//! Checking a host's transparency log on fetch (spec §14.8, oxygenhub#238).
//!
//! A host that keeps a log sends its latest checkpoint with the refs, and
//! for each bookmark's attestation its entry and an inclusion proof. This
//! user remembers, per hub, the log's key and the last checkpoint seen
//! (`<oxy home>/tlog/<hub>`): the key on first sight, then never another.
//! Each new checkpoint must extend the last one, which oxy proves from the
//! hub's public tiles (C2SP tlog-tiles); one that does not means the hub
//! showed two histories, and the fetch fails. An attestation that is not
//! in the log yet, or whose proof does not verify, is only reported while
//! the log is new (oxygenhub docs/transparency.md, decision 3).

use super::*;
use crate::credentials::hub_of;
use crate::protocol::http_get;
use crate::tlog::{self, Checkpoint, Entry, Hash, HashReader, Note, Tile, TlogError};
use std::cell::RefCell;

/// Tiles fetched, by level and number, with the width fetched.
type TileCache = RefCell<HashMap<(u32, u64), (Tile, Vec<u8>)>>;

/// A hub's log as its tiles serve it, for a tree of `size` entries.
struct Tiles<'a> {
    hub: &'a str,
    size: u64,
    options: &'a RemoteOptions,
    cache: TileCache,
}

impl HashReader for Tiles<'_> {
    fn read_hashes(&self, indexes: &[u64]) -> std::result::Result<Vec<Hash>, TlogError> {
        let mut out = Vec::with_capacity(indexes.len());
        for &i in indexes {
            let (t, ..) = Tile::for_index(i);
            let key = (t.level, t.n);
            if !self.cache.borrow().contains_key(&key) {
                let held =
                    (self.size >> (tlog::TILE_HEIGHT * t.level)) - (t.n << tlog::TILE_HEIGHT);
                let tile = Tile {
                    width: held.min(tlog::TILE_WIDTH) as u32,
                    ..t
                };
                let data = http_get(&format!("{}/log/{}", self.hub, tile.path()), self.options)
                    .map_err(|e| TlogError::Read(e.to_string()))?;
                self.cache.borrow_mut().insert(key, (tile, data));
            }
            let cache = self.cache.borrow();
            let (tile, data) = &cache[&key];
            out.push(tile.hash_at(data, i)?);
        }
        Ok(out)
    }
}

/// What this user last saw of a hub's log.
struct Seen {
    identity: Identity,
    checkpoint: Checkpoint,
}

fn file_name(hub: &str) -> String {
    hub.chars()
        .map(|c| {
            if c.is_ascii_alphanumeric() || c == '.' || c == '-' {
                c
            } else {
                '_'
            }
        })
        .collect()
}

impl Repo {
    fn tlog_file(&self, hub: &str) -> PathBuf {
        self.user_config
            .parent()
            .map(Path::to_path_buf)
            .unwrap_or_else(default_home)
            .join("tlog")
            .join(file_name(hub))
    }

    fn seen_log(&self, hub: &str) -> Option<Seen> {
        let text = fs::read_to_string(self.tlog_file(hub)).ok()?;
        let (key, note) = text.split_once('\n')?;
        let identity = Identity::decode(&crate::dsse::base64_decode(key)?).ok()?;
        let note = Note::parse(note).ok()?;
        Some(Seen {
            checkpoint: Checkpoint::parse(&note.text).ok()?,
            identity,
        })
    }

    fn remember_log(&self, hub: &str, identity: &Identity, note: &Note) -> Result<()> {
        let path = self.tlog_file(hub);
        if let Some(dir) = path.parent() {
            fs::create_dir_all(dir)?;
        }
        let text = format!(
            "{}\n{}",
            crate::dsse::base64(&identity.encode()),
            note.encode()
        );
        crate::durable::write_atomic(&path, text.as_bytes())?;
        Ok(())
    }

    /// Check what a refs answer from `url` says about its host's log.
    /// Fails only when the host showed two histories (or a log key other
    /// than the one it showed before); everything else is a note.
    pub(crate) fn check_log(
        &self,
        url: &str,
        ca: Option<&Path>,
        resp: &RefsResponse,
    ) -> Result<Vec<String>> {
        let (Some(text), Some(hub)) = (&resp.log, hub_of(url)) else {
            return Ok(vec![]);
        };
        let mut notes = Vec::new();
        let unchecked = |why: String| {
            Ok(vec![format!(
                "{hub}'s transparency log was not checked: {why}"
            )])
        };
        let note = match Note::parse(text) {
            Ok(n) => n,
            Err(e) => return unchecked(e.to_string()),
        };
        let cp = match Checkpoint::parse(&note.text) {
            Ok(c) => c,
            Err(e) => return unchecked(e.to_string()),
        };
        let options = RemoteOptions {
            ca: ca.map(Path::to_path_buf),
            ..RemoteOptions::default()
        };
        let seen = self.seen_log(&hub);
        let identity = match &seen {
            Some(s) => s.identity.clone(),
            None => match http_get(&format!("{hub}/.well-known/oxyhub-key"), &options)
                .and_then(|b| Ok(Identity::decode(&b)?))
            {
                Ok(i) => i,
                Err(e) => return unchecked(format!("its key could not be fetched ({e})")),
            },
        };
        let two = |why: String| {
            fail(format!(
                "{hub} showed two histories: {why}. Do not trust what it serves; tell its operator. \
                 (What this user saw of its log is in {}.)",
                self.tlog_file(&hub).display()
            ))
        };
        if let Err(e) = note.verify(&cp.origin, &identity) {
            return match &seen {
                Some(_) => two(format!(
                    "its checkpoint is not signed by the log key it showed before ({e})"
                )),
                None => unchecked(e.to_string()),
            };
        }
        if let Some(s) = &seen {
            let old = &s.checkpoint;
            if old.origin != cp.origin {
                return two(format!("its log is now {}, not {}", cp.origin, old.origin));
            }
            if cp.size < old.size {
                return two(format!(
                    "its log has {} entries, fewer than the {} it showed before",
                    cp.size, old.size
                ));
            }
            if cp.size == old.size && cp.root != old.root {
                return two(format!("two different logs of {} entries", cp.size));
            }
            if cp.size > old.size && old.size > 0 {
                let tiles = Tiles {
                    hub: &hub,
                    size: cp.size,
                    options: &options,
                    cache: RefCell::new(HashMap::new()),
                };
                match tlog::prove_consistency(cp.size, old.size, &tiles).and_then(|p| {
                    tlog::check_consistency(&p, cp.size, &cp.root, old.size, &old.root)
                }) {
                    Ok(()) => {}
                    Err(TlogError::ProofFailed) | Err(TlogError::BadIndex(_)) => {
                        return two(format!(
                            "its log of {} entries does not extend the one of {} it showed before",
                            cp.size, old.size
                        ))
                    }
                    // Keep the older checkpoint: the next fetch tries again.
                    Err(e) => return unchecked(format!("its tiles could not be read ({e})")),
                }
            }
        }
        self.remember_log(&hub, &identity, &note)?;
        let mut missing = 0;
        for r in &resp.refs {
            let Some(a) = &r.attestation else { continue };
            let Some(l) = &r.logged else {
                missing += 1;
                continue;
            };
            let fits = Entry::decode(&l.entry).is_ok_and(|e| {
                e.kind == "ref" && e.object == a.attest.hash() && e.repo == a.attest.repo
            });
            let included = fits
                && tlog::check_inclusion(
                    &l.proof,
                    cp.size,
                    &cp.root,
                    l.index,
                    &tlog::leaf_hash(&l.entry),
                )
                .is_ok();
            if !included {
                notes.push(format!(
                    "{hub}'s log entry {} for {} does not prove its attestation is in the log",
                    l.index, r.name
                ));
            }
        }
        if missing > 0 {
            notes.push(format!(
                "{} of {hub}'s attestations {} not in its transparency log yet (logged within seconds of a push; checked on a later fetch)",
                missing,
                if missing == 1 { "is" } else { "are" }
            ));
        }
        Ok(notes)
    }
}