-- lib/gitano/markdown.lua
--
-- Markdown implementation used in Gitano
--
-- Copyright 2008 Niklas Frykholm <niklas@frykholm.se>
-- Portions copyright 2012 Daniel Silverstone <dsilvers@digital-scurf.org>
--
-- For licencing terms, see COPYING
--
-- Conversion to Lua 5.2 compatible module syntax by Daniel Silverstone
-- Header ID functionality by Daniel Silverstone

----------------------------------------------------------------------
-- Utility functions
----------------------------------------------------------------------

-- Returns the result of mapping the values in table t through the function f
local function map(t, f)
	local out = {}
	for k,v in pairs(t) do out[k] = f(v,k) end
	return out
end

-- The identity function, useful as a placeholder.
local function identity(text) return text end

-- Functional style if statement. (NOTE: no short circuit evaluation)
local function iff(t, a, b) if t then return a else return b end end

-- Splits the text into an array of separate lines.
local function split(text, sep)
	sep = sep or "\n"
	local lines = {}
	local pos = 1
	while true do
		local b,e = text:find(sep, pos)
		if not b then table.insert(lines, text:sub(pos)) break end
		table.insert(lines, text:sub(pos, b-1))
		pos = e + 1
	end
	return lines
end

-- Converts tabs to spaces
local function detab(text)
	local tab_width = 4
	local function rep(match)
		local spaces = -match:len()
		while spaces<1 do spaces = spaces + tab_width end
		return match .. string.rep(" ", spaces)
	end
	text = text:gsub("([^\n]-)\t", rep)
	return text
end

-- Applies string.find for every pattern in the list and returns the first match
local function find_first(s, patterns, index)
	local res = {}
	for _,p in ipairs(patterns) do
		local match = {s:find(p, index)}
		if #match>0 and (#res==0 or match[1] < res[1]) then res = match end
	end
	return unpack(res)
end

-- If a replacement array is specified, the range [start, stop] in the array is replaced
-- with the replacement array and the resulting array is returned. Without a replacement
-- array the section of the array between start and stop is returned.
local function splice(array, start, stop, replacement)
	if replacement then
		local n = stop - start + 1
		while n > 0 do
			table.remove(array, start)
			n = n - 1
		end
		for i,v in ipairs(replacement) do
			table.insert(array, start, v)
		end
		return array
	else
		local res = {}
		for i = start,stop do
			table.insert(res, array[i])
		end
		return res
	end
end

-- Outdents the text one step.
local function outdent(text)
	text = "\n" .. text
	text = text:gsub("\n  ? ? ?", "\n")
	text = text:sub(2)
	return text
end

-- Indents the text one step.
local function indent(text)
	text = text:gsub("\n", "\n    ")
	return text
end

-- Does a simple tokenization of html data. Returns the data as a list of tokens. 
-- Each token is a table with a type field (which is either "tag" or "text") and
-- a text field (which contains the original token data).
local function tokenize_html(html)
	local tokens = {}
	local pos = 1
	while true do
		local start = find_first(html, {"<!%-%-", "<[a-z/!$]", "<%?"}, pos)
		if not start then
			table.insert(tokens, {type="text", text=html:sub(pos)})
			break
		end
		if start ~= pos then table.insert(tokens, {type="text", text = html:sub(pos, start-1)}) end
		
		local _, stop
		if html:match("^<!%-%-", start) then
			_,stop = html:find("%-%->", start)
		elseif html:match("^<%?", start) then
			_,stop = html:find("?>", start)
		else
			_,stop = html:find("%b<>", start)
		end
		if not stop then
			-- error("Could not match html tag " .. html:sub(start,start+30)) 
		 	table.insert(tokens, {type="text", text=html:sub(start, start)})
			pos = start + 1
		else
			table.insert(tokens, {type="tag", text=html:sub(start, stop)})
			pos = stop + 1
		end
	end
	return tokens
end

----------------------------------------------------------------------
-- Hash
----------------------------------------------------------------------

-- This is used to "hash" data into alphanumeric strings that are unique
-- in the document. (Note that this is not cryptographic hash, the hash
-- function is not one-way.) The hash procedure is used to protect parts
-- of the document from further processing.

local HASH = {
	-- Has the hash been inited.
	inited = false,
	
	-- The unique string prepended to all hash values. This is to ensure
	-- that hash values do not accidently coincide with an actual existing
	-- string in the document.
	identifier = "",
	
	-- Counter that counts up for each new hash instance.
	counter = 0,
	
	-- Hash table.
	table = {}
}

-- Inits hashing. Creates a hash_identifier that doesn't occur anywhere
-- in the text.
local function init_hash(text)
	HASH.inited = true
	HASH.identifier = ""
	HASH.counter = 0
	HASH.table = {}
	
	local s = "HASH"
	local counter = 0
	local id
	while true do
		id  = s .. counter
		if not text:find(id, 1, true) then break end
		counter = counter + 1
	end
	HASH.identifier = id
end

-- Returns the hashed value for s.
local function hash(s)
	assert(HASH.inited)
	if not HASH.table[s] then
		HASH.counter = HASH.counter + 1
		local id = HASH.identifier .. HASH.counter .. "X"
		HASH.table[s] = id
	end
	return HASH.table[s]
end

----------------------------------------------------------------------
-- Protection
----------------------------------------------------------------------

-- The protection module is used to "protect" parts of a document
-- so that they are not modified by subsequent processing steps. 
-- Protected parts are saved in a table for later unprotection

-- Protection data
local PD = {
	-- Saved blocks that have been converted
	blocks = {},

	-- Block level tags that will be protected
	tags = {"p", "div", "h1", "h2", "h3", "h4", "h5", "h6", "blockquote",
	"pre", "table", "dl", "ol", "ul", "script", "noscript", "form", "fieldset",
	"iframe", "math", "ins", "del"}
}

-- Pattern for matching a block tag that begins and ends in the leftmost
-- column and may contain indented subtags, i.e.
-- <div>
--    A nested block.
--    <div>
--        Nested data.
--     </div>
-- </div>
local function block_pattern(tag)
	return "\n<" .. tag .. ".-\n</" .. tag .. ">[ \t]*\n"
end

-- Pattern for matching a block tag that begins and ends with a newline
local function line_pattern(tag)
	return "\n<" .. tag .. ".-</" .. tag .. ">[ \t]*\n"
end

-- Protects the range of characters from start to stop in the text and
-- returns the protected string.
local function protect_range(text, start, stop)
	local s = text:sub(start, stop)
	local h = hash(s)
	PD.blocks[h] = s
	text = text:sub(1,start) .. h .. text:sub(stop)
	return text
end

-- Protect every part of the text that matches any of the patterns. The first
-- matching pattern is protected first, etc.
local function protect_matches(text, patterns)
	while true do
		local start, stop = find_first(text, patterns)
		if not start then break end
		text = protect_range(text, start, stop)
	end
	return text
end

-- Protects blocklevel tags in the specified text
local function protect(text)
	-- First protect potentially nested block tags
	text = protect_matches(text, map(PD.tags, block_pattern))
	-- Then protect block tags at the line level.
	text = protect_matches(text, map(PD.tags, line_pattern))
	-- Protect <hr> and comment tags
	text = protect_matches(text, {"\n<hr[^>]->[ \t]*\n"})
	text = protect_matches(text, {"\n<!%-%-.-%-%->[ \t]*\n"})
	return text
end

-- Returns true if the string s is a hash resulting from protection
local function is_protected(s)
	return PD.blocks[s]
end

-- Unprotects the specified text by expanding all the nonces
local function unprotect(text)
	for k,v in pairs(PD.blocks) do
		v = v:gsub("%%", "%%%%")
		text = text:gsub(k, v)
	end
	return text
end


----------------------------------------------------------------------
-- Block transform
----------------------------------------------------------------------

-- Forward decls
local span_transform, blocks_to_html, encode_code

-- The block transform functions transform the text on the block level.
-- They work with the text as an array of lines rather than as individual
-- characters.

-- Returns true if the line is a ruler of (char) characters.
-- The line must contain at least three char characters and contain only spaces and
-- char characters.
local function is_ruler_of(line, char)
	if not line:match("^[ %" .. char .. "]*$") then return false end
	if not line:match("%" .. char .. ".*%" .. char .. ".*%" .. char) then return false end
	return true
end

-- Identifies the block level formatting present in the line
local function classify(line)
	local info = {line = line, text = line}
	
	if line:match("^    ") then
		info.type = "indented"
		info.outdented = line:sub(5)
		return info
	end
	
	for _,c in ipairs({'*', '-', '_', '='}) do
		if is_ruler_of(line, c) then
			info.type = "ruler"
			info.ruler_char = c
			return info
		end
	end
	
	if line == "" then
		info.type = "blank"
		return info
	end
	
	if line:match("^(#+)[ \t]*(.-)[ \t]*#*[ \t]*$") then
		local m1, m2 = line:match("^(#+)[ \t]*(.-)[ \t]*#*[ \t]*$")
		info.type = "header"
		info.level = m1:len()
		info.text = m2
		return info
	end
	
	if line:match("^ ? ? ?(%d+)%.[ \t]+(.+)") then
		local number, text = line:match("^ ? ? ?(%d+)%.[ \t]+(.+)")
		info.type = "list_item"
		info.list_type = "numeric"
		info.number = 0 + number
		info.text = text
		return info
	end
	
	if line:match("^ ? ? ?([%*%+%-])[ \t]+(.+)") then
		local bullet, text = line:match("^ ? ? ?([%*%+%-])[ \t]+(.+)")
		info.type = "list_item"
		info.list_type = "bullet"
		info.bullet = bullet
		info.text= text
		return info
	end
	
	if line:match("^>[ \t]?(.*)") then
		info.type = "blockquote"
		info.text = line:match("^>[ \t]?(.*)")
		return info
	end
	
	if is_protected(line) then
		info.type = "raw"
		info.html = unprotect(line)
		return info
	end
	
	info.type = "normal"
	return info
end

-- Find headers constisting of a normal line followed by a ruler and converts them to
-- header entries.
local function headers(array)
	local i = 1
	while i <= #array - 1 do
		if array[i].type  == "normal" and array[i+1].type == "ruler" and 
			(array[i+1].ruler_char == "-" or array[i+1].ruler_char == "=") then
			local info = {line = array[i].line}
			info.text = info.line
			info.type = "header"
			info.level = iff(array[i+1].ruler_char == "=", 1, 2)
			table.remove(array, i+1)
			array[i] = info
		end
		i = i + 1
	end
	return array
end

-- Find list blocks and convert them to protected data blocks
local function lists(array, sublist)
	local function process_list(arr)
		local function any_blanks(arr)
			for i = 1, #arr do
				if arr[i].type == "blank" then return true end
			end
			return false
		end
		
		local function split_list_items(arr)
			local acc = {arr[1]}
			local res = {}
			for i=2,#arr do
				if arr[i].type == "list_item" then
					table.insert(res, acc)
					acc = {arr[i]}
				else
					table.insert(acc, arr[i])
				end
			end
			table.insert(res, acc)
			return res
		end
		
		local function process_list_item(lines, block)
			while lines[#lines].type == "blank" do
				table.remove(lines)
			end
			
			local itemtext = lines[1].text
			for i=2,#lines do
				itemtext = itemtext .. "\n" .. outdent(lines[i].line)
			end
			if block then
				itemtext = block_transform(itemtext, true)
				if not itemtext:find("<pre>") then itemtext = indent(itemtext) end
				return "    <li>" .. itemtext .. "</li>"
			else
				local lines = split(itemtext)
				lines = map(lines, classify)
				lines = lists(lines, true)
				lines = blocks_to_html(lines, true)
				itemtext = table.concat(lines, "\n")
				if not itemtext:find("<pre>") then itemtext = indent(itemtext) end
				return "    <li>" .. itemtext .. "</li>"
			end
		end
		
		local block_list = any_blanks(arr)
		local items = split_list_items(arr)
		local out = ""
		for _, item in ipairs(items) do
			out = out .. process_list_item(item, block_list) .. "\n"
		end
		if arr[1].list_type == "numeric" then
			return "<ol>\n" .. out .. "</ol>"
		else
			return "<ul>\n" .. out .. "</ul>"
		end
	end
	
	-- Finds the range of lines composing the first list in the array. A list
	-- starts with (^ list_item) or (blank list_item) and ends with
	-- (blank* $) or (blank normal).
	--
	-- A sublist can start with just (list_item) does not need a blank...
	local function find_list(array, sublist)
		local function find_list_start(array, sublist)
			if array[1].type == "list_item" then return 1 end
			if sublist then
				for i = 1,#array do
					if array[i].type == "list_item" then return i end
				end
			else
				for i = 1, #array-1 do
					if array[i].type == "blank" and array[i+1].type == "list_item" then
						return i+1
					end
				end
			end
			return nil
		end
		local function find_list_end(array, start)
			local pos = #array
			for i = start, #array-1 do
				if array[i].type == "blank" and array[i+1].type ~= "list_item" 
					and array[i+1].type ~= "indented" and array[i+1].type ~= "blank" then
					pos = i-1
					break
				end
			end
			while pos > start and array[pos].type == "blank" do
				pos = pos - 1
			end
			return pos
		end
		
		local start = find_list_start(array, sublist)
		if not start then return nil end
		return start, find_list_end(array, start)
	end
	
	while true do
		local start, stop = find_list(array, sublist)
		if not start then break end
		local text = process_list(splice(array, start, stop))
		local info = {
			line = text,
			type = "raw",
			html = text
		}
		array = splice(array, start, stop, {info})
	end
	
	-- Convert any remaining list items to normal
	for _,line in ipairs(array) do
		if line.type == "list_item" then line.type = "normal" end
	end
	
	return array
end

-- Find and convert blockquote markers.
local function blockquotes(lines)
	local function find_blockquote(lines)
		local start
		for i,line in ipairs(lines) do
			if line.type == "blockquote" then
				start = i
				break
			end
		end
		if not start then return nil end
		
		local stop = #lines
		for i = start+1, #lines do
			if lines[i].type == "blank" or lines[i].type == "blockquote" then
			elseif lines[i].type == "normal" then
				if lines[i-1].type == "blank" then stop = i-1 break end
			else
				stop = i-1 break
			end
		end
		while lines[stop].type == "blank" do stop = stop - 1 end
		return start, stop
	end
	
	local function process_blockquote(lines)
		local raw = lines[1].text
		for i = 2,#lines do
			raw = raw .. "\n" .. lines[i].text
		end
		local bt = block_transform(raw)
		if not bt:find("<pre>") then bt = indent(bt) end
		return "<blockquote>\n    " .. bt ..
			"\n</blockquote>"
	end
	
	while true do
		local start, stop = find_blockquote(lines)
		if not start then break end
		local text = process_blockquote(splice(lines, start, stop))
		local info = {
			line = text,
			type = "raw",
			html = text
		}
		lines = splice(lines, start, stop, {info})
	end
	return lines
end

-- Find and convert codeblocks.
local function codeblocks(lines)
	local function find_codeblock(lines)
		local start
		for i,line in ipairs(lines) do
			if line.type == "indented" then start = i break end
		end
		if not start then return nil end
		
		local stop = #lines
		for i = start+1, #lines do
			if lines[i].type ~= "indented" and lines[i].type ~= "blank" then
				stop = i-1
				break
			end
		end
		while lines[stop].type == "blank" do stop = stop - 1 end
		return start, stop
	end
	
	local function process_codeblock(lines)
		local raw = detab(encode_code(outdent(lines[1].line)))
		for i = 2,#lines do
			raw = raw .. "\n" .. detab(encode_code(outdent(lines[i].line)))
		end
		return "<pre><code>" .. raw .. "\n</code></pre>"
	end
	
	while true do
		local start, stop = find_codeblock(lines)
		if not start then break end
		local text = process_codeblock(splice(lines, start, stop))
		local info = {
			line = text,
			type = "raw",
			html = text
		}
		lines = splice(lines, start, stop, {info})
	end
	return lines
end

local function make_id(text)
   -- What Pandoc does, is:
   --   Remove all formatting, links, etc.
   --   Remove all punctuation, except underscores, hyphens, and periods.
   --   Replace all spaces and newlines with hyphens.
   --   Convert all alphabetic characters to lowercase.
   --   Remove everything up to the first letter (identifiers may not begin
   --                                       with a number or punctuation mark).
   --   If nothing is left after this, use the identifier 'section'.

   -- Pandoc then appends -1, -2 etc *iff* the identifier is repeated.
   -- Since we're pretty much in-flow we can't easily do that so we don't
   -- bother.

   -- Since we cannot easily do the format stripping, we do the following,
   -- which is almost compatible.

   -- Lowercase the text and remove anything not letter, number, underscore,
   -- dot, dash, or whitespae
   local id = text:lower():gsub("[^a-z0-9_%.%- \t\n]", "")
   -- Replace all whitespace with hyphens
   id = id:gsub("%s", "-")
   -- Remove any leading non-letters
   id = id:gsub("^[^a-z]*", "")
   -- Return the id, or 'section' if empty
   return (id == "") and "section" or id
end

-- Convert lines to html code
function blocks_to_html(lines, no_paragraphs)
	local out = {}
	local i = 1
	while i <= #lines do
		local line = lines[i]
		if line.type == "ruler" then
			table.insert(out, "<hr/>")
		elseif line.type == "raw" then
			table.insert(out, line.html)
		elseif line.type == "normal" then
			local s = line.line
			
			while i+1 <= #lines and lines[i+1].type == "normal" do
				i = i + 1
				s = s .. "\n" .. lines[i].line
			end
			
			if no_paragraphs then
				table.insert(out, span_transform(s))
			else
				table.insert(out, "<p>" .. span_transform(s) .. "</p>")
			end
		elseif line.type == "header" then
			local id = make_id(line.text)
			local s = "<h" .. line.level .. " id=\"" .. id .."\">" .. span_transform(line.text) .. "</h" .. line.level .. ">"
			table.insert(out, s)
		else
			table.insert(out, line.line)
		end
		i = i + 1
	end
	return out
end

-- Perform all the block level transforms
local function block_transform(text, sublist)
	local lines = split(text)
	lines = map(lines, classify)
	lines = headers(lines)
	lines = lists(lines, sublist)
	lines = codeblocks(lines)
	lines = blockquotes(lines)
	lines = blocks_to_html(lines)
	local text = table.concat(lines, "\n")
	return text
end

----------------------------------------------------------------------
-- Span transform
----------------------------------------------------------------------

-- Functions for transforming the text at the span level.

-- These characters may need to be escaped because they have a special
-- meaning in markdown.
local escape_chars = "'\\`*_{}[]()>#+-.!'"
local escape_table = {}

local function init_escape_table()
	escape_table = {}
	for i = 1,#escape_chars do
		local c = escape_chars:sub(i,i)
		escape_table[c] = hash(c)
	end
end

-- Adds a new escape to the escape table.
local function add_escape(text)
	if not escape_table[text] then
		escape_table[text] = hash(text)
	end
	return escape_table[text]
end	

-- Encode backspace-escaped characters in the markdown source.
local function encode_backslash_escapes(t)
	for i=1,escape_chars:len() do
		local c = escape_chars:sub(i,i)
		t = t:gsub("\\%" .. c, escape_table[c])
	end
	return t
end

-- Escape characters that should not be disturbed by markdown.
local function escape_special_chars(text)
	local tokens = tokenize_html(text)
	
	local out = ""
	for _, token in ipairs(tokens) do
		local t = token.text
		if token.type == "tag" then
			-- In tags, encode * and _ so they don't conflict with their use in markdown.
			t = t:gsub("%*", escape_table["*"])
			t = t:gsub("%_", escape_table["_"])
		else
			t = encode_backslash_escapes(t)
		end
		out = out .. t
	end
	return out
end

-- Unescape characters that have been encoded.
local function unescape_special_chars(t)
	local tin = t
	for k,v in pairs(escape_table) do
		k = k:gsub("%%", "%%%%")
		t = t:gsub(v,k)
	end
	if t ~= tin then t = unescape_special_chars(t) end
	return t
end

-- Encode/escape certain characters inside Markdown code runs.
-- The point is that in code, these characters are literals,
-- and lose their special Markdown meanings.
function encode_code(s)
	s = s:gsub("%&", "&amp;")
	s = s:gsub("<", "&lt;")
	s = s:gsub(">", "&gt;")
	for k,v in pairs(escape_table) do
		s = s:gsub("%"..k, v)
	end
	return s
end

-- Handle backtick blocks.
local function code_spans(s)
	s = s:gsub("\\\\", escape_table["\\"])
	s = s:gsub("\\`", escape_table["`"])

	local pos = 1
	while true do
		local start, stop = s:find("`+", pos)
		if not start then return s end
		local count = stop - start + 1
		-- Find a matching numbert of backticks
		local estart, estop = s:find(string.rep("`", count), stop+1)
		local brstart = s:find("\n", stop+1)
		if estart and (not brstart or estart < brstart) then
			local code = s:sub(stop+1, estart-1)
			code = code:gsub("^[ \t]+", "")
			code = code:gsub("[ \t]+$", "")
			code = code:gsub(escape_table["\\"], escape_table["\\"] .. escape_table["\\"])
			code = code:gsub(escape_table["`"], escape_table["\\"] .. escape_table["`"])
			code = "<code>" .. encode_code(code) .. "</code>"
			code = add_escape(code)
			s = s:sub(1, start-1) .. code .. s:sub(estop+1)
			pos = start + code:len()
		else
			pos = stop + 1
		end
	end
	return s
end

-- Encode alt text... enodes &, and ".
local function encode_alt(s)
	if not s then return s end
	s = s:gsub('&', '&amp;')
	s = s:gsub('"', '&quot;')
	s = s:gsub('<', '&lt;')
	return s
end

-- Handle image references
local function images(text)
	local function reference_link(alt, id)
		alt = encode_alt(alt:match("%b[]"):sub(2,-2))
		id = id:match("%[(.*)%]"):lower()
		if id == "" then id = text:lower() end
		link_database[id] = link_database[id] or {}
		if not link_database[id].url then return nil end
		local url = link_database[id].url or id
		url = encode_alt(url)
		local title = encode_alt(link_database[id].title)
		if title then title = " title=\"" .. title .. "\"" else title = "" end
		return add_escape ('<img src="' .. url .. '" alt="' .. alt .. '"' .. title .. "/>")
	end
	
	local function inline_link(alt, link)
		alt = encode_alt(alt:match("%b[]"):sub(2,-2))
		local url, title = link:match("%(<?(.-)>?[ \t]*['\"](.+)['\"]")
		url = url or link:match("%(<?(.-)>?%)")
		url = encode_alt(url)
		title = encode_alt(title)
		if title then
			return add_escape('<img src="' .. url .. '" alt="' .. alt .. '" title="' .. title .. '"/>')
		else
			return add_escape('<img src="' .. url .. '" alt="' .. alt .. '"/>')
		end
	end
	
	text = text:gsub("!(%b[])[ \t]*\n?[ \t]*(%b[])", reference_link)
	text = text:gsub("!(%b[])(%b())", inline_link)
	return text
end

-- Handle anchor references
local function anchors(text)
	local function reference_link(text, id)
		text = text:match("%b[]"):sub(2,-2)
		id = id:match("%b[]"):sub(2,-2):lower()
		if id == "" then id = text:lower() end
		link_database[id] = link_database[id] or {}
		if not link_database[id].url then return nil end
		local url = link_database[id].url or id
		url = encode_alt(url)
		local title = encode_alt(link_database[id].title)
		if title then title = " title=\"" .. title .. "\"" else title = "" end
		return add_escape("<a href=\"" .. url .. "\"" .. title .. ">") .. text .. add_escape("</a>")
	end
	
	local function inline_link(text, link)
		text = text:match("%b[]"):sub(2,-2)
		local url, title = link:match("%(<?(.-)>?[ \t]*['\"](.+)['\"]")
		title = encode_alt(title)
		url  = url or  link:match("%(<?(.-)>?%)") or ""
		url = encode_alt(url)
		if title then
			return add_escape("<a href=\"" .. url .. "\" title=\"" .. title .. "\">") .. text .. "</a>"
		else
			return add_escape("<a href=\"" .. url .. "\">") .. text .. add_escape("</a>")
		end
	end
	
	text = text:gsub("(%b[])[ \t]*\n?[ \t]*(%b[])", reference_link)
	text = text:gsub("(%b[])(%b())", inline_link)
	return text
end

-- Handle auto links, i.e. <http://www.google.com/>.
local function auto_links(text)
	local function link(s)
		return add_escape("<a href=\"" .. s .. "\">") .. s .. "</a>"
	end
	-- Encode chars as a mix of dec and hex entitites to (perhaps) fool
	-- spambots.
	local function encode_email_address(s)
		-- Use a deterministic encoding to make unit testing possible.
		-- Code 45% hex, 45% dec, 10% plain.
		local hex = {code = function(c) return "&#x" .. string.format("%x", c:byte()) .. ";" end, count = 1, rate = 0.45}
		local dec = {code = function(c) return "&#" .. c:byte() .. ";" end, count = 0, rate = 0.45}
		local plain = {code = function(c) return c end, count = 0, rate = 0.1}
		local codes = {hex, dec, plain}
		local function swap(t,k1,k2) local temp = t[k2] t[k2] = t[k1] t[k1] = temp end
		
		local out = ""
		for i = 1,s:len() do
			for _,code in ipairs(codes) do code.count = code.count + code.rate end
			if codes[1].count < codes[2].count then swap(codes,1,2) end
			if codes[2].count < codes[3].count then swap(codes,2,3) end
			if codes[1].count < codes[2].count then swap(codes,1,2) end
			
			local code = codes[1]
			local c = s:sub(i,i)
			-- Force encoding of "@" to make email address more invisible.
			if c == "@" and code == plain then code = codes[2] end
			out = out .. code.code(c)
			code.count = code.count - 1
		end	
		return out
	end
	local function mail(s)
		s = unescape_special_chars(s)
		local address = encode_email_address("mailto:" .. s)
		local text = encode_email_address(s)
		return add_escape("<a href=\"" .. address .. "\">") .. text .. "</a>"
	end
	-- links
	text = text:gsub("<(https?:[^'\">%s]+)>", link)
	text = text:gsub("<(ftp:[^'\">%s]+)>", link)
	
	-- mail
	text = text:gsub("<mailto:([^'\">%s]+)>", mail)
	text = text:gsub("<([-.%w]+%@[-.%w]+)>", mail)
	return text
end

-- Encode free standing amps (&) and angles (<)... note that this does not
-- encode free >.
local function amps_and_angles(s)
	-- encode amps not part of &..; expression
	local pos = 1
	while true do
		local amp = s:find("&", pos)
		if not amp then break end
		local semi = s:find(";", amp+1)
		local stop = s:find("[ \t\n&]", amp+1)
		if not semi or (stop and stop < semi) or (semi - amp) > 15 then
			s = s:sub(1,amp-1) .. "&amp;" .. s:sub(amp+1)
			pos = amp+1
		else
			pos = amp+1
		end
	end
	
	-- encode naked <'s
	s = s:gsub("<([^a-zA-Z/?$!])", "&lt;%1")
	s = s:gsub("<$", "&lt;")
	
	-- what about >, nothing done in the original markdown source to handle them
	return s
end

-- Handles emphasis markers (* and _) in the text.
local function emphasis(text)
	for _, s in ipairs {"%*%*", "%_%_"} do
		text = text:gsub(s .. "([^%s][%*%_]?)" .. s, "<strong>%1</strong>")
		text = text:gsub(s .. "([^%s][^<>]-[^%s][%*%_]?)" .. s, "<strong>%1</strong>")
	end
	for _, s in ipairs {"%*", "%_"} do
		text = text:gsub(s .. "([^%s_])" .. s, "<em>%1</em>")
		text = text:gsub(s .. "(<strong>[^%s_]</strong>)" .. s, "<em>%1</em>")
		text = text:gsub(s .. "([^%s_][^<>_]-[^%s_])" .. s, "<em>%1</em>")
		text = text:gsub(s .. "([^<>_]-<strong>[^<>_]-</strong>[^<>_]-)" .. s, "<em>%1</em>")
	end
	return text
end

-- Handles line break markers in the text.
local function line_breaks(text)
	return text:gsub("  +\n", " <br/>\n")
end

-- Perform all span level transforms.
function span_transform(text)
	text = code_spans(text)
	text = escape_special_chars(text)
	text = images(text)
	text = anchors(text)
	text = auto_links(text)
	text = amps_and_angles(text)
	text = emphasis(text)
	text = line_breaks(text)
	return text
end

----------------------------------------------------------------------
-- Markdown
----------------------------------------------------------------------

-- Cleanup the text by normalizing some possible variations to make further
-- processing easier.
local function cleanup(text)
	-- Standardize line endings
	text = text:gsub("\r\n", "\n")  -- DOS to UNIX
	text = text:gsub("\r", "\n")    -- Mac to UNIX
	
	-- Convert all tabs to spaces
	text = detab(text)
	
	-- Strip lines with only spaces and tabs
	while true do
		local subs
		text, subs = text:gsub("\n[ \t]+\n", "\n\n")
		if subs == 0 then break end
	end
	
	return "\n" .. text .. "\n"
end

-- Strips link definitions from the text and stores the data in a lookup table.
local function strip_link_definitions(text)
	local linkdb = {}
	
	local function link_def(id, url, title)
		id = id:match("%[(.+)%]"):lower()
		linkdb[id] = linkdb[id] or {}
		linkdb[id].url = url or linkdb[id].url
		linkdb[id].title = title or linkdb[id].title
		return ""
	end

	local def_no_title = "\n ? ? ?(%b[]):[ \t]*\n?[ \t]*<?([^%s>]+)>?[ \t]*"
	local def_title1 = def_no_title .. "[ \t]+\n?[ \t]*[\"'(]([^\n]+)[\"')][ \t]*"
	local def_title2 = def_no_title .. "[ \t]*\n[ \t]*[\"'(]([^\n]+)[\"')][ \t]*"
	local def_title3 = def_no_title .. "[ \t]*\n?[ \t]+[\"'(]([^\n]+)[\"')][ \t]*"
	
	text = text:gsub(def_title1, link_def)
	text = text:gsub(def_title2, link_def)
	text = text:gsub(def_title3, link_def)
	text = text:gsub(def_no_title, link_def)
	return text, linkdb
end

link_database = {}

-- Main markdown processing function
local function markdown(text)
	init_hash(text)
	init_escape_table()
	
	text = cleanup(text)
	text = protect(text)
	text, link_database = strip_link_definitions(text)
	text = block_transform(text)
	text = unescape_special_chars(text)
	return text
end

----------------------------------------------------------------------
-- End of module
----------------------------------------------------------------------


return {
   convert_to_html = markdown,
   make_id = make_id,
}