Module:string/gline

local error = error
local find = string.find
local gmatch = string.gmatch
local match = string.match
local sub = string.sub

--[==[
Iterates over the lines in a string, treating {"\n"}, {"\r"} and {"\r\n"} as new lines.

The optional {skip} parameter determines whether certain lines are skipped:
* {NONE}: none (default).
* {EMPTY}: empty lines: lines with a length of 0, consisting of no characters.
* {BLANK}: blank lines: empty lines and lines which only consist of whitespace characters.]==]
return function(str, skip)
	-- Use gmatch() for EMPTY and BLANK, for speed.
	if skip == "EMPTY" then
		return gmatch(str, "+")
	elseif skip == "BLANK" then
		return gmatch(str, "-%S*")
	elseif not (skip == nil or skip == "NONE") then
		error("bad argument #2 to 'string/gline' (expected NONE, EMPTY or BLANK)", 2)
	end
	local start, linechar, flag = 1
	if not find(str, "\n", nil, true) then
		if not find(str, "\r", nil, true) then
			-- No newlines: return `str` on the first iteration then end.
			return function()
				if flag then
					return
				end
				flag = true
				return str
			end
		end
		linechar = "\r"
	elseif find(str, "\r", nil, true) then
		-- Newline could be "\n", "\r" or "\r\n" (but not "\n\r").
		return function()
			if not flag then
				local line, nl, pos = match(str, "^(*)(\n?)()", start)
				if not line then
					flag = true
					return sub(str, start)
				end
				-- If `nl` is "\r\n" it should be treated as a single newline,
				-- but if it's "\n\n" it needs to be treated as two separate
				-- newlines, so set `flag` to 2 to avoid an unnecessary match()
				-- call on the next iteration.
				if nl == "\n\n" then
					flag = 2
				end
				start = pos
				return line
			elseif flag == 2 then
				flag = nil
				return ""
			end
		end
	else
		linechar = "\n"
	end
	-- Newline is either "\n" or "\r", depending on which has been found.
	return function()
		if flag then
			return
		end
		local pos = find(str, linechar, start, true)
		if not pos then
			flag = true
			return sub(str, start)
		end
		local line = sub(str, start, pos - 1)
		start = pos + 1
		return line
	end
end