--[[
A parser that approximates 8chan's markup:

Surround text with double single-quotes(') to make text italic
Surround text with triple single-quotes to make text bold
Surround text with underscores(_) to make it underlined
Surround text with double asterisks(*) to make it spoilered
Surround text with tildes(~) to make it strike through
Begin a line with a greater-than followed by a a space to make it
>greentext
Begin a line with a less-than followed by a space to make it
<pinktext
Surround text with forum-style [spoiler] and [/spoiler] tags as a second way to spoiler
Surround text with forum-style [code] and [/code] tags to make it preformatted and monospace

]]

local lpeg = require("lpeg")
lpeg.locale(lpeg)
local V,P,C,S,B,Cs = lpeg.V,lpeg.P,lpeg.C,lpeg.S,lpeg.B,lpeg.Cs
--Characters to escape in the body text
local escapes = {
	["&"] = "&amp;",
	["<"] = "&lt;",
	[">"] = "&gt;",
}
local esctbl = {}
for char,_ in pairs(escapes) do
	table.insert(esctbl,char)
end
local escapematch = string.format("([%s])",table.concat(esctbl))
local function sanitize_item(capture)
	return escapes[capture] or capture
end
local function sanitize(text)
	local ret,_ = string.gsub(text,escapematch,sanitize_item)
	return ret
end

--Grammar
local space = S" \t\r"^0
local special = P{
	P"**" + P"''" + P"'''" +
	P"__" + P"==" + P"~~" +
	P"\n>" + P"\n<" + P"\n" +
	P"[code]" + P"[spoiler]"
}
local word = Cs((1 - special)^1) * space / sanitize

--Generates a pattern that formats text inside matching 'seq' tags with format
--ex wrap("^^",[[<sup>%s</sup>]])
--will wrap text "5^^3^^" as "5<sup>3</sup>"
--The third argument is nessessary to stop exponential backtracking. This removes
--a DOS vulnerability: If tags are nested really deep, the parser can lock up,
--potentially locking up all processes.
--[[
local function wrap(seq,format,V"sup")
	return P(seq) * Cs(((V"marked" + word + P"\n"))^1) * P(seq) / function(a)
		return string.format(format,a)
	end
end
]]

local function wrap(seq,format,s)
	return P(seq) * Cs((((V"marked" - s) + word + P"\n"))^0) * P(seq) / function(a)
		return string.format(format,a)
	end
end

--Generates a pattern that formats text inside opening and closing "name" tags
--with a format, BB forum style
local function tag(name,format)
	local start_tag = P(string.format("[%s]",name))
	local end_tag = P(string.format("[/%s]",name))
	return start_tag * Cs(((1 - end_tag))^0) * end_tag / function(a)
		return string.format(format,sanitize(a))
	end
end

local grammar = P{
	"chunk";
	--regular
	spoiler = wrap("**",[[<span class="spoiler">%s</span>]],V"spoiler"),
	spoiler2 = tag("spoiler",[[<span class="spoiler2">%s</span>]]),
	italic = wrap("''",[[<i>%s</i>]], V"italic"),
	bold = wrap("'''",[[<b>%s</b>]], V"bold"),
	underline = wrap("__",[[<u>%s</u>]], V"underline"),
	heading = wrap("==",[[<h2>%s</h2>]], V"heading"),
	strike = wrap("~~",[[<s>%s</s>]], V"strike"),
	code = tag("code",[[<pre><code>%s</code></pre>]]),
	greentext = P">" * (B"\n>" + B">") * Cs((V"marked" + word)^0) / function(a)
		return string.format([[<span class="greentext">&gt;%s</span>]],a)
	end,
	pinktext = P"<" * (B"\n<" + B"<") * Cs((V"marked" + word)^0) / function(a)
		return string.format([[<span class="pinktext">&lt;%s</span>]],a)
	end,
	marked = V"spoiler" + V"bold" + V"italic" + V"underline" + V"heading" + V"strike" + V"spoiler2" + V"code",
	plainline = (V"marked" + word)^0,
	line = Cs(V"greentext" + V"pinktext" + V"plainline" + P"") * P"\n" / function(a)
		if a == "\r" then
			return "<br/>"
		else
			return string.format("<p>%s</p>",a)
		end
	end,
	ending = C(P(1)^0) / function(a) return sanitize(a) end,
	chunk = V"line"^0 * V"plainline" * V"ending"
}

return function(text)
	return table.concat({grammar:match(text .. "\n")}," ")
end