// Copyright 2024 Saorsa Labs Ltd.
//
// This Saorsa Network Software is licensed under the General Public License (GPL), version 3.
// Please see the file LICENSE-GPL, or visit <http://www.gnu.org/licenses/> for the full text.
//
// Full details available at https://saorsalabs.com/licenses


use std::{
    collections::HashSet,
    f64::consts::LN_2,
    hash::{BuildHasher, Hasher},
    mem::{size_of, take},
    sync::Mutex,
};

use fastbloom::BloomFilter;
use rustc_hash::FxBuildHasher;
use tracing::{trace, warn};

use crate::{Duration, SystemTime, TokenLog, TokenReuseError, UNIX_EPOCH};

/// Bloom filter-based [`TokenLog`]
///
/// Parameterizable over an approximate maximum number of bytes to allocate. Starts out by storing
/// used tokens in a hash set. Once the hash set becomes too large, converts it to a bloom filter.
/// This achieves a memory profile of linear growth with an upper bound.
///
/// Divides time into periods based on `lifetime` and stores two filters at any given moment, for
/// each of the two periods currently non-expired tokens could expire in. As such, turns over
/// filters as time goes on to avoid bloom filter false positive rate increasing infinitely over
/// time.
pub struct BloomTokenLog(Mutex<State>);

impl BloomTokenLog {
    /// Construct with an approximate maximum memory usage and expected number of validation token
    /// usages per expiration period
    ///
    /// Calculates the optimal bloom filter k number automatically.
    pub fn new_expected_items(max_bytes: usize, expected_hits: u64) -> Self {
        Self::new(max_bytes, optimal_k_num(max_bytes, expected_hits))
    }

    /// Construct with an approximate maximum memory usage and a [bloom filter k number][bloom]
    ///
    /// [bloom]: https://en.wikipedia.org/wiki/Bloom_filter
    ///
    /// If choosing a custom k number, note that `BloomTokenLog` always maintains two filters
    /// between them and divides the allocation budget of `max_bytes` evenly between them. As such,
    /// each bloom filter will contain `max_bytes * 4` bits.
    pub fn new(max_bytes: usize, k_num: u32) -> Self {
        Self(Mutex::new(State {
            config: FilterConfig {
                filter_max_bytes: max_bytes / 2,
                k_num,
            },
            period_1_start: UNIX_EPOCH,
            filter_1: Filter::default(),
            filter_2: Filter::default(),
        }))
    }
}

impl TokenLog for BloomTokenLog {
    fn check_and_insert(
        &self,
        nonce: u128,
        issued: SystemTime,
        lifetime: Duration,
    ) -> Result<(), TokenReuseError> {
        trace!(%nonce, "check_and_insert");

        if lifetime.is_zero() {
            // avoid divide-by-zero if lifetime is zero
            return Err(TokenReuseError);
        }

        let mut guard = self.0.lock().unwrap();
        let state = &mut *guard;

        // calculate how many periods past period 1 the token expires
        let expires_at = issued + lifetime;
        let Ok(periods_forward) = expires_at
            .duration_since(state.period_1_start)
            .map(|duration| duration.as_nanos() / lifetime.as_nanos())
        else {
            // shouldn't happen unless time travels backwards or lifetime changes or the current
            // system time is before the Unix epoch
            warn!("BloomTokenLog presented with token too far in past");
            return Err(TokenReuseError);
        };

        // get relevant filter
        let filter = match periods_forward {
            0 => &mut state.filter_1,
            1 => &mut state.filter_2,
            2 => {
                // turn over filter 1
                state.filter_1 = take(&mut state.filter_2);
                state.period_1_start += lifetime;
                &mut state.filter_2
            }
            _ => {
                // turn over both filters
                state.filter_1 = Filter::default();
                state.filter_2 = Filter::default();
                state.period_1_start = expires_at;
                &mut state.filter_1
            }
        };

        // insert into the filter
        //
        // the token's nonce needs to guarantee uniqueness because of the role it plays in the
        // encryption of the tokens, so it is 128 bits. but since the token log can tolerate false
        // positives, we trim it down to 64 bits, which would still only have a small collision
        // rate even at significant amounts of usage, while allowing us to store twice as many in
        // the hash set variant.
        //
        // token nonce values are uniformly randomly generated server-side and cryptographically
        // integrity-checked, so we don't need to employ secure hashing to trim it down to 64 bits,
        // we can simply truncate.
        //
        // per the Rust reference, we can truncate by simply casting:
        // https://doc.rust-lang.org/stable/reference/expressions/operator-expr.html#numeric-cast
        filter.check_and_insert(nonce as u64, &state.config)
    }
}

/// Default to 20 MiB max memory consumption and expected one million hits
///
/// With the default validation token lifetime of 2 weeks, this corresponds to one token usage per
/// 1.21 seconds.
impl Default for BloomTokenLog {
    fn default() -> Self {
        Self::new_expected_items(DEFAULT_MAX_BYTES, DEFAULT_EXPECTED_HITS)
    }
}

/// Lockable state of [`BloomTokenLog`]
struct State {
    config: FilterConfig,
    // filter_1 covers tokens that expire in the period starting at period_1_start and extending
    // lifetime after. filter_2 covers tokens for the next lifetime after that.
    period_1_start: SystemTime,
    filter_1: Filter,
    filter_2: Filter,
}

/// Unchanging parameters governing [`Filter`] behavior
struct FilterConfig {
    filter_max_bytes: usize,
    k_num: u32,
}

/// Period filter within [`State`]
enum Filter {
    Set(HashSet<u64, IdentityBuildHasher>),
    Bloom(BloomFilter<FxBuildHasher>),
}

impl Filter {
    fn check_and_insert(
        &mut self,
        fingerprint: u64,
        config: &FilterConfig,
    ) -> Result<(), TokenReuseError> {
        match self {
            Self::Set(hset) => {
                if !hset.insert(fingerprint) {
                    return Err(TokenReuseError);
                }

                if hset.capacity() * size_of::<u64>() <= config.filter_max_bytes {
                    return Ok(());
                }

                // convert to bloom
                // avoid panicking if user passed in filter_max_bytes of 0. we document that this
                // limit is approximate, so just fudge it up to 1.
                let mut bloom = BloomFilter::with_num_bits((config.filter_max_bytes * 8).max(1))
                    .hasher(FxBuildHasher)
                    .hashes(config.k_num);
                for item in &*hset {
                    bloom.insert(item);
                }
                *self = Self::Bloom(bloom);
            }
            Self::Bloom(bloom) => {
                if bloom.insert(&fingerprint) {
                    return Err(TokenReuseError);
                }
            }
        }
        Ok(())
    }
}

impl Default for Filter {
    fn default() -> Self {
        Self::Set(HashSet::default())
    }
}

/// `BuildHasher` of `IdentityHasher`
#[derive(Default)]
struct IdentityBuildHasher;

impl BuildHasher for IdentityBuildHasher {
    type Hasher = IdentityHasher;

    fn build_hasher(&self) -> Self::Hasher {
        IdentityHasher::default()
    }
}

/// Hasher that is the identity operation--it assumes that exactly 8 bytes will be hashed, and the
/// resultant hash is those bytes as a `u64`
#[derive(Default)]
struct IdentityHasher {
    data: [u8; 8],
    #[cfg(debug_assertions)]
    wrote_8_byte_slice: bool,
}

impl Hasher for IdentityHasher {
    fn write(&mut self, bytes: &[u8]) {
        #[cfg(debug_assertions)]
        {
            assert!(!self.wrote_8_byte_slice);
            assert_eq!(bytes.len(), 8);
            self.wrote_8_byte_slice = true;
        }
        self.data.copy_from_slice(bytes);
    }

    fn finish(&self) -> u64 {
        #[cfg(debug_assertions)]
        assert!(self.wrote_8_byte_slice);
        u64::from_ne_bytes(self.data)
    }
}

fn optimal_k_num(num_bytes: usize, expected_hits: u64) -> u32 {
    // be more forgiving rather than panickey here. excessively high num_bits may occur if the user
    // wishes it to be unbounded, so just saturate. expected_hits of 0 would cause divide-by-zero,
    // so just fudge it up to 1 in that case.
    let num_bits = (num_bytes as u64).saturating_mul(8);
    let expected_hits = expected_hits.max(1);
    // reference for this formula: https://programming.guide/bloom-filter-calculator.html
    // optimal k = (m ln 2) / n
    // wherein m is the number of bits, and n is the number of elements in the set.
    //
    // we also impose a minimum return value of 1, to avoid making the bloom filter entirely
    // useless in the case that the user provided an absurdly high ratio of hits / bytes.
    (((num_bits as f64 / expected_hits as f64) * LN_2).round() as u32).max(1)
}

// remember to change the doc comment for `impl Default for BloomTokenLog` if these ever change
const DEFAULT_MAX_BYTES: usize = 10 << 20;
const DEFAULT_EXPECTED_HITS: u64 = 1_000_000;

#[cfg(test)]
mod test {
    use super::*;
    use rand::prelude::*;
    use rand_pcg::Pcg32;

    fn new_rng() -> impl Rng {
        Pcg32::from_seed(0xdeadbeefdeadbeefdeadbeefdeadbeef_u128.to_le_bytes())
    }

    #[test]
    fn identity_hash_test() {
        let mut rng = new_rng();
        let builder = IdentityBuildHasher;
        for _ in 0..100 {
            let n = rng.random::<u64>();
            let hash = builder.hash_one(n);
            assert_eq!(hash, n);
        }
    }

    #[test]
    fn optimal_k_num_test() {
        assert_eq!(optimal_k_num(10 << 20, 1_000_000), 58);
        assert_eq!(optimal_k_num(10 << 20, 1_000_000_000_000_000), 1);
        // assert that these don't panic:
        optimal_k_num(10 << 20, 0);
        optimal_k_num(usize::MAX, 1_000_000);
    }

    #[test]
    fn bloom_token_log_conversion() {
        let mut rng = new_rng();
        let mut log = BloomTokenLog::new_expected_items(800, 200);

        let issued = SystemTime::now();
        let lifetime = Duration::from_secs(1_000_000);

        for i in 0..200 {
            let token = rng.random::<u128>();
            let result = log.check_and_insert(token, issued, lifetime);
            {
                let filter = &log.0.lock().unwrap().filter_1;
                if let Filter::Set(ref hset) = *filter {
                    assert!(hset.capacity() * size_of::<u64>() <= 800);
                    assert_eq!(hset.len(), i + 1);
                    assert!(result.is_ok());
                } else {
                    assert!(i > 10, "definitely bloomed too early");
                }
            }
            assert!(log.check_and_insert(token, issued, lifetime).is_err());
        }

        assert!(
            matches!(log.0.get_mut().unwrap().filter_1, Filter::Bloom { .. }),
            "didn't bloom"
        );
    }

    #[test]
    fn turn_over() {
        let mut rng = new_rng();
        let log = BloomTokenLog::new_expected_items(800, 200);
        let lifetime = Duration::from_secs(1_000);
        let mut old = Vec::default();
        let mut accepted = 0;

        for i in 0..200 {
            let token = rng.random::<u128>();
            let now = UNIX_EPOCH + lifetime * 10 + lifetime * i / 10;
            let issued = now - lifetime.mul_f32(rng.gen_range(0.0..3.0));
            let result = log.check_and_insert(token, issued, lifetime);
            if result.is_ok() {
                accepted += 1;
            }
            old.push((token, issued));
            let old_idx = rng.gen_range(0..old.len());
            let (old_token, old_issued) = old[old_idx];
            assert!(
                log.check_and_insert(old_token, old_issued, lifetime)
                    .is_err()
            );
        }
        assert!(accepted > 0);
    }

    fn test_doesnt_panic(log: BloomTokenLog) {
        let mut rng = new_rng();

        let issued = SystemTime::now();
        let lifetime = Duration::from_secs(1_000_000);

        for _ in 0..200 {
            let _ = log.check_and_insert(rng.random::<u128>(), issued, lifetime);
        }
    }

    #[test]
    fn max_bytes_zero() {
        // "max bytes" is documented to be approximate. but make sure it doesn't panic.
        test_doesnt_panic(BloomTokenLog::new_expected_items(0, 200));
    }

    #[test]
    fn expected_hits_zero() {
        test_doesnt_panic(BloomTokenLog::new_expected_items(100, 0));
    }

    #[test]
    fn k_num_zero() {
        test_doesnt_panic(BloomTokenLog::new(100, 0));
    }
}
