function format_date(date)
    if date == nil 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 year == nil then return nil end

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

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

    for key, value in pairs(t) do
        l:insert(pandoc.MetaMap({key = key, value = value}))
    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 ~= nil then
            local out = insert(groups[f], f, item)
            if out ~= nil 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 apply_path_rewrites(rewrites, str)
    for i = 1, #rewrites.path do
        local r = rewrites.path[i]
        str = str:gsub(pandoc.utils.stringify(r.from),
                       pandoc.utils.stringify(r.to))
    end
    return str
end

function get_file_out(rewrites, content_dir, output_dir, file_in)
    local file_out = file_in:gsub("^" .. content_dir, ""):gsub("%.md$", ".html")

    if file_out:match(".html$") and not file_out:match("/index%.html$") then
        file_out = file_out:gsub("/(.*)%.html$", "/%1/index.html")
    end

    file_out = apply_path_rewrites(rewrites, file_out)

    return pandoc.MetaString(output_dir .. file_out)
end

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

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

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

function resolve_section(content_dir, file_in)
    local section = file_in:gsub("^" .. content_dir, ""):match("^/(.-)[/.]") or
                        "index"
    return pandoc.MetaMap({
        id = pandoc.MetaString(section),
        ["is_" .. section] = pandoc.MetaBool(true)
    })
end

function resolve_category(categories, category)
    if categories and category then
        category = pandoc.utils.stringify(category)
        return pandoc.MetaMap({
            id = pandoc.MetaString(category),
            name = pandoc.MetaString(categories[category] or category)
        })
    end
end

function prep_main_menu(rewrites, section, main_menu)
    local active_item = nil

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

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

function organize_subpages(site_url, output_dir, ref_file, pages)
    for i = 1, #pages do
        local page = pages[i]
        page.url = resolve_url(site_url, output_dir, ref_file,
                               pandoc.utils.stringify(page.file_out))
    end

    local pages_grouped_date =
        group_by(pages, function(p) return not p.date end)

    local pages_undated = pages_grouped_date[true] or pandoc.MetaList({})
    pages_undated:sort(function(p1, p2)
        return pandoc.utils.stringify(p1.title) <
                   pandoc.utils.stringify(p2.title)
    end)

    local pages_dated = pages_grouped_date[false] or pandoc.MetaList({})
    pages_dated:sort(function(p1, p2)
        return pandoc.utils.stringify(p1.date.yyyy_mm_dd) >
                   pandoc.utils.stringify(p2.date.yyyy_mm_dd)
    end)

    local pages_categorized = pages:filter(
                                  function(p) return p.category ~= nil end)
    pages_categorized:sort(function(p1, p2)
        return pandoc.utils.stringify(p1.title) >
                   pandoc.utils.stringify(p2.title)
    end)

    local pages_by_year = group_by(pages_dated, function(p)
        return pandoc.utils.stringify(p.date.yyyy)
    end)
    pages_by_year = pandoc.MetaList(table_to_list(pages_by_year,
                                                  function(i1, i2)
        return i1.key > i2.key
    end))

    local pages_by_category = group_by(pages_categorized, function(p)
        return pandoc.utils.stringify(p.category.id)
    end)
    pages_by_category = pandoc.MetaList(table_to_list(pages_by_category,
                                                      function(i1, i2)
        return i1.key > i2.key
    end))

    local pages_data = pandoc.MetaMap({
        all_dated = pages_dated,
        all_undated = pages_undated,
        by_year = pages_by_year,
        by_category = pages_by_category,
        last_update = #pages_dated ~= 0 and pages_dated[1].last_update
    })

    local categories_data = group_by(pages_categorized, function(p)
        return p.category and pandoc.utils.stringify(p.category.id)
    end, function(stats, _, p)
        if not stats then
            return {name = pandoc.MetaString(p.category.name), count = 1}
        else
            stats.count = stats.count + 1
        end
    end)
    categories_data = pandoc.MetaList(table_to_list(categories_data))

    for i = 1, #categories_data do
        categories_data[i].value.count =
            pandoc.MetaString(("%d"):format(categories_data[i].value.count))
    end

    return pages_data, categories_data
end

function Meta(meta)
    meta.content_dir = meta.content_dir:gsub("/$", "")
    meta.output_dir = meta.output_dir:gsub("/$", "")
    meta.site.url = pandoc.utils.stringify(meta.site.url):gsub("/$", "")
    meta.rewrites = meta.rewrites or
                        pandoc.MetaMap({path = pandoc.MetaList({})})
    meta.page_type = meta.page_type or "page"
    meta.layout = resolve_layout(meta.layout)
    meta.section = resolve_section(meta.content_dir, meta.file_in)

    meta.file_out = get_file_out(meta.rewrites, meta.content_dir,
                                 meta.output_dir, meta.file_in)

    meta.date = format_date(meta.date)
    if meta.last_update ~= nil then
        meta.last_update = format_date(meta.last_update)
    else
        meta.last_update = meta.date
    end

    meta.category = resolve_category(meta.categories[meta.section.id],
                                     meta.category)
    meta.categories = nil

    if meta.page_type == "feed" then
        meta.page = pandoc.MetaMap({
            url = resolve_url(meta.site.url, meta.output_dir, meta.file_out,
                              meta.file_out:gsub("%.xml$", ".html"))
        })
    end

    if meta.create_feed then
        meta.feed = pandoc.MetaMap({
            url = resolve_url(meta.site.url, meta.output_dir, meta.file_out,
                              meta.file_out:gsub("%.html$", ".xml"))
        })
    end

    if meta.menus and meta.menus.main then
        meta.menus.main = prep_main_menu(meta.rewrites, meta.section,
                                         meta.menus.main)
    end

    if meta.pages then
        local pages, categories = organize_subpages(meta.site.url,
                                                    meta.output_dir,
                                                    meta.file_out, meta.pages)
        meta.pages = pages
        meta.categories = categories
    end

    return meta
end