markdown.lua
#!/usr/bin/env lua

--[[
# markdown.lua -- version 0.40

**Author:** Niklas Frykholm, <niklas@frykholm.se>
**Date:** 31 May 2008

See the following page for changes to the original file
https://github.com/speedata/luamarkdown


This is an implementation of the popular text markup language Markdown in pure Lua.
Markdown can convert documents written in a simple and easy to read text format
to well-formatted HTML. For a more thourough description of Markdown and the Markdown
syntax, see <http://daringfireball.net/projects/markdown>.

The original Markdown source is written in Perl and makes heavy use of advanced
regular expression techniques (such as negative look-ahead, etc) which are not available
in Lua's simple regex engine. Therefore this Lua port has been rewritten from the ground
up. It is probably not completely bug free. If you notice any bugs, please report them to
me. A unit test that exposes the error is helpful.

## Usage

    local markdown = require "markdown"
    markdown.markdown(source)

``markdown.lua`` returns a table with a single function named ``markdown(s)`` which applies the
Markdown transformation to the specified string.

``markdown.lua`` can also be used directly from the command line:

    lua markdown.lua test.md

Creates a file ``test.html`` with the converted content of ``test.md``. Run:

    lua markdown.lua -h

For a description of the command-line options.

``markdown.lua`` uses the same license as Lua, the MIT license.

## License

Copyright &copy; 2008 Niklas Frykholm.

Permission is hereby granted, free of charge, to any person obtaining a copy of this
software and associated documentation files (the "Software"), to deal in the Software
without restriction, including without limitation the rights to use, copy, modify, merge,
publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies
or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
THE SOFTWARE.

## Version history

-   **0.40** -- 17 Apr 2014
    - Compatibility for Lua 5.1 and 5.2
-   **0.32** -- 31 May 2008
    - Fix for links containing brackets
-   **0.31** -- 1 Mar 2008
    -   Fix for link definitions followed by spaces
-   **0.30** -- 25 Feb 2008
    -   Consistent behavior with Markdown when the same link reference is reused
-   **0.29** -- 24 Feb 2008
    -   Fix for <pre> blocks with spaces in them
-   **0.28** -- 18 Feb 2008
    -   Fix for link encoding
-   **0.27** -- 14 Feb 2008
    -   Fix for link database links with ()
-   **0.26** -- 06 Feb 2008
    -   Fix for nested italic and bold markers
-   **0.25** -- 24 Jan 2008
    -   Fix for encoding of naked <
-   **0.24** -- 21 Jan 2008
    -   Fix for link behavior.
-   **0.23** -- 10 Jan 2008
    -   Fix for a regression bug in longer expressions in italic or bold.
-   **0.22** -- 27 Dec 2007
    -   Fix for crash when processing blocks with a percent sign in them.
-   **0.21** -- 27 Dec 2007
    -   Fix for combined strong and emphasis tags
-   **0.20** -- 13 Oct 2007
    -   Fix for < as well in image titles, now matches Dingus behavior
-   **0.19** -- 28 Sep 2007
    -   Fix for quotation marks " and ampersands & in link and image titles.
-   **0.18** -- 28 Jul 2007
    -   Does not crash on unmatched tags (behaves like standard markdown)
-   **0.17** -- 12 Apr 2007
    -   Fix for links with %20 in them.
-   **0.16** -- 12 Apr 2007
    -   Do not require arg global to exist.
-   **0.15** -- 28 Aug 2006
    -   Better handling of links with underscores in them.
-   **0.14** -- 22 Aug 2006
    -   Bug for *`foo()`*
-   **0.13** -- 12 Aug 2006
    -   Added -l option for including stylesheet inline in document.
    -   Fixed bug in -s flag.
    -   Fixed emphasis bug.
-   **0.12** -- 15 May 2006
    -   Fixed several bugs to comply with MarkdownTest 1.0 <http://six.pairlist.net/pipermail/markdown-discuss/2004-December/000909.html>
-   **0.11** -- 12 May 2006
    -   Fixed bug for escaping `*` and `_` inside code spans.
    -   Added license terms.
    -   Changed join() to table.concat().
-   **0.10** -- 3 May 2006
    -   Initial public release.

// Niklas
]]


local unpack = unpack or table.unpack

local span_transform, encode_backslash_escapes, block_transform, blocks_to_html, blocks_to_html

----------------------------------------------------------------------
-- 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
----------------------------------------------------------------------

-- 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

-- 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 s = "<h" .. line.level .. ">" .. 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
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

-- Debug function for printing a line array to see the result
-- of partial transforms.
local function print_lines(lines)
    for i, line in ipairs(lines) do
        print(i, line.type, line.text or line.line)
    end
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.
escape_chars = "'\\`*_{}[]()>#+-.!'"
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

-- 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

-- Encode backspace-escaped characters in the markdown source.
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

-- 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
----------------------------------------------------------------------

-- Expose markdown function to the world
md = { markdown = markdown }

-- Class for parsing command-line options
local OptionParser = {}
OptionParser.__index = OptionParser

-- Creates a new option parser
function OptionParser:new()
    local o = {short = {}, long = {}}
    setmetatable(o, self)
    return o
end

-- Calls f() whenever a flag with specified short and long name is encountered
function OptionParser:flag(short, long, f)
    local info = {type = "flag", f = f}
    if short then self.short[short] = info end
    if long then self.long[long] = info end
end

-- Calls f(param) whenever a parameter flag with specified short and long name is encountered
function OptionParser:param(short, long, f)
    local info = {type = "param", f = f}
    if short then self.short[short] = info end
    if long then self.long[long] = info end
end

-- Calls f(v) for each non-flag argument
function OptionParser:arg(f)
    self.arg = f
end

-- Runs the option parser for the specified set of arguments. Returns true if all arguments
-- where successfully parsed and false otherwise.
function OptionParser:run(args)
    local pos = 1
    while pos <= #args do
        local arg = args[pos]
        if arg == "--" then
            for i=pos+1,#args do
                if self.arg then self.arg(args[i]) end
                return true
            end
        end
        if arg:match("^%-%-") then
            local info = self.long[arg:sub(3)]
            if not info then print("Unknown flag: " .. arg) return false end
            if info.type == "flag" then
                info.f()
                pos = pos + 1
            else
                param = args[pos+1]
                if not param then print("No parameter for flag: " .. arg) return false end
                info.f(param)
                pos = pos+2
            end
        elseif arg:match("^%-") then
            for i=2,arg:len() do
                local c = arg:sub(i,i)
                local info = self.short[c]
                if not info then print("Unknown flag: -" .. c) return false end
                if info.type == "flag" then
                    info.f()
                else
                    if i == arg:len() then
                        param = args[pos+1]
                        if not param then print("No parameter for flag: -" .. c) return false end
                        info.f(param)
                        pos = pos + 1
                    else
                        param = arg:sub(i+1)
                        info.f(param)
                    end
                    break
                end
            end
            pos = pos + 1
        else
            if self.arg then self.arg(arg) end
            pos = pos + 1
        end
    end
    return true
end

-- Handles the case when markdown is run from the command line
local function run_command_line(arg)
    -- Generate output for input s given options
    local function run(s, options)
        s = markdown(s)
        if not options.wrap_header then return s end
        local header = ""
        if options.header then
            local f = io.open(options.header) or error("Could not open file: " .. options.header)
            header = f:read("*a")
            f:close()
        else
            header = [[
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
    <meta http-equiv="content-type" content="text/html; charset=CHARSET" />
    <title>TITLE</title>
    <link rel="stylesheet" type="text/css" href="STYLESHEET" />
</head>
<body>
]]
            local title = options.title or s:match("<h1>(.-)</h1>") or s:match("<h2>(.-)</h2>") or
                s:match("<h3>(.-)</h3>") or "Untitled"
            header = header:gsub("TITLE", title)
            if options.inline_style then
                local style = ""
                local f = io.open(options.stylesheet)
                if f then
                    style = f:read("*a") f:close()
                else
                    error("Could not include style sheet " .. options.stylesheet .. ": File not found")
                end
                header = header:gsub('<link rel="stylesheet" type="text/css" href="STYLESHEET" />',
                    "<style type=\"text/css\"><!--\n" .. style .. "\n--></style>")
            else
                header = header:gsub("STYLESHEET", options.stylesheet)
            end
            header = header:gsub("CHARSET", options.charset)
        end
        local footer = "</body></html>"
        if options.footer then
            local f = io.open(options.footer) or error("Could not open file: " .. options.footer)
            footer = f:read("*a")
            f:close()
        end
        return header .. s .. footer
    end

    -- Generate output path name from input path name given options.
    local function outpath(path, options)
        if options.append then return path .. ".html" end
        local m = path:match("^(.+%.html)[^/\\]+$") if m then return m end
        m = path:match("^(.+%.)[^/\\]*$") if m and path ~= m .. "html" then return m .. "html" end
        return path .. ".html"
    end

    -- Default commandline options
    local options = {
        wrap_header = true,
        header = nil,
        footer = nil,
        charset = "utf-8",
        title = nil,
        stylesheet = "default.css",
        inline_style = false
    }
    local help = [[
Usage: markdown.lua [OPTION] [FILE]
Runs the markdown text markup to HTML converter on each file specified on the
command line. If no files are specified, runs on standard input.

No header:
    -n, --no-wrap        Don't wrap the output in <html>... tags.
Custom header:
    -e, --header FILE    Use content of FILE for header.
    -f, --footer FILE    Use content of FILE for footer.
Generated header:
    -c, --charset SET    Specifies charset (default utf-8).
    -i, --title TITLE    Specifies title (default from first <h1> tag).
    -s, --style STYLE    Specifies style sheet file (default default.css).
    -l, --inline-style   Include the style sheet file inline in the header.
Generated files:
    -a, --append         Append .html extension (instead of replacing).
Other options:
    -h, --help           Print this help text.
    -t, --test           Run the unit tests.
]]

    local run_stdin = true
    local op = OptionParser:new()
    op:flag("n", "no-wrap", function () options.wrap_header = false end)
    op:param("e", "header", function (x) options.header = x end)
    op:param("f", "footer", function (x) options.footer = x end)
    op:param("c", "charset", function (x) options.charset = x end)
    op:param("i", "title", function(x) options.title = x end)
    op:param("s", "style", function(x) options.stylesheet = x end)
    op:flag("l", "inline-style", function(x) options.inline_style = true end)
    op:flag("a", "append", function() options.append = true end)
    op:flag("t", "test", function()
        local n = arg[0]:gsub("markdown.lua", "markdown-tests.lua")
        local f = io.open(n)
        if f then
            f:close() dofile(n)
        else
            error("Cannot find markdown-tests.lua")
        end
        run_stdin = false
    end)
    op:flag("h", "help", function() print(help) run_stdin = false end)
    op:arg(function(path)
            local file = io.open(path) or error("Could not open file: " .. path)
            local s = file:read("*a")
            file:close()
            s = run(s, options)
            file = io.open(outpath(path, options), "w") or error("Could not open output file: " .. outpath(path, options))
            file:write(s)
            file:close()
            run_stdin = false
        end
    )

    if not op:run(arg) then
        print(help)
        run_stdin = false
    end

    if run_stdin then
        local s = io.read("*a")
        s = run(s, options)
        io.write(s)
    end
end

-- If we are being run from the command-line, act accordingly
if arg and arg[0]:find("markdown%.lua$") then
    run_command_line(arg)
else
    return md
end
