// The Construct
//
// Copyright (C) The Construct Developers, Authors & Contributors
// Copyright (C) 2016-2020 Jason Volk <jason@zemos.net>
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted, provided that the above
// copyright notice and this permission notice is present in all copies. The
// full license for this software is available in the LICENSE file.

namespace ircd::m::dbs
{
	static size_t _prefetch_event_refs_m_room_redaction(const event &, const write_opts &);
	static void _index_event_refs_m_room_redaction(db::txn &, const event &, const write_opts &); //query
	static size_t _prefetch_event_refs_m_receipt_m_read(const event &, const write_opts &);
	static void _index_event_refs_m_receipt_m_read(db::txn &, const event &, const write_opts &); //query
	static size_t _prefetch_event_refs_m_relates_m_reply(const event &, const write_opts &);
	static void _index_event_refs_m_relates_m_reply(db::txn &, const event &, const write_opts &); //query
	static size_t _prefetch_event_refs_m_relates(const event &, const write_opts &);
	static void _index_event_refs_m_relates(db::txn &, const event &, const write_opts &); //query
	static size_t _prefetch_event_refs_state(const event &, const write_opts &);
	static void _index_event_refs_state(db::txn &, const event &, const write_opts &); // query
	static size_t _prefetch_event_refs_auth(const event &, const write_opts &);
	static void _index_event_refs_auth(db::txn &, const event &, const write_opts &); //query
	static size_t _prefetch_event_refs_prev(const event &, const write_opts &);
	static void _index_event_refs_prev(db::txn &, const event &, const write_opts &); //query
	static bool event_refs__cmp_less(const string_view &a, const string_view &b);
}

decltype(ircd::m::dbs::event_refs)
ircd::m::dbs::event_refs;

decltype(ircd::m::dbs::desc::event_refs__comp)
ircd::m::dbs::desc::event_refs__comp
{
	{ "name",     "ircd.m.dbs._event_refs.comp" },
	{ "default",  "default"                     },
};

decltype(ircd::m::dbs::desc::event_refs__block__size)
ircd::m::dbs::desc::event_refs__block__size
{
	{ "name",     "ircd.m.dbs._event_refs.block.size" },
	{ "default",  512L                                },
};

decltype(ircd::m::dbs::desc::event_refs__meta_block__size)
ircd::m::dbs::desc::event_refs__meta_block__size
{
	{ "name",     "ircd.m.dbs._event_refs.meta_block.size" },
	{ "default",  512L                                     },
};

decltype(ircd::m::dbs::desc::event_refs__cache__size)
ircd::m::dbs::desc::event_refs__cache__size
{
	{
		{ "name",     "ircd.m.dbs._event_refs.cache.size" },
		{ "default",  long(32_MiB)                        },
	}, []
	{
		const size_t &value{event_refs__cache__size};
		db::capacity(db::cache(dbs::event_refs), value);
	}
};

decltype(ircd::m::dbs::desc::event_refs__cache_comp__size)
ircd::m::dbs::desc::event_refs__cache_comp__size
{
	{
		{ "name",     "ircd.m.dbs._event_refs.cache_comp.size" },
		{ "default",  long(0_MiB)                              },
	}, []
	{
		const size_t &value{event_refs__cache_comp__size};
		db::capacity(db::cache_compressed(dbs::event_refs), value);
	}
};

const ircd::db::prefix_transform
ircd::m::dbs::desc::event_refs__pfx
{
	"_event_refs",
	[](const string_view &key)
	{
		return size(key) >= sizeof(event::idx) * 2;
	},

	[](const string_view &key)
	{
		assert(size(key) >= sizeof(event::idx));
		return string_view
		{
			data(key), data(key) + sizeof(event::idx)
		};
	}
};

const ircd::db::comparator
ircd::m::dbs::desc::event_refs__cmp
{
	"_event_refs",
	event_refs__cmp_less,
	db::cmp_string_view::equal,
};

const ircd::db::descriptor
ircd::m::dbs::desc::event_refs
{
	// name
	"_event_refs",

	// explanation
	R"(Inverse reference graph of events.

	event_idx | ref, event_idx => --

	The first part of the key is the event being referenced. The second part
	of the key is the event which refers to the first event somewhere in its
	prev_events references. The event_idx in the second part of the key also
	contains a dbs::ref type in its highest order byte so we can store
	different kinds of references.

	The prefix transform is in effect; an event may be referenced multiple
	times. We can find all the events we have which reference a target, and
	why. The database must already contain both events (hence they have
	event::idx numbers).

	The value is currently unused/empty; we may eventually store metadata with
	information about this reference (i.e. is depth adjacent? is the ref
	redundant with another in the same event and should not be made? etc).

	)",

	// typing (key, value)
	{
		typeid(uint64_t), typeid(string_view)
	},

	// options
	{},

	// comparator
	event_refs__cmp,

	// prefix transform
	event_refs__pfx,

	// drop column
	false,

	// cache size
	bool(cache_enable)? -1 : 0, //uses conf item

	// cache size for compressed assets
	bool(cache_comp_enable)? -1 : 0,

	// bloom filter bits
	0,

	// expect queries hit
	true,

	// block size
	size_t(event_refs__block__size),

	// meta_block size
	size_t(event_refs__meta_block__size),

	// compression
	string_view{event_refs__comp},

	// compactor
	{},

	// compaction priority algorithm
	"kOldestSmallestSeqFirst"s,
};

//
// indexers
//

void
ircd::m::dbs::_index_event_refs(db::txn &txn,
                                const event &event,
                                const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));

	if(opts.event_refs.test(uint(ref::NEXT)))
		_index_event_refs_prev(txn, event, opts);

	if(opts.event_refs.test(uint(ref::NEXT_AUTH)))
		_index_event_refs_auth(txn, event, opts);

	if(opts.event_refs.test(uint(ref::NEXT_STATE)) ||
	   opts.event_refs.test(uint(ref::PREV_STATE)))
		_index_event_refs_state(txn, event, opts);

	if(opts.event_refs.test(uint(ref::M_RECEIPT__M_READ)))
		_index_event_refs_m_receipt_m_read(txn, event, opts);

	if(opts.event_refs.test(uint(ref::M_RELATES)))
		_index_event_refs_m_relates(txn, event, opts);

	if(opts.event_refs.test(uint(ref::M_RELATES)))
		_index_event_refs_m_relates_m_reply(txn, event, opts);

	if(opts.event_refs.test(uint(ref::M_ROOM_REDACTION)))
		_index_event_refs_m_room_redaction(txn, event, opts);
}

size_t
ircd::m::dbs::_prefetch_event_refs(const event &event,
                                   const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));

	size_t ret(0);
	if(opts.event_refs.test(uint(ref::NEXT)))
		ret += _prefetch_event_refs_prev(event, opts);

	if(opts.event_refs.test(uint(ref::NEXT_AUTH)))
		ret += _prefetch_event_refs_auth(event, opts);

	if(opts.event_refs.test(uint(ref::NEXT_STATE)) ||
	   opts.event_refs.test(uint(ref::PREV_STATE)))
		ret += _prefetch_event_refs_state(event, opts);

	if(opts.event_refs.test(uint(ref::M_RECEIPT__M_READ)))
		ret += _prefetch_event_refs_m_receipt_m_read(event, opts);

	if(opts.event_refs.test(uint(ref::M_RELATES)))
		ret += _prefetch_event_refs_m_relates(event, opts);

	if(opts.event_refs.test(uint(ref::M_RELATES)))
		ret += _prefetch_event_refs_m_relates_m_reply(event, opts);

	if(opts.event_refs.test(uint(ref::M_ROOM_REDACTION)))
		ret += _prefetch_event_refs_m_room_redaction(event, opts);

	return ret;
}

// NOTE: QUERY
void
ircd::m::dbs::_index_event_refs_prev(db::txn &txn,
                                     const event &event,
                                     const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::NEXT)));

	const event::prev prev
	{
		event
	};

	const size_t count
	{
		std::min(prev.prev_events_count(), event::prev::MAX)
	};

	event::id prev_id[count];
	event::idx prev_idx[count];
	for(size_t i(0); i < count; ++i)
		prev_id[i] = prev.prev_event(i),
		prev_idx[i] = 0;

	const size_t found
	{
		find_event_idx({prev_idx, count}, {prev_id, count}, opts)
	};

	for(size_t i(0); i < count; ++i)
	{
		if(opts.appendix.test(appendix::EVENT_HORIZON) && !prev_idx[i])
		{
			_index_event_horizon(txn, event, opts, prev_id[i]);
			continue;
		}
		else if(!prev_idx[i])
		{
			log::dwarning
			{
				log, "No index found to ref %s PREV of %s",
				string_view{prev_id[i]},
				string_view{event.event_id},
			};

			continue;
		}

		assert(opts.event_idx != prev_idx[i]);
		assert(prev_idx[i] != 0 && opts.event_idx != 0);
		thread_local char buf[EVENT_REFS_KEY_MAX_SIZE];
		const string_view &key
		{
			event_refs_key(buf, prev_idx[i], ref::NEXT, opts.event_idx)
		};

		db::txn::append
		{
			txn, dbs::event_refs,
			{
				opts.op, key
			}
		};
	}
}

size_t
ircd::m::dbs::_prefetch_event_refs_prev(const event &event,
                                        const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::NEXT)));

	const event::prev prev
	{
		event
	};

	const size_t count
	{
		std::min(prev.prev_events_count(), event::prev::MAX)
	};

	event::id prev_id[count];
	for(size_t i(0); i < count; ++i)
		prev_id[i] = prev.prev_event(i);

	return prefetch_event_idx({prev_id, count}, opts);
}

// NOTE: QUERY
void
ircd::m::dbs::_index_event_refs_auth(db::txn &txn,
                                     const event &event,
                                     const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::NEXT_AUTH)));

	if(!m::room::auth::is_power_event(event))
		return;

	const event::auth prev
	{
		event
	};

	const size_t count
	{
		std::min(prev.auth_events_count(), event::prev::MAX)
	};

	event::id auth_id[count];
	event::idx auth_idx[count];
	for(size_t i(0); i < count; ++i)
		auth_id[i] = prev.auth_event(i);

	const auto found
	{
		find_event_idx({auth_idx, count}, {auth_id, count}, opts)
	};

	for(size_t i(0); i < count; ++i)
	{
		if(unlikely(!auth_idx[i]))
		{
			if(opts.appendix.test(appendix::EVENT_HORIZON))
				_index_event_horizon(txn, event, opts, auth_id[i]);

			log::error
			{
				log, "No index found to ref %s AUTH of %s",
				string_view{auth_id[i]},
				string_view{event.event_id}
			};

			continue;
		}

		thread_local char buf[EVENT_REFS_KEY_MAX_SIZE];
		assert(opts.event_idx != 0 && auth_idx[i] != 0);
		assert(opts.event_idx != auth_idx[i]);
		const string_view &key
		{
			event_refs_key(buf, auth_idx[i], ref::NEXT_AUTH, opts.event_idx)
		};

		db::txn::append
		{
			txn, dbs::event_refs,
			{
				opts.op, key
			}
		};
	}
}

size_t
ircd::m::dbs::_prefetch_event_refs_auth(const event &event,
                                        const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::NEXT_AUTH)));

	if(!m::room::auth::is_power_event(event))
		return false;

	const event::auth prev
	{
		event
	};

	const size_t count
	{
		std::min(prev.auth_events_count(), event::prev::MAX)
	};

	event::id auth_id[count];
	for(size_t i(0); i < count; ++i)
		auth_id[i] = prev.auth_event(i);

	return prefetch_event_idx({auth_id, count}, opts);
}

// NOTE: QUERY
void
ircd::m::dbs::_index_event_refs_state(db::txn &txn,
                                      const event &event,
                                      const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::NEXT_STATE)) ||
	       opts.event_refs.test(uint(ref::PREV_STATE)));

	if(!json::get<"room_id"_>(event))
		return;

	if(!json::get<"state_key"_>(event))
		return;

	const m::room room
	{
		at<"room_id"_>(event) //TODO: ABA ABA ABA ABA
	};

	const m::room::state state
	{
		room
	};

	const event::idx &prev_state_idx
	{
		opts.allow_queries?
			state.get(std::nothrow, at<"type"_>(event), at<"state_key"_>(event)): // query
			0UL
	};

	// No previous state; nothing to do.
	if(!prev_state_idx)
		return;

	// If the previous state's event_idx is greater than the event_idx of the
	// event we're transacting this is almost surely a replay/rewrite. Bail
	// out for now rather than corrupting the graph.
	if(unlikely(prev_state_idx >= opts.event_idx))
		return;

	thread_local char buf[EVENT_REFS_KEY_MAX_SIZE];
	assert(opts.event_idx != 0 && prev_state_idx != 0);
	assert(opts.event_idx != prev_state_idx);

	if(opts.event_refs.test(uint(ref::NEXT_STATE)))
	{
		const string_view &key
		{
			event_refs_key(buf, prev_state_idx, ref::NEXT_STATE, opts.event_idx)
		};

		db::txn::append
		{
			txn, dbs::event_refs,
			{
				opts.op, key
			}
		};
	}

	if(opts.event_refs.test(uint(ref::PREV_STATE)))
	{
		const string_view &key
		{
			event_refs_key(buf, opts.event_idx, ref::PREV_STATE, prev_state_idx)
		};

		db::txn::append
		{
			txn, dbs::event_refs,
			{
				opts.op, key
			}
		};
	}
}

size_t
ircd::m::dbs::_prefetch_event_refs_state(const event &event,
                                         const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::NEXT_STATE)) ||
	       opts.event_refs.test(uint(ref::PREV_STATE)));

	assert(json::get<"type"_>(event));
	assert(json::get<"room_id"_>(event));
	if(!json::get<"state_key"_>(event))
		return false;

	const m::room room
	{
		json::get<"room_id"_>(event)
	};

	const m::room::state state
	{
		room
	};

	return state.prefetch(json::get<"type"_>(event), json::get<"state_key"_>(event));
}

// NOTE: QUERY
void
ircd::m::dbs::_index_event_refs_m_receipt_m_read(db::txn &txn,
                                                 const event &event,
                                                 const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::M_RECEIPT__M_READ)));

	if(json::get<"type"_>(event) != "ircd.read")
		return;

	if(!my_host(json::get<"origin"_>(event)))
		return;

	//TODO: disallow local forge?

	const json::string &event_id
	{
		json::get<"content"_>(event).get("event_id")
	};

	if(!valid(m::id::EVENT, event_id))
		return;

	const event::idx &event_idx
	{
		find_event_idx(event_id, opts)
	};

	if(opts.appendix.test(appendix::EVENT_HORIZON) && !event_idx)
	{
		_index_event_horizon(txn, event, opts, event_id);
		return;
	}
	else if(!event_idx)
	{
		log::dwarning
		{
			log, "No index found to ref %s M_RECEIPT__M_READ of %s",
			string_view{event_id},
			string_view{event.event_id}
		};

		return;
	}

	thread_local char buf[EVENT_REFS_KEY_MAX_SIZE];
	assert(opts.event_idx != 0 && event_idx != 0);
	assert(opts.event_idx != event_idx);
	const string_view &key
	{
		event_refs_key(buf, event_idx, ref::M_RECEIPT__M_READ, opts.event_idx)
	};

	db::txn::append
	{
		txn, dbs::event_refs,
		{
			opts.op, key
		}
	};
}

size_t
ircd::m::dbs::_prefetch_event_refs_m_receipt_m_read(const event &event,
                                                    const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::M_RECEIPT__M_READ)));

	if(json::get<"type"_>(event) != "ircd.read")
		return false;

	if(!my_host(json::get<"origin"_>(event)))
		return false;

	const json::string &event_id
	{
		json::get<"content"_>(event).get("event_id")
	};

	if(unlikely(!valid(m::id::EVENT, event_id)))
		return false;

	return prefetch_event_idx(event_id, opts);
}

// NOTE: QUERY
void
ircd::m::dbs::_index_event_refs_m_relates(db::txn &txn,
                                          const event &event,
                                          const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::M_RELATES)));

	const auto &content
	{
		json::get<"content"_>(event)
	};

	if(!content.has("m.relates_to", json::OBJECT))
		return;

	const json::object &m_relates_to
	{
		content["m.relates_to"]
	};

	const json::string &event_id
	{
		m_relates_to["event_id"]
	};

	if(!event_id)
		return;

	if(unlikely(!valid(m::id::EVENT, event_id)))
	{
		log::derror
		{
			log, "Cannot index m.relates_to in %s; '%s' is not an event_id.",
			string_view{event.event_id},
			string_view{event_id}
		};

		return;
	}

	const event::idx &event_idx
	{
		find_event_idx(event_id, opts)
	};

	if(opts.appendix.test(appendix::EVENT_HORIZON) && !event_idx)
	{
		// If we don't have the event being related to yet, place a marker in
		// the event_horizon indicating need for re-evaluation later.
		_index_event_horizon(txn, event, opts, event_id);
		return;
	}
	else if(!event_idx)
	{
		log::derror
		{
			log, "Cannot index m.relates_to in %s; referenced %s not found.",
			string_view{event.event_id},
			string_view{event_id}
		};

		return;
	}

	thread_local char buf[EVENT_REFS_KEY_MAX_SIZE];
	assert(opts.event_idx != 0 && event_idx != 0);
	assert(opts.event_idx != event_idx);
	const string_view &key
	{
		event_refs_key(buf, event_idx, ref::M_RELATES, opts.event_idx)
	};

	db::txn::append
	{
		txn, dbs::event_refs,
		{
			opts.op, key
		}
	};
}

size_t
ircd::m::dbs::_prefetch_event_refs_m_relates(const event &event,
                                             const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::M_RELATES)));

	const auto &content
	{
		json::get<"content"_>(event)
	};

	if(!content.has("m.relates_to", json::OBJECT))
		return false;

	const json::object &m_relates_to
	{
		content["m.relates_to"]
	};

	const json::string &event_id
	{
		m_relates_to["event_id"]
	};

	if(unlikely(!valid(m::id::EVENT, event_id)))
		return false;

	return prefetch_event_idx(event_id, opts);
}

// NOTE: QUERY
void
ircd::m::dbs::_index_event_refs_m_relates_m_reply(db::txn &txn,
                                                  const event &event,
                                                  const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::M_RELATES)));

	if(json::get<"type"_>(event) != "m.room.message")
		return;

	const auto &content
	{
		json::get<"content"_>(event)
	};

	if(!content.has("m.relates_to", json::OBJECT))
		return;

	const json::object &m_relates_to
	{
		content["m.relates_to"]
	};

	if(!m_relates_to.has("m.in_reply_to", json::OBJECT))
		return;

	const json::object &m_in_reply_to
	{
		m_relates_to["m.in_reply_to"]
	};

	const json::string &event_id
	{
		m_in_reply_to["event_id"]
	};

	if(unlikely(!valid(m::id::EVENT, event_id)))
	{
		log::derror
		{
			log, "Cannot index m.in_reply_to in %s; '%s' is not an event_id.",
			string_view{event.event_id},
			string_view{event_id}
		};

		return;
	}

	const event::idx &event_idx
	{
		find_event_idx(event_id, opts)
	};

	if(opts.appendix.test(appendix::EVENT_HORIZON) && !event_idx)
	{
		_index_event_horizon(txn, event, opts, event_id);
		return;
	}
	else if(!event_idx)
	{
		log::dwarning
		{
			log, "Cannot index m.in_reply_to in %s; referenced %s not found.",
			string_view{event.event_id},
			string_view{event_id}
		};

		return;
	}

	thread_local char buf[EVENT_REFS_KEY_MAX_SIZE];
	assert(opts.event_idx != 0 && event_idx != 0);
	assert(opts.event_idx != event_idx);
	const string_view &key
	{
		event_refs_key(buf, event_idx, ref::M_RELATES, opts.event_idx)
	};

	db::txn::append
	{
		txn, dbs::event_refs,
		{
			opts.op, key
		}
	};
}

size_t
ircd::m::dbs::_prefetch_event_refs_m_relates_m_reply(const event &event,
                                                     const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::M_RELATES)));

	if(json::get<"type"_>(event) != "m.room.message")
		return false;

	const auto &content
	{
		json::get<"content"_>(event)
	};

	if(!content.has("m.relates_to", json::OBJECT))
		return false;

	const json::object &m_relates_to
	{
		content["m.relates_to"]
	};

	if(!m_relates_to.has("m.in_reply_to", json::OBJECT))
		return false;

	const json::object &m_in_reply_to
	{
		m_relates_to["m.in_reply_to"]
	};

	const json::string &event_id
	{
		m_in_reply_to["event_id"]
	};

	if(unlikely(!valid(m::id::EVENT, event_id)))
		return false;

	return prefetch_event_idx(event_id, opts);
}

// NOTE: QUERY
void
ircd::m::dbs::_index_event_refs_m_room_redaction(db::txn &txn,
                                                 const event &event,
                                                 const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::M_ROOM_REDACTION)));

	if(json::get<"type"_>(event) != "m.room.redaction")
		return;

	if(!valid(m::id::EVENT, json::get<"redacts"_>(event)))
		return;

	const auto &event_id
	{
		json::get<"redacts"_>(event)
	};

	const event::idx &event_idx
	{
		find_event_idx(event_id, opts)
	};

	if(opts.appendix.test(appendix::EVENT_HORIZON) && !event_idx)
	{
		_index_event_horizon(txn, event, opts, event_id);
		return;
	}
	else if(!event_idx)
	{
		log::dwarning
		{
			log, "Cannot index m.room.redaction in %s; referenced %s not found.",
			string_view{event.event_id},
			string_view{event_id}
		};

		return;
	}

	thread_local char buf[EVENT_REFS_KEY_MAX_SIZE];
	assert(opts.event_idx != 0 && event_idx != 0);
	assert(opts.event_idx != event_idx);
	const string_view &key
	{
		event_refs_key(buf, event_idx, ref::M_ROOM_REDACTION, opts.event_idx)
	};

	db::txn::append
	{
		txn, dbs::event_refs,
		{
			opts.op, key
		}
	};
}

size_t
ircd::m::dbs::_prefetch_event_refs_m_room_redaction(const event &event,
                                                    const write_opts &opts)
{
	assert(opts.appendix.test(appendix::EVENT_REFS));
	assert(opts.event_refs.test(uint(ref::M_ROOM_REDACTION)));

	if(json::get<"type"_>(event) != "m.room.redaction")
		return false;

	const auto &event_id
	{
		json::get<"redacts"_>(event)
	};

	if(!valid(m::id::EVENT, event_id))
		return false;

	return prefetch_event_idx(event_id, opts);
}

//
// cmp
//

bool
ircd::m::dbs::event_refs__cmp_less(const string_view &a,
                                   const string_view &b)
{
	static const size_t half(sizeof(event::idx));
	static const size_t full(half * 2);

	assert(size(a) >= half);
	assert(size(b) >= half);
	const event::idx *const key[2]
	{
		reinterpret_cast<const event::idx *>(data(a)),
		reinterpret_cast<const event::idx *>(data(b)),
	};

	return
		key[0][0] < key[1][0]?   true:
		key[0][0] > key[1][0]?   false:
		size(a) < size(b)?       true:
		size(a) > size(b)?       false:
		size(a) == half?         false:
		key[0][1] < key[1][1]?   true:
		                         false;
}

//
// key
//

std::tuple<ircd::m::dbs::ref, ircd::m::event::idx>
ircd::m::dbs::event_refs_key(const string_view &amalgam)
{
	const event::idx key
	{
		byte_view<event::idx>{amalgam}
	};

	return
	{
		ref(key >> ref_shift), key & ~ref_mask
	};
}

ircd::string_view
ircd::m::dbs::event_refs_key(const mutable_buffer &out,
                             const event::idx &tgt,
                             const ref &type,
                             const event::idx &src)
{
	assert((src & ref_mask) == 0);
	assert(size(out) >= sizeof(event::idx) * 2);
	event::idx *const &key
	{
		reinterpret_cast<event::idx *>(data(out))
	};

	key[0] = tgt;
	key[1] = src;
	key[1] |= uint64_t(type) << ref_shift;
	return string_view
	{
		data(out), data(out) + sizeof(event::idx) * 2
	};
}

//
// util
//

ircd::string_view
ircd::m::dbs::reflect(const ref &type)
{
	switch(type)
	{
		case ref::NEXT:                  return "NEXT";
		case ref::NEXT_AUTH:             return "NEXT_AUTH";
		case ref::NEXT_STATE:            return "NEXT_STATE";
		case ref::PREV_STATE:            return "PREV_STATE";
		case ref::M_RECEIPT__M_READ:     return "M_RECEIPT__M_READ";
		case ref::M_RELATES:             return "M_RELATES";
		case ref::M_ROOM_REDACTION:      return "M_ROOM_REDACTION";
	}

	return "????";
}