//! Packet builders and endpoint construction.

use alloc::{collections::BTreeSet, string::String, vec::Vec};

use crate::protocol::tree::{HookKey, PendingHook};
use crate::protocol::{
    CallMessage, DataMessage, FrameBytes, HookTarget, PacketHeader, PacketType, ValidationError,
    encode_packet, validate_call, validate_header, validate_procedure_id,
};

use super::super::{CompiledRoutes, RouteDecision};
use super::core::{ChildRoute, EndpointError, EndpointOutcome, ProtocolEndpoint};
use crate::protocol::tree::LeafSpec;

impl ProtocolEndpoint {
    fn prepare_call(
        &self,
        dst_path: Vec<String>,
        dst_leaf: Option<String>,
        procedure_id: impl Into<String>,
        response_hook_id: Option<u64>,
        data: Vec<u8>,
    ) -> Result<(PacketHeader, CallMessage), EndpointError> {
        let procedure_id = procedure_id.into();
        validate_procedure_id(&procedure_id)?;

        let response_hook = response_hook_id.map(|hook_id| HookTarget {
            hook_id,
            return_path: self.path.clone(),
        });
        let header = PacketHeader {
            packet_type: PacketType::Call,
            src_path: self.path.clone(),
            dst_path,
            dst_leaf,
            hook_id: None,
        };
        let call = CallMessage {
            procedure_id,
            data,
            response_hook,
        };

        validate_header(&header)?;
        validate_call(&header, &call)?;
        Ok((header, call))
    }

    fn prepare_data(
        &self,
        dst_path: Vec<String>,
        hook_id: u64,
        procedure_id: impl Into<String>,
        data: Vec<u8>,
        end_hook: bool,
    ) -> Result<(PacketHeader, DataMessage), EndpointError> {
        let procedure_id = procedure_id.into();
        validate_procedure_id(&procedure_id)?;

        let header = PacketHeader {
            packet_type: PacketType::Data,
            src_path: self.path.clone(),
            dst_path,
            dst_leaf: None,
            hook_id: Some(hook_id),
        };
        let message = DataMessage {
            procedure_id,
            data,
            end_hook,
        };

        validate_header(&header)?;
        Ok((header, message))
    }

    fn register_outbound_call_hook(
        &mut self,
        header: &PacketHeader,
        call: &CallMessage,
    ) -> Result<(), EndpointError> {
        // Outbound calls reserve their response hook before the frame is emitted so
        // the endpoint can attribute returned Fault packets even before the callee
        // accepts the call. The hook only becomes active once valid hook traffic
        // comes back from the expected peer.
        if let Some(hook) = &call.response_hook
            && let key = HookKey::new(hook.return_path.clone(), hook.hook_id)
            && self
                .hooks
                .insert_pending(
                    key,
                    PendingHook {
                        caller_src_path: header.dst_path.clone(),
                        procedure_id: call.procedure_id.clone(),
                        local_ended: false,
                    },
                )
                .is_err()
        {
            return Err(EndpointError::Validation(ValidationError::InvalidHookId));
        }
        Ok(())
    }

    #[must_use]
    /// Creates an endpoint with compiled routing tables for its current topology.
    ///
    /// `parent_path` is currently used only as a presence flag. The endpoint stores its own
    /// absolute `path`, and routing only needs to know whether an upward route exists.
    ///
    /// # Example
    /// ```rust
    /// use unshell::protocol::tree::{ChildRoute, LeafSpec, ProtocolEndpoint};
    /// let endpoint = ProtocolEndpoint::new(
    ///     vec!["worker".into()],
    ///     Some(Vec::new()),
    ///     vec![ChildRoute::registered(vec!["worker".into(), "child".into()])],
    ///     vec![LeafSpec {
    ///         name: "service".into(),
    ///         procedures: vec!["example.service.v1.invoke".into()],
    ///     }],
    /// );
    /// let _ = endpoint;
    /// ```
    pub fn new(
        path: Vec<String>,
        parent_path: Option<Vec<String>>,
        children: Vec<ChildRoute>,
        leaves: Vec<LeafSpec>,
    ) -> Self {
        let registered_child_paths = children
            .iter()
            .filter(|child| child.registered)
            .map(|child| child.path.clone())
            .collect::<Vec<_>>();

        Self {
            routing: CompiledRoutes::new(&path, &registered_child_paths, parent_path.is_some()),
            path,
            children,
            leaves: leaves
                .into_iter()
                .map(|leaf| (leaf.name.clone(), leaf))
                .collect(),
            endpoint_procedures: BTreeSet::new(),
            hooks: Default::default(),
        }
    }

    /// Registers a procedure that is handled directly by the endpoint.
    ///
    /// Endpoint-level procedures exist for protocol services that are not attached to one leaf,
    /// such as built-in runtime behavior.
    ///
    /// # Example
    /// ```rust
    /// use unshell::protocol::tree::ProtocolEndpoint;
    /// let mut endpoint = ProtocolEndpoint::new(Vec::new(), None, Vec::new(), Vec::new());
    /// endpoint.add_endpoint_procedure("example.endpoint.v1.health")?;
    /// # Ok::<(), unshell::protocol::tree::EndpointError>(())
    /// ```
    pub fn add_endpoint_procedure(
        &mut self,
        procedure_id: impl Into<String>,
    ) -> Result<(), EndpointError> {
        let procedure_id = procedure_id.into();
        validate_procedure_id(&procedure_id)?;
        self.endpoint_procedures.insert(procedure_id);
        Ok(())
    }

    #[must_use]
    /// Allocates a hook id scoped to this endpoint path.
    ///
    /// # Example
    /// ```rust
    /// use unshell::protocol::tree::ProtocolEndpoint;
    /// let mut endpoint = ProtocolEndpoint::new(Vec::new(), None, Vec::new(), Vec::new());
    /// let hook_id = endpoint.allocate_hook_id();
    /// assert_ne!(hook_id, 0);
    /// ```
    pub fn allocate_hook_id(&mut self) -> u64 {
        self.hooks.allocate_hook_id(&self.path)
    }

    /// Encodes a call frame without routing it through the local endpoint.
    ///
    /// This exists for callers that want a fully encoded outbound frame while handling transport
    /// themselves.
    ///
    /// # Example
    /// ```rust
    /// use unshell::protocol::tree::ProtocolEndpoint;
    /// let mut endpoint = ProtocolEndpoint::new(Vec::new(), None, Vec::new(), Vec::new());
    /// let frame = endpoint.make_call(
    ///     vec!["worker".into()],
    ///     Some("service".into()),
    ///     "example.service.v1.invoke",
    ///     None,
    ///     vec![1, 2, 3],
    /// )?;
    /// assert!(!frame.is_empty());
    /// # Ok::<(), unshell::protocol::tree::EndpointError>(())
    /// ```
    pub fn make_call(
        &mut self,
        dst_path: Vec<String>,
        dst_leaf: Option<String>,
        procedure_id: impl Into<String>,
        response_hook_id: Option<u64>,
        data: Vec<u8>,
    ) -> Result<FrameBytes, EndpointError> {
        let (header, call) =
            self.prepare_call(dst_path, dst_leaf, procedure_id, response_hook_id, data)?;
        self.register_outbound_call_hook(&header, &call)?;
        Ok(encode_packet(&header, &call)?)
    }

    /// Builds and immediately routes a call, producing either a forward or a local event.
    ///
    /// # Example
    /// ```rust
    /// use unshell::protocol::tree::{ChildRoute, EndpointOutcome, ProtocolEndpoint};
    /// let mut endpoint = ProtocolEndpoint::new(
    ///     Vec::new(),
    ///     None,
    ///     vec![ChildRoute::registered(vec!["worker".into()])],
    ///     Vec::new(),
    /// );
    /// let outcome = endpoint.send_call(
    ///     vec!["worker".into()],
    ///     Some("service".into()),
    ///     "example.service.v1.invoke",
    ///     None,
    ///     vec![],
    /// )?;
    /// assert!(matches!(outcome, EndpointOutcome::Forward { .. } | EndpointOutcome::Dropped | EndpointOutcome::Local(_)));
    /// # Ok::<(), unshell::protocol::tree::EndpointError>(())
    /// ```
    pub fn send_call(
        &mut self,
        dst_path: Vec<String>,
        dst_leaf: Option<String>,
        procedure_id: impl Into<String>,
        response_hook_id: Option<u64>,
        data: Vec<u8>,
    ) -> Result<EndpointOutcome, EndpointError> {
        let (header, call) =
            self.prepare_call(dst_path, dst_leaf, procedure_id, response_hook_id, data)?;
        self.register_outbound_call_hook(&header, &call)?;

        match self.decide_route(&header.dst_path) {
            RouteDecision::Local => self.handle_local_call(header, call),
            RouteDecision::Drop => {
                self.rollback_pending_call_hook(&call);
                Ok(EndpointOutcome::Dropped)
            }
            route => Ok(EndpointOutcome::Forward {
                route,
                frame: encode_packet(&header, &call)?,
            }),
        }
    }

    /// Encodes a data frame without routing it through the local endpoint.
    ///
    /// # Example
    /// ```rust
    /// use unshell::protocol::tree::ProtocolEndpoint;
    /// let endpoint = ProtocolEndpoint::new(Vec::new(), None, Vec::new(), Vec::new());
    /// let frame = endpoint.make_data(vec!["root".into()], 7, "example.service.v1.invoke", vec![1], false)?;
    /// assert!(!frame.is_empty());
    /// # Ok::<(), unshell::protocol::tree::EndpointError>(())
    /// ```
    pub fn make_data(
        &self,
        dst_path: Vec<String>,
        hook_id: u64,
        procedure_id: impl Into<String>,
        data: Vec<u8>,
        end_hook: bool,
    ) -> Result<FrameBytes, EndpointError> {
        let (header, message) =
            self.prepare_data(dst_path, hook_id, procedure_id, data, end_hook)?;
        Ok(encode_packet(&header, &message)?)
    }

    /// Builds and immediately routes a data packet, updating local hook state for end-of-stream.
    ///
    /// # Example
    /// ```rust
    /// use unshell::protocol::tree::ProtocolEndpoint;
    /// let mut endpoint = ProtocolEndpoint::new(Vec::new(), None, Vec::new(), Vec::new());
    /// let _ = endpoint.send_data(vec!["root".into()], 7, "example.service.v1.invoke", vec![], false);
    /// # Ok::<(), unshell::protocol::tree::EndpointError>(())
    /// ```
    pub fn send_data(
        &mut self,
        dst_path: Vec<String>,
        hook_id: u64,
        procedure_id: impl Into<String>,
        data: Vec<u8>,
        end_hook: bool,
    ) -> Result<EndpointOutcome, EndpointError> {
        if let Some(active_key) = self
            .hooks
            .resolve_active_key(&dst_path, hook_id, &self.path)
            && self
                .hooks
                .active(&active_key)
                .is_some_and(|active| active.local_ended)
        {
            return Err(EndpointError::Validation(ValidationError::HookInvariant(
                "local side already closed this hook",
            )));
        }

        let local_end_dst_path = dst_path.clone();
        let host_key = HookKey::new(self.path.clone(), hook_id);
        let (header, message) =
            self.prepare_data(dst_path, hook_id, procedure_id, data, end_hook)?;

        if end_hook {
            self.mark_local_stream_end(&local_end_dst_path, hook_id, &host_key);
        }

        match self.decide_route(&header.dst_path) {
            RouteDecision::Local => self.handle_local_data(header, message),
            RouteDecision::Drop => Ok(EndpointOutcome::Dropped),
            route => Ok(EndpointOutcome::Forward {
                route,
                frame: encode_packet(&header, &message)?,
            }),
        }
    }

    fn rollback_pending_call_hook(&mut self, call: &CallMessage) {
        if let Some(hook) = &call.response_hook {
            self.hooks
                .remove_pending(&HookKey::new(hook.return_path.clone(), hook.hook_id));
        }
    }

    fn mark_local_stream_end(&mut self, dst_path: &[String], hook_id: u64, host_key: &HookKey) {
        // Locally-originated streams may not have been resolved against a peer yet, so fall
        // back to the endpoint's own hook key shape when closing them.
        let local_hook_key = self
            .hooks
            .resolve_active_key(dst_path, hook_id, &self.path)
            .unwrap_or_else(|| host_key.clone());
        if self.hooks.pending(host_key).is_some() {
            self.hooks.mark_pending_local_end(host_key);
        } else if self.hooks.mark_local_end(&local_hook_key) {
            self.hooks.remove_active(&local_hook_key);
        }
    }
}