Select range of bits from string?

Right, seems like I have came up with some seemingly efficient solution which returns the bits as table of booleans.

First thing you should do, is find the bytes that fall within the begin and end bit range, so no processing power is wasted converting unnecessary bytes to bits. After that is done, you can start converting the bytes into bits.

function getBits( str, beginBit, endBit )
	-- A reference table for bit values(reading from table is significantly faster than using power operator)
	local byte_t = { 1, 2, 4, 8, 16, 32, 64, 128 }

	-- Finding out actual bytes that we are interested in
	local beginChar = math.ceil( beginBit/8 )
	local endChar = math.ceil( endBit/8 )
	local range = str:sub( beginChar, endChar )

	-- I will convert and directly set the captured bits, so there is no reason to loop several times and what not
	local bits = {}
	-- Since I will directly store the result, I need to know from which bit should I start storing
	local beginOffset = beginBit%8 - 1
	-- Same applies to the end
	local endOffset = endBit - beginOffset + 1

	for num = 1,#range do
		local byte = str:sub( num, num ):byte()
		-- Figure out how far in the bits table we are
		local offset = (num-1)*8 - beginOffset

		-- I won't really go into details of byte to bit conversion, as I lack experience with these sort of operations in order to explain them properly
		for bit=8, 1, -1 do
			-- Some micro optimisation, probably no big performance gain, but might make a change on bigger captures
			local sub = byte - byte_t[bit]
			-- Current output bit
			local pos = offset + bit

			-- If current bit is smaller than byte, subtract bit from it and set the current bit to true.
			-- Otherwise simply set current output bit to false
			if sub >= 0 then
				-- Save only the bits in the capture range
				if pos > 0 and pos < endOffset then
					bits[pos] = true
				end

				-- But it's still necessary to subtract byte to continue converting it
				byte = sub
			else
				if pos > 0 and pos < endOffset then
					bits[pos] = false
				end
			end
		end
	end

	return bits
end

print( unpack( getBits( "Some content string, serving simply testing purpose", 56, 131  ) ) )

Tried my best to comment it, but I don't have much experience with these sort of manipulations and I'm too sleepy right now.