#include "diff.h"

#include <algorithm>
#include <sstream>

#include <boost/property_tree/xml_parser.hpp>

namespace pt = boost::property_tree;

Diff::Diff()
{
}

Diff::Diff(const std::string& old_version, const std::string& new_version)
{
 create(old_version, new_version);
}

std::string Diff::apply(const std::string& old_version) const
{
 std::string result{old_version};

 result.erase(m_pos0, m_pos1 - m_pos0);

 result.insert(m_pos0, m_data);
 
 return result;
}

void Diff::create(const std::string& old_version, const std::string& new_version)
{
 auto front_mismatch{std::mismatch(old_version.cbegin(), old_version.cend(), new_version.cbegin(), new_version.cend())};
 std::string::difference_type old_pos0 {front_mismatch.first - old_version.cbegin()};
 auto& new_pos0 {old_pos0};

 // equal
 if (old_pos0 == old_version.size() && new_pos0 == new_version.size()) {
  m_pos0 = 0;
  m_pos1 = 0;
  m_data.clear();
  return;
 }

 // append at end
 if (old_pos0 == old_version.size()) {
  m_pos0 = old_pos0;
  m_pos1 = old_pos0;
  m_data = new_version.substr(new_pos0);
  return;
 }

 // remove from end
 if (new_pos0 == new_version.size()) {
  m_pos0 = old_pos0;
  m_pos1 = old_version.size();
  m_data.clear();
  return;
 }

 auto back_mismatch{std::mismatch(old_version.crbegin(), old_version.crend(), new_version.crbegin(), new_version.crend())};
 // i.e. the indices starting from which we can assume equality
 size_t old_pos1 {old_version.size() - (back_mismatch.first - old_version.crbegin())};
 size_t new_pos1 {new_version.size() - (back_mismatch.second - new_version.crbegin())};

 // complete equality is already handled above
 
 // insert at start
 if (old_pos1 == 0) {
  m_pos0 = 0;
  m_pos1 = 0;
  m_data = new_version.substr(0, new_pos1);
  return;
 }

 // remove from start
 if (new_pos1 == 0) {
  m_pos0 = 0;
  m_pos1 = old_pos1;
  m_data.clear();
  return;
 }

 // insert in the middle
 if (old_pos0 == old_pos1) {
  m_pos0 = old_pos0;
  m_pos1 = old_pos0;
  m_data = new_version.substr(new_pos0, new_pos1 - new_pos0);
  return;
 }
 
 // remove from the middle
 if (new_pos0 == new_pos1) {
  m_pos0 = old_pos0;
  m_pos1 = old_pos1;
  m_data.clear();
  return;
 }

 // last resort: remove and add in the middle
 m_pos0 = old_pos0;
 m_pos1 = old_pos1;
 m_data = new_version.substr(old_pos0, new_pos1 - new_pos0);
}

Diff::Diff(const std::string& xml)
{
 create(xml);
}

void Diff::create(const std::string& xml)
{
 pt::ptree tree;
 std::istringstream ss{xml};
 pt::read_xml(ss, tree, pt::xml_parser::no_comments | pt::xml_parser::trim_whitespace);

 m_pos0 = tree.get<int>("diff.start");
 m_pos1 = tree.get<int>("diff.end");

 if (m_pos0 > m_pos1)
  throw std::runtime_error("Bad range in diff");

 m_data = tree.get<std::string>("diff.data");
}

boost::property_tree::ptree Diff::get_structure() const
{
 pt::ptree ptree;
 ptree.put("diff.start", std::to_string(m_pos0));
 ptree.put("diff.end", std::to_string(m_pos1));
 ptree.put("diff.data", m_data);

 return ptree;
}

std::string Diff::get_xml() const
{
 pt::ptree ptree{get_structure()};

 std::ostringstream oss;
 // write_xml_element instead of write_xml to omit <!xml...> header
 //pt::xml_parser::write_xml(oss, xml);
 pt::xml_parser::write_xml_element(oss, {}, ptree, -1, boost::property_tree::xml_writer_settings<pt::ptree::key_type>{});
 return oss.str();
}

extern "C" {

 const char* diff_create(const char* old_version, const char* new_version)
 {
  Diff diff{old_version, new_version};
  return strdup(diff.get_xml().c_str());
 }

 const char* diff_apply(const char* old_version, const char* diff)
 {
  Diff d{diff};

  return strdup(d.apply(old_version).c_str());
 }

}