# The `DODB::Storage` abstract class defines the specifications of
# subsequent DODB databases (uncached, cached, RAM-only, etc.).
abstract class DODB::Storage(V)
	# List of triggers (basic indexes, partitions, tags, etc.).
	property triggers = [] of Trigger(V)

	property cached_last_key : Int32

	# Directory where data and triggers will be written.
	property directory_name : String

	# Creates a database.
	#
	# A DODB database is instanciated with a *path* where data will be written.
	# Another directory is created where locks can be written.
	# In case the database is empty, the *last_key* is set to *-1*.
	def initialize(@directory_name : String)
		Dir.mkdir_p data_path
		Dir.mkdir_p locks_directory

		@cached_last_key = init_last_key
	end

	# Requests a (named) lock.
	# Locks prevent concurrent access to the same data.
	#
	# In case of a request for a lock that is already in use,
	# wait for a millisecond then retry, loop until it works.
	# A lock is simply an opened file with the `LibC::O_EXCL` flag.
	def request_lock(name, subname = nil)
		r = -1
		file_path = get_lock_file_path name, subname
		file_perms = 0o644

		flags = LibC::O_EXCL | LibC::O_CREAT
		while (r = LibC.open file_path, flags, file_perms) == -1
			sleep 1.milliseconds
		end

		LibC.close r
	end

	# Releases a (named) lock.
	#
	# The implementation is simple, it just removes the file.
	def release_lock(name, subname = nil)
		File.delete get_lock_file_path name, subname
	end

	private def key_file
		"#{@directory_name}/last-key"
	end

	# Reads the last database *key* from the storage device.
	def init_last_key : Int32
		File.read(key_file).to_i
	rescue
		-1
	end

	# Reads the (cached) last key.
	def last_key : Int32
		@cached_last_key
	end

	# Changes the last *key* in the database.
	def last_key=(x : Int32)
		file = File.open(key_file, "w")
		file << x.to_s
		file.close

		@cached_last_key = x

		x
	rescue
		raise Exception.new "could not update last-key file"
	end

	# Take a database key and convert it in a formated string. Example: 343 -> "0000000343"
	def stringify_key(key : Int32)
		# Negative numbers give strange results with Crystal’s printf.
		if key >= 0
			"%010i" % key
		else
			key.to_s
		end
	end

	# Adds a value to the database without a locking mechanism.
	#
	# For a thread-safe version, use `#<<`.
	#
	# WARNING: not thread-safe.
	def unsafe_add(item : V)
		key = last_key + 1

		self[key] = item
		self.last_key = key

		key # FIXME: Should we really return the internal key?
	rescue e
		raise e
	end

	# Adds a value to the database, with a locking mechanism to prevent race conditions.
	#
	# This operation should be thread-safe since a lock is required before tinkering with the database.
	# Because of the file-system operations, this function may be a bit slow.
	# For single-thread applications, use the `#unsafe_add` operation instead.
	def <<(item : V)
		request_lock "key"
		key = init_last_key + 1
		self[key] = item
		self.last_key = key

		release_lock "key"

		key # FIXME: Should we really return the internal key?
	rescue e
		release_lock "key"

		raise e
	end

	# Lists all entries in the database.
	#
	# WARNING: Very slow. Try not to use.
	def each(reversed : Bool = false, offset = 0, limit : Int32? = nil)
		each_with_key(
			reversed: reversed,
			offset: offset,
			limit: limit
		) do |item, key|
			yield item
		end
	end

	# Converts all the database into an array.
	#
	# WARNING: Very slow. Try not to use.
	def to_a(reversed : Bool = false, offset = 0, limit : Int32? = nil)
		array = ::Array(V).new

		each(
			reversed: reversed,
			offset: offset,
			limit: limit
		) do |value|
			array << value
		end

		array
	end

	# Converts the entire database into a hash.
	#
	# WARNING: Very slow. Try not to use.
	def to_h(reversed : Bool = false, offset = 0, limit : Int32? = nil)
		hash = ::Hash(Int32, V).new

		each_with_key(
			reversed: reversed,
			offset: offset,
			limit: limit
		) do |element, key|
			hash[key] = element
		end

		hash
	end

	# Run triggers (indexes, partitions, tags, etc.) for a value.
	def run_triggers(key : Int32, value : V)
		@triggers.each &.index(stringify_key(key), value)
	end

	# Creates a new basic index **with a cache**.
	# The *name* parameter is the name of the directory that will be created.
	def new_index(name : String, &block : Proc(V, String | DODB::NoIndex))
		Trigger::IndexCached(V).new(self, @directory_name, name, block).tap do |trigger|
			@triggers << trigger
		end
	end

	# Creates a new basic index **without a cache**.
	# The *name* parameter is the name of the directory that will be created.
	#
	# NOTE: this will be a lot slower than the cached version.
	def new_uncached_index(name : String, &block : Proc(V, String | DODB::NoIndex))
		Trigger::Index(V).new(self, @directory_name, name, block).tap do |trigger|
			@triggers << trigger
		end
	end

	# Creates a new basic index **only in RAM**.
	# The *name* parameter is the name of the directory that will be created.
	#
	# NOTE: this index is the fastest, but doesn't have a file-system representation.
	def new_RAM_index(name : String, &block : Proc(V, String | DODB::NoIndex))
		Trigger::IndexRAMOnly(V).new(self, @directory_name, name, block).tap do |trigger|
			@triggers << trigger
		end
	end

	# Gets an *index object* based on its name.
	def get_index(name : String, key)
		index = @triggers.find &.name.==(name)
		index.not_nil!.as(Trigger).get key
	end

	# Creates a new partition **with a cache**.
	# The *name* parameter is the name of the directory that will be created.
	def new_partition(name : String, &block : Proc(V, String | DODB::NoIndex))
		Trigger::PartitionCached(V).new(self, @directory_name, name, block).tap do |table|
			@triggers << table
		end
	end

	# Creates a new partition **without a cache**.
	# The *name* parameter is the name of the directory that will be created.
	#
	# NOTE: this will be a lot slower than the cached version.
	def new_uncached_partition(name : String, &block : Proc(V, String | DODB::NoIndex))
		Trigger::Partition(V).new(self, @directory_name, name, block).tap do |table|
			@triggers << table
		end
	end

	# Creates a new partition **only in RAM**.
	# The *name* parameter is the name of the directory that will be created.
	#
	# NOTE: this partition index is the fastest but doesn't have a file-system representation.
	def new_RAM_partition(name : String, &block : Proc(V, String | DODB::NoIndex))
		Trigger::PartitionRAMOnly(V).new(self, @directory_name, name, block).tap do |table|
			@triggers << table
		end
	end

	# Gets an *index (partition) object* based on its name.
	def get_partition(table_name : String, partition_name : String)
		partition = @triggers.find &.name.==(table_name)
		partition.not_nil!.as(DODB::Partition).get partition_name
	end

	# Creates a new tag **with a cache**.
	# The *name* parameter is the name of the directory that will be created.
	def new_tags(name : String, &block : Proc(V, Array(String) | DODB::NoIndex))
		Trigger::TagsCached(V).new(self, @directory_name, name, block).tap do |tags|
			@triggers << tags
		end
	end

	# Creates a new tag **without a cache**.
	# The *name* parameter is the name of the directory that will be created.
	#
	# NOTE: this will be a lot slower than the cached version.
	def new_uncached_tags(name : String, &block : Proc(V, Array(String) | DODB::NoIndex))
		Trigger::Tags(V).new(self, @directory_name, name, block).tap do |tags|
			@triggers << tags
		end
	end

	# Creates a new partition **only in RAM**.
	# The *name* parameter is the name of the directory that will be created.
	#
	# NOTE: this tag index is the fastest but doesn't have a file-system representation.
	def new_RAM_tags(name : String, &block : Proc(V, Array(String) | DODB::NoIndex))
		Trigger::TagsRAMOnly(V).new(self, @directory_name, name, block).tap do |tags|
			@triggers << tags
		end
	end

	# Gets an *index (tag) object* based on its name.
	def get_tags(name, key : String)
		tag = @triggers.find &.name.==(name)
		tag.not_nil!.as(DODB::Tags).get name, key
	end

	# WARNING: directed graphs haven't been reviewed in YEARS, assume as dead code.
	def new_directed_graph(name : String, index : DODB::Trigger(V), &block : Proc(V, Array(String))) : DirectedGraph(V)
		Trigger::DirectedGraph(V).new(self, @directory_name, index, name, block).tap do |table|
			@triggers << table
		end
	end

	# Checks for collisions in the indexes.
	def check_collisions!(key : Int32, value : V, old_value : V?)
		@triggers.each &.check!(stringify_key(key), value, old_value)
	end

	# Retrieves a value and remove it from the database.
	def pop
		request_lock "key"

		key = last_key

		# Some entries may have been removed. We’ll skip over those.
		# Not the most efficient if a large number of indices are empty.
		while key >= 0 && self[key]?.nil?
			key = key - 1
		end

		if key < 0
			return nil
		end

		poped = self[key]

		self.unsafe_delete key

		last_key = key - 1

		release_lock "key"

		poped
	end

	private def data_path
		"#{@directory_name}/data"
	end

	private def file_path(key : Int32)
		"#{data_path}/%010i" % key
	end

	private def locks_directory : String
		"#{@directory_name}/locks"
	end

	private def get_lock_file_path(name : String, subname : String? = nil)
		if subname
			"#{locks_directory}/#{name}-#{subname}.lock" # FIXME: Separator that causes less collisions?
		else
			"#{locks_directory}/#{name}.lock"
		end
	end

	private def read(file_path : String)
		V.from_json ::File.read file_path
	end

	private def remove_data!
		FileUtils.rm_rf data_path
		Dir.mkdir_p data_path
	end

	private def nuke_triggers!
		@triggers.each do |trigger|
			trigger.nuke_trigger
		end
	end

	private def clear_storage_cache!
		puts "DODB::Storage(V) clear_storage_cache! (no cache)"
		# There's no cache by default.
		# This function has to be changed in storage implementations.
	end

	# NOTE: clears all caches (`storage` and `triggers`).
	def clear_cache!
		clear_storage_cache!
		@triggers.each do |trigger|
			trigger.clear_cache!
		end
	end

	# Removes all indices and then rewrites them all.
	#
	# WARNING: slow operation.
	# NOTE: clears all caches (`storage` and `triggers`).
	def reindex_everything!
		clear_cache!
		nuke_triggers!

		each_with_key() do |item, key|
			run_triggers key, item
		end
	end

	# Removes all indexes of a value.
	def remove_triggers(key : Int32, value : V)
		@triggers.each &.deindex(stringify_key(key), value)
	end

	# Gets the data with the *key*.
	# In case the data is missing, returns *nil*.
	def []?(key : Int32) : V?
		self[key]
	rescue MissingEntry
		# FIXME: Only rescue JSON and “no such file” errors.
		return nil
	end

	# Gets the data with the *key*.
	# In case the data is missing, returns an exception `DODB::MissingEntry`.
	def [](key : Int32) : V
		raise MissingEntry.new(key) unless ::File.exists? file_path key
		read file_path key
	end

	# Replaces the data with the *key*.
	# In case the data is missing, returns an exception `DODB::MissingEntry`.
	def []=(key : Int32, value : V)
		old_value = self.[key]?

		check_collisions! key, value, old_value

		# Removes any old indices or partitions pointing to a value about
		# to be replaced.
		if old_value
			remove_triggers key, old_value
		end

		# Avoids corruption in case the application crashes while writing.
		file_path(key).tap do |path|
			::File.write "#{path}.new", value.to_json
			::FileUtils.mv "#{path}.new", path
		end

		run_triggers key, value

		if key > @cached_last_key
			self.last_key = key
		end
	end

	# Deletes the data with the *key* but doesn't request for a lock.
	# This function is required since `#pop` is already locked before trying to remove an entry,
	# thus calling `#unsafe_delete`.
	def unsafe_delete(key : Int32)
		value = self[key]?

		return if value.nil?

		begin
			::File.delete file_path key
		rescue File::NotFoundError
		end

		remove_triggers key, value

		value
	end

	# Deletes the data with the *key*.
	def delete(key : Int32)
		request_lock "key"
		value = unsafe_delete key
		release_lock "key"

		value
	end

	# Lists all the keys in the database.
	def each_key_from_fs(reversed = false)
		# Removes the first two "." and ".." directories.
		keys = Dir.children(data_path).map(&.to_i).sort
		(reversed ? keys.reverse : keys).each do |key|
			yield key
		end
	end

	# Lists all database entries with their key.
	# Can be useful for making dumps or to restore a database.
	#
	# WARNING: Very slow. Try not to use.
	def each_with_key(reversed : Bool = false, offset = 0, limit : Int32? = -1)
		limit = if l = limit
			l
		else
			-1
		end

		each_key_from_fs(reversed) do |key|
			offset -= 1 if offset >= 0
			next if offset >= 0

			return if limit == 0
			limit -= 1 if limit > 0

			begin
				# FIXME: Only intercept JSON parsing errors.
				value = self[key]
			rescue
				next
			end

			yield value, key
		end
	end
end

require "./storage/*"