//! ECFP (Extended Connectivity Fingerprints) based on the Morgan algorithm.
//!
//! Uses FNV-1a 64-bit hashing for reproducibility and WASM-compatibility.

use chematic_core::{AtomIdx, BondOrder, Molecule, implicit_hcount};
use chematic_perception::find_sssr;
use rustc_hash::FxHashMap;
use smallvec::SmallVec;

use crate::bitvec::BitVec2048;

const FNV_OFFSET: u64 = 14695981039346656037;
const FNV_PRIME: u64 = 1099511628211;

/// Compute the FNV-1a 64-bit hash of `bytes`.
pub(crate) fn fnv1a(bytes: &[u8]) -> u64 {
    let mut h = FNV_OFFSET;
    for &b in bytes {
        h ^= b as u64;
        h = h.wrapping_mul(FNV_PRIME);
    }
    h
}

/// Hash one atom's neighbourhood at iteration `r` of the Morgan expansion.
///
/// Byte layout: `[r as u8, self_id (8 bytes), (bond_type (1) ++ nb_id (8))*]`
/// Neighbours are sorted before hashing to make the result order-independent.
fn expand_atom_id(mol: &Molecule, i: usize, r: u32, ids: &[u64]) -> u64 {
    let idx = AtomIdx(i as u32);
    // SmallVec<6>: typical atoms have ≤4 heavy neighbors; avoids heap alloc for ~95% of calls.
    let mut neighbor_info: SmallVec<[(u8, u64); 6]> = mol
        .neighbors(idx)
        .map(|(nb_idx, bond_idx)| {
            (
                bond_type_int(mol.bond(bond_idx).order),
                ids[nb_idx.0 as usize],
            )
        })
        .collect();
    neighbor_info.sort_unstable();

    // 1 (radius) + 8 (self id) + up to 6 × 9 (bond_type + nb_id) = 63 bytes max on stack.
    let mut bytes: SmallVec<[u8; 64]> = SmallVec::new();
    bytes.push(r as u8);
    bytes.extend_from_slice(&ids[i].to_le_bytes());
    for (btype, nb_id) in &neighbor_info {
        bytes.push(*btype);
        bytes.extend_from_slice(&nb_id.to_le_bytes());
    }
    fnv1a(&bytes)
}

/// Compute the FNV-1a atom identifier for iteration 0 of the Morgan algorithm.
///
/// The six-byte invariant covers: atomic number, degree, implicit H count, formal
/// charge (clamped to byte range), ring membership, and aromaticity.  When
/// `use_chirality` is true an extra chirality byte is appended; this preserves
/// bit-compatibility with the default (`use_chirality=false`) fingerprints.
pub(crate) fn initial_atom_id(
    mol: &Molecule,
    idx: AtomIdx,
    ring_set: &chematic_perception::RingSet,
    use_chirality: bool,
) -> u64 {
    let atom = mol.atom(idx);
    let charge_adjusted = (atom.charge as i16 + 8).clamp(0, 255) as u8;
    let base_bytes = [
        atom.element.atomic_number(),
        mol.neighbors(idx).count().min(255) as u8,
        implicit_hcount(mol, idx),
        charge_adjusted,
        ring_set.contains_atom(idx) as u8,
        atom.aromatic as u8,
    ];
    if use_chirality {
        use chematic_core::Chirality;
        let chirality_byte = match atom.chirality {
            Chirality::None => 0u8,
            Chirality::CounterClockwise => 1u8,
            Chirality::Clockwise => 2u8,
        };
        let mut chiral_bytes = base_bytes.to_vec();
        chiral_bytes.push(chirality_byte);
        fnv1a(&chiral_bytes)
    } else {
        fnv1a(&base_bytes)
    }
}

/// Configuration for ECFP computation.
#[derive(Debug, Clone)]
pub struct EcfpConfig {
    /// Number of iterations (radius). ECFP4 = 2, ECFP6 = 3.
    pub radius: u32,
    /// Output bitvector size (default 2048).
    pub nbits: usize,
    /// When `true`, include tetrahedral chirality in the initial atom hash so
    /// that R and S enantiomers produce different fingerprints.
    ///
    /// Defaults to `false` (chirality ignored, matching RDKit's `useChirality=False`).
    pub use_chirality: bool,
    /// When `true`, each hash sets two bit positions (using single and double-folded hash)
    /// to reduce bitvector collisions. This reduces collision probability but changes
    /// fingerprint values — **not backwards-compatible** with stored fingerprints.
    ///
    /// Defaults to `false` (single-bit folding, current behavior preserved).
    pub use_double_fold: bool,
}

impl Default for EcfpConfig {
    fn default() -> Self {
        Self {
            radius: 2,
            nbits: 2048,
            use_chirality: false,
            use_double_fold: false,
        }
    }
}

/// Map a `BondOrder` to the integer code used in the ECFP hash.
///
/// - Single / Up / Down → 1
/// - Double            → 2
/// - Triple            → 3
/// - Aromatic          → 4
/// - Quadruple         → 5  (not in standard ECFP; assigned a distinct value)
#[inline]
pub(crate) fn bond_type_int(order: BondOrder) -> u8 {
    match order {
        BondOrder::Single | BondOrder::Up | BondOrder::Down | BondOrder::Dative => 1,
        BondOrder::Double => 2,
        BondOrder::Triple => 3,
        BondOrder::Aromatic => 4,
        BondOrder::Quadruple => 5,
        BondOrder::Zero => 0,
        BondOrder::QueryAny => 6,
        BondOrder::QuerySingleOrDouble => 7,
        BondOrder::QuerySingleOrAromatic => 8,
        BondOrder::QueryDoubleOrAromatic => 9,
    }
}

/// Compute an ECFP fingerprint for `mol` using the given configuration.
///
/// # Algorithm overview
/// 1. Compute initial atom identifiers from atomic properties.
/// 2. Iteratively expand each identifier by incorporating neighbour identifiers
///    (with their bond types) for `config.radius` rounds.
/// 3. After each iteration (including iteration 0), map every identifier to a
///    bit in the output bitvector.
///
/// Maximum supported radius for `ecfp`.  Matches the cap in `morgan_fp_counts`.
/// Beyond this, `r as u8` would silently truncate, producing hash collisions.
pub const MAX_ECFP_RADIUS: u32 = 20;

pub fn ecfp(mol: &Molecule, config: &EcfpConfig) -> BitVec2048 {
    let n = mol.atom_count();
    let nbits = config.nbits;
    // Cap radius to prevent `r as u8` truncation at r > 255 (hash collision bug).
    let config = &EcfpConfig {
        radius: config.radius.min(MAX_ECFP_RADIUS),
        ..*config
    };
    let mut fp = BitVec2048::new();

    if n == 0 {
        return fp;
    }

    let ring_set = find_sssr(mol);

    // Step 1: initial atom identifiers (iteration 0).
    let mut ids: Vec<u64> = Vec::with_capacity(n);
    for i in 0..n {
        let idx = AtomIdx(i as u32);
        let id = initial_atom_id(mol, idx, &ring_set, config.use_chirality);
        fp.set((id % nbits as u64) as usize);
        if config.use_double_fold {
            fp.set(((id >> 11) % nbits as u64) as usize);
        }
        ids.push(id);
    }

    // Step 2: iterative expansion.
    let mut new_ids: Vec<u64> = vec![0u64; n];
    for r in 1..=config.radius {
        for (i, slot) in new_ids.iter_mut().enumerate() {
            let new_id = expand_atom_id(mol, i, r, &ids);
            *slot = new_id;
            fp.set((new_id % nbits as u64) as usize);
            if config.use_double_fold {
                fp.set(((new_id >> 11) % nbits as u64) as usize);
            }
        }
        core::mem::swap(&mut ids, &mut new_ids);
    }

    fp
}

/// Count-based Morgan fingerprint: returns a map of `hash → count` for all
/// atom environments up to `radius` iterations.
///
/// Each (atom, iteration) pair contributes its hash to the map.  Unlike the
/// default RDKit behavior, redundant (duplicate) environments are **not**
/// suppressed — every atom contributes at every iteration level.
///
/// This corresponds to `GetMorganFingerprint(mol, radius,
/// useFeatures=False, includeRedundantEnvironments=True)` in RDKit.
pub fn morgan_fp_counts(mol: &Molecule, radius: u32) -> FxHashMap<u64, u32> {
    const MAX_RADIUS: u32 = 20;
    let radius = radius.min(MAX_RADIUS);

    let n = mol.atom_count();
    let mut counts: FxHashMap<u64, u32> = FxHashMap::default();

    if n == 0 {
        return counts;
    }

    let ring_set = find_sssr(mol);

    // Radius-0: initial atom identifiers.
    let mut ids: Vec<u64> = (0..n)
        .map(|i| initial_atom_id(mol, AtomIdx(i as u32), &ring_set, false))
        .collect();

    for &id in &ids {
        *counts.entry(id).or_insert(0) += 1;
    }

    // Radius 1..=radius: iterative expansion (same hash scheme as ecfp).
    let mut new_ids = vec![0u64; n];
    for r in 1..=radius {
        for (i, slot) in new_ids.iter_mut().enumerate() {
            let new_id = expand_atom_id(mol, i, r, &ids);
            *slot = new_id;
            *counts.entry(new_id).or_insert(0) += 1;
        }
        core::mem::swap(&mut ids, &mut new_ids);
    }

    counts
}

/// ECFP4 fingerprint (radius = 2, 2048 bits).
pub fn ecfp4(mol: &Molecule) -> BitVec2048 {
    ecfp(mol, &EcfpConfig::default())
}

/// ECFP6 fingerprint (radius = 3, 2048 bits).
pub fn ecfp6(mol: &Molecule) -> BitVec2048 {
    ecfp(
        mol,
        &EcfpConfig {
            radius: 3,
            ..EcfpConfig::default()
        },
    )
}

/// Tanimoto similarity between two molecules using ECFP4.
pub fn tanimoto_ecfp4(a: &Molecule, b: &Molecule) -> f64 {
    ecfp4(a).tanimoto(&ecfp4(b))
}

#[cfg(test)]
mod tests {
    use super::*;
    use chematic_smiles::parse;

    fn benzene() -> Molecule {
        parse("c1ccccc1").unwrap()
    }

    fn ethane() -> Molecule {
        parse("CC").unwrap()
    }

    fn toluene() -> Molecule {
        parse("Cc1ccccc1").unwrap()
    }

    fn aspirin() -> Molecule {
        // Acetylsalicylic acid
        parse("CC(=O)Oc1ccccc1C(=O)O").unwrap()
    }

    fn methane() -> Molecule {
        parse("C").unwrap()
    }

    fn water() -> Molecule {
        parse("O").unwrap()
    }

    #[test]
    fn benzene_ecfp4_nonzero() {
        let fp = ecfp4(&benzene());
        assert!(fp.popcount() > 0, "benzene ECFP4 must be non-zero");
    }

    #[test]
    fn benzene_ecfp4_deterministic() {
        let fp1 = ecfp4(&benzene());
        let fp2 = ecfp4(&benzene());
        assert_eq!(fp1, fp2, "ECFP4 must be deterministic");
    }

    #[test]
    fn ethane_vs_benzene_tanimoto_lt1() {
        let t = tanimoto_ecfp4(&ethane(), &benzene());
        assert!(t < 1.0, "ethane and benzene must differ (tanimoto={t})");
    }

    #[test]
    fn benzene_vs_benzene_tanimoto_eq1() {
        let t = tanimoto_ecfp4(&benzene(), &benzene());
        assert_eq!(t, 1.0, "identical molecules must have tanimoto == 1.0");
    }

    #[test]
    fn benzene_vs_toluene_tanimoto_between() {
        let t = tanimoto_ecfp4(&benzene(), &toluene());
        assert!(t > 0.0, "benzene and toluene share bits (tanimoto={t})");
        assert!(
            t < 1.0,
            "benzene and toluene are not identical (tanimoto={t})"
        );
    }

    #[test]
    fn aspirin_ecfp4_many_bits() {
        let fp = ecfp4(&aspirin());
        assert!(
            fp.popcount() > 5,
            "aspirin ECFP4 must have more than 5 bits set (got {})",
            fp.popcount()
        );
    }

    #[test]
    fn ecfp6_vs_ecfp4_benzene_differ() {
        let fp4 = ecfp4(&benzene());
        let fp6 = ecfp6(&benzene());
        // Larger radius explores more environment — the bit counts should differ
        // because radius-3 adds new hash values not present at radius-2.
        assert_ne!(
            fp4.popcount(),
            fp6.popcount(),
            "ECFP6 and ECFP4 should produce different bit counts for benzene"
        );
    }

    #[test]
    fn methane_ecfp4_nonzero() {
        let fp = ecfp4(&methane());
        assert!(fp.popcount() > 0, "methane ECFP4 must be non-zero");
    }

    #[test]
    fn water_ecfp4_nonzero() {
        let fp = ecfp4(&water());
        assert!(fp.popcount() > 0, "water ECFP4 must be non-zero");
    }

    #[test]
    fn tanimoto_ecfp4_benzene_self_is_one() {
        let t = tanimoto_ecfp4(&benzene(), &benzene());
        assert_eq!(t, 1.0, "tanimoto_ecfp4 of identical molecules must be 1.0");
    }

    #[test]
    fn tanimoto_ecfp4_methane_vs_benzene_lt_half() {
        let t = tanimoto_ecfp4(&methane(), &benzene());
        assert!(
            t < 0.5,
            "methane and benzene should be very dissimilar (tanimoto={t})"
        );
    }

    // ── morgan_fp_counts ─────────────────────────────────────────────────────

    #[test]
    fn morgan_counts_radius0_atom_count() {
        // At radius 0, one hash per atom.
        let m = benzene();
        let counts = morgan_fp_counts(&m, 0);
        let total: u32 = counts.values().sum();
        assert_eq!(
            total,
            m.atom_count() as u32,
            "radius-0 total count == atom_count"
        );
    }

    #[test]
    fn morgan_counts_radius2_total_grows() {
        // Each additional radius adds one hash per atom → total = n * (radius+1).
        let m = methane();
        let n = m.atom_count() as u32;
        let r = 2u32;
        let counts = morgan_fp_counts(&m, r);
        let total: u32 = counts.values().sum();
        assert_eq!(
            total,
            n * (r + 1),
            "methane total = atom_count * (radius+1)"
        );
    }

    #[test]
    fn morgan_counts_benzene_symmetry() {
        // All 6 benzene C atoms are equivalent → radius-0 yields 1 unique hash.
        let m = benzene();
        let counts = morgan_fp_counts(&m, 0);
        assert_eq!(
            counts.len(),
            1,
            "benzene has one unique radius-0 environment"
        );
        assert_eq!(
            *counts.values().next().unwrap(),
            6,
            "that environment appears 6 times"
        );
    }

    #[test]
    fn morgan_counts_empty_mol_is_empty() {
        use chematic_core::MoleculeBuilder;
        let m = MoleculeBuilder::new().build();
        let counts = morgan_fp_counts(&m, 2);
        assert!(counts.is_empty(), "empty molecule yields empty count map");
    }

    #[test]
    fn morgan_counts_deterministic() {
        let m = aspirin();
        let c1 = morgan_fp_counts(&m, 2);
        let c2 = morgan_fp_counts(&m, 2);
        assert_eq!(c1, c2, "morgan_fp_counts must be deterministic");
    }

    #[test]
    fn morgan_counts_consistent_with_ecfp_bits() {
        // Every hash in the count map should be reachable from the ecfp bit set
        // (after folding to 2048 bits).  This checks the same hash scheme.
        let m = toluene();
        let fp = ecfp(
            &m,
            &EcfpConfig {
                radius: 2,
                nbits: 2048,
                use_chirality: false,
                use_double_fold: false,
            },
        );
        let counts = morgan_fp_counts(&m, 2);
        for &hash in counts.keys() {
            let bit = (hash % 2048) as usize;
            assert!(
                fp.get(bit),
                "bit {bit} from count map not set in ECFP bitvec"
            );
        }
    }

    // -- Chirality tests ------------------------------------------------------

    #[test]
    fn ecfp4_ignores_chirality_by_default() {
        // L-alanine and D-alanine should produce the same ECFP4 when
        // use_chirality=false (default), since chirality is not in the hash.
        let l_ala = parse("N[C@@H](C)C(=O)O").unwrap();
        let d_ala = parse("N[C@H](C)C(=O)O").unwrap();
        let fp_l = ecfp4(&l_ala);
        let fp_d = ecfp4(&d_ala);
        assert_eq!(
            fp_l, fp_d,
            "L/D-alanine ECFP4 should be identical when use_chirality=false"
        );
    }

    #[test]
    fn ecfp4_distinguishes_enantiomers_with_chirality() {
        // With use_chirality=true, L-alanine and D-alanine must have different FPs.
        let l_ala = parse("N[C@@H](C)C(=O)O").unwrap();
        let d_ala = parse("N[C@H](C)C(=O)O").unwrap();
        let config = EcfpConfig {
            radius: 2,
            nbits: 2048,
            use_chirality: true,
            use_double_fold: false,
        };
        let fp_l = ecfp(&l_ala, &config);
        let fp_d = ecfp(&d_ala, &config);
        assert_ne!(
            fp_l, fp_d,
            "L/D-alanine ECFP4 must differ when use_chirality=true"
        );
        // Tanimoto < 1.0 confirms they are not identical.
        assert!(
            fp_l.tanimoto(&fp_d) < 1.0,
            "Tanimoto of L/D-alanine must be < 1.0 with use_chirality"
        );
    }

    #[test]
    fn ecfp4_non_chiral_generates_with_chirality_flag() {
        // Non-chiral molecules like benzene should generate valid ECFP with use_chirality flag.
        // This test verifies that use_chirality=true doesn't break on achiral molecules.
        let mol = parse("c1ccccc1").unwrap(); // Benzene — no stereo centers.
        let config = EcfpConfig {
            radius: 2,
            nbits: 2048,
            use_chirality: true,
            use_double_fold: false,
        };
        let fp = ecfp(&mol, &config);
        assert!(
            fp.popcount() > 0,
            "Benzene should generate non-empty ECFP4 with use_chirality=true"
        );
    }

    // -----------------------------------------------------------------------
    // Implicit vs. explicit hydrogen representation (CDK issue #1084 pattern)
    // -----------------------------------------------------------------------
    //
    // chematic's molecule model stores only heavy atoms; implicit H counts are
    // computed on demand via `implicit_hcount()`.  When a SMILES contains
    // explicit H atoms (e.g. `[H]O` or `[OH2]`), those atoms ARE stored in
    // the molecular graph and WILL change the ECFP invariant for the heavy
    // atom they're bonded to (its degree increases and its implicit_hcount
    // decreases).  This is the expected behaviour for a graph-based fingerprint
    // and mirrors CDK / RDKit behaviour.  These tests document it explicitly.

    #[test]
    fn ecfp4_implicit_h_water_vs_no_atoms() {
        // "O" has 1 heavy atom (O) with 2 implicit H.
        // "[OH2]" is the same molecule: O with explicit H count = 2 but still
        // only 1 heavy atom (implicit H also = 0 because OH2 bracket sets it).
        // Both should produce the same fingerprint.
        let implicit = parse("O").unwrap();
        let bracketed = parse("[OH2]").unwrap();
        assert_eq!(
            ecfp4(&implicit),
            ecfp4(&bracketed),
            "[OH2] and O should give the same ECFP4 (same heavy-atom graph)"
        );
    }

    #[test]
    fn ecfp4_explicit_h_atom_changes_fingerprint() {
        // "[H]O[H]" parses as 3 atoms: H-O-H.  The O atom now has degree=2
        // and implicit_hcount=0, so its Morgan invariant differs from the
        // single-atom "O" (degree=0, implicit_hcount=2).  The fingerprint
        // must differ — this documents the expected behaviour when explicit
        // H atoms are present in the molecular graph.
        let implicit = parse("O").unwrap();
        let explicit_h = parse("[H]O[H]").unwrap();
        assert_ne!(
            ecfp4(&implicit),
            ecfp4(&explicit_h),
            "explicit H atoms in the graph change the ECFP4"
        );
    }

    #[test]
    fn ecfp4_implicit_vs_explicit_h_in_organic_molecule() {
        // Methanol "CO" vs "C([H])([H])([H])O" — the second form has 3
        // explicit H atoms on C, which changes C's degree and implicit_hcount.
        // These are different molecular graphs → different ECFP4.
        let implicit = parse("CO").unwrap();
        let explicit_h = parse("C([H])([H])([H])O").unwrap();
        assert_ne!(
            ecfp4(&implicit),
            ecfp4(&explicit_h),
            "methanol with explicit H atoms has a different heavy-atom neighbourhood"
        );
    }
}
