//! Rules by path every node judges a change by, the same way, before it applies it
//! (`judge`): who may write which part of the document. A change is judged by what it
//! touches (`document::inspect`), by who made it (its committer, the node a signature
//! proves, `document::signatures`; and the user its message names, `User:`), and by the
//! document as its writer saw it: at its dependencies (`document::at`), so a later
//! revocation doesn't refuse what was allowed when it was written, and every node,
//! whatever it holds since, judges alike.
//!
//! * A change without dependencies is an epoch's root: the genesis change, the first a
//!   document holds, or a remake's snapshot, whose hash is the epoch's ID. Nothing else
//!   is judged of it: it writes the protocol data it starts with.
//! * Any other is its committer's, a member at its dependencies, or registering itself.
//! * Protocol data (`_continuum`, `document::meta`): a member's entry and its votes by
//!   that member only (removing an entry also by a user who may write there); epochs
//!   and the swarm ID in a root only; a remake's decision by the coordinator its
//!   proposal names, the rest of a record by any member; a user's tokens revoked by that
//!   user too (logging out); the rest by a user who may write there.
//! * User data: by a user who may write there.
//!
//! In a swarm without a user registry (`access::registry`), whatever a user would need,
//! any member may.

use std::collections::BTreeSet;

use crate::access::Right;
use crate::access::registry;
use crate::document::at::At;
use crate::document::inspect::Path;
use crate::document::meta;
use crate::document::replica::Epoch;
use crate::document::{ChangeHash, Document, store};

/// A change to judge, as its bytes say.
#[derive(Debug, Clone)]
pub(crate) struct Change<'a> {
    pub(crate) hash: ChangeHash,
    pub(crate) deps: &'a [ChangeHash],
    /// The heads to judge it at: its dependencies, or, for those not applied yet, the
    /// changes of the document they build on, where nothing between changed what rules
    /// read (`access::screen`).
    pub(crate) seen: &'a [ChangeHash],
    /// The node that made it (its node ID), as its signature proves.
    pub(crate) committer: Option<&'a str>,
    /// The user it names (`User:`), if any.
    pub(crate) user: Option<&'a str>,
    /// What it touches (`inspect::Inspector`).
    pub(crate) paths: &'a BTreeSet<Path>,
}

/// Why a change is refused.
#[derive(Debug, Clone, PartialEq, Eq)]
pub(crate) enum Refusal {
    /// Its dependencies aren't all in the document: it can't be judged yet.
    Unready,
    /// A rule refuses it: which, and what it touched.
    Rule(String),
}

impl std::fmt::Display for Refusal {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        match self {
            Refusal::Unready => f.write_str("its dependencies aren't all here"),
            Refusal::Rule(why) => f.write_str(why),
        }
    }
}

/// Judges `change`, about to be applied to `doc` (of `epoch`).
pub(crate) fn judge(doc: &Document, epoch: Epoch, change: &Change<'_>) -> Result<(), Refusal> {
    if change.deps.is_empty() {
        let root = if epoch.generation == 0 {
            doc.get_heads().is_empty()
        } else {
            change.hash.0 == epoch.id
        };
        return if root {
            Ok(())
        } else {
            Err(Refusal::Rule(
                "a change without dependencies that isn't the epoch's root".to_owned(),
            ))
        };
    }
    let at = At::new(doc, change.seen).ok_or(Refusal::Unready)?;
    let refuse = |why: String| Err(Refusal::Rule(why));
    let Some(node) = change.committer else {
        return refuse("it names no node".to_owned());
    };
    let own_entry = meta::path(&[meta::MEMBERS, node]);
    let member = store::read_path(&at, &own_entry).ok().flatten().is_some();
    let registering = change
        .paths
        .iter()
        .all(|path| path.starts_with(&own_entry) || *path == meta::path(&[meta::MEMBERS]));
    if !member && !registering {
        return refuse(format!("{node} isn't a member"));
    }
    let registry = registry::exists(&at);
    // Whether the user may write `path`: as the registry at its dependencies says, or,
    // without one, any member.
    let user_may = |path: &Path| match (registry, change.user) {
        (false, _) => true,
        (true, Some(user)) => registry::caps_of(&at, user).allows(Right::Write, path),
        (true, None) => false,
    };
    for path in change.paths {
        if !meta::is_protocol(path) {
            if !user_may(path) {
                return refuse(format!("{} may not write {}", who(change), shown(path)));
            }
            continue;
        }
        let part = path.get(1).map(String::as_str);
        let owner = path.get(2).map(String::as_str);
        let allowed = match (part, owner) {
            // A member's entry, its votes: its own. Removing an entry, also a user's.
            (Some(meta::MEMBERS), Some(owner)) => owner == node || user_may(path),
            (Some(meta::VOTES), Some(owner)) => owner == node,
            // What holds them, made by whichever member first writes in it.
            (Some(meta::MEMBERS | meta::VOTES | meta::REMAKES), None) => true,
            // Written by a root only.
            (Some(meta::EPOCHS | meta::SWARM), _) => false,
            // Logging out: a user revokes its own tokens (`POST /auth/logout`).
            (Some(meta::USERS), Some(owner))
                if path.get(3).map(String::as_str) == Some("tokensNotBefore")
                    && change.user == Some(owner) =>
            {
                true
            }
            (Some(meta::REMAKES), Some(id)) => match path.get(3).map(String::as_str) {
                // Its coordinator's; or a record carried into a new epoch whole, by the
                // node that coordinated it (`remake::switch`): its proposal written with it.
                Some("decision") => match coordinator(&at, id) {
                    Some(coordinator) => coordinator == node,
                    None => {
                        change
                            .paths
                            .iter()
                            .any(|other| other == &meta::path(&[meta::REMAKES, id, "proposal"]))
                            || change
                                .paths
                                .iter()
                                .any(|other| other == &meta::path(&[meta::REMAKES, id]))
                    }
                },
                _ => true,
            },
            _ => user_may(path),
        };
        if !allowed {
            return refuse(format!("{} may not write {}", who(change), shown(path)));
        }
    }
    Ok(())
}

/// Who wrote it, for a refusal.
fn who(change: &Change<'_>) -> String {
    match (change.committer, change.user) {
        (Some(node), Some(user)) => format!("{user} at {node}"),
        (Some(node), None) => node.to_owned(),
        (None, _) => "nobody".to_owned(),
    }
}

fn shown(path: &Path) -> String {
    format!("/{}", path.join("/"))
}

/// The coordinator remake `id`'s proposal names, at `at`.
fn coordinator(at: &At<'_>, id: &str) -> Option<String> {
    let proposal = store::read_path(at, &meta::path(&[meta::REMAKES, id, "proposal"]))
        .ok()
        .flatten()?;
    proposal
        .get("coordinator")
        .and_then(serde_json::Value::as_str)
        .map(str::to_owned)
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::clock::UnixMillis;
    use crate::document::replica::{Writer, transact};
    use serde_json::json;

    fn path(text: &str) -> Path {
        text.split('/').map(str::to_owned).collect()
    }

    fn paths(texts: &[&str]) -> BTreeSet<Path> {
        texts.iter().map(|text| path(text)).collect()
    }

    fn write(doc: &mut Document, at: &str, value: serde_json::Value) {
        let writer = Writer {
            node: "genesis",
            user: None,
        };
        let at = path(at);
        transact(doc, UnixMillis(1), &writer, |doc| {
            store::write_path(doc, &at, &value)
        })
        .unwrap();
    }

    /// A swarm with members `a` and `b`, a remake proposed by `a`, and, if `users`,
    /// user `alice` who may write `notes` and `bob` who may not write at all.
    fn swarm(users: bool) -> Document {
        let mut doc = Document::new();
        write(&mut doc, "_continuum/swarm", json!("s"));
        write(&mut doc, "_continuum/members/a", json!({"since": 1}));
        write(&mut doc, "_continuum/members/b", json!({"since": 1}));
        write(
            &mut doc,
            "_continuum/remakes/r1/proposal",
            json!({"coordinator": "a"}),
        );
        if users {
            let alice = registry::entry("a", 1, &[(path("notes"), &[Right::Write][..])]);
            write(&mut doc, "_continuum/users/alice", alice);
            write(
                &mut doc,
                "_continuum/users/bob",
                registry::entry("a", 1, &[]),
            );
        }
        doc
    }

    fn judged(
        doc: &Document,
        committer: &str,
        user: Option<&str>,
        touched: &[&str],
    ) -> Result<(), Refusal> {
        let deps = doc.get_heads();
        let touched = paths(touched);
        judge(
            doc,
            Epoch::GENESIS,
            &Change {
                hash: ChangeHash([9; 32]),
                deps: &deps,
                seen: &deps,
                committer: Some(committer),
                user,
                paths: &touched,
            },
        )
    }

    fn refused(result: Result<(), Refusal>) -> bool {
        matches!(result, Err(Refusal::Rule(_)))
    }

    /// A member's entry and votes are its own; epochs and the swarm ID a root's.
    #[test]
    fn protocol_data() {
        let doc = swarm(true);
        assert!(judged(&doc, "a", None, &["_continuum/members/a/alive"]).is_ok());
        assert!(refused(judged(
            &doc,
            "a",
            None,
            &["_continuum/members/b/alive"]
        )));
        assert!(judged(&doc, "b", None, &["_continuum/votes/b"]).is_ok());
        assert!(refused(judged(&doc, "a", None, &["_continuum/votes/b"])));
        assert!(refused(judged(&doc, "a", None, &["_continuum/swarm"])));
        assert!(refused(judged(&doc, "a", None, &["_continuum/epochs/1"])));
        // A remake's decision: its coordinator's; the rest of the record, any member's.
        assert!(judged(&doc, "a", None, &["_continuum/remakes/r1/decision"]).is_ok());
        assert!(refused(judged(
            &doc,
            "b",
            None,
            &["_continuum/remakes/r1/decision"]
        )));
        assert!(judged(&doc, "b", None, &["_continuum/remakes/r1/abortRequested"]).is_ok());
        // A record carried into a new epoch whole: its proposal with its decision.
        assert!(
            judged(
                &doc,
                "b",
                None,
                &[
                    "_continuum/remakes/r2",
                    "_continuum/remakes/r2/proposal",
                    "_continuum/remakes/r2/decision"
                ]
            )
            .is_ok()
        );
        assert!(refused(judged(
            &doc,
            "b",
            None,
            &["_continuum/remakes/r2/decision"]
        )));
        // The rest: a user's who may write there.
        assert!(refused(judged(&doc, "a", None, &["_continuum/neighbours"])));
        assert!(refused(judged(
            &doc,
            "a",
            Some("alice"),
            &["_continuum/neighbours"]
        )));
        // Logging out: its own tokens only.
        let tokens = "_continuum/users/bob/tokensNotBefore";
        assert!(judged(&doc, "a", Some("bob"), &[tokens]).is_ok());
        assert!(refused(judged(&doc, "a", Some("alice"), &[tokens])));
    }

    /// A node that isn't a member may only register itself.
    #[test]
    fn strangers_register_themselves_only() {
        let doc = swarm(false);
        assert!(
            judged(
                &doc,
                "c",
                None,
                &[
                    "_continuum/members",
                    "_continuum/members/c",
                    "_continuum/members/c/alive"
                ]
            )
            .is_ok()
        );
        assert!(refused(judged(&doc, "c", None, &["notes/x"])));
        assert!(refused(judged(&doc, "c", None, &["_continuum/members/a"])));
    }

    /// User data: as the registry at the change's dependencies says; without one, any
    /// member's.
    #[test]
    fn user_data() {
        let doc = swarm(true);
        assert!(judged(&doc, "a", Some("alice"), &["notes/x"]).is_ok());
        assert!(refused(judged(&doc, "a", Some("alice"), &["config"])));
        assert!(refused(judged(&doc, "a", Some("bob"), &["notes/x"])));
        assert!(refused(judged(&doc, "a", None, &["notes/x"])));
        assert!(refused(judged(&doc, "a", Some("nobody"), &["notes/x"])));
        let legacy = swarm(false);
        assert!(judged(&legacy, "a", None, &["notes/x"]).is_ok());
        assert!(judged(&legacy, "a", None, &["_continuum/neighbours"]).is_ok());
    }

    /// Judged by the registry its writer saw: a revocation since refuses nothing it
    /// allowed then.
    #[test]
    fn judged_as_its_writer_saw_it() {
        let mut doc = swarm(true);
        let then = doc.get_heads();
        write(
            &mut doc,
            "_continuum/users/alice",
            registry::entry("a", 2, &[]),
        );
        let touched = paths(&["notes/x"]);
        let change = |deps: &[ChangeHash]| {
            judge(
                &doc,
                Epoch::GENESIS,
                &Change {
                    hash: ChangeHash([9; 32]),
                    deps,
                    seen: deps,
                    committer: Some("a"),
                    user: Some("alice"),
                    paths: &touched,
                },
            )
        };
        assert!(change(&then).is_ok());
        assert!(refused(change(&doc.get_heads())));
        // Unknown dependencies: not judged yet.
        assert_eq!(change(&[ChangeHash([7; 32])]), Err(Refusal::Unready));
    }

    /// Roots: the first change of a swarm's first epoch, a later epoch's snapshot.
    #[test]
    fn roots() {
        let root = |doc: &Document, epoch: Epoch, hash: [u8; 32]| {
            let touched = paths(&["_continuum/swarm"]);
            judge(
                doc,
                epoch,
                &Change {
                    hash: ChangeHash(hash),
                    deps: &[],
                    seen: &[],
                    committer: None,
                    user: None,
                    paths: &touched,
                },
            )
        };
        assert!(root(&Document::new(), Epoch::GENESIS, [1; 32]).is_ok());
        assert!(refused(root(&swarm(false), Epoch::GENESIS, [1; 32])));
        let epoch = Epoch {
            generation: 3,
            id: [5; 32],
        };
        assert!(root(&swarm(false), epoch, [5; 32]).is_ok());
        assert!(refused(root(&swarm(false), epoch, [6; 32])));
    }
}