEND_ASSERT(prefix != NULL);
    return zend_hash_str_find_ptr(&map->ht, (const char *) prefix, prefix_len);
}
 
static zend_always_inline bool dom_xml_local_prefix_map_conflicts(
    const dom_xml_local_prefix_map *map,
    const xmlChar *prefix,
    size_t prefix_len,
    const xmlChar *ns
)
{
    const xmlChar *result = dom_xml_local_prefix_map_find(map, prefix, prefix_len);
    if (result == NULL) {
        return false;
    }
    return !dom_xml_str_equals_treat_nulls_as_empty(result, ns);
}
 
static zend_always_inline bool dom_xml_local_prefix_map_contains(
    const dom_xml_local_prefix_map *map,
    const xmlChar *prefix,
    size_t prefix_len
)
{
    return dom_xml_local_prefix_map_find(map, prefix, prefix_len) != NULL;
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-add */
static void dom_xml_ns_prefix_map_add(
    dom_xml_ns_prefix_map *map,
    const xmlChar *prefix,
    bool prefix_owned,
    const xmlChar *ns,
    size_t ns_length
)
{
    ZEND_ASSERT(map->ht != NULL);
    ZEND_ASSERT(prefix != NULL);
 
    if (ns == NULL) {
        ns = BAD_CAST "";
    }
 
    if (GC_REFCOUNT(map->ht) > 1) {
        GC_DELREF(map->ht);
        map->ht = zend_array_dup(map->ht);
 
        HashTable *list;
        ZEND_HASH_MAP_FOREACH_PTR(map->ht, list) {
            GC_ADDREF(list);
        } ZEND_HASH_FOREACH_END();
    }
 
    /* 1. Let candidates list be the result of retrieving a list from map where there exists a key in map
    *     that matches the value of ns
     *    or if there is no such key, then let candidates list be null. */
    HashTable *list = zend_hash_str_find_ptr(map->ht, (const char *) ns, ns_length);
 
    /* 2. If candidates list is null, then create a new list with prefix as the only item in the list,
     *    and associate that list with a new key ns in map. */
    if (list == NULL) {
        ALLOC_HASHTABLE(list);
        zend_hash_init(list, 8, NULL, NULL, false);
        zend_hash_str_add_new_ptr(map->ht, (const char *) ns, ns_length, list);
    } else if (GC_REFCOUNT(list) > 1) {
        GC_DELREF(list);
        list = zend_array_dup(list);
        zend_hash_str_update_ptr(map->ht, (const char *) ns, ns_length, list);
    }
 
    /* 3. (Otherwise), append prefix to the end of candidates list. */
    zval tmp;
    if (prefix_owned) {
        DOM_Z_OWNED(&tmp, prefix);
    } else {
        DOM_Z_UNOWNED(&tmp, prefix);
    }
    zend_hash_next_index_insert_new(list, &tmp);
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-found */
static zend_always_inline HashTable *dom_get_candidates_list(dom_xml_ns_prefix_map *map, const xmlChar *ns, size_t ns_length)
{
    ZEND_ASSERT(map->ht != NULL);
 
    /* 1. Let candidates list be the result of retrieving a list from map where there exists a key in map that matches
     *    the value of ns
     *    or if there is no such key, then let candidates list be null. */
    return zend_hash_str_find_ptr(map->ht, (const char *) ns, ns_length);
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-found */
static zend_always_inline bool dom_prefix_in_candidate_list(const HashTable *list, const xmlChar *prefix)
{
    ZEND_ASSERT(prefix != NULL);
 
    if (list == NULL) {
        return false;
    }
 
    /* 2. If the value of prefix occurs at least once in candidates list, return true, otherwise return false. */
    const char *tmp;
    ZEND_HASH_PACKED_FOREACH_PTR(list, tmp) {
        if (dom_xml_str_equals_treat_nulls_as_empty(BAD_CAST tmp, prefix)) {
            return true;
        }
    } ZEND_HASH_FOREACH_END();
 
    return false;
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-found */
static zend_always_inline bool dom_prefix_found_in_ns_prefix_map(
    dom_xml_ns_prefix_map *map,
    const xmlChar *prefix,
    const xmlChar *ns,
    size_t ns_length
)
{
    ZEND_ASSERT(ns != NULL);
    HashTable *list = dom_get_candidates_list(map, ns, ns_length);
    return dom_prefix_in_candidate_list(list, prefix);
}
 
/* Helper to get the attribute value, will return "" instead of NULL for empty values, to mimic getAttribute()'s behaviour. */
static zend_always_inline const xmlChar *dom_get_attribute_value(const xmlAttr *attr)
{
    if (attr->children == NULL) {
        return BAD_CAST "";
    }
    return attr->children->content ? attr->children->content : BAD_CAST "";
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-recording-the-namespace-information */
static const xmlChar *dom_recording_the_namespace_information(
    dom_xml_ns_prefix_map *namespace_prefix_map,
    dom_xml_local_prefix_map *local_prefixes_map,
    xmlNodePtr element
)
{
    ZEND_ASSERT(element->type == XML_ELEMENT_NODE);
 
    /* 1. Let default namespace attr value be null. */
    const xmlChar *default_namespace_attr_value = NULL;
 
    /* 2. [MAIN] For each attribute attr in element's attributes, in the order they are specified in the element's attribute list: */
    for (xmlAttrPtr attr = element->properties; attr != NULL; attr = attr->next) {
        /* Steps 2.1-2.2 fetch namespace information from the attribute, but let's defer that for simplicity to the if's body. */
 
        /* 2.3. If the attribute namespace is the XMLNS namespace, then: */
        if (php_dom_ns_is_fast((xmlNodePtr) attr, php_dom_ns_is_xmlns_magic_token)) {
            /* 2.3.1. If attribute prefix is null, then attr is a default namespace declaration.
             *        Set the default namespace attr value to attr's value and stop running these steps,
             *        returning to Main to visit the next attribute. */
            if (attr->ns->prefix == NULL) {
                default_namespace_attr_value = dom_get_attribute_value(attr);
                continue;
            }
            /* 2.3.2. Otherwise, the attribute prefix is not null and attr is a namespace prefix definition.
             *        Run the following steps: */
            else {
                /* 2.3.2.1. Let prefix definition be the value of attr's localName. */
                const xmlChar *prefix_definition = attr->name;
                ZEND_ASSERT(prefix_definition != NULL);
 
                /* 2.3.2.2. Let namespace definition be the value of attr's value. */
                const xmlChar *namespace_definition = dom_get_attribute_value(attr);
                ZEND_ASSERT(namespace_definition != NULL);
 
                /* 2.3.2.3. If namespace definition is the XML namespace, then stop running these steps,
                 *          and return to Main to visit the next attribute. */
                if (strcmp((const char *) namespace_definition, DOM_XML_NS_URI) == 0) {
                    continue;
                }
 
                /* 2.3.2.4. If namespace definition is the empty string (the declarative form of having no namespace),
                 *          then let namespace definition be null instead.
                 *          => This gets delayed until later down. */
 
                size_t namespace_definition_length = strlen((const char *) namespace_definition);
 
                /* 2.3.2.5. If prefix definition is found in map given the namespace namespace definition,
                 *          then stop running these steps, and return to Main to visit the next attribute. */
                if (dom_prefix_found_in_ns_prefix_map(namespace_prefix_map, prefix_definition, namespace_definition, namespace_definition_length)) {
                    continue;
                }
 
                /* Delayed step 2.3.2.4 */
                if (*namespace_definition == '\0') {
                    namespace_definition = NULL;
                }
 
                /* 2.3.2.6. Add the prefix prefix definition to map given namespace namespace definition. */
                dom_xml_ns_prefix_map_add(namespace_prefix_map, prefix_definition, false, namespace_definition, namespace_definition_length);
 
                /* 2.3.2.7. Add the value of prefix definition as a new key to the local prefixes map,
                 *          with the namespace definition as the key's value replacing the value of null with the empty string if applicable. */
                size_t prefix_definition_length = strlen((const char *) prefix_definition);
                namespace_definition = namespace_definition == NULL ? BAD_CAST "" : namespace_definition;
                dom_xml_local_prefix_map_add(local_prefixes_map, prefix_definition, prefix_definition_length, namespace_definition);
            }
        }
    }
 
    /* 3. Return the value of default namespace attr value. */
    return default_namespace_attr_value;
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-retrieving-a-preferred-prefix-string */
static const xmlChar *dom_retrieve_a_preferred_prefix_string(
    dom_xml_ns_prefix_map *namespace_prefix_map,
    dom_xml_local_prefix_map *local_prefixes_map,
    const xmlChar *preferred_prefix,
    const xmlChar *ns,
    size_t ns_length
)
{
    ZEND_ASSERT(namespace_prefix_map->ht != NULL);
 
    if (ns == NULL) {
        ns = BAD_CAST "";
    }
 
    /* 1. Let candidates list be the result of retrieving a list from map where there exists a key in map that matches
     *    the value of ns or if there is no such key, then stop running these steps, and return the null value. */
    HashTable *list = dom_get_candidates_list(namespace_prefix_map, ns, ns_length);
    if (list == NULL) {
        return NULL;
    }
 
    /* 2. Otherwise, for each prefix value prefix in candidates list, iterating from beginning to end: */
    const xmlChar *prefix = NULL;
    const xmlChar *last_non_conflicting_in_list = NULL;
 
    /* Reverse so that the "nearest" ns gets priority: https://github.com/w3c/DOM-Parsing/issues/45 */
    ZEND_HASH_PACKED_REVERSE_FOREACH_PTR(list, prefix) {
        ZEND_ASSERT(prefix != NULL);
 
        /* 2.1. If prefix matches preferred prefix, then stop running these steps and return prefix. */
        /* Adapted for https://github.com/w3c/DOM-Parsing/issues/45 */
        if (!dom_xml_local_prefix_map_conflicts(local_prefixes_map, prefix, strlen((const char *) prefix), ns)) {
            if (dom_xml_str_equals_treat_nulls_as_empty(preferred_prefix, prefix)) {
                return prefix;
            }
 
            if (last_non_conflicting_in_list == NULL) {
                last_non_conflicting_in_list = prefix;
            }
        }
    } ZEND_HASH_FOREACH_END();
 
    /* 2.2. If prefix is the last item in the candidates list, then stop running these steps and return prefix. */
    /* Note: previously the last item was "prefix", but we loop backwards now. */
    return last_non_conflicting_in_list;
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-generating-a-prefix */
static xmlChar *dom_xml_generate_a_prefix(
    dom_xml_ns_prefix_map *map,
    dom_xml_local_prefix_map *local_prefixes_map,
    const xmlChar *new_namespace,
    size_t new_namespace_length,
    unsigned int *prefix_index
)
{
    /* 1. Let generated prefix be the concatenation of the string "ns" and the current numerical value of prefix index. */
    char buffer[32];
    buffer[0] = 'n';
    buffer[1] = 's';
    size_t length;
    do {
        length = snprintf(buffer + 2, sizeof(buffer) - 2, "%u", *prefix_index) + 2;
 
        /* 2. Let the value of prefix index be incremented by one. */
        (*prefix_index)++;
 
        /* Loop condition is for https://github.com/w3c/DOM-Parsing/issues/44 */
    } while (dom_xml_local_prefix_map_contains(local_prefixes_map, (const xmlChar *) buffer, length));
 
    xmlChar *generated_prefix = emalloc(length + 1);
    memcpy(generated_prefix, buffer, length + 1);
 
    /* 3. Add to map the generated prefix given the new namespace namespace. */
    dom_xml_ns_prefix_map_add(map, generated_prefix, true, new_namespace, new_namespace_length);
    /* Continuation of https://github.com/w3c/DOM-Parsing/issues/44 */
    dom_xml_local_prefix_map_add(local_prefixes_map, generated_prefix, length, new_namespace);
 
    /* 4. Return the value of generated prefix. */
    return generated_prefix;
}
 
static int dom_xml_output_qname(xmlOutputBufferPtr out, const dom_qname_pair *qname)
{
    if (qname->prefix != NULL) {
        TRY(xmlOutputBufferWriteString(out, (const char *) qname->prefix));
        TRY(xmlOutputBufferWriteLit(out, ":"));
    }
    return xmlOutputBufferWriteString(out, (const char *) qname->name);
}
 
/* This is a utility method used by both
 * https://w3c.github.io/DOM-Parsing/#dfn-xml-serializing-an-element-node
 * and https://w3c.github.io/DOM-Parsing/#dfn-serializing-an-attribute-value */
static int dom_xml_common_text_serialization(xmlOutputBufferPtr out, const char *content, bool attribute_mode)
{
    if (content == NULL) {
        return 0;
    }
 
    const char *last_output = content;
    const char *mask = attribute_mode ? "&<>\"\t\n\r" : "&<>";
 
    while (true) {
        size_t chunk_length = strcspn(content, mask);
 
        content += chunk_length;
        if (*content == '\0') {
            break;
        }
 
        TRY(xmlOutputBufferWrite(out, content - last_output, last_output));
 
        switch (*content) {
            case '&': {
                TRY(xmlOutputBufferWriteLit(out, "&amp;"));
                break;
            }
 
            case '<': {
                TRY(xmlOutputBufferWriteLit(out, "&lt;"));
                break;
            }
 
            case '>': {
                TRY(xmlOutputBufferWriteLit(out, "&gt;"));
                break;
            }
 
            case '"': {
                TRY(xmlOutputBufferWriteLit(out, "&quot;"));
                break;
            }
 
            /* The following three are added to address https://github.com/w3c/DOM-Parsing/issues/59 */
 
            case '\t': {
                TRY(xmlOutputBufferWriteLit(out, "&#9;"));
                break;
            }
 
            case '\n': {
                TRY(xmlOutputBufferWriteLit(out, "&#10;"));
                break;
            }
 
            case '\r': {
                TRY(xmlOutputBufferWriteLit(out, "&#13;"));
                break;
            }
        }
 
        content++;
        last_output = content;
    }
 
    return xmlOutputBufferWrite(out, content - last_output, last_output);
}
 
static int dom_xml_check_char_production(const xmlChar *content)
{
    // TODO: optimization idea: fast-pass for ASCII-only data
 
    const xmlChar *ptr = content;
    while (*ptr != '\0') {
        int len = 4;
        int c = xmlGetUTF8Char(ptr, &len);
        if (c < 0 || !xmlIsCharQ(c)) {
            return -1;
        }
        ptr += len;
    }
 
    return 0;
}
 
/* https://w3c.github.io/DOM-Parsing/#xml-serializing-a-text-node */
static zend_always_inline int dom_xml_serialize_text_node(xmlOutputBufferPtr out, xmlNodePtr text, bool require_well_formed)
{
    /* 1. If the require well-formed flag is set and node's data contains characters that are not matched by the XML Char production,
     *    then throw an exception. */
    if (require_well_formed && text->content != NULL) {
        TRY(dom_xml_check_char_production(text->content));
    }
 
    return dom_xml_common_text_serialization(out, (const char *) text->content, false);
}
 
static zend_always_inline const xmlChar *dom_xml_attribute_namespace(const xmlAttr *attr)
{
    return attr->ns == NULL ? NULL : attr->ns->href;
}
 
static int dom_xml_serialize_attribute_node_value(xmlOutputBufferPtr out, xmlAttrPtr attr)
{
    TRY(xmlOutputBufferWriteString(out, (const char *) attr->name));
    TRY(xmlOutputBufferWriteLit(out, "=\""));
    for (xmlNodePtr child = attr->children; child != NULL; child = child->next) {
        if (child->type == XML_TEXT_NODE) {
            if (child->content != NULL) {
                TRY(dom_xml_common_text_serialization(out, (const char *) child->content, true));
            }
        } else if (child->type == XML_ENTITY_REF_NODE) {
            TRY(xmlOutputBufferWriteLit(out, "&"));
            TRY(dom_xml_common_text_serialization(out, (const char *) child->name, true));
            TRY(xmlOutputBufferWriteLit(out, ";"));
        }
    }
    return xmlOutputBufferWriteLit(out, "\"");
}
 
/* These steps are from the attribute serialization algorithm's well-formed checks.
 * Note that this does not return a boolean but an int to be compatible with the TRY/TRY_CLEANUP interface
 * that we do for compatibility with libxml's interfaces. */
static zend_always_inline int dom_xml_check_xmlns_attribute_requirements(const xmlAttr *attr, const xmlChar *candidate_prefix)
{
    const xmlChar *attr_value = dom_get_attribute_value(attr);
 
    /* 3.5.2.2. If the require well-formed flag is set and the value of attr's value attribute matches the XMLNS namespace, then throw an exception */
    if (strcmp((const char *) attr_value, DOM_XMLNS_NS_URI) == 0) {
        return -1;
    }
 
    /* 3.5.2.3. If the require well-formed flag is set and the value of attr's value attribute is the empty string.
     * Errata: an "xmlns" attribute is allowed but not one with a prefix, so the idea in the spec is right but the description isn't. */
    if (*attr_value == '\0' && candidate_prefix != NULL) {
        return -1;
    }
 
    return 0;
}
 
/* Spec says to do nothing, but that's inconsistent/wrong, see https://github.com/w3c/DOM-Parsing/issues/28
 * This does not have a require_well_formed argument because the only way to get here is via saveXML(), which has it off. */
static int dom_xml_serialize_attribute_node(xmlOutputBufferPtr out, xmlNodePtr attr)
{
    if (attr->ns != NULL && attr->ns->prefix != NULL) {
        TRY(xmlOutputBufferWriteString(out, (const char *) attr->ns->prefix));
        TRY(xmlOutputBufferWriteLit(out, ":"));
    }
    return dom_xml_serialize_attribute_node_value(out, (xmlAttrPtr) attr);
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-xml-serializing-a-comment-node */
static int dom_xml_serialize_comment_node(xmlOutputBufferPtr out, xmlNodePtr comment, bool require_well_formed)
{
    /* Step 1 deals with well-formed flag */
    if (require_well_formed) {
        /* node's data contains characters that are not matched by the XML Char production or contains "--"
         * (two adjacent U+002D HYPHEN-MINUS characters) or that ends with a "-" (U+002D HYPHEN-MINUS) character,
         * then throw an exception */
        const xmlChar *ptr = comment->content;
        if (ptr != NULL) {
            TRY(dom_xml_check_char_production(ptr));
            if (strstr((const char *) ptr, "--") != NULL || ptr[strlen((const char *) ptr) - 1] == '-') {
                return -1;
            }
        }
    }
 
    TRY(xmlOutputBufferWriteLit(out, "<!--"));
    if (EXPECTED(comment->content != NULL)) {
        TRY(xmlOutputBufferWriteString(out, (const char *) comment->content));
    }
    return xmlOutputBufferWriteLit(out, "-->");
}
 
/* https://w3c.github.io/DOM-Parsing/#xml-serializing-a-processinginstruction-node */
static int dom_xml_serialize_processing_instruction(xmlOutputBufferPtr out, xmlNodePtr pi, bool require_well_formed)
{
    /* Steps 1-2 deal with well-formed flag */
    if (require_well_formed) {
        /* target contains a ":" (U+003A COLON) character or is an ASCII case-insensitive match for the string "xml", then throw an exception */
        if (strchr((const char *) pi->name, ':') != NULL || strcasecmp((const char *) pi->name, "xml") == 0) {
            return -1;
        }
 
        /* node's data contains characters that are not matched by the XML Char production or contains the string "?>"
         * (U+003F QUESTION MARK, U+003E GREATER-THAN SIGN), then throw an exception */
        if (pi->content != NULL) {
            TRY(dom_xml_check_char_production(pi->content));
            if (strstr((const char *) pi->content, "?>") != NULL) {
                return -1;
            }
        }
    }
 
    TRY(xmlOutputBufferWriteLit(out, "<?"));
    TRY(xmlOutputBufferWriteString(out, (const char *) pi->name));
    TRY(xmlOutputBufferWriteLit(out, " "));
    if (EXPECTED(pi->content != NULL)) {
        TRY(xmlOutputBufferWriteString(out, (const char *) pi->content));
    }
    return xmlOutputBufferWriteLit(out, "?>");
}
 
/* https://github.com/w3c/DOM-Parsing/issues/38
 * and https://github.com/w3c/DOM-Parsing/blob/ab8d1ac9699ed43ae6de9f4be2b0f3cfc5f3709e/index.html#L1510 */
static int dom_xml_serialize_cdata_section_node(xmlOutputBufferPtr out, xmlNodePtr cdata)
{
    TRY(xmlOutputBufferWriteLit(out, "<![CDATA["));
    if (EXPECTED(cdata->content != NULL)) {
        TRY(xmlOutputBufferWriteString(out, (const char *) cdata->content));
    }
    return xmlOutputBufferWriteLit(out, "]]>");
}
 
static zend_string *dom_xml_create_localname_set_key(const xmlAttr *attr)
{
    if (attr->ns == NULL || attr->ns->href == NULL) {
        return zend_string_init((const char *) attr->name, strlen((const char *) attr->name), false);
    }
 
    /* Spec requires us to create a tuple as a key, however HashTable doesn't support that natively.
     * Fortunately, href and name cannot have embedded NUL bytes in them, so we can create a
     * "tuple" by concatenating them against each other, separated by a \0 byte.
     */
    return zend_string_concat3(
        (const char *) attr->ns->href, strlen((const char *) attr->ns->href),
        "", 1, /* include the \0 */
        (const char *) attr->name, strlen((const char *) attr->name)
    );
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-xml-serialization-of-the-attributes */
static int dom_xml_serialize_attributes(
    xmlOutputBufferPtr out,
    xmlNodePtr element,
    dom_xml_ns_prefix_map *map,
    dom_xml_local_prefix_map *local_prefixes_map,
    unsigned int *prefix_index,
    bool ignore_namespace_definition_attribute,
    bool require_well_formed
)
{
    /* 1. Let result be the empty string.
     *    => We're going to write directly to the output buffer. */
 
    /* 2. Let localname set be a new empty namespace localname set.
     *    We can do this unconditionally even if we don't use it, because this doesn't allocate memory anyway. */
    HashTable localname_set;
    zend_hash_init(&localname_set, 8, NULL, NULL, false);
 
    /* 3. [LOOP] For each attribute attr in element's attributes, in the order they are specified in the element's attribute list: */
    for (xmlAttrPtr attr = element->properties; attr != NULL; attr = attr->next) {
        if (require_well_formed) {
            zend_string *key = dom_xml_create_localname_set_key(attr);
            /* 3.1. If the require well-formed flag is set and the localname set contains a tuple whose values match those of a
             *      new tuple consisting of attr's namespaceURI attribute and localName attribute, then throw an exception
             * 3.2. Create a new tuple consisting of attr's namespaceURI attribute and localName attribute, and add it to the localname set. */
            bool duplicate = zend_hash_add_empty_element(&localname_set, key) == NULL;
            zend_string_release_ex(key, false);
            if (duplicate) {
                goto cleanup;
            }
        }
 
        /* 3.3. Let attribute namespace be the value of attr's namespaceURI value. */
        const xmlChar *attribute_namespace = dom_xml_attribute_namespace(attr);
 
        /* 3.4. Let candidate prefix be null. */
        const xmlChar *candidate_prefix = NULL;
 
        /* 3.5. If attribute namespace is not null, then run these sub-steps: */
        if (attribute_namespace != NULL) {
            /* 3.5.1. Let candidate prefix be the result of retrieving a preferred prefix string from map
             *        given namespace attribute namespace with preferred prefix being attr's prefix value. */
            candidate_prefix = dom_retrieve_a_preferred_prefix_string(
                map,
                local_prefixes_map,
                attr->ns->prefix,
                attribute_namespace,
                strlen((const char *) attribute_namespace)
            );
 
            /* 3.5.2. If the value of attribute namespace is the XMLNS namespace, then run these steps: */
            if (php_dom_ns_is_fast((xmlNodePtr) attr, php_dom_ns_is_xmlns_magic_token)) {
                const xmlChar *attr_value = dom_get_attribute_value(attr);
 
                /* 3.5.2.1. If any of the following are true, then stop running these steps and goto Loop to visit the next attribute: */
                /* the attr's value is the XML namespace; */
                if (strcmp((const char *) attr_value, DOM_XML_NS_URI) == 0) {
                    continue;
                }
                /* the attr's prefix is null and the ignore namespace definition attribute flag is true */
                if (ignore_namespace_definition_attribute && attr->ns->prefix == NULL) {
                    /* https://github.com/w3c/DOM-Parsing/issues/47 */
                    if (!dom_xml_str_equals_treat_nulls_as_empty(element->ns == NULL ? NULL : element->ns->href, attr_value)) {
                        continue;
                    }
                }
                /* the attr's prefix is not null and either */
                if (attr->ns->prefix != NULL) {
                    /* the attr's localName is not a key contained in the local prefixes map
                     * or the attr's localName is present in the local prefixes map but the value of the key does not match attr's value
                     * and furthermore that the attr's localName (as the prefix to find) is found in the namespace prefix map
                     * given the namespace consisting of the attr's value */
                    const xmlChar *value = dom_xml_local_prefix_map_find(local_prefixes_map, attr->name, strlen((const char *) attr->name));
                    if (value == NULL || strcmp((const char *) value, (const char *) attr_value) != 0) {
                        if (dom_prefix_found_in_ns_prefix_map(map, attr->name, attr_value, strlen((const char *) attr_value))) {
                            continue;
                        }
                    }
                }
 
                /* 3.5.2.4. the attr's prefix matches the string "xmlns", then let candidate prefix be the string "xmlns". */
                if (attr->ns->prefix != NULL && strcmp((const char *) attr->ns->prefix, "xmlns") == 0) {
                    candidate_prefix = BAD_CAST "xmlns";
                }
 
                /* Errata: step 3.5.2.3 can only really be checked if we already know the candidate prefix. */
                if (require_well_formed) {
                    /* 3.5.2.2 and 3.5.2.3 are done by this call. */
                    TRY_OR_CLEANUP(dom_xml_check_xmlns_attribute_requirements(attr, candidate_prefix));
                }
            }
            /* 3.5.3. Otherwise, the attribute namespace in not the XMLNS namespace. Run these steps: */
            else if (candidate_prefix == NULL) { /* https://github.com/w3c/DOM-Parsing/issues/29 */
                /* Continuation of https://github.com/w3c/DOM-Parsing/issues/29 */
                if (attr->ns->prefix == NULL
                    || dom_xml_local_prefix_map_contains(local_prefixes_map, attr->ns->prefix, strlen((const char *) attr->ns->prefix))) {
                    /* 3.5.3.1. Let candidate prefix be the result of generating a prefix providing map,
                     *          attribute namespace, and prefix index as input. */
                    candidate_prefix = dom_xml_generate_a_prefix(
                        map,
                        local_prefixes_map,
                        attribute_namespace,
                        strlen((const char *) attribute_namespace),
                        prefix_index
                    );
                } else {
                    candidate_prefix = attr->ns->prefix;
                    /* Continuation of https://github.com/w3c/DOM-Parsing/issues/29 */
                    dom_xml_ns_prefix_map_add(
                        map,
                        candidate_prefix,
                        false,
                        attribute_namespace,
                        strlen((const char *) attribute_namespace)
                    );
                    dom_xml_local_prefix_map_add(
                        local_prefixes_map,
                        candidate_prefix,
                        strlen((const char *) candidate_prefix),
                        attribute_namespace
                    );
                }
 
                /* 3.5.3.2. Append the following to result, in the order listed: */
                TRY_OR_CLEANUP(xmlOutputBufferWriteLit(out, " xmlns:"));
                TRY_OR_CLEANUP(xmlOutputBufferWriteString(out, (const char *) candidate_prefix));
                TRY_OR_CLEANUP(xmlOutputBufferWriteLit(out, "=\""));
                TRY_OR_CLEANUP(dom_xml_common_text_serialization(out, (const char *) attribute_namespace, true));
                TRY_OR_CLEANUP(xmlOutputBufferWriteLit(out, "\""));
            }
        }
 
        /* 3.6. Append a " " (U+0020 SPACE) to result. */
        TRY_OR_CLEANUP(xmlOutputBufferWriteLit(out, " "));
 
        /* 3.7. If candidate prefix is not null, then append to result the concatenation of candidate prefix with ":" (U+003A COLON). */
        if (candidate_prefix != NULL) {
            TRY_OR_CLEANUP(xmlOutputBufferWriteString(out, (const char *) candidate_prefix));
            TRY_OR_CLEANUP(xmlOutputBufferWriteLit(out, ":"));
        }
 
        if (require_well_formed) {
            /* 3.8. If the require well-formed flag is set and
             *      this attr's localName attribute contains the character ":" (U+003A COLON)
             *      or does not match the XML Name production
             *      or equals "xmlns" and attribute namespace is null */
            if (xmlValidateNCName(attr->name, /* space */ 0) != 0
                || (strcmp((const char *) attr->name, "xmlns") == 0 && dom_xml_attribute_namespace(attr) == NULL)) {
                goto cleanup;
            }
        }
 
        /* 3.9. Append the following strings to result, in the order listed: */
        TRY_OR_CLEANUP(dom_xml_serialize_attribute_node_value(out, attr));
    }
 
    /* 4. Return the value of result.
     *    => We're writing directly to the output buffer. */
 
    zend_hash_destroy(&localname_set);
    return 0;
 
cleanup:
    zend_hash_destroy(&localname_set);
    return -1;
}
 
/* Only format output if there are no text/entityrefs/cdata nodes as children. */
static bool dom_xml_should_format_element(xmlNodePtr element)
{
    xmlNodePtr child = element->children;
    ZEND_ASSERT(child != NULL);
    do {
        if (child->type == XML_TEXT_NODE || child->type == XML_ENTITY_REF_NODE || child->type == XML_CDATA_SECTION_NODE) {
            return false;
        }
        child = child->next;
    } while (child != NULL);
    return true;
}
 
static int dom_xml_output_indents(xmlOutputBufferPtr out, int indent)
{
    TRY(xmlOutputBufferWriteLit(out, "\n"));
    for (int i = 0; i < indent; i++) {
        TRY(xmlOutputBufferWriteLit(out, "  "));
    }
    return 0;
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-xml-serializing-an-element-node */
static int dom_xml_serialize_element_node(
    dom_xml_serialize_ctx *ctx,
    const xmlChar *namespace,
    dom_xml_ns_prefix_map *namespace_prefix_map,
    xmlNodePtr element,
    unsigned int *prefix_index,
    int indent,
    bool require_well_formed
)
{
    /* 1. If the require well-formed flag is set and this node's localName attribute contains
     *    the character ":" (U+003A COLON) or does not match the XML Name production, then throw an exception. */
    if (require_well_formed) {
        if (xmlValidateNCName(element->name, /* space */ 0) != 0) {
            return -1;
        }
    }
 
    bool should_format = indent >= 0 && element->children != NULL && dom_xml_should_format_element(element);
 
    /* 2. Let markup be the string "<" (U+003C LESS-THAN SIGN). */
    TRY(xmlOutputBufferWriteLit(ctx->out, "<"));
 
    /* 3. Let qualified name be an empty string.
     *    => We're going to do it a bit differently.
     *       To avoid string allocations, we're going to store the qualified name separately as prefix+name.
     *       If the prefix is NULL then the qualified name will be == name, otherwise == prefix:name. */
    dom_qname_pair qualified_name = { NULL, NULL };
 
    /* 4. Let skip end tag be a boolean flag with value false. */
    bool skip_end_tag = false;
 
    /* 5. Let ignore namespace definition attribute be a boolean flag with value false. */
    bool ignore_namespace_definition_attribute = false;
 
    /* 6. Given prefix map, copy a namespace prefix map and let map be the result. */
    dom_xml_ns_prefix_map map;
    dom_xml_ns_prefix_map_copy(&map, namespace_prefix_map);
 
    /* 7. Let local prefixes map be an empty map. */
    dom_xml_local_prefix_map local_prefixes_map;
    dom_xml_local_prefix_map_ctor(&local_prefixes_map);
 
    /* 8. Let local default namespace be the result of recording the namespace information for node given map and local prefixes map. */
    const xmlChar *local_default_namespace = dom_recording_the_namespace_information(&map, &local_prefixes_map, element);
 
    /* 9. Let inherited ns be a copy of namespace. */
    const xmlChar *inherited_ns = namespace;
 
    /* 10. Let ns be the value of node's namespaceURI attribute. */
    const xmlChar *const ns = element->ns == NULL ? NULL : element->ns->href;
 
    /* 11. If inherited ns is equal to ns, then: */
    if (dom_xml_str_equals_treat_nulls_as_nulls(inherited_ns, ns)) {
        /* 11.1. If local default namespace is not null, then set ignore namespace definition attribute to true. */
        if (local_default_namespace != NULL) {
            ignore_namespace_definition_attribute = true;
        }
 
        /* 11.2. If ns is the XML namespace,
         *       then append to qualified name the concatenation of the string "xml:" and the value of node's localName. */
        if (php_dom_ns_is_fast(element, php_dom_ns_is_xml_magic_token)) {
            qualified_name.prefix = BAD_CAST "xml";
            qualified_name.name = element->name;
        }
        /* 11.3. Otherwise, append to qualified name the value of node's localName. */
        else {
            qualified_name.name = element->name;
        }
 
        /* 11.4. Append the value of qualified name to markup. */
        TRY_OR_CLEANUP(dom_xml_output_qname(ctx->out, &qualified_name));
    }
    /* 12. Otherwise, inherited ns is not equal to ns */
    else {
        /* 12.1. Let prefix be the value of node's prefix attribute. */
        const xmlChar *prefix = element->ns == NULL ? NULL : element->ns->prefix;
 
        /* 12.2. Let candidate prefix be the result of retrieving a preferred prefix string prefix from map given namespace ns. */
        /* https://github.com/w3c/DOM-Parsing/issues/52 */
        const xmlChar *candidate_prefix;
        if (prefix == NULL && dom_xml_str_equals_treat_nulls_as_empty(ns, local_default_namespace)) {
            candidate_prefix = NULL;
        } else {
            size_t ns_length = ns == NULL ? 0 : strlen((const char *) ns);
            candidate_prefix = dom_retrieve_a_preferred_prefix_string(&map, &local_prefixes_map, prefix, ns, ns_length);
        }
 
        /* 12.3. If the value of prefix matches "xmlns", then run the following steps: */
        if (prefix != NULL && strcmp((const char *) prefix, "xmlns") == 0) {
            /* 12.3.1. If the require well-formed flag is set, then throw an error. */
            if (require_well_formed) {
                goto cleanup;
            }
 
            /* 12.3.2. Let candidate prefix be the value of prefix. */
            candidate_prefix = prefix;
        }
 
        /* 12.4. if candidate prefix is not null (a namespace prefix is defined which maps to ns), then: */
        if (candidate_prefix != NULL) {
            /* 12.4.1. Append to qualified name the concatenation of candidate prefix, ":" (U+003A COLON), and node's localName. */
            qualified_name.prefix = candidate_prefix;
            qualified_name.name = element->name;
 
            /* 12.4.2. If the local default namespace is not null (there exists a locally-defined default namespace declaration attribute)
             *         and its value is not the XML namespace ... */
            if (local_default_namespace != NULL && strcmp((const char *) local_default_namespace, DOM_XML_NS_URI) != 0) {
                if (*local_default_namespace == '\0') {
                    inherited_ns = NULL;
                } else {
                    inherited_ns = local_default_namespace;
                }
            }
 
            /* 12.4.3. Append the value of qualified name to markup. */
            TRY_OR_CLEANUP(dom_xml_output_qname(ctx->out, &qualified_name));
        }
        /* 12.5. Otherwise, if prefix is not null, then: */
        else if (prefix != NULL) {
            size_t ns_length = ns == NULL ? 0 : strlen((const char *) ns);
 
            /* 12.5.1. If the local prefixes map contains a key matching prefix, ... */
            size_t prefix_length = strlen((const char *) prefix);
            if (dom_xml_local_prefix_map_contains(&local_prefixes_map, prefix, prefix_length)) {
                prefix = dom_xml_generate_a_prefix(&map, &local_prefixes_map, ns, ns_length, prefix_index);
            } else { /* else branch fixes spec issue: generating a prefix already adds it to the maps. */
                /* 12.5.2. Add prefix to map given namespace ns. */
                dom_xml_ns_prefix_map_add(&map, prefix, false, ns, ns_length);
                /* This is not spelled out in spec, but we have to do this to avoid conflicts (see default_namespace_move.phpt). */
                dom_xml_local_prefix_map_add(&local_prefixes_map, prefix, prefix_length, ns);
            }
 
            /* 12.5.3. Append to qualified name the concatenation of prefix, ":" (U+003A COLON), and node's localName. */
            qualified_name.prefix = prefix;
            qualified_name.name = element->name;
 
            /* 12.5.4. Append the value of qualified name to markup. */
            TRY_OR_CLEANUP(dom_xml_output_qname(ctx->out, &qualified_name));
 
            /* 12.5.5. Append the following to markup, in the order listed: ... */
            TRY_OR_CLEANUP(xmlOutputBufferWriteLit(ctx->out, " xmlns:")); /* 12.5.5.1 - 12.5.5.2 */
            TRY_OR_CLEANUP(xmlOutputBufferWriteString(ctx->out, (const char *) prefix));
            TRY_OR_CLEANUP(xmlOutputBufferWriteLit(ctx->out, "=\""));
            TRY_OR_CLEANUP(dom_xml_common_text_serialization(ctx->out, (const char *) ns, true));
            TRY_OR_CLEANUP(xmlOutputBufferWriteLit(ctx->out, "\""));
 
            /* 12.5.6. If local default namespace is not null ... (editorial numbering error: https://github.com/w3c/DOM-Parsing/issues/43) */
            if (local_default_namespace != NULL) {
                if (*local_default_namespace == '\0') {
                    inherited_ns = NULL;
                } else {
                    inherited_ns = local_default_namespace;
                }
            }
        }
        /* 12.6. Otherwise, if local default namespace is null, or local default namespace is not null and its value is not equal to ns, then: */
        /* Note: https://github.com/w3c/DOM-Parsing/issues/47 */
        else if (local_default_namespace == NULL || !dom_xml_str_equals_treat_nulls_as_empty(local_default_namespace, ns)) {
            /* 12.6.1. Set the ignore namespace definition attribute flag to true. */
            ignore_namespace_definition_attribute = true;
 
            /* 12.6.2. Append to qualified name the value of node's localName. */
            qualified_name.name = element->name;
 
            /* 12.6.3. Let the value of inherited ns be ns. */
            inherited_ns = ns;
 
            /* 12.6.4. Append the value of qualified name to markup. */
            TRY_OR_CLEANUP(dom_xml_output_qname(ctx->out, &qualified_name));
 
            /* 12.6.5. Append the following to markup, in the order listed: ... */
            TRY_OR_CLEANUP(xmlOutputBufferWriteLit(ctx->out, " xmlns=\"")); /* 12.6.5.1 - 12.6.5.2 */
            TRY_OR_CLEANUP(dom_xml_common_text_serialization(ctx->out, (const char *) ns, true));
            TRY_OR_CLEANUP(xmlOutputBufferWriteLit(ctx->out, "\""));
        }
        /* 12.7. Otherwise, the node has a local default namespace that matches ns ... */
        else {
            qualified_name.name = element->name;
            inherited_ns = ns;
            TRY_OR_CLEANUP(dom_xml_output_qname(ctx->out, &qualified_name));
        }
    }
 
    /* 13. Append to markup the result of the XML serialization of node's attributes given map, prefix index,
     *     local prefixes map, ignore namespace definition attribute flag, and require well-formed flag. */
    TRY_OR_CLEANUP(dom_xml_serialize_attributes(ctx->out, element, &map, &local_prefixes_map, prefix_index, ignore_namespace_definition_attribute, require_well_formed));
 
    /* 14. If ns is the HTML namespace, and the node's list of children is empty, and the node's localName matches
     *     any one of the following void elements: ... */
    if (element->children == NULL) {
        if (xmlSaveNoEmptyTags) {
            /* Do nothing, use the <x></x> closing style. */
        } else if (php_dom_ns_is_fast(element, php_dom_ns_is_html_magic_token)) {
            size_t name_length = strlen((const char *) element->name);
            if (dom_local_name_compare_ex(element, "area", strlen("area"), name_length)
                || dom_local_name_compare_ex(element, "base", strlen("base"), name_length)
                || dom_local_name_compare_ex(element, "basefont", strlen("basefont"), name_length)
                || dom_local_name_compare_ex(element, "bgsound", strlen("bgsound"), name_length)
                || dom_local_name_compare_ex(element, "br", strlen("br"), name_length)
                || dom_local_name_compare_ex(element, "col", strlen("col"), name_length)
                || dom_local_name_compare_ex(element, "embed", strlen("embed"), name_length)
                || dom_local_name_compare_ex(element, "frame", strlen("frame"), name_length)
                || dom_local_name_compare_ex(element, "hr", strlen("hr"), name_length)
                || dom_local_name_compare_ex(element, "img", strlen("img"), name_length)
                || dom_local_name_compare_ex(element, "input", strlen("input"), name_length)
                || dom_local_name_compare_ex(element, "keygen", strlen("keygen"), name_length)
                || dom_local_name_compare_ex(element, "link", strlen("link"), name_length)
                || dom_local_name_compare_ex(element, "menuitem", strlen("menuitem"), name_length)
                || dom_local_name_compare_ex(element, "meta", strlen("meta"), name_length)
                || dom_local_name_compare_ex(element, "param", strlen("param"), name_length)
                || dom_local_name_compare_ex(element, "source", strlen("source"), name_length)
                || dom_local_name_compare_ex(element, "track", strlen("track"), name_length)
                || dom_local_name_compare_ex(element, "wbr", strlen("wbr"), name_length)) {
                TRY_OR_CLEANUP(xmlOutputBufferWriteLit(ctx->out, " /"));
                skip_end_tag = true;
            }
        } else {
            /* 15. If ns is not the HTML namespace, and the node's list of children is empty,
             *     then append "/" (U+002F SOLIDUS) to markup and set the skip end tag flag to true. */
            TRY_OR_CLEANUP(xmlOutputBufferWriteLit(ctx->out, "/"));
            skip_end_tag = true;
        }
    }
 
    /* 16. Append ">" (U+003E GREATER-THAN SIGN) to markup. */
    TRY_OR_CLEANUP(xmlOutputBufferWriteLit(ctx->out, ">"));
 
    /* 17. If the value of skip end tag is true, then return the value of markup and skip the remaining steps. */
    if (!skip_end_tag) {
        if (should_format) {
            indent++;
        } else {
            indent = -1;
        }
 
        /* 18. If ns is the HTML namespace, and the node's localName matches the string "template",
         *     then this is a template element.
         *     Append to markup the result of XML serializing a DocumentFragment node. */
        xmlNodePtr child = NULL;
        if (php_dom_ns_is_fast(element, php_dom_ns_is_html_magic_token) && xmlStrEqual(element->name, BAD_CAST "template")) {
            if (ctx->private_data != NULL) {
                child = php_dom_retrieve_templated_content(ctx->private_data, element);
            }
        } else {
            child = element->children;
        }
 
        /* 19. Otherwise, append to markup the result of running the XML serialization algorithm on each of node's children. */
        for (; child != NULL; child = child->next) {
            if (should_format) {
                TRY_OR_CLEANUP(dom_xml_output_indents(ctx->out, indent));
            }
            TRY_OR_CLEANUP(dom_xml_serialization_algorithm(ctx, &map, child, inherited_ns, prefix_index, indent, require_well_formed));
        }
 
        if (should_format) {
            indent--;
            TRY_OR_CLEANUP(dom_xml_output_indents(ctx->out, indent));
        }
 
        /* 20. Append the following to markup, in the order listed: */
        TRY_OR_CLEANUP(xmlOutputBufferWriteLit(ctx->out, "</"));
        TRY_OR_CLEANUP(dom_xml_output_qname(ctx->out, &qualified_name));
        TRY_OR_CLEANUP(xmlOutputBufferWriteLit(ctx->out, ">"));
    }
 
    /* 21. Return the value of markup.
     *     => We use the output buffer instead. */
    dom_xml_ns_prefix_map_dtor(&map);
    dom_xml_local_prefix_map_dtor(&local_prefixes_map);
    return 0;
 
cleanup:
    dom_xml_ns_prefix_map_dtor(&map);
    dom_xml_local_prefix_map_dtor(&local_prefixes_map);
    return -1;
}
 
/* https://w3c.github.io/DOM-Parsing/#xml-serializing-a-documentfragment-node */
static int dom_xml_serializing_a_document_fragment_node(
    dom_xml_serialize_ctx *ctx,
    dom_xml_ns_prefix_map *namespace_prefix_map,
    xmlNodePtr node,
    const xmlChar *namespace,
    unsigned int *prefix_index,
    int indent,
    bool require_well_formed
)
{
    /* 1. Let markup the empty string.
     *    => We use the output buffer instead. */
 
    /* 2. For each child child of node, in tree order, run the XML serialization algorithm on the child ... */
    xmlNodePtr child = node->children;
    while (child != NULL) {
        TRY(dom_xml_serialization_algorithm(ctx, namespace_prefix_map, child, namespace, prefix_index, indent, require_well_formed));
        child = child->next;
    }
 
    /* 3. Return the value of markup
     *    => We use the output buffer instead. */
    return 0;
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-xml-serializing-a-document-node */
static int dom_xml_serializing_a_document_node(
    dom_xml_serialize_ctx *ctx,
    dom_xml_ns_prefix_map *namespace_prefix_map,
    xmlNodePtr node,
    const xmlChar *namespace,
    unsigned int *prefix_index,
    int indent,
    bool require_well_formed
)
{
    /* 1. Let serialized document be an empty string.
     *    => We use the output buffer instead. */
 
    xmlNodePtr child = node->children;
    node->children = NULL;
 
    /* https://github.com/w3c/DOM-Parsing/issues/50 */
    TRY(xmlOutputBufferFlush(ctx->out));
    TRY(xmlSaveDoc(ctx->ctxt, (xmlDocPtr) node));
    TRY(xmlSaveFlush(ctx->ctxt));
 
    node->children = child;
 
    /* 2. For each child child of node, in tree order, run the XML serialization algorithm on the child passing along the provided arguments,
     *    and append the result to serialized document. */
    while (child != NULL) {
        TRY(dom_xml_serialization_algorithm(ctx, namespace_prefix_map, child, namespace, prefix_index, indent, require_well_formed));
        child = child->next;
    }
 
    /* 3. Return the value of serialized document.
     *    => We use the output buffer instead. */
    return 0;
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-xml-serialization-algorithm */
static int dom_xml_serialization_algorithm(
    dom_xml_serialize_ctx *ctx,
    dom_xml_ns_prefix_map *namespace_prefix_map,
    xmlNodePtr node,
    const xmlChar *namespace,
    unsigned int *prefix_index,
    int indent,
    bool require_well_formed
)
{
    /* If node's interface is: */
    switch (node->type) {
        case XML_ELEMENT_NODE:
            return dom_xml_serialize_element_node(ctx, namespace, namespace_prefix_map, node, prefix_index, indent, require_well_formed);
 
        case XML_DOCUMENT_FRAG_NODE:
            return dom_xml_serializing_a_document_fragment_node(ctx, namespace_prefix_map, node, namespace, prefix_index, indent, require_well_formed);
 
        case XML_HTML_DOCUMENT_NODE:
        case XML_DOCUMENT_NODE:
            return dom_xml_serializing_a_document_node(ctx, namespace_prefix_map, node, namespace, prefix_index, indent, require_well_formed);
 
        case XML_TEXT_NODE:
            return dom_xml_serialize_text_node(ctx->out, node, require_well_formed);
 
        case XML_COMMENT_NODE:
            return dom_xml_serialize_comment_node(ctx->out, node, require_well_formed);
 
        case XML_PI_NODE:
            return dom_xml_serialize_processing_instruction(ctx->out, node, require_well_formed);
 
        case XML_CDATA_SECTION_NODE:
            return dom_xml_serialize_cdata_section_node(ctx->out, node);
 
        case XML_ATTRIBUTE_NODE:
            return dom_xml_serialize_attribute_node(ctx->out, node);
 
        default:
            TRY(xmlOutputBufferFlush(ctx->out));
            TRY(xmlSaveTree(ctx->ctxt, node));
            TRY(xmlSaveFlush(ctx->ctxt));
            if (node->type == XML_DTD_NODE) {
                return xmlOutputBufferWriteLit(ctx->out, "\n");
            }
            return 0;
    }
 
    ZEND_UNREACHABLE();
}
 
/* https://w3c.github.io/DOM-Parsing/#dfn-xml-serialization */
int dom_xml_serialize(xmlSaveCtxtPtr ctxt, xmlOutputBufferPtr out, xmlNodePtr node, bool format, bool require_well_formed, php_dom_private_data *private_data)
{
    /* 1. Let namespace be a context namespace with value null. */
    const xmlChar *namespace = NULL;
 
    /* 2. Let prefix map be a new namespace prefix map. */
    dom_xml_ns_prefix_map namespace_prefix_map;
    dom_xml_ns_prefix_map_ctor(&namespace_prefix_map);
 
    /* 3. Add the XML namespace with prefix value "xml" to prefix map. */
    dom_xml_ns_prefix_map_add(&namespace_prefix_map, BAD_CAST "xml", false, BAD_CAST DOM_XML_NS_URI, strlen(DOM_XML_NS_URI));
 
    /* 4. Let prefix index be a generated namespace prefix index with value 1. */
    unsigned int prefix_index = 1;
 
    /* 5. Return the result of running the XML serialization algorithm ... */
    dom_xml_serialize_ctx ctx;
    ctx.out = out;
    ctx.ctxt = ctxt;
    ctx.private_data = private_data;
    int indent = format ? 0 : -1;
    int result = dom_xml_serialization_algorithm(&ctx, &namespace_prefix_map, node, namespace, &prefix_index, indent, require_well_formed);
 
    dom_xml_ns_prefix_map_dtor(&namespace_prefix_map);
 
    return result;
}
 
#endif  /* HAVE_LIBXML && HAVE_DOM */