darkrc/bin/linify

#!/usr/bin/env lua

-- Ajdacent characters numbering:
--   1
-- 2 + 3
--   4
-- '+' = it's another plus
-- '.' = it's something else

local adjacency_char_map = {
	["...."] = "┼";
	["...+"] = "│";
	["..+."] = "─";
	["..++"] = "┌";
	[".+.."] = "─";
	[".+.+"] = "┐";
	[".++."] = "─";
	[".+++"] = "┬";
	["+..."] = "│";
	["+..+"] = "│";
	["+.+."] = "└";
	["+.++"] = "├";
	["++.."] = "┘";
	["++.+"] = "┤";
	["+++."] = "┴";
	["++++"] = "┼";
}

local input do
	input = setmetatable({}, {__index = function(self, key)
		if type(key) == "number" then
			return {}
		end
	end})

	for line in io.stdin:lines() do
		local buf = {}
		for p, char in utf8.codes(line) do
			table.insert(buf, utf8.char(char))
		end
		table.insert(input, buf)
	end
end

local function rawipairs(tab)
	return function(tab, i)
		i = i + 1
		if rawget(tab, i) then
			return i, rawget(tab, i)
		end
	end, tab, 0
end

local output = {}
for lnum, line in rawipairs(input) do
	output[lnum] = {}
	for cnum, char in rawipairs(line) do
		if char == "+" then
			local adjacency = 
				(input[lnum-1][cnum] == "+" and "+" or ".")
				..(line[cnum-1] == "+" and "+" or ".")
				..(line[cnum+1] == "+" and "+" or ".")
				..(input[lnum+1][cnum] == "+" and "+" or ".")
			output[lnum][cnum] = adjacency_char_map[adjacency]
		else
			output[lnum][cnum] = char
		end
	end
end

for _, line in ipairs(output) do
	print(table.concat(line))
end