--- lam.utf8 --- I did not write this code.  I did lightly edit it to work with
-- this project.  This module was written by GitHub user meepen and is under the
-- CC0 1.0 license.  The source can be found here:
-- https://github.com/meepen/Lua-5.1-UTF-8/
local utf8 = {}
local bit = bit
local error = error
local ipairs = ipairs
local string = string
local table = table
local unpack = table.unpack or unpack

-- Pattern that can be used with the string library to match a single UTF-8
-- byte-sequence.  This expects the string to contain valid UTF-8 data.
utf8.charpattern = "[%z\x01-\x7F\xC2-\xF4][\x80-\xBF]*"

-- Transforms indexes of a string to be positive.  Negative indices will wrap
-- around like the string library's functions.
local function strRelToAbs(str, ...)
	local args = { ... }
	for k, v in ipairs(args) do
		v = v > 0 and v or #str + v + 1
		if v < 1 or v > #str then
			error("bad index to string (out of range)", 3)
		end
		args[ k ] = v
	end
	return unpack(args)
end

-- Decodes a single UTF-8 byte-sequence from a string, ensuring it is valid.
-- Returns the index of the first and last character of the sequence
local function decode(str, startPos)
	startPos = strRelToAbs(str, startPos or 1)
	local b1 = str:byte(startPos, startPos)
	-- Single-byte sequence
	if b1 < 0x80 then
		return startPos, startPos
	end
	-- Validate first byte of multi-byte sequence
	if b1 > 0xF4 or b1 < 0xC2 then
		return nil
	end
	-- Get 'supposed' amount of continuation bytes from primary byte
	local contByteCount =	b1 >= 0xF0 and 3 or
		b1 >= 0xE0 and 2 or
		b1 >= 0xC0 and 1
	local endPos = startPos + contByteCount
	-- Validate our continuation bytes
	for _, bX in ipairs { str:byte(startPos + 1, endPos) } do
		if bit.band(bX, 0xC0) ~= 0x80 then
			return nil
		end
	end
	return startPos, endPos
end

-- Takes zero or more integers and returns a string containing the UTF-8
-- representation of each
function utf8.char(...)
	local buf = {}
	for k, v in ipairs { ... } do
		if v < 0 or v > 0x10FFFF then
			error("bad argument #" .. k ..
			       " to char (out of range)", 2)
		end
		local b1, b2, b3, b4 = nil, nil, nil, nil
		if v < 0x80 then -- Single-byte sequence
			table.insert(buf, string.char(v))
		elseif v < 0x800 then -- Two-byte sequence
			b1 = bit.bor(0xC0, bit.band(bit.rshift(v, 6), 0x1F))
			b2 = bit.bor(0x80, bit.band(v, 0x3F))
			table.insert(buf, string.char(b1, b2))
		elseif v < 0x10000 then -- Three-byte sequence
			b1 = bit.bor(0xE0, bit.band(bit.rshift(v, 12), 0x0F))
			b2 = bit.bor(0x80, bit.band(bit.rshift(v, 6), 0x3F))
			b3 = bit.bor(0x80, bit.band(v, 0x3F))
			table.insert(buf, string.char(b1, b2, b3))
		else -- Four-byte sequence
			b1 = bit.bor(0xF0, bit.band(bit.rshift(v, 18), 0x07))
			b2 = bit.bor(0x80, bit.band(bit.rshift(v, 12), 0x3F))
			b3 = bit.bor(0x80, bit.band(bit.rshift(v, 6), 0x3F))
			b4 = bit.bor(0x80, bit.band(v, 0x3F))
			table.insert(buf, string.char(b1, b2, b3, b4))
		end
	end
	return table.concat(buf, "")
end

-- Iterates over a UTF-8 string similarly to pairs.
-- k = index of sequence, v = string value of sequence
function utf8.codes(str)
	local i = 1
	return function()
		-- Have we hit the end of the iteration set?
		if i > #str then
			return nil
		end
		local startPos, endPos = decode(str, i)
		if not startPos then
			error("invalid UTF-8 code", 2)
		end
		i = endPos + 1
		return startPos, str:sub(startPos, endPos)
	end
end

-- Returns an integer-representation of the UTF-8 sequence(s) in a string
-- startPos defaults to 1, endPos defaults to startPos
function utf8.codepoint(str, startPos, endPos)
	startPos, endPos = strRelToAbs(str,
				       startPos or 1,
				       endPos or startPos or 1)
	local ret = {}
	repeat
		local seqStartPos, seqEndPos = decode(str, startPos)
		if not seqStartPos then
			error("invalid UTF-8 code", 2)
		end
		-- Increment current string index
		startPos = seqEndPos + 1
		-- Amount of bytes making up our sequence
		local len = seqEndPos - seqStartPos + 1
		if len == 1 then -- Single-byte codepoint
			table.insert(ret, str:byte(seqStartPos))
		else -- Multi-byte codepoint
			local b1 = str:byte(seqStartPos)
			local cp = 0
			for i = seqStartPos + 1, seqEndPos do
				local bX = str:byte(i)
				cp = bit.bor(bit.lshift(cp, 6),
					     bit.band(bX, 0x3F))
				b1 = bit.lshift(b1, 1)
			end
			cp = bit.bor(cp, bit.lshift(bit.band(b1, 0x7F),
						    (len - 1) * 5))
			table.insert(ret, cp)
		end
	until seqEndPos >= endPos
	return unpack(ret)
end

-- Returns the length of a UTF-8 string. false, index is returned if an invalid
-- sequence is hit startPos defaults to 1, endPos defaults to -1
function utf8.len(str, startPos, endPos)
	startPos, endPos = strRelToAbs(str, startPos or 1, endPos or -1)
	local len = 0
	repeat
		local seqStartPos, seqEndPos = decode(str, startPos)
		-- Hit an invalid sequence?
		if not seqStartPos then
			return false, startPos
		end
		-- Increment current string pointer
		startPos = seqEndPos + 1
		-- Increment length
		len = len + 1
	until seqEndPos >= endPos
	return len
end

-- Returns the byte-index of the n'th UTF-8-character after the given byte-index
-- (nil if none).  startPos defaults to 1 when n is positive and -1 when n is
-- negative.  If 0 is zero, this function instead returns the byte-index of the
-- UTF-8-character startPos lies within.
function utf8.offset(str, n, startPos)
	startPos = strRelToAbs(str, startPos or (n >= 0 and 1) or #str)
	-- Find the beginning of the sequence over startPos
	if n == 0 then
		for i = startPos, 1, -1 do
			local seqStartPos, seqEndPos = decode(str, i)
			if seqStartPos then
				return seqStartPos
			end
		end
		return nil
	end
	if not decode(str, startPos) then
		error("initial position is not beginning of a valid sequence",
		      2)
	end
	local itStart, itEnd, itStep = nil, nil, nil
	if n > 0 then -- Find the beginning of the n'th sequence forwards
		itStart = startPos
		itEnd = #str
		itStep = 1
	else -- Find the beginning of the n'th sequence backwards
		n = -n
		itStart = startPos
		itEnd = 1
		itStep = -1
	end
	for i = itStart, itEnd, itStep do
		local seqStartPos, seqEndPos = decode(str, i)
		if seqStartPos then
			n = n - 1
			if n == 0 then
				return seqStartPos
			end
		end
	end
	return nil
end

-- Forces a string to contain only valid UTF-8 data.
-- Invalid sequences are replaced with U+FFFD.
function utf8.force(str)
	local buf = {}
	local curPos, endPos = 1, #str
	repeat
		local seqStartPos, seqEndPos = decode(str, curPos)
		if not seqStartPos then
			table.insert(buf, char(0xFFFD))
			curPos = curPos + 1
		else
			table.insert(buf, str:sub(seqStartPos, seqEndPos))
			curPos = seqEndPos + 1
		end
	until curPos > endPos
	return table.concat(buf, "")
end

---
return utf8