//! Changes signed by the node that committed them: an Ed25519 signature by its node
//! key over the change's hash (`message`), kept beside the document, not in it (a
//! change's hash covers its message, so it can't carry its own signature).
//!
//! One signature vouches for more than its change: a change's hash covers its
//! dependencies, and a node's change builds on its previous one, so a signed change of
//! an actor vouches for all that actor's changes up to it (`check`: the actor and seq
//! it verifies up to). So not every change needs one: a change is signed when it
//! touches protocol data (`_continuum`) or a path the swarm lists as signed
//! (`_continuum/access/signed`, `listed`), or when its write asks for it (`Sign`); a
//! change left unsigned is vouched for by the node's next signed one.

use std::collections::HashMap;

use super::{ChangeHash, Document};

/// Where the swarm lists the paths whose writes are signed: a list of paths, each a
/// list of segments, as capabilities name them (`access.md`).
pub(crate) const SIGNED_PATHS: [&str; 3] = ["_continuum", "access", "signed"];

/// What a signature is made over: a domain, then the change's hash.
pub(crate) fn message(hash: &ChangeHash) -> Vec<u8> {
    let mut message = b"continuum/change/1\0".to_vec();
    message.extend_from_slice(&hash.0);
    message
}

/// Whether a write is signed (`wanted`).
#[derive(Debug, Clone)]
pub(crate) enum Sign {
    /// The node's own work: protocol data, always signed.
    Protocol,
    /// A write through the API at `path`, signed if `asked` (`?signed=true`) or if the
    /// path is protocol data or listed as signed.
    Write { path: Vec<String>, asked: bool },
}

impl Sign {
    /// Whether a change made so, in `doc`, is to be signed.
    pub(crate) fn wanted(&self, doc: &Document) -> bool {
        match self {
            Sign::Protocol => true,
            Sign::Write { path, asked } => {
                *asked
                    || path.first().map(String::as_str) == Some(SIGNED_PATHS[0])
                    || listed(doc).iter().any(|signed| related(signed, path))
            }
        }
    }
}

/// Whether a write at `path` touches what is at `signed`: within it, or above it
/// (replacing or deleting what holds it).
fn related(signed: &[String], path: &[String]) -> bool {
    path.starts_with(signed) || signed.starts_with(path)
}

/// The paths `doc` lists as signed (`SIGNED_PATHS`); an entry that isn't a list of
/// strings is left out.
pub(crate) fn listed(doc: &Document) -> Vec<Vec<String>> {
    let at: Vec<String> = SIGNED_PATHS.iter().map(|s| (*s).to_owned()).collect();
    let Ok(Some(serde_json::Value::Array(entries))) = super::store::read_path(doc, &at) else {
        return Vec::new();
    };
    entries
        .iter()
        .filter_map(|entry| {
            entry
                .as_array()?
                .iter()
                .map(|segment| segment.as_str().map(str::to_owned))
                .collect()
        })
        .collect()
}

/// A document's signatures, by change.
#[derive(Debug, Default)]
pub(crate) struct Signatures {
    by_change: HashMap<ChangeHash, [u8; 64]>,
}

impl Signatures {
    pub(crate) fn insert(&mut self, hash: ChangeHash, signature: [u8; 64]) {
        self.by_change.insert(hash, signature);
    }

    /// Signs `hash` with `key` and keeps the signature.
    pub(crate) fn sign(&mut self, hash: ChangeHash, key: &crate::identity::NodeKey) {
        self.insert(hash, key.sign(&message(&hash)));
    }
}

/// What a valid signature of a change in `doc` verifies: its actor's changes up to its
/// seq, committed by the node its author names (its node ID, whose key it is). None if
/// `doc` doesn't hold the change, its author isn't a node ID, or the signature isn't
/// that node's over it.
#[cfg_attr(not(test), allow(dead_code))]
pub(crate) fn check(
    doc: &Document,
    hash: &ChangeHash,
    signature: &[u8; 64],
) -> Option<(super::ActorId, u64)> {
    let meta = doc.get_change_meta_by_hash(hash)?;
    let author = std::str::from_utf8(meta.author.as_ref()?.as_bytes()).ok()?;
    let key = crate::identity::public_of(author)?;
    crate::identity::verify(&key, &message(hash), signature)
        .then(|| (meta.actor.into_owned(), meta.seq))
}

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

    fn write(doc: &mut Document, node: &str, path: &[&str], value: serde_json::Value) {
        let path: Vec<String> = path.iter().map(|s| (*s).to_owned()).collect();
        let writer = Writer { node, user: None };
        transact(doc, UnixMillis(1), &writer, |doc| {
            store::write_path(doc, &path, &value)
        })
        .unwrap();
    }

    /// A signature verifies its actor's changes up to it, by the node its author names;
    /// another node's key, or another change, doesn't.
    #[test]
    fn a_signature_vouches_for_its_actors_changes_up_to_it() {
        let key = NodeKey::from_seed([1; 32]);
        let other = NodeKey::from_seed([2; 32]);
        let node = key.id();
        let mut doc = Document::new();
        write(&mut doc, &node, &["a"], json!(1));
        write(&mut doc, &node, &["b"], json!(2));
        let last = doc.get_heads()[0];
        let (actor, seq) = check(&doc, &last, &key.sign(&message(&last))).unwrap();
        assert_eq!(&actor, doc.get_actor());
        assert_eq!(seq, 2);
        assert!(check(&doc, &last, &other.sign(&message(&last))).is_none());
        let first = doc.get_changes(&[])[0].hash();
        assert!(check(&doc, &last, &key.sign(&message(&first))).is_none());
        // A change whose author isn't a node ID: nobody's to check.
        let mut named = Document::new();
        write(&mut named, "node", &["a"], json!(1));
        let hash = named.get_heads()[0];
        assert!(check(&named, &hash, &key.sign(&message(&hash))).is_none());
    }

    /// Protocol data, listed paths (within or above them) and asked-for writes are
    /// signed; others aren't.
    #[test]
    fn which_writes_are_signed() {
        let mut doc = Document::new();
        let write_at = |path: &[&str], asked| Sign::Write {
            path: path.iter().map(|s| (*s).to_owned()).collect(),
            asked,
        };
        assert!(Sign::Protocol.wanted(&doc));
        assert!(write_at(&["_continuum", "users", "a"], false).wanted(&doc));
        assert!(!write_at(&["metrics", "cpu"], false).wanted(&doc));
        assert!(write_at(&["metrics", "cpu"], true).wanted(&doc));
        write(
            &mut doc,
            "node",
            &SIGNED_PATHS,
            json!([["config"], ["servers", "eu"], "junk", [1]]),
        );
        assert_eq!(
            listed(&doc),
            vec![
                vec!["config".to_owned()],
                vec!["servers".to_owned(), "eu".to_owned()]
            ]
        );
        assert!(write_at(&["config", "x"], false).wanted(&doc));
        assert!(write_at(&["servers"], false).wanted(&doc));
        assert!(!write_at(&["servers", "us"], false).wanted(&doc));
        assert!(!write_at(&["configs"], false).wanted(&doc));
    }
}