-- Config
local steps_per_tick = 1      -- process 1 surface×force per tick
local flush_interval_ticks = 60 * 15  -- flush every n ticks

-- State
local queue = {}
local buffer = {}
local last_flush_tick = 0

-- Initialize queue with all surface×force pairs
local function prepare_queue()
    queue = {}
    for surface_name, _ in pairs(game.surfaces) do
        for force_name, _ in pairs(game.forces) do
            table.insert(queue, {surface_name = surface_name, force_name = force_name})
        end
    end
end

-- Process a few items from the queue each tick
local function process_queue()
    for i = 1, steps_per_tick do
        local entry = table.remove(queue, 1)
        if not entry then break end

        local surface_name, force_name = entry.surface_name, entry.force_name
        local force = game.forces[force_name]

        -- Items
        local item_stats = force.get_item_production_statistics(surface_name)
        for item, count in pairs(item_stats.input_counts) do
            table.insert(buffer, string.format(
                "production{type=\"item\",surface=\"%s\",force=\"%s\",name=\"%s\"} %d",
                surface_name, force_name, item, count
            ))
        end
        for item, count in pairs(item_stats.output_counts) do
            table.insert(buffer, string.format(
                "consumption{type=\"item\",surface=\"%s\",force=\"%s\",name=\"%s\"} %d",
                surface_name, force_name, item, count
            ))
        end

        -- Fluids
        local fluid_stats = force.get_fluid_production_statistics(surface_name)
        for fluid, count in pairs(fluid_stats.input_counts) do
            table.insert(buffer, string.format(
                "production{type=\"fluid\",surface=\"%s\",force=\"%s\",name=\"%s\"} %d",
                surface_name, force_name, fluid, count
            ))
        end
        for fluid, count in pairs(fluid_stats.output_counts) do
            table.insert(buffer, string.format(
                "consumption{type=\"fluid\",surface=\"%s\",force=\"%s\",name=\"%s\"} %d",
                surface_name, force_name, fluid, count
            ))
        end
    end
end

-- Flush buffer to file periodically
local function maybe_flush()
    if #queue == 0 and game.tick - last_flush_tick >= flush_interval_ticks then
        if #buffer > 0 then
            helpers.write_file("metrics.prom", table.concat(buffer, "\n"), false)
            buffer = {}
        end
        last_flush_tick = game.tick
        prepare_queue()
    end
end

-- Main tick handler
script.on_event(defines.events.on_tick, function()
    process_queue()
    maybe_flush()
end)

-- Bootstrap
script.on_init(function()
    last_flush_tick = game.tick
    prepare_queue()
end)