//! Inter-agent **mailbox** port (Option 1 « Terminal + MCP », lot B-1).
//!
//! A delegating agent calls `idea_ask_agent(target, task)` and **blocks** until the
//! target renders a result via `idea_reply(result)`. Between those two MCP calls,
//! IdeA must (1) hand the task to the target's single FIFO input and (2) hold the
//! caller's await on a reply slot that the target's later `idea_reply` resolves.
//!
//! This module owns the **pure** contract of that rendezvous: the [`AgentMailbox`]
//! port plus its value objects ([`Ticket`], [`TicketId`], [`MailboxError`]) and the
//! [`PendingReply`] handle the caller awaits. It is **I/O-free**: the actual queue,
//! its mutex and the one-shot reply channel are an infrastructure concern (the
//! `InMemoryMailbox` adapter). Keeping the contract here means the FIFO/resolution
//! invariants are expressed against a port the application layer depends on, never
//! against a concrete channel type (hexagonal + DIP).
//!
//! ## Model
//!
//! - **One queue per target agent** (`AgentId`). `enqueue` appends a [`Ticket`] and
//!   returns a [`PendingReply`] the caller awaits.
//! - **`resolve(agent, result)` corrèle implicitement** the result with the ticket
//!   at the **head** of that agent's queue — the agent is processing one task at a
//!   time (1 agent = 1 employee, FIFO input), so its current `idea_reply` answers
//!   the task it is currently working on. No ticket id is exposed to the model.
//! - **Timeout** is the caller's concern: it drops its [`PendingReply`] and calls
//!   [`AgentMailbox::cancel_head`] to retire the stuck head ticket so the queue
//!   advances.

use std::future::Future;
use std::pin::Pin;
use std::task::{Context, Poll};

use crate::conversation::ConversationId;
use crate::ids::AgentId;
use crate::input::InputSource;

/// A read-only, cloned view of one queued [`Ticket`] in a target agent's FIFO.
///
/// Carries **only the data** of a ticket (never the one-shot reply sender, which
/// stays inside the adapter), plus its `position` in the FIFO at snapshot time
/// (`0` = head). Pure value object used by the [`AgentQueueSnapshot`] read port so
/// the work-state read model can list pending delegations without touching the
/// mutating [`AgentMailbox`] surface (ISP).
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct QueuedTicketSnapshot {
    /// Stable id of the queued ticket.
    pub id: TicketId,
    /// The origin of the input (Human or a delegating Agent).
    pub source: InputSource,
    /// The conversation thread this task enters.
    pub conversation: ConversationId,
    /// Display name (or id) of the requester.
    pub requester: String,
    /// The full task/message text (preview/truncation is the caller's concern).
    pub task: String,
    /// Position in the FIFO at snapshot time (`0` = head, recomputed each call).
    pub position: u32,
}

/// Read-only inspection port over the per-agent delegation FIFO.
///
/// Segregated from the mutating [`AgentMailbox`] (Interface Segregation): the
/// work-state read model depends on this port to *observe* the queue, never on
/// `enqueue`/`resolve`/`cancel`. Implementations return **cloned** snapshots in
/// FIFO order with recomputed positions; observing the queue must never mutate it.
///
/// Object-safe (`&self`) so the application layer holds it as
/// `Arc<dyn AgentQueueSnapshot>`; one concrete `InMemoryMailbox` can be shared as
/// both an [`AgentMailbox`] (mutation) and an [`AgentQueueSnapshot`] (read) view.
pub trait AgentQueueSnapshot: Send + Sync {
    /// Returns a cloned, FIFO-ordered snapshot of `agent`'s queued tickets.
    ///
    /// An agent with no queue yields an empty `Vec`. Positions are recomputed from
    /// the current order (`0` = head). Pure read: the queue is left untouched.
    fn queue_for(&self, agent: AgentId) -> Vec<QueuedTicketSnapshot>;
}

/// Identifies one queued [`Ticket`] within a target agent's mailbox.
///
/// Newtype around [`uuid::Uuid`]; minted by the adapter on `enqueue`. It is **never
/// exposed to the model** (the MCP `idea_reply` schema carries only `result`):
/// correlation is positional (head of the FIFO), the id only lets the caller name
/// the exact ticket it wants retired on timeout ([`AgentMailbox::cancel_head`]).
#[derive(
    Debug, Clone, Copy, PartialEq, Eq, Hash, PartialOrd, Ord, serde::Serialize, serde::Deserialize,
)]
#[serde(transparent)]
pub struct TicketId(pub uuid::Uuid);

impl TicketId {
    /// Wraps an existing [`uuid::Uuid`].
    #[must_use]
    pub const fn from_uuid(id: uuid::Uuid) -> Self {
        Self(id)
    }

    /// Mints a fresh random ticket id.
    #[must_use]
    pub fn new_random() -> Self {
        Self(uuid::Uuid::new_v4())
    }

    /// Returns the inner [`uuid::Uuid`].
    #[must_use]
    pub const fn as_uuid(&self) -> uuid::Uuid {
        self.0
    }
}

impl std::fmt::Display for TicketId {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        write!(f, "{}", self.0)
    }
}

/// A delegation request queued for a target agent: who asked, and what for.
///
/// The `id` lets the caller retire **this** ticket on timeout; `requester` and
/// `task` are carried so the application layer can prefix the task with the asking
/// agent's identity when it writes to the target's input (`[IdeA · tâche de A ·
/// ticket …]`). Pure value object (no behaviour, no I/O).
///
/// **Origin & target** (cadrage C1): a ticket also carries its [`InputSource`]
/// (Human or Agent — the *source of truth* for the requester, the `requester`
/// string being a derived display label) and the [`ConversationId`] of the thread
/// it enters. These are added through **additive** constructors ([`Ticket::from_human`],
/// [`Ticket::from_agent`]); [`Ticket::new`] keeps its signature (Open/Closed) and
/// defaults to a `Human` source with a nil conversation for back-compat.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Ticket {
    /// Stable id of this queued request (minted by the adapter on `enqueue`).
    pub id: TicketId,
    /// The origin of this input (Human or a delegating Agent).
    pub source: InputSource,
    /// The conversation thread this task enters.
    pub conversation: ConversationId,
    /// Display name (or id) of the agent that issued the `idea_ask_agent`.
    pub requester: String,
    /// The task/message to deliver to the target agent.
    pub task: String,
}

impl Ticket {
    /// Builds a ticket from its parts (back-compat: `Human` source, nil conversation).
    ///
    /// Preserved for existing call sites. Prefer [`Ticket::from_human`] /
    /// [`Ticket::from_agent`] when the source and conversation are known.
    #[must_use]
    pub fn new(id: TicketId, requester: impl Into<String>, task: impl Into<String>) -> Self {
        Self {
            id,
            source: InputSource::Human,
            conversation: ConversationId::from_uuid(uuid::Uuid::nil()),
            requester: requester.into(),
            task: task.into(),
        }
    }

    /// Builds a ticket originating from the **human** operator, for a given thread.
    #[must_use]
    pub fn from_human(
        id: TicketId,
        conversation: ConversationId,
        requester: impl Into<String>,
        task: impl Into<String>,
    ) -> Self {
        Self {
            id,
            source: InputSource::Human,
            conversation,
            requester: requester.into(),
            task: task.into(),
        }
    }

    /// Builds a ticket originating from a delegating **agent**, for a given thread.
    #[must_use]
    pub fn from_agent(
        id: TicketId,
        source: AgentId,
        conversation: ConversationId,
        requester: impl Into<String>,
        task: impl Into<String>,
    ) -> Self {
        Self {
            id,
            source: InputSource::agent(source),
            conversation,
            requester: requester.into(),
            task: task.into(),
        }
    }
}

/// Errors raised by the [`AgentMailbox`] port.
#[derive(Debug, Clone, PartialEq, Eq, thiserror::Error)]
pub enum MailboxError {
    /// `resolve` was called for an agent whose queue is **empty** — an
    /// `idea_reply` with no matching `idea_ask_agent` in flight. Surfaced as a typed
    /// error (never a panic) so the offending agent can read it and adjust.
    #[error("no pending request to reply to for agent {0}")]
    NoPendingRequest(AgentId),
    /// The awaited reply channel closed before a result arrived — the target's
    /// session ended (or was killed) without rendering an `idea_reply`. The caller's
    /// await resolves to this instead of hanging forever.
    #[error("reply channel closed before a result was rendered")]
    Cancelled,
}

/// A handle the caller awaits to receive a target agent's reply.
///
/// Returned by [`AgentMailbox::enqueue`]. Awaiting it yields the `String` result a
/// later [`AgentMailbox::resolve`] feeds into this ticket's slot, or
/// [`MailboxError::Cancelled`] if the reply channel closes first. The future is
/// **opaque** (the adapter builds it from its one-shot receiver): the domain stays
/// free of any concrete channel type, and the await point lives in the application
/// layer's `ask_agent`.
pub struct PendingReply {
    inner: Pin<Box<dyn Future<Output = Result<String, MailboxError>> + Send>>,
}

impl PendingReply {
    /// Wraps a reply future built by the adapter (e.g. over a one-shot receiver).
    #[must_use]
    pub fn new(inner: Pin<Box<dyn Future<Output = Result<String, MailboxError>> + Send>>) -> Self {
        Self { inner }
    }
}

impl Future for PendingReply {
    type Output = Result<String, MailboxError>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        self.inner.as_mut().poll(cx)
    }
}

impl std::fmt::Debug for PendingReply {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        f.debug_struct("PendingReply").finish_non_exhaustive()
    }
}

/// The inter-agent rendezvous port: a per-agent FIFO of delegation tickets, each
/// awaiting a one-shot reply (Option 1, lot B-1).
///
/// **Per-agent FIFO** (1 agent = 1 employee): tickets for the **same** target are
/// served head-first; tickets for **different** targets never block one another.
/// `enqueue` returns the [`PendingReply`] the caller awaits; the target's later
/// `idea_reply` lands in [`AgentMailbox::resolve`], which corrèle it with the head
/// of that agent's queue.
///
/// Kept object-safe (`&self`, owned/`Copy` args) so the application layer holds it
/// as `Arc<dyn AgentMailbox>`; the implementation owns all interior mutability.
pub trait AgentMailbox: Send + Sync {
    /// Appends `ticket` to `agent`'s FIFO and returns the handle to await its reply.
    ///
    /// The returned [`PendingReply`] resolves when a later [`AgentMailbox::resolve`]
    /// feeds a result to this ticket (once it reaches the head and is answered), or
    /// to [`MailboxError::Cancelled`] if the reply channel closes first.
    fn enqueue(&self, agent: AgentId, ticket: Ticket) -> PendingReply;

    /// Resolves the request at the **head** of `agent`'s FIFO with `result`,
    /// waking its awaiting [`PendingReply`] and removing it from the queue.
    ///
    /// Positional correlation: the agent processes one task at a time, so its
    /// current `idea_reply` answers the head ticket.
    ///
    /// # Errors
    /// [`MailboxError::NoPendingRequest`] when `agent` has no queued ticket (an
    /// `idea_reply` with no matching ask in flight).
    fn resolve(&self, agent: AgentId, result: String) -> Result<(), MailboxError>;

    /// Resolves the request identified by `ticket_id` **anywhere** in `agent`'s FIFO
    /// with `result`, waking its awaiting [`PendingReply`] and removing it from the
    /// queue (cadrage C3 §3.3 — deterministic, multi-thread correlation).
    ///
    /// This is the **by-ticket** counterpart of [`AgentMailbox::resolve`]: when an
    /// agent answers a delegation it echoes the ticket id from the `[IdeA · … ·
    /// ticket <id>]` prefix, so IdeA correlates exactly that request even when the
    /// agent has several threads in flight.
    ///
    /// The default implementation falls back to [`AgentMailbox::resolve`] (head of
    /// queue), so a mailbox that does not track ids stays correct for mono-thread
    /// agents. The in-memory adapter overrides it with true id-keyed resolution.
    ///
    /// # Errors
    /// [`MailboxError::NoPendingRequest`] when no queued ticket matches `ticket_id`
    /// (and, for the default, when `agent`'s queue is empty).
    fn resolve_ticket(
        &self,
        agent: AgentId,
        _ticket_id: TicketId,
        result: String,
    ) -> Result<(), MailboxError> {
        self.resolve(agent, result)
    }

    /// Retires the ticket `ticket_id` **iff** it is currently at the head of
    /// `agent`'s FIFO (the caller timed out waiting for it), so the queue advances.
    ///
    /// A no-op when the head is a different ticket (the timed-out one was already
    /// resolved, or another caller's ticket is now in front) — idempotent and safe.
    fn cancel_head(&self, agent: AgentId, ticket_id: TicketId);
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn ticket_carries_its_parts() {
        let id = TicketId::new_random();
        let t = Ticket::new(id, "Main", "do X");
        assert_eq!(t.id, id);
        assert_eq!(t.requester, "Main");
        assert_eq!(t.task, "do X");
        // Back-compat default: human source, nil conversation.
        assert_eq!(t.source, InputSource::Human);
        assert_eq!(t.conversation, ConversationId::from_uuid(uuid::Uuid::nil()));
    }

    #[test]
    fn from_human_carries_source_and_conversation() {
        let conv = ConversationId::from_uuid(uuid::Uuid::from_u128(5));
        let t = Ticket::from_human(TicketId::new_random(), conv, "User", "do X");
        assert_eq!(t.source, InputSource::Human);
        assert_eq!(t.conversation, conv);
        assert_eq!(t.task, "do X");
    }

    #[test]
    fn from_agent_carries_agent_source_and_conversation() {
        let conv = ConversationId::from_uuid(uuid::Uuid::from_u128(6));
        let from = AgentId::from_uuid(uuid::Uuid::from_u128(2));
        let t = Ticket::from_agent(TicketId::new_random(), from, conv, "A", "delegate");
        assert_eq!(t.source, InputSource::agent(from));
        assert_eq!(t.source.as_agent(), Some(from));
        assert_eq!(t.conversation, conv);
    }

    #[test]
    fn ticket_id_roundtrips_through_uuid() {
        let u = uuid::Uuid::from_u128(7);
        let id = TicketId::from_uuid(u);
        assert_eq!(id.as_uuid(), u);
        assert_eq!(id.to_string(), u.to_string());
    }

    #[test]
    fn mailbox_errors_are_distinct_and_typed() {
        let a = AgentId::from_uuid(uuid::Uuid::from_u128(1));
        assert_ne!(MailboxError::NoPendingRequest(a), MailboxError::Cancelled);
    }

    #[test]
    fn queued_ticket_snapshot_carries_ticket_data_and_position() {
        let conv = ConversationId::from_uuid(uuid::Uuid::from_u128(3));
        let from = AgentId::from_uuid(uuid::Uuid::from_u128(4));
        let snap = QueuedTicketSnapshot {
            id: TicketId::from_uuid(uuid::Uuid::from_u128(8)),
            source: InputSource::agent(from),
            conversation: conv,
            requester: "Main".to_owned(),
            task: "delegate".to_owned(),
            position: 2,
        };
        assert_eq!(snap.source.as_agent(), Some(from));
        assert_eq!(snap.conversation, conv);
        assert_eq!(snap.position, 2);
        assert_eq!(snap.task, "delegate");
    }
}