//! Verifies the audit log's hash chain. A pure state machine: the caller reads the files and feeds
//! it lines. Shared by `brokerd`'s startup check and `bxctl audit verify`.
//!
//! Each line is held back until the next one has been seen, because a `Recovery` record says
//! that the line before it is not a record at all.

use std::collections::BTreeSet;

use crate::{AuditEvent, AuditRecord, DecisionRecord, Hash32, Timestamp, sha256};

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Location {
    pub file: String,
    pub line: u64,
}

/// The first failure since the last accepted break, with what an `AcceptedBreak` record appended
/// now would have to carry.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ChainFailure {
    pub file: String,
    pub line: u64,
    pub what: String,
    pub last_good: Hash32,
    pub break_prev: Hash32,
    pub break_seq: u64,
    /// The last line fed has no newline; it must be ended before a break record is appended.
    pub tail_torn: bool,
}

/// The last line of the last file when it needs recovery, with what its `Recovery` record
/// would have to carry.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct TornTail {
    pub at: Location,
    pub has_newline: bool,
    pub bytes: u64,
    pub sha256: Hash32,
    pub recovery_prev: Hash32,
    pub recovery_seq: u64,
}

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct ChainReport {
    pub records: u64,
    pub head: Option<Hash32>,
    pub next_seq: u64,
    pub failure: Option<ChainFailure>,
    pub recoveries: Vec<Location>,
    pub accepted_breaks: Vec<Location>,
    /// `seq` of `ask` decisions with no `Approval` after them.
    pub abandoned: Vec<u64>,
    /// `seq` of decisions that were allowed, at once or by approval, with no `Result` after them.
    pub unfinished: Vec<u64>,
    pub clock_warnings: Vec<Location>,
    pub torn_tail: Option<TornTail>,
}

#[derive(Debug)]
struct Held {
    at: Location,
    hash: Hash32,
    bytes: u64,
    has_newline: bool,
    record: Option<AuditRecord>,
}

#[derive(Debug)]
struct Broken {
    file: String,
    line: u64,
    what: String,
    last_good: Hash32,
    expected_seq: u64,
    /// Lines from the failing line up to the last one judged, the failing line included.
    lines: u64,
}

#[derive(Debug)]
pub struct ChainVerifier {
    resumed: bool,
    first_file: Option<String>,
    file: String,
    line_no: u64,
    next_seq: u64,
    prev: Hash32,
    head: Option<Hash32>,
    /// Hash of the line before the held one, whether or not it verified.
    before_held: Hash32,
    held: Option<Held>,
    /// The held line is a `Recovery` record that described the line before it.
    held_recovers: bool,
    broken: Option<Broken>,
    last_time: Option<Timestamp>,
    records: u64,
    open_asks: BTreeSet<u64>,
    open_runs: BTreeSet<u64>,
    recoveries: Vec<Location>,
    accepted_breaks: Vec<Location>,
    clock_warnings: Vec<Location>,
}

impl Default for ChainVerifier {
    fn default() -> Self {
        Self::new()
    }
}

impl ChainVerifier {
    /// Expects `seq` 0 and an all-zero `prev`.
    pub fn new() -> Self {
        Self::start(false, 0, Hash32::ZERO)
    }

    /// Continues from a known point: the `seq` after, and the hash of, the last line of the file
    /// before the first one fed.
    pub fn resume(next_seq: u64, prev: Hash32) -> Self {
        Self::start(true, next_seq, prev)
    }

    fn start(resumed: bool, next_seq: u64, prev: Hash32) -> Self {
        ChainVerifier {
            resumed,
            first_file: None,
            file: String::new(),
            line_no: 0,
            next_seq,
            prev,
            head: None,
            before_held: prev,
            held: None,
            held_recovers: false,
            broken: None,
            last_time: None,
            records: 0,
            open_asks: BTreeSet::new(),
            open_runs: BTreeSet::new(),
            recoveries: Vec::new(),
            accepted_breaks: Vec::new(),
            clock_warnings: Vec::new(),
        }
    }

    /// Starts the next file. Files are fed in name order.
    pub fn file(&mut self, name: &str) {
        if self.first_file.is_none() {
            self.first_file = Some(name.to_string());
        }
        self.file = name.to_string();
        self.line_no = 0;
    }

    /// Feeds a whole file: `file(name)`, then every line of `content`.
    pub fn feed(&mut self, name: &str, content: &[u8]) {
        self.file(name);
        let mut rest = content;
        while !rest.is_empty() {
            match rest.iter().position(|b| *b == b'\n') {
                Some(end) => {
                    let (line, tail) = rest.split_at(end);
                    self.line(line, true);
                    rest = tail.get(1..).unwrap_or_default();
                }
                None => {
                    self.line(rest, false);
                    rest = &[];
                }
            }
        }
    }

    /// One line without its newline. `has_newline` is false only for a file's last line.
    pub fn line(&mut self, bytes: &[u8], has_newline: bool) {
        self.line_no = self.line_no.saturating_add(1);
        let at = Location {
            file: self.file.clone(),
            line: self.line_no,
        };
        let Ok(hash) = sha256(bytes) else {
            // The hasher fails only on an input too long to count; such a line is no record.
            self.judge_held(None);
            self.fail(&at, "the line is too long to hash".to_string());
            return;
        };
        let current = Held {
            at,
            hash,
            bytes: u64::try_from(bytes.len()).unwrap_or(u64::MAX),
            has_newline,
            record: serde_json::from_slice(bytes).ok(),
        };
        self.judge_held(Some(&current));
        self.held = Some(current);
    }

    /// Judges the held line now that the line after it (if any) is known.
    fn judge_held(&mut self, next: Option<&Held>) {
        let Some(held) = self.held.take() else {
            return;
        };
        let recovers = std::mem::replace(&mut self.held_recovers, false);
        if self.broken.is_some() {
            self.judge_in_region(&held);
        } else if self.is_recovered_by(&held, next) {
            self.recoveries.push(held.at.clone());
            self.held_recovers = true;
        } else {
            self.judge_record(&held, recovers);
        }
        self.before_held = held.hash;
    }

    /// True if `next` is a `Recovery` record that describes `held` and chains from the line
    /// before it. Whether `held` parses makes no difference.
    fn is_recovered_by(&self, held: &Held, next: Option<&Held>) -> bool {
        let Some(AuditRecord {
            seq,
            prev,
            event:
                AuditEvent::Recovery {
                    torn_bytes,
                    torn_sha256,
                },
            ..
        }) = next.and_then(|n| n.record.as_ref())
        else {
            return false;
        };
        *torn_bytes == held.bytes
            && *torn_sha256 == held.hash
            && *prev == self.prev
            && *seq == self.next_seq
    }

    fn judge_record(&mut self, held: &Held, recovers: bool) {
        let Some(record) = &held.record else {
            self.fail(&held.at, "does not parse as an audit record".to_string());
            return;
        };
        if let AuditEvent::AcceptedBreak { file, .. } = &record.event {
            // Only a resumed verifier may meet a break with no failure before it: the failure
            // is in a file it was not given. The full verification judges the rest.
            if self.names_earlier_file(file) && record.prev == self.prev {
                self.accept_break(held, record);
            } else {
                self.fail(
                    &held.at,
                    "an accepted break with no failure before it".to_string(),
                );
            }
            return;
        }
        if record.seq != self.next_seq {
            let what = format!("seq is {}, expected {}", record.seq, self.next_seq);
            self.fail(&held.at, what);
            return;
        }
        if record.prev != self.prev {
            let what = if held.at.line == 1 && (self.resumed || self.head.is_some()) {
                "does not chain from the last line of the file before"
            } else {
                "prev is not the hash of the line before"
            };
            self.fail(&held.at, what.to_string());
            return;
        }
        if matches!(record.event, AuditEvent::Recovery { .. }) && !recovers {
            self.fail(
                &held.at,
                "a recovery record that does not describe the line before it".to_string(),
            );
            return;
        }
        self.advance(held, record);
    }

    /// A line after a failure. It is read only to see whether it is the break record.
    fn judge_in_region(&mut self, held: &Held) {
        let Some(broken) = &mut self.broken else {
            return;
        };
        if let Some(record) = &held.record
            && let AuditEvent::AcceptedBreak {
                file,
                line,
                last_good,
            } = &record.event
            && record.prev == self.before_held
        {
            let names_this = *file == broken.file
                && *line == broken.line
                && *last_good == broken.last_good
                && broken.expected_seq.checked_add(broken.lines) == Some(record.seq);
            if names_this
                || (self.resumed
                    && self
                        .first_file
                        .as_deref()
                        .is_some_and(|f| file.as_str() < f))
            {
                self.broken = None;
                self.accept_break(held, record);
                return;
            }
        }
        broken.lines = broken.lines.saturating_add(1);
    }

    fn names_earlier_file(&self, file: &str) -> bool {
        self.resumed && self.first_file.as_deref().is_some_and(|first| file < first)
    }

    fn accept_break(&mut self, held: &Held, record: &AuditRecord) {
        self.accepted_breaks.push(held.at.clone());
        self.advance(held, record);
    }

    /// `held` is a verified record: it becomes the head.
    fn advance(&mut self, held: &Held, record: &AuditRecord) {
        self.records = self.records.saturating_add(1);
        self.head = Some(held.hash);
        self.prev = held.hash;
        self.next_seq = record.seq.saturating_add(1);
        if self.last_time.is_some_and(|last| record.time < last) {
            self.clock_warnings.push(held.at.clone());
        }
        self.last_time = Some(record.time);
        match &record.event {
            AuditEvent::Decision { outcome, .. } => match outcome {
                DecisionRecord::Allowed {} => {
                    self.open_runs.insert(record.seq);
                }
                DecisionRecord::Ask {} => {
                    self.open_asks.insert(record.seq);
                }
                DecisionRecord::Denied { .. } => {}
            },
            AuditEvent::Approval {
                decision, outcome, ..
            } => {
                self.open_asks.remove(decision);
                if matches!(outcome, DecisionRecord::Allowed {}) {
                    self.open_runs.insert(*decision);
                }
            }
            AuditEvent::Result { decision, .. } => {
                self.open_runs.remove(decision);
            }
            AuditEvent::Recovery { .. } | AuditEvent::AcceptedBreak { .. } => {}
        }
    }

    fn fail(&mut self, at: &Location, what: String) {
        if self.broken.is_some() {
            return;
        }
        self.broken = Some(Broken {
            file: at.file.clone(),
            line: at.line,
            what,
            last_good: self.prev,
            expected_seq: self.next_seq,
            lines: 1,
        });
    }

    pub fn finish(mut self) -> ChainReport {
        let mut torn_tail = None;
        let mut tail_torn = false;
        let mut last_line = self.before_held;
        if let Some(held) = self.held.take() {
            last_line = held.hash;
            tail_torn = !held.has_newline;
            let needs_recovery = !held.has_newline || held.record.is_none();
            if self.broken.is_none() && needs_recovery {
                torn_tail = Some(TornTail {
                    at: held.at.clone(),
                    has_newline: held.has_newline,
                    bytes: held.bytes,
                    sha256: held.hash,
                    recovery_prev: self.prev,
                    recovery_seq: self.next_seq,
                });
            } else {
                self.held = Some(held);
                self.judge_held(None);
            }
        }
        let failure = self.broken.map(|b| ChainFailure {
            file: b.file,
            line: b.line,
            what: b.what,
            last_good: b.last_good,
            break_prev: last_line,
            break_seq: b.expected_seq.saturating_add(b.lines),
            tail_torn,
        });
        ChainReport {
            records: self.records,
            head: self.head,
            next_seq: self.next_seq,
            failure,
            recoveries: self.recoveries,
            accepted_breaks: self.accepted_breaks,
            abandoned: self.open_asks.into_iter().collect(),
            unfinished: self.open_runs.into_iter().collect(),
            clock_warnings: self.clock_warnings,
            torn_tail,
        }
    }
}