local function resolve(base, relative)
    if relative:sub(1, 1) == "/" then
        return relative
    end

    local combined = base .. "/" .. relative

    local segments = {}
    for segment in combined:gmatch("[^/]+") do
        if segment == ".." then
            if #segments > 0 and segments[#segments] ~= ".." then
                table.remove(segments)
            else
                table.insert(segments, "..")
            end
        elseif segment ~= "." and segment ~= "" then
            table.insert(segments, segment)
        end
    end

    local prefix = (base:sub(1, 1) == "/" or relative:sub(1, 1) == "/") and "/" or ""
    return prefix .. table.concat(segments, "/")
end

-- The branch from the query string (?h=...), or nil.  cgit doesn't pass it
-- to filters, but Lua filters run in cgit's process, which has the CGI
-- environment.
local function query_head()
    local query = os.getenv("QUERY_STRING") or ""
    for key, value in query:gmatch("([^&=]+)=([^&]*)") do
        if key == "h" then
            value = value:gsub("%+", " "):gsub("%%(%x%x)", function(hex)
                return string.char(tonumber(hex, 16))
            end)
            if value ~= "" then
                return value
            end
        end
    end
    return nil
end

local function url_escape(s)
    return (s:gsub("[^%w._~/-]", function(c)
        return string.format("%%%02X", c:byte())
    end))
end

-- cgit provides html() to Lua filters.  It writes in order with cgit's own
-- output, which io.write's buffered stdout doesn't.  io.write is for running
-- the filter outside of cgit.
local emit = html or io.write

-- Run md2html and return its output.
local function md2html(markdown)
    local input = os.tmpname()
    local file = assert(io.open(input, "w"))
    file:write(markdown)
    file:close()
    local handle = io.popen("@md2html@ --github < " .. input, "r")
    local output = handle and handle:read("*a")
    if handle then
        handle:close()
    end
    os.remove(input)
    return output
end

local buffer = ""
local should_render = true
local current_dir = "" -- Stores the directory prefix of the file being processed
local current_file = ""

function filter_open(...)
    local args = {...}
    current_file = args[1] or ""

    if current_file == "" then
        current_file = "README.md"
    end

    -- Strict route enforcement: Allow README.md OR any file starting with docs/ and ending in .md
    if current_file == "README.md" or (current_file:sub(1, 5) == "docs/" and current_file:sub(-3) == ".md") then
        should_render = true

        -- Extract directory context (e.g., "docs/ops.md" -> "docs/")
        -- If it's the root README.md, current_dir stays an empty string ""
        if current_file ~= "README.md" then
            current_dir = current_file:match("(.*/)") or ""
        else
            current_dir = ""
        end
    else
        should_render = false
    end

    buffer = ""
end

function filter_write(str)
    if should_render then
        buffer = buffer .. str
    end
end

function filter_close()
    if not should_render then
        emit("<p><em>Error: Access Denied. Only README.md and documentation markdown files are viewable here.</em></p>")
        return
    end

    local repo = os.getenv("CGIT_REPO_URL") or ""

    local head = query_head()
    local query = ""
    if head and head ~= os.getenv("CGIT_REPO_DEFBRANCH") then
        query = "?h=" .. url_escape(head)
    end

    local processed = buffer:gsub("%[([^%]]+)%]%(([^:)]+)%)", function(text, path)
        -- Ignore protocols (http, https, etc.), absolute paths (/), and anchors (#)
        if path:sub(1, 1) == "/" or path:sub(1, 1) == "#" or path:match("^%w+:") then
            return string.format("[%s](%s)", text, path)
        end

        local fragment
        path, fragment = path:match("^([^#]*)(.*)$")

        path = resolve(current_dir, path)

        if path:sub(-3) == ".md" then
          return string.format("[%s](/%s/about/%s%s%s)", text, repo, path, query, fragment)
        else
          return string.format("[%s](/%s/tree/%s%s%s)", text, repo, path, query, fragment)
        end
    end)

    local output = md2html(processed)
    if output then
        emit(output)
    else
        emit("<p>Error: Could not execute md2html.</p>")
    end
end