function format_date(date)
    if not date then return date end

    date = pandoc.utils.normalize_date(pandoc.utils.stringify(date))
    local year, month, day = date:match("(%d%d%d%d)-(%d%d)-(%d%d)")
    if not year then return nil end

    local time = os.time({ year = tonumber(year), month = tonumber(month), day = tonumber(day) })
    return {
        yyyy_mm_dd = os.date("%F", time),
        yyyy = os.date("%Y", time),
        mm = os.date("%m", time),
        dd = os.date("%d", time),
        rfc3339 = os.date("%FT%T+00:00", time),
        long = os.date("%B %d, %Y", time),
        short = os.date("%b %d, %y", time),
    }
end

function table_to_list(t, kv, cmp)
    local l = pandoc.List()

    if kv then
        for key, value in pairs(t) do l:insert({ key = key, value = value }) end
    else
        for _, value in pairs(t) do l:insert(value) end
    end

    l:sort(cmp or function(i1, i2) return i1.key < i2.key end)

    return l
end

function group_by(l, field, insert)
    insert = insert or function(group, _, item)
        if not group then
            group = l:new({ item })
            return group
        end
        group:insert(item)
    end

    local groups = {}

    for i = 1, #l do
        local item = l[i]
        local f = field(item)

        if f then
            local out = insert(groups[f], f, item)
            if out then groups[f] = out end
        end
    end

    return groups
end

function splitstr(input, sep)
    sep = sep or "%s"
    local t = {}
    for str in input:gmatch("([^" .. sep .. "]+)") do table.insert(t, str) end
    return t
end

function relative_to(dir, target)
    dir = splitstr(dir, "/")
    target = splitstr(target, "/")

    local prefix = true
    local path = ""

    for i = 1, math.min(#dir, #target) do
        local t = target[i]
        if prefix then
            if dir[i] ~= t then
                prefix = false
                path = "../" .. t
            end
        else
            path = "../" .. path .. "/" .. t
        end
    end

    if #dir < #target then
        for i = #dir + 1, #target do path = path .. (path == "" and "" or "/") .. target[i] end
    elseif #dir > #target then
        for _ = #target + 1, #dir do path = "../" .. path end
    end

    return path
end

function make_absolute(rel, base)
    return rel:find("^/") and rel or base:gsub("^(.*)/.-$", "%1") .. "/" .. rel
end

function resolve_url(site_url, ref_file, target_file)
    target_file = target_file:gsub("/index%.html$", "/")

    local ref_base_dir = ref_file:gsub("^(.*)/.-$", "%1")
    local abs = target_file
    local rel = relative_to(ref_base_dir, abs):gsub("/index%.html$", "/")

    return { abs = abs, rel = rel, full = (site_url .. abs) }
end

function resolve_layout(layout)
    if layout then
        layout = pandoc.utils.stringify(layout)
        return { id = layout, ["is_" .. layout] = true }
    end
end

function resolve_namespace(namespace)
    namespace = pandoc.utils.stringify(namespace)

    local root = "index"
    if namespace ~= "" then root = namespace:gsub("^/([^/]*).*$", "%1") end

    return { root = { id = root, ["is_" .. root] = true }, full = namespace }
end

function prep_menu(active_id, main_menu)
    local active_item = nil

    for i = 1, #main_menu do
        local item = main_menu[i]
        local active = pandoc.utils.stringify(item.id) == active_id
        item.active = active
        if active then active_item = item end
    end

    return {
        items = main_menu:filter(function(item) return not item.hidden or item.active end),
        active = active_item,
    }
end

function process_pages(global, pages_by_id)
    if not pages_by_id then return nil end

    local pages_list = pandoc.List()

    for _, page in pairs(pages_by_id) do pages_list:insert(process(global, page)) end

    pages_list:sort(function(p1, p2)
        if p1.position then
            if p2.position then
                return p1.position < p2.position
            else
                return true
            end
        elseif p2.position then
            return false
        elseif p1.date then
            if p2.date then
                return p1.date.yyyy_mm_dd > p2.date.yyyy_mm_dd
            else
                return true
            end
        elseif p2.date then
            return false
        else
            return p1.title < p2.title
        end
    end)

    local pages_by_category = pages_list:filter(function(p) return p.category ~= nil end)
    pages_by_category = group_by(pages_by_category, function(p) return p.category.id end,
        function(data, _, p)
            if not data then
                local l = pandoc.List()
                l:insert(p)
                return { name = pandoc.utils.stringify(p.category.name), pages = l }
            else
                data.pages:insert(p)
            end
        end)
    pages_by_category = table_to_list(pages_by_category, false,
        function(i1, i2) return i1.name < i2.name end)

    local pages_data = { all = pages_list, by_id = pages_by_id, by_category = pages_by_category }

    return pages_data
end

function pages_last_update(all_pages)
    local last_update = format_date("1990-01-01")

    for i = 1, #all_pages do
        local page = all_pages[i]
        if page.last_update and page.last_update.yyyy_mm_dd > last_update.yyyy_mm_dd then
            last_update = page.last_update
        end
    end

    return last_update
end

function process(global, meta)
    meta.namespace = resolve_namespace(meta.namespace)
    meta.file_out = pandoc.utils.stringify(meta.file_out):gsub("^out", "")
    meta.layout = resolve_layout(meta.layout)
    meta.url = resolve_url(global.site.url, global.file_out, meta.file_out)
    meta.title = (meta.title and pandoc.utils.stringify(meta.title)) or ""

    if meta.position then meta.position = pandoc.utils.stringify(meta.position) end

    if meta.create_feed then
        if meta.file_out:match(".html$") then
            meta.feed = {
                url = resolve_url(global.site.url, global.file_out,
                    meta.file_out:gsub(".html$", ".xml")),
            }
        else
            meta.page = {
                url = resolve_url(global.site.url, global.file_out,
                    meta.file_out:gsub(".xml$", ".html")),
            }
        end
    end

    if meta.preview then
        meta.preview = make_absolute(pandoc.utils.stringify(meta.preview), meta.file_out)
        meta.preview = resolve_url(global.site.url, global.file_out, meta.preview)
    end

    if meta.date then meta.date = format_date(meta.date) end

    if meta.menus and meta.menus.main then
        meta.menus.main = prep_menu(meta.namespace.root.id, meta.menus.main)
    end

    meta.pages = process_pages(global, meta.pages)

    if meta.last_update then
        meta.last_update = format_date(meta.last_update)
    elseif meta.date then
        meta.last_update = meta.date
    elseif meta.pages then
        meta.last_update = pages_last_update(meta.pages.all)
    end

    return meta
end

function Meta(meta)
    meta.site.url = pandoc.utils.stringify(meta.site.url):gsub("/$", "")

    return process(meta, meta)
end