lz4.lua

--!strict
-- metatablecat 2022

local lz4 = {}

type Streamer = {
	Offset: number,
	Source: string,
	Length: number,
	IsFinished: boolean,
	LastUnreadBytes: number,

	read: (Streamer, len: number?, shiftOffset: boolean?) -> string,
	seek: (Streamer, len: number) -> (),
	append: (Streamer, newData: string) -> (),
	toEnd: (Streamer) -> ()
}

type BlockData = {
	[number]: {
		Literal: string,
		LiteralLength: number,
		MatchOffset: number?,
		MatchLength: number?
	}
}

local function plainFind(str, pat)
	return string.find(str, pat, 0, true)
end

local function streamer(str): Streamer
	local Stream = {}
	Stream.Offset = 0
	Stream.Source = str
	Stream.Length = string.len(str)
	Stream.IsFinished = false	
	Stream.LastUnreadBytes = 0

	function Stream.read(self: Streamer, len: number?, shift: boolean?): string
		local len = len or 1
		local shift = if shift ~= nil then shift else true
		local dat = string.sub(self.Source, self.Offset + 1, self.Offset + len)

		local dataLength = string.len(dat)
		local unreadBytes = len - dataLength

		if shift then
			self:seek(len)
		end

		self.LastUnreadBytes = unreadBytes
		return dat
	end

	function Stream.seek(self: Streamer, len: number)
		local len = len or 1

		self.Offset = math.clamp(self.Offset + len, 0, self.Length)
		self.IsFinished = self.Offset >= self.Length
	end

	function Stream.append(self: Streamer, newData: string)
		-- adds new data to the end of a stream
		self.Source ..= newData
		self.Length = string.len(self.Source)
		self:seek(0) --hacky but forces a recalculation of the isFinished flag
	end

	function Stream.toEnd(self: Streamer)
		self:seek(self.Length)
	end

	return Stream
end

function lz4.compress(str: string): string
	local blocks: BlockData = {}
	local iostream = streamer(str)

	if iostream.Length > 12 then
		local firstFour = iostream:read(4)

		local processed = firstFour
		local lit = firstFour
		local match = ""
		local LiteralPushValue = ""
		local pushToLiteral = true

		repeat
			pushToLiteral = true
			local nextByte = iostream:read()

			if plainFind(processed, nextByte) then
				local next3 = iostream:read(3, false)

				if string.len(next3) < 3 then
					--push bytes to literal block then break
					LiteralPushValue = nextByte .. next3
					iostream:seek(3)
				else
					match = nextByte .. next3

					local matchPos = plainFind(processed, match)
					if matchPos then
						iostream:seek(3)
						repeat
							local nextMatchByte = iostream:read(1, false)
							local newResult = match .. nextMatchByte

							local repos = plainFind(processed, newResult) 
							if repos then
								match = newResult
								matchPos = repos
								iostream:seek(1)
							end
						until not plainFind(processed, newResult) or iostream.IsFinished

						local matchLen = string.len(match)
						local pushMatch = true

						if iostream.Length - iostream.Offset <= 5 then
							LiteralPushValue = match
							pushMatch = false
							--better safe here, dont bother pushing to match ever
						end

						if pushMatch then
							pushToLiteral = false

							-- gets the position from the end of processed, then slaps it onto processed
							local realPosition = string.len(processed) - matchPos
							processed = processed .. match

							table.insert(blocks, {
								Literal = lit,
								LiteralLength = string.len(lit),
								MatchOffset = realPosition + 1,
								MatchLength = matchLen,
							})
							lit = ""
						end
					else
						LiteralPushValue = nextByte
					end
				end
			else
				LiteralPushValue = nextByte
			end

			if pushToLiteral then
				lit = lit .. LiteralPushValue
				processed = processed .. nextByte
			end
		until iostream.IsFinished
		table.insert(blocks, {
			Literal = lit,
			LiteralLength = string.len(lit)
		})
	else
		local str = iostream.Source
		blocks[1] = {
			Literal = str,
			LiteralLength = string.len(str)
		}
	end

	-- generate the output chunk
	-- %s is for adding header
	local output = string.rep("\x00", 4)
	local function write(char)
		output = output .. char
	end
	-- begin working through chunks
	for chunkNum, chunk in blocks do
		local litLen = chunk.LiteralLength
		local matLen = (chunk.MatchLength or 4) - 4

		-- create token
		local tokenLit = math.clamp(litLen, 0, 15)
		local tokenMat = math.clamp(matLen, 0, 15)

		local token = bit32.lshift(tokenLit, 4) + tokenMat
		write(string.pack("<I1", token))

		if litLen >= 15 then
			litLen = litLen - 15
			--begin packing extra bytes
			repeat
				local nextToken = math.clamp(litLen, 0, 0xFF)
				write(string.pack("<I1", nextToken))
				if nextToken == 0xFF then
					litLen = litLen - 255
				end
			until nextToken < 0xFF
		end

		-- push raw lit data
		write(chunk.Literal)

		if chunkNum ~= #blocks then
			-- push offset as u16
			write(string.pack("<I2", chunk.MatchOffset))

			-- pack extra match bytes
			if matLen >= 15 then
				matLen = matLen - 15

				repeat
					local nextToken = math.clamp(matLen, 0, 0xFF)
					write(string.pack("<I1", nextToken))
					if nextToken == 0xFF then
						matLen = matLen - 255
					end
				until nextToken < 0xFF
			end
		end
	end
	--append chunks
	local compLen = string.len(output) - 4
	local decompLen = iostream.Length

	return string.pack("<I4", compLen) .. string.pack("<I4", decompLen) .. output
end

function lz4.decompress(lz4data: string): string
	local inputStream = streamer(lz4data)

	local compressedLen = string.unpack("<I4", inputStream:read(4))
	local decompressedLen = string.unpack("<I4", inputStream:read(4))
	local reserved = string.unpack("<I4", inputStream:read(4))

	if compressedLen == 0 then
		return inputStream:read(decompressedLen)
	end

	local outputStream = streamer("")

	repeat
		local token = string.byte(inputStream:read())
		local litLen = bit32.rshift(token, 4)
		local matLen = bit32.band(token, 15) + 4

		if litLen >= 15 then
			repeat
				local nextByte = string.byte(inputStream:read())
				litLen += nextByte
			until nextByte ~= 0xFF
		end

		local literal = inputStream:read(litLen)
		outputStream:append(literal)
		outputStream:toEnd()
		if outputStream.Length < decompressedLen then
			--match
			local offset = string.unpack("<I2", inputStream:read(2))
			if matLen >= 19 then
				repeat
					local nextByte = string.byte(inputStream:read())
					matLen += nextByte
				until nextByte ~= 0xFF
			end

			outputStream:seek(-offset)
			local pos = outputStream.Offset
			local match = outputStream:read(matLen)
			local unreadBytes = outputStream.LastUnreadBytes
			local extra
			if unreadBytes then
				repeat
					outputStream.Offset = pos
					extra = outputStream:read(unreadBytes)
					unreadBytes = outputStream.LastUnreadBytes
					match ..= extra
				until unreadBytes <= 0
			end

			outputStream:append(match)
			outputStream:toEnd()
		end

	until outputStream.Length >= decompressedLen

	return outputStream.Source
end

return lz4

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