Aegisub/automation/include/unicode.lua

--[[
 Copyright (c) 2007, Niels Martin Hansen, Rodrigo Braz Monteiro
 All rights reserved.

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-- Unicode (UTF-8) support functions for Aegisub Automation 4 Lua
-- http://www.ietf.org/rfc/rfc2279.txt

module("unicode")

-- Return the number of bytes occupied by the character starting at the i'th byte in s
function charwidth(s, i)
	local b = s:byte(i)
	if b < 128 then
		return 1
	elseif b < 224 then
		return 2
	elseif b < 240 then
		return 3
	elseif b < 248 then
		return 4
	elseif b < 252 then
		return 5
	else
		return 6
	end
	-- Actually there are more possibilities, but those aren't really legal
end

-- Returns an iterator function for iterating over the characters in s
function chars(s)
	local curchar, i = 0, 1
	
	local function itor()
		if i > s:len() then
			return nil
		end
		
		local width = charwidth(s, i)
		local j = i
		curchar = curchar + 1
		i = i + width
		return s:sub(j, i-1), curchar
	end
	
	return itor
end

-- Returns the number of characters in s
-- Runs in O(s:len()) time!
function len(s)
	local n = 0
	for c in chars(s) do
		n = n + 1
	end
	return n
end

-- Get codepoint of first char in s
function codepoint(s)
	-- Basic case, ASCII
	local b = s:byte(1)
	if s:byte(1) < 128 then
		return s:byte(1)
	end
	
	-- Use a naive decoding algorithm, and assume input is valid
	local res, w = 0
	
	if b < 224 then
		-- prefix byte is 110xxxxx
		res = b - 192
		w = 2
	elseif b < 240 then
		-- prefix byte is 11100000
		res = b - 224
		w = 3
	elseif b < 248 then
		-- prefix byte is 11110000
		res = b - 240
		w = 4
	elseif b < 252 then
		-- prefix byte is 11111000
		res = b - 248
		w = 5
	else
		-- prefix byte is 11111100
		res = b - 252
		w = 6
	end
	
	for i = 2, w do
		res = res*64 + s:byte(i) - 128
	end
	
	return res
end