//! Thinkers, and how to reach one.
//!
//! A node is an address, not a place. The same lance commander can be a closure
//! on this thread, a subprocess in this container, or a service on another
//! machine, and nothing above it changes: the tree is built from strings.
//!
//! That is the point of the indirection rather than a flourish. The interesting
//! nodes are not all going to be Rust functions - a planner backed by a language
//! model wants to run somewhere with a key and a GPU and a long timeout, and a
//! unit thinker wants to run a thousand times a second next to the state. Both
//! have to hang off the same tree or the tree is a lie.
//!
//! Three transports, one line protocol - a JSON document per line, in and out.
//! Same bytes whether it crosses a function call, a pipe or a socket, so a node
//! can be moved by editing a config string and nothing else.
//!
//! | address | where it runs |
//! |---|---|
//! | `local:brawler` | a thinker registered in this process |
//! | `proc:python3 bots/planner.py` | a child process, over its stdio |
//! | `tcp:10.0.0.4:7000` | anything listening on that socket |
//!
//! Deliberately not HTTP. The line protocol is the same on all three, so a
//! remote node is a small adapter rather than a second protocol to keep in
//! step; an HTTP transport can be added later without any caller learning about
//! it.

use std::collections::HashMap;
use std::sync::Arc;

use anyhow::{anyhow, Context, Result};
use async_trait::async_trait;
use sds_core::features::latch::Latches;
use sds_core::plan::{ForceDecision, UnitThought};
use sds_core::stance::{Grit, Stance};
use sds_core::wire::{Board, Observation};
use serde::{Deserialize, Serialize};

mod process;
mod tcp;

pub use process::ProcessNode;
pub use tcp::TcpNode;

/// What a node is asked.
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(tag = "ask", rename_all = "snake_case")]
pub enum Request {
    /// Unit level: what could this machine do, and what is it worth.
    Propose {
        observation: Observation,
        /// The map. Carried in the request rather than assumed to be cached,
        /// because a unit thinker may be a process on another machine that has
        /// never seen this match before.
        #[serde(default)]
        board: Board,
        unit: i32,
        /// What the force is currently trying to do. A hint - a unit that can
        /// only serve the stance badly should say so rather than pretend.
        stance: Stance,
        /// What the force refuses to forget. Carried rather than kept here: a
        /// unit thinker may be a process that has seen none of this match.
        #[serde(default)]
        latches: Latches,
    },
    /// Force level, at the top of a round: what would you be doing?
    ///
    /// Asked before movement, and answered without one hex having been looked
    /// at - see `sds_core::situation` for why that is a hard boundary rather
    /// than a convention. The force proposes; the coordinator harmonises the
    /// answers and hands each force an order back on [`Request::Command`].
    Consider {
        force: String,
        round: i32,
        /// This force's own machines, still standing.
        #[serde(default)]
        ours: Vec<sds_core::wire::Unit>,
        /// Every enemy machine this side can see.
        #[serde(default)]
        theirs: Vec<sds_core::wire::Unit>,
        /// The tactic the force is on now, by name, or `None` in its first
        /// round.
        #[serde(default)]
        held: Option<String>,
    },
    /// Force level: here is everything below you; decide.
    Command {
        force: String,
        thoughts: Vec<UnitThought>,
        stance: Stance,
        grit: Grit,
        round: i32,
        /// What each enemy has left, by unit id: armour plus structure. The
        /// denominator `sds_core::reconcile` normalises its target terms by,
        /// and the reason two units can decide that one of them finishes a
        /// machine rather than both emptying into it.
        ///
        /// Empty when the caller did not say. A target the map does not name
        /// is left out of the coordination terms rather than assumed healthy:
        /// an unknown denominator is not a large one.
        #[serde(default)]
        remaining: std::collections::BTreeMap<i32, f32>,
        /// Every friendly machine still standing, whichever force it belongs
        /// to.
        ///
        /// What `sds_core::features::incoming` shares the enemy's fire out
        /// over. All of them rather than this force's: two lances that each
        /// normalise over their own units are two lances that each believe the
        /// whole enemy line is pointed at them, which is the count the
        /// allocation exists to remove.
        ///
        /// Empty when the caller did not say, and then no unit is given an
        /// incoming term at all - a feature nobody measured contributes
        /// nothing, and that is better than a denominator nobody has.
        #[serde(default)]
        ours: Vec<sds_core::wire::Unit>,
        /// What this force proposed on [`Request::Consider`], carried back down
        /// so the decision it produces records what was asked for beside what
        /// was ordered. `None` when the caller did not harmonise.
        #[serde(default)]
        proposal: Option<sds_core::situation::Proposal>,
        /// The tactic the coordinator has put this force on for this round.
        ///
        /// An order, not a preference: the force proposed on
        /// [`Request::Consider`] and the coordinator has already weighed that
        /// against what it was on. `None` when the caller did not harmonise,
        /// and then the force falls back to its formation's opening, which is
        /// what it did before there was a coordinator to ask.
        #[serde(default)]
        tactic: Option<String>,
    },
}

/// What a node answers.
#[derive(Debug, Clone, Serialize, Deserialize)]
#[serde(tag = "answer", rename_all = "snake_case")]
pub enum Response {
    Thought(Box<UnitThought>),
    Decision(Box<ForceDecision>),
    /// What a force would be doing, and everything it scored getting there.
    Considered(Box<sds_core::situation::Considered>),
}

impl Response {
    pub fn into_thought(self) -> Result<UnitThought> {
        match self {
            Response::Thought(t) => Ok(*t),
            other => Err(anyhow!("asked a unit to propose, got {}", other.kind())),
        }
    }

    pub fn into_decision(self) -> Result<ForceDecision> {
        match self {
            Response::Decision(d) => Ok(*d),
            other => Err(anyhow!("asked a force to command, got {}", other.kind())),
        }
    }

    pub fn into_considered(self) -> Result<sds_core::situation::Considered> {
        match self {
            Response::Considered(c) => Ok(*c),
            other => Err(anyhow!("asked a force to consider, got {}", other.kind())),
        }
    }

    fn kind(&self) -> &'static str {
        match self {
            Response::Thought(_) => "a unit thought",
            Response::Decision(_) => "a force decision",
            Response::Considered(_) => "a tactic proposal",
        }
    }
}

/// Something that can think. Implement this to add a node that runs in-process.
#[async_trait]
pub trait Thinker: Send + Sync {
    async fn think(&self, request: Request) -> Result<Response>;
}

/// A reachable thinker.
#[async_trait]
pub trait Node: Send + Sync {
    fn address(&self) -> &str;
    async fn think(&self, request: Request) -> Result<Response>;
}

/// A thinker in this process, reached by name.
pub struct LocalNode {
    address: String,
    thinker: Arc<dyn Thinker>,
}

impl LocalNode {
    pub fn new(name: &str, thinker: Arc<dyn Thinker>) -> Self {
        Self {
            address: format!("local:{name}"),
            thinker,
        }
    }
}

#[async_trait]
impl Node for LocalNode {
    fn address(&self) -> &str {
        &self.address
    }

    async fn think(&self, request: Request) -> Result<Response> {
        self.thinker.think(request).await
    }
}

/// Builds nodes from address strings.
///
/// The registry holds the in-process thinkers a `local:` address can name.
/// Everything else is constructed on demand, so a config file naming a machine
/// that is not there fails when it is dialled rather than at start-up - which
/// is the behaviour you want when one remote planner is optional and the rest
/// of the tree is not.
#[derive(Default)]
pub struct Registry {
    thinkers: HashMap<String, Arc<dyn Thinker>>,
}

impl Registry {
    pub fn new() -> Self {
        Self::default()
    }

    pub fn register(&mut self, name: &str, thinker: Arc<dyn Thinker>) -> &mut Self {
        self.thinkers.insert(name.to_string(), thinker);
        self
    }

    /// Resolve an address into a node.
    pub fn resolve(&self, address: &str) -> Result<Arc<dyn Node>> {
        let (scheme, rest) = address
            .split_once(':')
            .with_context(|| format!("address '{address}' has no scheme"))?;
        match scheme {
            "local" => {
                let thinker = self
                    .thinkers
                    .get(rest)
                    .with_context(|| format!("no thinker registered as '{rest}'"))?;
                Ok(Arc::new(LocalNode::new(rest, thinker.clone())))
            }
            "proc" => Ok(Arc::new(ProcessNode::spawn(rest)?)),
            "tcp" => Ok(Arc::new(TcpNode::new(rest))),
            other => Err(anyhow!("unknown node scheme '{other}' in '{address}'")),
        }
    }
}