/// The tunable settings for the runtime.
pub struct RuntimeConfig {
    pub max_actors: usize,
    pub channel_buffer_size: usize,
    /// Maximum messages processed per actor per tick.
    /// Prevents a single actor with a large mailbox from starving others.
    /// `0` means unlimited (drain entire mailbox).
    pub actor_message_budget: usize,
    /// Number of logical workers created with the runtime.
    ///
    /// Actors are pinned to one logical worker at spawn and never moved.
    /// Defaults to `1` to preserve single-worker behavior unless a caller opts
    /// into more workers. `0` is rejected during runtime construction.
    pub worker_count: usize,
    /// Maximum number of items consumed by one ingress drain (transfer, spawn,
    /// admin) per worker pass. `0` means unlimited.
    ///
    /// Each drain operation is bounded independently, so a backlog in one queue
    /// cannot permanently block another. Defaults to `1024`.
    pub worker_ingress_budget: usize,
}

/// 8kB for the `Box<..>` before counting the rest of the memory
const DEFAULT_MAX_ACTORS: usize = 1_000;

/// Pre-allocated ring buffer capacity for each channel (transfer, spawn, inbox).
/// When the ring is full, messages overflow into an unbounded backup queue.
const DEFAULT_CHANNEL_BUFFER_SIZE: usize = 1_000;

/// Default per-actor message budget per tick.
/// Inspired by BEAM's reduction budget (4000) and tokio's cooperative budget (128).
/// 64 is a good default: high enough for throughput, low enough for fairness.
const DEFAULT_ACTOR_MESSAGE_BUDGET: usize = 64;

/// Default logical worker count. Preserves the historic single-worker runtime
/// unless a caller explicitly opts into more workers.
const DEFAULT_WORKER_COUNT: usize = 1;

/// Default per-drain ingress budget. `0` would mean unlimited, so the default
/// is a finite, generous cap that keeps every worker pass bounded.
const DEFAULT_WORKER_INGRESS_BUDGET: usize = 1_024;

impl Default for RuntimeConfig {
    fn default() -> Self {
        Self {
            max_actors: DEFAULT_MAX_ACTORS,
            channel_buffer_size: DEFAULT_CHANNEL_BUFFER_SIZE,
            actor_message_budget: DEFAULT_ACTOR_MESSAGE_BUDGET,
            worker_count: DEFAULT_WORKER_COUNT,
            worker_ingress_budget: DEFAULT_WORKER_INGRESS_BUDGET,
        }
    }
}