#include "sax_processor.hh"

#include "buffer.hh"
#include "guessing_decoder.hh"
#include "processor.hh"
#include "sax_attributes.hh"
#include "sax_decoder.hh"
#include "sax_decoder_factory.hh"
#include "sax_delegate.hh"
#include "utf8.hh"
#include "utf_error.hh"
#include "utils.hh"

#include <algorithm>
#include <cassert>
#include <charconv>
#include <format>
#include <map>
#include <optional>
#include <utility>
#include <vector>

using namespace std::string_view_literals;

namespace modxml {
namespace sax {

namespace {

constexpr std::size_t kDefaultBufferSize = 8192;
constexpr std::size_t kMinBufferSize = 128;

inline bool is_digit(char c) {
  return c >= '0' && c <= '9';
}

// 2.2 Characters
// [2] Char ::= #x9 | #xA | #xD | [#x20-#xD7FF] | [#xE000-#xFFFD] | [#x10000-#x10FFFF]

inline bool valid_char(uint32_t c) {
  // Assume valid unicode (U+0 - U+10ffff except surrogate blocks)
  if (c >= 0x20 && c <= 0xfffd)
    return true;
  if (c == 0x9 || c == 0xa || c == 0xd)
    return true;
  return c >= 0x10000;
}

// 2.3 Common Syntactic Constructs
// [3] S ::= (#x20 | #x9 | #xD | #xA)+

inline bool is_ws(uint32_t c) {
  // Assume we already checked for valid_char.
  return c <= 0x20;
}

// [4] NameStartChar ::= ":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF] | [#x370-#x37D] | [#x37F-#x1FFF] | [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] | [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]
// [4a]	NameChar ::= NameStartChar | "-" | "." | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]

inline bool is_namestartchar(uint32_t c) {
  if (c < 0x41 /* A */)
    return c == 0x3a /* : */;
  if (c <= 0x5a /* Z */)
    return true;
  if (c < 0x61 /* a */)
    return c == 0x5f /* _ */;
  if (c <= 0x7a /* z */)
    return true;
  if (c < 0xc0)
    return false;
  if (c < 0x300)
    return c != 0xd7 && c != 0xf7;
  if (c > 0x37d && c < 0x37f)
    return false;
  if (c > 0x1fff && c < 0x200c)
    return false;
  if (c > 0x200d && c < 0x2070)
    return false;
  if (c > 0x218f && c < 0x2c00)
    return false;
  if (c > 0x2fef && c < 0x3001)
    return false;
  // Already valid_char so don't check for surrogate pair here.
  if (c > 0xdfff && c < 0xf900)
    return false;
  if (c > 0xfdcf && c < 0xfdf0)
    return false;
  if (c > 0xfffd && c < 0x10000)
    return false;
  return true;
}

inline bool is_namechar(uint32_t c) {
  return is_namestartchar(c) || (c >= 0x2d /* - */ && c <= 0x2e /* . */) ||
      (c >= 0x30 /* 0 */ && c <= 0x39 /* 9 */) || (c == 0xb7) ||
      (c >= 0x300 && c <= 0x36f) || (c >= 0x203f && c <= 0x2040);
}

/*
[5]   	Name	   ::=   	NameStartChar (NameChar)*
[6]   	Names	   ::=   	Name (#x20 Name)*
[7]   	Nmtoken	   ::=   	(NameChar)+
[8]   	Nmtokens	   ::=   	Nmtoken (#x20 Nmtoken)*
*/

inline bool ascii_lowercase(char c) {
  return (c >= 'A' & c <= 'Z') ? (c | 0x20) : c;
}

bool eq_lowercase(std::string_view a, std::string_view b) {
  if (a.size() != b.size())
    return false;
  for (std::size_t i = 0; i < a.size(); ++i)
    if (ascii_lowercase(a[i]) != b[i])
      return false;
  return true;
}

inline std::string_view make_string_view(std::span<uint8_t const> span) {
  return std::string_view(reinterpret_cast<char const*>(span.data()),
                          span.size());
}

class Entities {
 public:
  Entities() {
    data_.emplace("lt", "<");
    data_.emplace("gt", ">");
    data_.emplace("amp", "&");
    data_.emplace("apos", "'");
    data_.emplace("quot", "\"");
  }

  std::optional<std::string> get(std::string const& entity) const {
    if (entity.empty())
      return std::nullopt;
    if (entity.front() == '#') {
      if (entity.size() == 1)
        return std::nullopt;
      int base;
      char const* start;
      char const* end = entity.data() + entity.size();
      if (entity[1] == 'x') {
        start = entity.data() + 2;
        base = 16;
      } else {
        start = entity.data() + 1;
        base = 10;
      }
      uint32_t value;
      auto [ptr, ec] = std::from_chars(start, end, value, base);
      if (ec == std::errc() && ptr == end) {
        uint8_t tmp[4];
        std::size_t offset = 0;
        utf::write8(value, tmp, offset);
        return std::string(reinterpret_cast<char*>(tmp), offset);
      }
    }
    auto it = data_.find(entity);
    if (it == data_.end())
      return std::nullopt;
    return it->second;
  }

 private:
  std::map<std::string, std::string> data_;
};

bool deamp(Entities const& entities, std::string& str, std::size_t last = 0) {
  while (true) {
    auto next = str.find('&', last);
    if (next == std::string::npos)
      break;
    next += 1;
    auto semicolon = str.find(';', next);
    if (semicolon == std::string::npos)
      return false;
    auto replacement = entities.get(str.substr(next, semicolon - next));
    if (!replacement.has_value())
      return false;
  }
  return true;
}

std::optional<std::string> unquote(Entities const& entities,
                                   std::string_view quoted) {
  assert(quoted.size() >= 2);
  assert(quoted.front() == quoted.back());
  std::string ret(quoted.substr(1, quoted.size() - 2));
  if (deamp(entities, ret))
    return ret;
  return std::nullopt;
}

std::optional<std::string_view> unquote_if_needed(Entities const& entities,
                                                  std::string_view quoted,
                                                  std::string& tmp) {
  assert(quoted.size() >= 2);
  assert(quoted.front() == quoted.back());
  auto input = quoted.substr(1, quoted.size() - 2);
  auto index = input.find('&');
  if (index == std::string_view::npos)
    return input;
  tmp.assign(input);
  if (deamp(entities, tmp, index))
    return tmp;
  return std::nullopt;
}

class AttributesImpl : public Attributes {
 public:
  AttributesImpl() = default;

  bool init(Entities const& entities,
            std::span<const uint8_t> data,
            std::vector<size_t> const& offsets,
            std::size_t first) {
    std::size_t a = first;
    attr_.reserve((offsets.size() - first) / 4);
    while (a + 4 <= offsets.size()) {
      auto name = make_string_view(data.subspan(offsets[a], offsets[a + 1]));
      std::string tmp;
      auto value = unquote_if_needed(
          entities,
          make_string_view(data.subspan(offsets[a + 2], offsets[a + 3])),
          tmp);
      if (!value.has_value())
        return false;
      if (tmp.empty()) {
        attr_.emplace_back(name, *value);
      } else {
        attr_.emplace_back(name, *value, std::move(tmp));
      }
      a += 4;
    }
    return true;
  }

  iterator begin() const override {
    return Iterator(this, 0);
  }

  iterator end() const override {
    return Iterator(this, attr_.size());
  }

  std::size_t size() const override {
    return attr_.size();
  }

  Attribute const& at(std::size_t index) const override {
    return attr_[index];
  }

 private:
  class Iterator : public iterator {
   public:
    Iterator(Attributes const* attributes, std::size_t index)
        : iterator(attributes, index) {}
  };

  struct AttributeImpl : public Attribute {
    AttributeImpl(std::string_view name, std::string_view value)
        : Attribute(name, value) {}

    AttributeImpl(std::string_view name, std::string_view value,
                  std::string&& tmp)
        : Attribute(name, value), tmp_(std::move(tmp)) {}

   private:
    std::string tmp_;
  };

  std::span<const uint8_t> data_;
  std::vector<AttributeImpl> attr_;
};

class ProcessorImpl : public Processor {
 public:
  ProcessorImpl(std::shared_ptr<Delegate> delegate,
                std::shared_ptr<DecoderFactory> decoder_factory,
                std::unique_ptr<Decoder> decoder,
                std::size_t default_buffer_size,
                std::size_t max_buffer_size)
      : delegate_(std::move(delegate)),
        decoder_factory_(std::move(decoder_factory)),
        decoder_(std::move(decoder)),
        forced_decoder_(decoder_),
        buffer_(make_buffer(default_buffer_size, max_buffer_size)) {
    if (!decoder_)
      decoder_ = create_guessing_decoder();

    expect_document();
  }

  std::size_t process(std::span<uint8_t const> data,
                      std::size_t offset) override {
    cmds_.emplace_back(Command::FILL_BUFFER, Count::ZERO_OR_ONE);

    std::size_t consumed = 0;

    while (true) {
      if (cmds_.empty()) {
        if (!buffer_->empty()) {
          delegate_->error("Extra data at end");
        }
        return consumed;
      }

      auto current = cmds_.back();
      auto const old_size = cmds_.size();
      cmds_.pop_back();
      Process ret;
      switch (current.command) {
        case Command::FILL_BUFFER:
          ret = fill_buffer(data, offset, consumed);
          break;
        case Command::MISC:
          ret = process_misc(current);
          break;
        case Command::SPACE:
          ret = process_space(current);
          break;
        case Command::ELEMENT:
          ret = process_element(current);
          break;
        case Command::COMMENT:
          ret = process_comment(current);
          break;
        case Command::PROCESSING_INSTRUCTION:
          ret = process_processing_instruction(current);
          break;
        case Command::XMLDECL:
          ret = process_xmldecl(current);
          break;
        case Command::ATTRIBUTE:
          ret = process_attribute(current);
          break;
        case Command::NAME:
          ret = process_name(current);
          break;
        case Command::ATTRIBUTE_VALUE:
          ret = process_attribute_value(current);
          break;
        case Command::EQUAL:
          ret = process_equal(current);
          break;
        case Command::START_OR_EMPTY_TAG:
          ret = process_start_or_empty_tag(current);
          break;
        case Command::END_TAG:
          ret = process_end_tag(current);
          break;
      }

      switch (ret) {
        case Process::NEED_MORE:
        case Process::ERROR:
          cmds_.push_back(current);
          assert(cmds_.size() == old_size);
          return consumed;
        case Process::CONTINUE:
          break;
      }
    }
  }

  uint64_t line() const override { return line_; }

  uint64_t column() const override { return column_; }

 private:
  enum class Process {
    NEED_MORE,
    ERROR,
    CONTINUE,
  };

  enum class Match {
    FULL_MATCH,
    PARTIAL_MATCH,
    NO_MATCH,
  };

  enum class Command {
    FILL_BUFFER,

    ATTRIBUTE,
    ATTRIBUTE_VALUE,
    COMMENT,
    ELEMENT,
    END_TAG,
    EQUAL,
    MISC,
    NAME,
    PROCESSING_INSTRUCTION,
    SPACE,
    START_OR_EMPTY_TAG,
    XMLDECL,
  };

  enum class Count {
    ONE,
    ONE_OR_MANY,
    ZERO_OR_ONE,
    ZERO_OR_MANY,
  };

  struct CommandItem {
    Command const command;
    Count const count;
    std::size_t offset;

    CommandItem(Command command, Count count, std::size_t offset = 0)
        : command(command), count(count), offset(offset) {}
  };

  struct StackItem {
    std::vector<std::size_t> offsets;
  };

  Process fill_buffer(std::span<uint8_t const> data,
                      std::size_t offset,
                      std::size_t& consumed) {
    if (offset >= data.size())
      return Process::NEED_MORE;

    std::size_t tmp = offset;
    auto wspan = buffer_->wspan(4);
    std::size_t wrote = 0;
    switch (decoder_->decode(data, tmp, wspan, wrote)) {
      case Decoder::State::GOOD:
        break;
      case Decoder::State::NEED_MORE:
        return Process::NEED_MORE;
      case Decoder::State::INVALID:
        delegate_->error("Invalid data");
        return Process::ERROR;
    }
    buffer_->commit(wrote);
    consumed = tmp - offset;
    return Process::CONTINUE;
  }

  void expect_document() {
    // document := prolog element Misc*
    expect_misc(Count::ZERO_OR_MANY);
    expect_element(Count::ONE);
    expect_prolog();
  }

  void expect_misc(Count count) {
    cmds_.emplace_back(Command::MISC, count);
  }

  void expect_element(Count count) {
    // element ::= EmptyElemTag | STag content ETag
    cmds_.emplace_back(Command::START_OR_EMPTY_TAG, count);
  }

  void expect_end_tag(Count count) {
    cmds_.emplace_back(Command::END_TAG, count);
  }

  void expect_prolog() {
    // prolog := XMLDecl? Misc* (doctypedecl Misc*)?
    expect_misc(Count::ZERO_OR_MANY);
    expect_doctypedecl(Count::ZERO_OR_ONE);
    expect_misc(Count::ZERO_OR_MANY);
    expect_xmldecl(Count::ZERO_OR_ONE);
  }

  void expect_xmldecl(Count count) {
    cmds_.emplace_back(Command::XMLDECL, count);
  }

  void expect_doctypedecl(Count) {
    // TODO
  }

  void expect_comment(Count count, std::size_t start_offset = 0) {
    // Comment should never be more than one, should be MISC that is repeated.
    assert(count == Count::ONE);
    cmds_.emplace_back(Command::COMMENT, count, start_offset);
  }

  void expect_content(Count) {
    // TODO
  }

  void expect_pi(Count count, std::size_t start_offset = 0) {
    // PI should never be more than one, should be MISC that is repeated.
    assert(count == Count::ONE);
    cmds_.emplace_back(Command::PROCESSING_INSTRUCTION, count, start_offset);
  }

  void expect_space(Count count) {
    // There is not way to have SS as S is continous, so we should never
    // ask for more than one or zero.
    assert(count == Count::ZERO_OR_ONE || count == Count::ONE);
    cmds_.emplace_back(Command::SPACE, count);
  }

  void expect_attribute(Count count) {
    switch (count) {
      case Count::ONE_OR_MANY:
        cmds_.emplace_back(Command::ATTRIBUTE, Count::ZERO_OR_MANY);
      case Count::ONE:
        // Attribute ::= Name Eq AttValue
        expect_attribute_value(Count::ONE);
        expect_equal(Count::ONE);
        expect_name(Count::ONE);
        expect_space(Count::ONE);
        break;
      case Count::ZERO_OR_ONE:
      case Count::ZERO_OR_MANY:
        cmds_.emplace_back(Command::ATTRIBUTE, count);
        break;
    }
  }

  void expect_attribute_value(Count count) {
    cmds_.emplace_back(Command::ATTRIBUTE_VALUE, count);
  }

  void expect_equal(Count count) {
    // Eq ::= S? '=' S?
    expect_space(Count::ZERO_OR_ONE);
    cmds_.emplace_back(Command::EQUAL, count);
    expect_space(Count::ZERO_OR_ONE);
  }

  void expect_name(Count count) {
    cmds_.emplace_back(Command::NAME, count);
  }

  Process process_misc(CommandItem const& item) {
    // Misc := Comment | PI | S
    assert(item.offset == 0);

    switch (match("<!--")) {
      case Match::FULL_MATCH:
        add_if_more(item);
        expect_comment(Count::ONE, 3);
        return Process::CONTINUE;
      case Match::PARTIAL_MATCH:
        return Process::NEED_MORE;
      case Match::NO_MATCH:
        break;
    }

    switch (match("<?")) {
      case Match::FULL_MATCH:
        add_if_more(item);
        expect_pi(Count::ONE, 2);
        return Process::CONTINUE;
      case Match::PARTIAL_MATCH:
        return Process::NEED_MORE;
      case Match::NO_MATCH:
        break;
    }

    switch (match_s()) {
      case Match::FULL_MATCH:
        add_if_more(item);
        expect_space(Count::ONE);
        return Process::CONTINUE;
      case Match::PARTIAL_MATCH:
        return Process::NEED_MORE;
      case Match::NO_MATCH:
        break;
    }

    return no_match(item);
  }

  Process process_attribute(CommandItem& item) {
    // This actually parses (S Attribute)* when followed by S?
    // for Attribute parsing see expect_attribute()
    // So we need to figure out if the S means start of attribute
    // or just an S. We do this by checking if the first non-S is
    // a namestart or something else. We consume the S.
    uint32_t last_char;
    auto ret = consume_space(item.offset, last_char);
    if (ret != Process::CONTINUE)
      return ret;

    // No S, cannot be followed by an attribute then.
    if (item.offset == 0)
      return no_match(item);

    // First character after S isn't a valid first character of a name,
    // cannot be followed by an attribute then.
    if (!is_namestartchar(last_char))
      return no_match(item);

    expect_attribute_value(Count::ONE);
    expect_equal(Count::ONE);
    expect_name(Count::ONE);
    return Process::CONTINUE;
  }

  Process process_equal(CommandItem const& item) {
    // Eq ::= S? '=' S?
    // Spacing added by expect_equal
    switch (match_consume("=")) {
      case Match::FULL_MATCH:
        add_if_more(item);
        return Process::CONTINUE;
      case Match::PARTIAL_MATCH:
        return Process::NEED_MORE;
      case Match::NO_MATCH:
        return no_match(item);
    }
  }

  Process process_name(CommandItem& item) {
	// Name ::= NameStartChar (NameChar)*
    auto data = buffer_->rspan(item.offset + 4);
    while (true) {
      std::size_t tmp = item.offset;
      auto c = utf::read8(data, tmp);
      if (c == utf::NEED_MORE)
        return Process::NEED_MORE;
      if (c == utf::INVALID || !valid_char(c))
        return invalid_char(data, tmp);
      if (item.offset == 0) {
        if (!is_namestartchar(c))
          return no_match(item);
      } else {
        if (!is_namechar(c))
          break;
      }
      item.offset = tmp;
    }

    assert(!stack_.empty());
    auto* read_view = static_cast<ReadViewBuffer*>(buffer_.get());
    stack_.back().offsets.push_back(read_view->consumed());
    stack_.back().offsets.push_back(item.offset);
    buffer_->consume(item.offset);
    return Process::CONTINUE;
  }

  Process process_attribute_value(CommandItem& item) {
    // AttValue ::= '"' ([^<&"] | Reference)* '"'
    //              | "'" ([^<&'] | Reference)* "'"

    uint32_t end_char;
    auto data = buffer_->rspan(item.offset + 4);

    if (item.offset == 0) {
      std::size_t tmp = item.offset;
      auto c = utf::read8(data, tmp);
      if (c == utf::NEED_MORE)
        return Process::NEED_MORE;
      if (c == utf::INVALID || !valid_char(c))
        return invalid_char(data, tmp);
      if (c != '"' && c != '\'')
        return no_match(item);
      item.offset = tmp;
      end_char = c;
    } else {
      assert(!data.empty());
      end_char = data[0];  // ok as both " and ' are ASCII
    }

    while (true) {
      auto c = utf::read8(data, item.offset);
      if (c == utf::NEED_MORE)
        return Process::NEED_MORE;
      if (c == utf::INVALID || !valid_char(c))
        return invalid_char(data, item.offset);
      if (c == end_char)
        break;
      // TODO: Should we validate reference already here or do we let
      // unquoute take care of that? As Reference can't contain end_char
      // only checking for end_char is safe here.
    }

    assert(!stack_.empty());
    auto* read_view = static_cast<ReadViewBuffer*>(buffer_.get());
    stack_.back().offsets.push_back(read_view->consumed());
    stack_.back().offsets.push_back(item.offset);
    buffer_->consume(item.offset);
    return Process::CONTINUE;
  }

  Process process_comment(CommandItem& item) {
    if (item.offset == 0) {
      switch (match_consume("<!--")) {
        case Match::FULL_MATCH:
          item.offset += 3;
          break;
        case Match::PARTIAL_MATCH:
          return Process::NEED_MORE;
        case Match::NO_MATCH:
          return no_match(item);
      }
    }

    auto match = find("-->", item.offset);
    switch (match) {
      case Match::FULL_MATCH: {
        auto data = buffer_->rspan(item.offset);
        assert(data.size() >= item.offset);
        delegate_->comment(
            make_string_view(data.subspan(3, item.offset - 3)));
        buffer_->consume(item.offset + 3);
        return Process::CONTINUE;
      }
      case Match::NO_MATCH:
      case Match::PARTIAL_MATCH:
        return Process::NEED_MORE;
    }
  }

  Process process_processing_instruction(CommandItem& item) {
    if (item.offset == 0) {
      switch (match_consume("<?")) {
        case Match::FULL_MATCH:
          item.offset += 2;
          break;
        case Match::PARTIAL_MATCH:
          return Process::NEED_MORE;
        case Match::NO_MATCH:
          return no_match(item);
      }
    }

    // TODO
    delegate_->error("PI not supported");
    return Process::ERROR;
  }

  void add_to_stack(CommandItem const& item, std::size_t offset) {
    cmds_.emplace_back(item.command, item.count, offset);
    stack_.emplace_back();
    buffer_ = make_read_view_buffer(std::move(buffer_));
    buffer_->consume(offset);
  }

  std::size_t pop_stack(std::vector<std::size_t>& attr) {
    assert(!stack_.empty());
    std::swap(attr, stack_.back().offsets);

    auto* read_view = static_cast<ReadViewBuffer*>(buffer_.get());
    auto consumed = read_view->consumed();

    buffer_ = read_view->release();
    stack_.pop_back();

    return consumed;
  }

  Process process_xmldecl(CommandItem const& item) {
    // XMLDecl ::= '<?xml' VersionInfo EncodingDecl? SDDecl? S? '?>'
    if (item.offset == 0) {
      switch (match("<?xml")) {
        case Match::FULL_MATCH:
          add_to_stack(item, /* offset */ 5);
          expect_space(Count::ZERO_OR_ONE);
          // Parsing as generic "Attribute" here and doing validation later.
          expect_attribute(Count::ONE_OR_MANY);
          return Process::CONTINUE;
        case Match::PARTIAL_MATCH:
          return Process::NEED_MORE;
        case Match::NO_MATCH:
          return no_match(item);
      }
    }

    assert(item.offset == 5);

    // Remember that this is still reading for the read view buffer.
    switch (match_consume("?>")) {
      case Match::FULL_MATCH:
        break;
      case Match::PARTIAL_MATCH:
        return Process::NEED_MORE;
      case Match::NO_MATCH:
        delegate_->error(std::format("Expected end of {}",
                                     command_name(item.command)));
        return Process::ERROR;
    }

    std::vector<std::size_t> attr;
    auto const consumed = pop_stack(attr);

    // Now we're back to the real buffer
    auto data = buffer_->rspan(consumed);
    std::size_t a = 0;

    if (a + 4 <= attr.size() &&
        make_string_view(data.subspan(attr[a + 0],
                                      attr[a + 1])) == "version") {
      auto version = make_string_view(data.subspan(attr[a + 2] + 1,
                                                   attr[a + 3] - 2));
      if (!valid_version(version)) {
        delegate_->error(std::format("Unsupported xmldecl version, {}",
                                     version));
        return Process::ERROR;
      }
      a += 4;
    } else {
      // No version
      delegate_->error("Invalid xmldecl, must have a version attribute first.");
      return Process::ERROR;
    }

    if (a + 4 <= attr.size() &&
        make_string_view(data.subspan(attr[a + 0],
                                      attr[a + 1])) == "encoding") {
      auto encoding = make_string_view(data.subspan(attr[a + 2] + 1,
                                                    attr[a + 3] - 2));
      if (forced_decoder_) {
        // encoding value is ignored
        // TODO: Should we check that it is valid anyway?
      } else {
        auto decoder = pick_decoder_for_encoding(encoding, nullptr);
        if (!decoder && decoder_factory_)
          decoder = decoder_factory_->create(encoding);
        if (!decoder) {
          delegate_->error(std::format("Unknown encoding {}", encoding));
          return Process::ERROR;
        }
        std::swap(decoder_, decoder);
        // TODO: Re-decode the rest of the buffer?
      }
      a += 4;
    }

    if (a + 4 <= attr.size() &&
        make_string_view(data.subspan(attr[a + 0],
                                      attr[a + 1])) == "standalone") {
      auto sd = make_string_view(data.subspan(attr[a + 2] + 1,
                                              attr[a + 3] - 2));
      if (sd == "yes") {
        // TODO: Handle standalone == yes
      } else if (sd == "no") {
        // TODO: Handle standalone == no
      } else {
        delegate_->error(std::format(
            "Invalid xmldecl, standalone attribute has unsupported value, {}",
            sd));
        return Process::ERROR;
      }
      a += 4;
    }

    if (a < attr.size()) {
      delegate_->error(
          std::format("Invalid xmldecl, unknown attribute, {}",
                      make_string_view(data.subspan(attr[a + 0],
                                                    attr[a + 1]))));
      return Process::ERROR;
    }

    buffer_->consume(consumed);
    return Process::CONTINUE;
  }

  Process process_start_or_empty_tag(CommandItem const& item) {
    // EmptyElemTag ::= '<' Name (S Attribute)* S? '/>'
    // STag         ::=	'<' Name (S Attribute)* S? '>'
    if (item.offset == 0) {
      switch (match("<")) {
        case Match::FULL_MATCH:
          add_to_stack(item, /* offset */ 1);
          expect_space(Count::ZERO_OR_ONE);
          expect_attribute(Count::ZERO_OR_MANY);
          expect_name(Count::ONE);
          return Process::CONTINUE;
        case Match::PARTIAL_MATCH:
          return Process::NEED_MORE;
        case Match::NO_MATCH:
          return no_match(item);
      }
    }

    assert(item.offset == 1);

    bool empty_tag;

    // Remember that this is still reading for the read view buffer.
    switch (match_consume("/>")) {
      case Match::FULL_MATCH:
        empty_tag = true;
        break;
      case Match::PARTIAL_MATCH:
        return Process::NEED_MORE;
      case Match::NO_MATCH:
        switch (match_consume(">")) {
          case Match::FULL_MATCH:
            empty_tag = false;
            break;
          case Match::PARTIAL_MATCH:
            return Process::NEED_MORE;
          case Match::NO_MATCH:
            delegate_->error(std::format("Expected end of {}",
                                         command_name(item.command)));
            return Process::ERROR;
        }
        break;
    }

    std::vector<std::size_t> attr;
    auto const consumed = pop_stack(attr);

    // Now we're back to the real buffer
    auto data = buffer_->rspan(consumed);

    assert(attr.size() >= 2);
    auto name = make_string_view(data.subspan(attr[0], attr[1]));

    AttributesImpl attributes;
    if (!attributes.init(entities_, data, std::move(attr), 2)) {
      delegate_->error("Invalid references in attribute values");
      return Process::ERROR;
    }

    add_if_more(item);

    if (empty_tag) {
      delegate_->empty_element(name, attributes);
    } else {
      delegate_->start_element(name, attributes);
      expect_end_tag(Count::ONE);
      expect_content(Count::ONE);
    }

    buffer_->consume(consumed);
    return Process::CONTINUE;
  }

  Process process_end_tag(CommandItem const& item) {
    // ETag ::=	'</' Name S? '>'
    if (item.offset == 0) {
      switch (match("</")) {
        case Match::FULL_MATCH:
          add_to_stack(item, /* offset */ 2);
          expect_space(Count::ZERO_OR_ONE);
          expect_name(Count::ONE);
          return Process::CONTINUE;
        case Match::PARTIAL_MATCH:
          return Process::NEED_MORE;
        case Match::NO_MATCH:
          return no_match(item);
      }
    }

    assert(item.offset == 1);

    // Remember that this is still reading for the read view buffer.
    switch (match_consume(">")) {
      case Match::FULL_MATCH:
        break;
      case Match::PARTIAL_MATCH:
        return Process::NEED_MORE;
      case Match::NO_MATCH:
        delegate_->error(std::format("Expected end of {}",
                                     command_name(item.command)));
        return Process::ERROR;
    }

    std::vector<std::size_t> attr;
    auto const consumed = pop_stack(attr);

    // Now we're back to the real buffer
    auto data = buffer_->rspan(consumed);

    assert(attr.size() == 2);
    auto name = make_string_view(data.subspan(attr[0], attr[1]));

    add_if_more(item);

    delegate_->end_element(name);

    buffer_->consume(consumed);
    return Process::CONTINUE;
  }

  static bool valid_version(std::string_view version) {
    if (version.size() < 3)
      return false;
    if (!version.starts_with("1."))
      return false;
    for (std::size_t i = 2; i < version.size(); ++i) {
      if (!is_digit(version[i]))
        return false;
    }
    return true;
  }

  Process process_element(CommandItem& item) {
    // TODO
    delegate_->error("Element is not yet supported");
    return Process::ERROR;
  }

  Process consume_space(std::size_t& count, uint32_t& last_char) {
    auto data = buffer_->rspan(4);
    std::size_t consumed = 0;
    while (true) {
      std::size_t offset = consumed;
      auto c = utf::read8(data, offset);
      if (c == utf::NEED_MORE) {
        buffer_->consume(consumed);
        return Process::NEED_MORE;
      }
      if (c == utf::INVALID || !valid_char(c))
        return invalid_char(data, offset);
      if (!is_ws(c)) {
        last_char = c;
        buffer_->consume(consumed);
        return Process::CONTINUE;
      }
      ++count;
      handle_ws(c);
      consumed = offset;
    }
  }

  Process process_space(CommandItem& item) {
    // S ::= (#x20 | #x9 | #xD | #xA)+
    // item.offset is only used to count spaces. We consume each space as it
    // is found so no offset in buffer.
    uint32_t unused;
    auto ret = consume_space(item.offset, unused);
    if (ret != Process::CONTINUE)
      return ret;

    if (item.offset == 0)
      return no_match(item);

    add_if_more(item);
    return Process::CONTINUE;
  }

  void add_if_more(CommandItem const& item) {
    switch (item.count) {
      case Count::ONE:
        break;
      case Count::ONE_OR_MANY:
        cmds_.emplace_back(item.command, Count::ZERO_OR_MANY);
        break;
      case Count::ZERO_OR_ONE:
        break;
      case Count::ZERO_OR_MANY:
        cmds_.emplace_back(item.command, item.count);
    }
  }

  Match find(std::string_view str, std::size_t& offset) {
    auto data = buffer_->rspan(offset + str.size());
    std::size_t i = 0;
    while (offset < data.size()) {
      if (str[i] == data[offset]) {
        ++i;
        if (i == str.size()) {
          offset -= i;
          return Match::FULL_MATCH;
        }
      } else {
        i = 0;
      }
      ++offset;
    }
    if (i > 0) {
      offset -= i;
      return Match::PARTIAL_MATCH;
    }
    return Match::NO_MATCH;
  }

  Match match(std::string_view str, std::size_t offset = 0) {
    auto data = buffer_->rspan(offset + str.size());
    if (data.size() <= offset)
      return Match::PARTIAL_MATCH;
    auto const avail = std::min(str.size(), data.size() - offset);
    for (std::size_t i = 0; i < avail; ++i) {
      if (str[i] != data[offset + i])
        return Match::NO_MATCH;
    }
    if (avail < str.size())
      return Match::PARTIAL_MATCH;
    return Match::FULL_MATCH;
  }

  Match match_consume(std::string_view str) {
    auto ret = match(str);
    if (ret == Match::FULL_MATCH)
      buffer_->consume(str.size());
    return ret;
  }

  Match match_s() {
    auto data = buffer_->rspan(4);
    std::size_t offset = 0;
    auto c = utf::read8(data, offset);
    if (c == utf::NEED_MORE)
      return data.size() == 0 ? Match::PARTIAL_MATCH : Match::NO_MATCH;
    if (c == utf::INVALID)
      return Match::NO_MATCH;
    if (!valid_char(c) || !is_ws(c))
      return Match::NO_MATCH;
    return Match::FULL_MATCH;
  }

  Process no_match(CommandItem const& item) {
    switch (item.count) {
      case Count::ONE:
      case Count::ONE_OR_MANY:
        delegate_->error(std::format("Expected {}",
                                     command_name(item.command)));
        return Process::ERROR;
      case Count::ZERO_OR_ONE:
      case Count::ZERO_OR_MANY:
        break;
    }
    return Process::CONTINUE;
  }

  void handle_ws(uint32_t c) {
    if (c == '\n') {
      ++line_;
      column_ = 0;
    } else {
      ++column_;
    }
  }

  Process invalid_char(std::span<uint8_t const> data, std::size_t offset) {
    delegate_->error(std::format("Invalid char {:02x}", data[offset]));
    return Process::ERROR;
  }

  static std::string_view command_name(Command command) {
    switch (command) {
      case Command::MISC:
        return "misc"sv;
      case Command::FILL_BUFFER:
        return "more data"sv;
      case Command::ELEMENT:
        return "element"sv;
      case Command::SPACE:
        return "whitespace"sv;
      case Command::COMMENT:
        return "comment"sv;
      case Command::PROCESSING_INSTRUCTION:
        return "processing instruction"sv;
      case Command::XMLDECL:
        return "xml declaration"sv;
      case Command::ATTRIBUTE:
        return "attribute"sv;
      case Command::ATTRIBUTE_VALUE:
        return "attribute value"sv;
      case Command::NAME:
        return "name"sv;
      case Command::EQUAL:
        return "equal sign (=)"sv;
      case Command::START_OR_EMPTY_TAG:
        return "element"sv;
      case Command::END_TAG:
        return "end tag"sv;
    }
    assert(false);
    return {};
  }

  std::shared_ptr<Delegate> delegate_;
  std::shared_ptr<DecoderFactory> decoder_factory_;
  std::unique_ptr<Decoder> decoder_;
  bool const forced_decoder_;
  std::unique_ptr<Buffer> buffer_;
  Entities entities_;
  std::vector<CommandItem> cmds_;
  std::vector<StackItem> stack_;
  uint64_t line_{1};
  uint64_t column_{0};
};

}  // namespace

std::unique_ptr<Processor> create_processor(
    std::shared_ptr<Delegate> delegate,
    std::shared_ptr<DecoderFactory> decoder_factory,
    std::optional<std::string> force_encoding,
    std::optional<std::size_t> opt_default_buffer_size,
    std::optional<std::size_t> opt_max_buffer_size) {

  std::unique_ptr<Decoder> decoder;
  if (force_encoding.has_value()) {
    decoder = pick_decoder_for_encoding(force_encoding.value(),
                                        decoder_factory.get());
  }

  std::size_t default_buffer_size = kDefaultBufferSize;
  if (opt_default_buffer_size.has_value())
    default_buffer_size = std::max(kMinBufferSize,
                                   opt_default_buffer_size.value());
  // This value is documented in public headers. Do NOT change.
  std::size_t max_buffer_size = 10 * 1024 * 1024;
  // No validation for user set value. If it is too small MAX_MEMORY_EXCEEDED
  // error will be thrown. If it is too large we will get OUT_OF_MEMORY or
  // crash depending on platform.
  if (opt_max_buffer_size.has_value())
    max_buffer_size = opt_max_buffer_size.value();

  return std::make_unique<ProcessorImpl>(std::move(delegate),
                                         std::move(decoder_factory),
                                         std::move(decoder),
                                         default_buffer_size,
                                         max_buffer_size);
}

std::unique_ptr<Processor>
Processor::create(std::shared_ptr<Delegate> delegate) {
  return create_processor(std::move(delegate), nullptr,
                          std::nullopt, std::nullopt, std::nullopt);
}

}  // namespace sax
}  // namespace modxml