//! Operations on one board. Requests describe intent independently of tool schemas.
//! Channel descriptions enforce their byte limit at construction and deserialization.

use crate::BoardReadView;
use crate::ChannelPage;
use crate::ChannelSummary;
use crate::Page;
use crate::PostContent;
use crate::PostMetadata;
use crate::PostPreview;
use crate::SubscriptionState;
use crate::ThreadPage;
use crate::ThreadSummary;
use codex_protocol::AgentPath;
use codex_protocol::SessionId;
use codex_protocol::ThreadId;
use codex_protocol::error::CodexErr;
use codex_protocol::error::Result;
use futures::future::BoxFuture;
use serde::Deserialize;
use serde::Serialize;
use std::collections::BTreeMap;
use std::num::NonZeroU32;
use std::sync::Arc;
use uuid::Uuid;

// Local and remote implementations must remain usable through the same handle.
const _: Option<&dyn AgentMessageBoard> = None;

/// Stores discussions and subscriptions for one board.
///
/// Every operation validates caller membership. IDs are scoped to this board;
/// an agent runtime's ThreadId is distinct from a discussion's root post UUID.
/// Mutation success acknowledges acceptance, not that recipients read a post.
/// Implementations enforce hard input/output limits and return backend failures
/// as errors. They own atomic subscription changes, posting and recipient
/// selection, excluding the post author even when explicitly targeted.
/// Notification delivery follows the host's scheduling and wake policy and may
/// be buffered for an idle recipient; see [`crate::MessageBoardHost`].
///
/// Boxed Send futures support an Arc<dyn AgentMessageBoard>, like AgentControl.
pub trait AgentMessageBoard: Send + Sync {
    fn identity(&self) -> SessionId;

    /// The host-selected storage namespace, if this backend identifies one.
    fn storage_namespace(&self) -> Option<&str> {
        None
    }

    /// Capture a live board's channel summaries and post boundary together for read-only access.
    fn read_view(&self, _caller: ThreadId) -> BoxFuture<'_, Result<Arc<dyn BoardReadView>>> {
        unsupported("capturing a board read view")
    }

    fn create_channel(
        &self,
        caller: ThreadId,
        request: CreateChannelRequest,
    ) -> BoxFuture<'_, Result<ChannelSummary>>;

    /// Creates a channel or replaces only its description. Preserves existing
    /// posts, authorship, creation time and subscriptions; never sends notices.
    /// Backends without upsert support must reject this rather than substitute creation.
    fn upsert_channel(
        &self,
        caller: ThreadId,
        request: UpsertChannelRequest,
    ) -> BoxFuture<'_, Result<()>>;

    fn list_channels(
        &self,
        caller: ThreadId,
        query: ChannelQuery,
    ) -> BoxFuture<'_, Result<ChannelPage>>;

    /// Uses the caller's configured clock; clock failures must not create a post.
    /// The request ID identifies a logical call across retries. A retry with
    /// different input is an error; a successful retry returns the same metadata.
    /// Creating a channel while posting also subscribes its author to new roots there.
    fn post(&self, caller: ThreadId, request: PostRequest) -> BoxFuture<'_, Result<PostMetadata>>;

    /// Writes without automatic subscriptions or notifications. Uses caller-scoped
    /// retry IDs shared with posting; changing the operation or payload is an error.
    /// Backends without silent-write support must reject this, never fall back to posting.
    fn append(
        &self,
        caller: ThreadId,
        request: AppendRequest,
    ) -> BoxFuture<'_, Result<PostMetadata>>;

    fn list_threads(
        &self,
        caller: ThreadId,
        query: ThreadQuery,
    ) -> BoxFuture<'_, Result<Page<ThreadSummary>>>;

    fn search_posts(
        &self,
        caller: ThreadId,
        query: PostQuery,
    ) -> BoxFuture<'_, Result<Page<PostPreview>>>;

    fn read_thread(
        &self,
        caller: ThreadId,
        request: ReadThreadRequest,
    ) -> BoxFuture<'_, Result<ThreadPage>>;

    fn read_post(
        &self,
        caller: ThreadId,
        request: ReadPostRequest,
    ) -> BoxFuture<'_, Result<PostContent>>;

    /// Channel subscriptions concern new roots; thread subscriptions concern
    /// replies. Changing one does not change the other. Any member may change
    /// another member's subscription. Posting subscribes its author to the thread
    /// by default, but preserves an explicit unsubscribe until subscribed again.
    fn set_subscription(
        &self,
        caller: ThreadId,
        request: SubscriptionRequest,
    ) -> BoxFuture<'_, Result<SubscriptionState>>;

    /// List a caller's current channel subscriptions on a live board.
    fn subscribed_channels(
        &self,
        _caller: ThreadId,
        _page: PageRequest,
    ) -> BoxFuture<'_, Result<Page<String>>> {
        unsupported("listing channel subscriptions")
    }

    /// Set the caller's membership in the supplied channels as one atomic change.
    /// Other channels and thread subscriptions retain their current membership.
    fn set_subscriptions<'a>(
        &'a self,
        _caller: ThreadId,
        _channels: &'a BTreeMap<&str, SubscriptionChange>,
    ) -> BoxFuture<'a, Result<()>> {
        unsupported("setting channel subscriptions in bulk")
    }
}

fn unsupported<T>(operation: &'static str) -> BoxFuture<'static, Result<T>> {
    Box::pin(async move { Err(CodexErr::UnsupportedOperation(operation.into())) })
}

#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct PageRequest {
    pub cursor: Option<String>,
    pub limit: NonZeroU32,
}

impl Default for PageRequest {
    fn default() -> Self {
        Self {
            cursor: None,
            limit: NonZeroU32::new(20).unwrap_or(NonZeroU32::MIN),
        }
    }
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub enum SortDirection {
    NewestFirst,
    OldestFirst,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub enum SubscriptionChange {
    Subscribe,
    Unsubscribe,
}

#[derive(Clone, Debug, Serialize, Deserialize)]
pub struct CreateChannelRequest {
    pub channel_name: String,
    /// Optional channel purpose, at most 512 UTF-8 bytes.
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub description: Option<ChannelDescription>,
    pub subscription: SubscriptionChange,
}

/// Creates or updates navigation metadata without changing stored evidence or delivery.
#[derive(Clone, Debug, Serialize, Deserialize)]
#[serde(deny_unknown_fields)]
pub struct UpsertChannelRequest {
    pub channel_name: String,
    pub description: ChannelDescription,
}

/// A channel purpose containing at most 512 UTF-8 bytes.
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
#[serde(try_from = "String")]
pub struct ChannelDescription(String);

impl TryFrom<String> for ChannelDescription {
    type Error = CodexErr;

    fn try_from(text: String) -> Result<Self> {
        if text.len() > 512 {
            return Err(CodexErr::InvalidRequest(
                "channel descriptions must contain at most 512 UTF-8 bytes".into(),
            ));
        }
        Ok(Self(text))
    }
}

impl ChannelDescription {
    pub fn as_str(&self) -> &str {
        &self.0
    }
}

impl From<ChannelDescription> for String {
    fn from(description: ChannelDescription) -> Self {
        description.0
    }
}

#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct ChannelQuery {
    pub query: Option<String>,
    pub direction: SortDirection,
    pub page: PageRequest,
}

/// Exactly one destination; discussion threads use their root post's ID.
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub enum PostDestination {
    Channel(String),
    NewChannel(String),
    Thread(Uuid),
}

#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct PostRequest {
    /// Host-generated tool invocation identity, never a model argument.
    pub request_id: String,
    pub destination: PostDestination,
    pub text: String,
    /// Resolved paths in this board's tree; duplicates notify only once.
    pub agents_to_notify: Vec<AgentPath>,
}

#[derive(Clone, Copy, PartialEq, Eq)]
pub(crate) enum PostBehavior {
    Discussion,
    SilentAppend,
}

/// Request to append evidence without subscribing its author or notifying anyone.
#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct AppendRequest {
    /// Caller-scoped invocation identity, shared with posting to reject conflicting retries.
    pub request_id: String,
    pub destination: PostDestination,
    pub text: String,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Serialize, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum ThreadSort {
    Created,
    Activity,
}

#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct ThreadQuery {
    pub channel_name: String,
    pub sort: ThreadSort,
    pub direction: SortDirection,
    pub page: PageRequest,
    pub max_chars_per_post: NonZeroU32,
}

#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct PostQuery {
    pub channel_name: Option<String>,
    pub query: Option<String>,
    pub after_message_id: Option<Uuid>,
    pub author: Option<AgentPath>,
    pub page: PageRequest,
    pub max_chars_per_post: NonZeroU32,
}

#[derive(Clone, Debug, PartialEq, Eq, Serialize, Deserialize)]
pub struct ReadThreadRequest {
    pub thread_id: Uuid,
    pub page: PageRequest,
    pub max_chars_per_post: NonZeroU32,
}

#[derive(Clone, Debug, Serialize, Deserialize)]
pub struct ReadPostRequest {
    pub message_id: Uuid,
    /// Offsets and lengths count Unicode scalar values, not UTF-8 bytes.
    pub offset_chars: u32,
    pub limit_chars: NonZeroU32,
}

#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Serialize, Deserialize)]
pub enum SubscriptionTarget {
    Channel(String),
    Thread(Uuid),
}

#[derive(Clone, Debug, Serialize, Deserialize)]
pub struct SubscriptionRequest {
    pub target: SubscriptionTarget,
    /// None changes the caller's subscription.
    pub target_agent: Option<AgentPath>,
    pub change: SubscriptionChange,
}