sci_form/

smiles.rs

use crate::graph::{Atom, Bond, BondOrder, BondStereo, ChiralType, Hybridization, Molecule};
use nalgebra::Vector3;
use petgraph::graph::NodeIndex;
use std::collections::HashMap;

/// Recursive-descent parser that converts a SMILES string into a [`Molecule`] graph.
///
/// Handles:
/// - Aliphatic & aromatic atoms, brackets, isotopes, formal charges
/// - Ring closure numbers (single and double-digit)
/// - Branching via `(` / `)`
/// - Stereo: `@`/`@@` for chiral centers, `/`/`\` for double-bond E/Z
/// - Implicit hydrogen addition and hybridization assignment
pub struct SmilesParser<'a> {
    input: &'a [u8],
    pos: usize,
    rings: HashMap<u32, (NodeIndex, BondOrder, BondStereo)>,
    mol: &'a mut Molecule,
}

impl<'a> SmilesParser<'a> {
    /// Create a new parser for `smiles`, writing atoms and bonds into `mol`.
    pub fn new(smiles: &'a str, mol: &'a mut Molecule) -> Self {
        SmilesParser {
            input: smiles.as_bytes(),
            pos: 0,
            rings: HashMap::new(),
            mol,
        }
    }

    /// Parse the SMILES string, populating the molecule.
    ///
    /// Returns `Err` with a human-readable message on invalid input.
    pub fn parse(&mut self) -> Result<(), String> {
        self.parse_chain(None)?;
        // After parsing, add implicit hydrogens
        self.add_implicit_hydrogens();
        Ok(())
    }

    fn peek(&self) -> Option<u8> {
        self.input.get(self.pos).copied()
    }

    fn advance(&mut self) {
        self.pos += 1;
    }

    fn parse_chain(&mut self, mut prev_node: Option<NodeIndex>) -> Result<(), String> {
        let mut prev_bond_order = BondOrder::Single;
        let mut prev_bond_stereo = BondStereo::None;

        let get_bond_order =
            |m: &Molecule, prev: NodeIndex, curr: NodeIndex, order: BondOrder| -> BondOrder {
                if order == BondOrder::Single
                    && m.graph[prev].hybridization == Hybridization::SP2
                    && m.graph[curr].hybridization == Hybridization::SP2
                {
                    BondOrder::Aromatic
                } else {
                    order
                }
            };

        while self.pos < self.input.len() {
            let c = self.peek().unwrap();
            match c {
                b'(' => {
                    self.advance();
                    self.parse_chain(prev_node)?;
                    if self.peek() == Some(b')') {
                        self.advance();
                    } else {
                        return Err("Expected ')'".to_string());
                    }
                }
                b')' => {
                    break;
                }
                b'-' => {
                    self.advance();
                    prev_bond_order = BondOrder::Single;
                }
                b'=' => {
                    self.advance();
                    prev_bond_order = BondOrder::Double;
                }
                b'#' => {
                    self.advance();
                    prev_bond_order = BondOrder::Triple;
                }
                b':' => {
                    self.advance();
                    prev_bond_order = BondOrder::Aromatic;
                }
                b'.' => {
                    // Disconnected fragment separator — no bond between previous and next atom
                    self.advance();
                    prev_node = None;
                    prev_bond_order = BondOrder::Single;
                    prev_bond_stereo = BondStereo::None;
                }
                b'/' => {
                    self.advance();
                    prev_bond_order = BondOrder::Single;
                    prev_bond_stereo = BondStereo::E;
                } // Simplify E/Z for now
                b'\\' => {
                    self.advance();
                    prev_bond_order = BondOrder::Single;
                    prev_bond_stereo = BondStereo::Z;
                }
                b'%' | b'0'..=b'9' => {
                    let mut ring_num = 0;
                    if c == b'%' {
                        self.advance();
                        if let (Some(d1), Some(d2)) = (self.peek(), self.input.get(self.pos + 1)) {
                            if d1.is_ascii_digit() && d2.is_ascii_digit() {
                                ring_num = ((d1 - b'0') * 10 + (d2 - b'0')) as u32;
                                self.advance();
                                self.advance();
                            }
                        }
                    } else {
                        ring_num = (c - b'0') as u32;
                        self.advance();
                    }
                    if ring_num > 0 {
                        if let Some(prev) = prev_node {
                            if let Some((target, order, stereo)) = self.rings.remove(&ring_num) {
                                let mut final_order = if prev_bond_order != BondOrder::Single {
                                    prev_bond_order
                                } else {
                                    order
                                };
                                final_order = get_bond_order(self.mol, prev, target, final_order);
                                let final_stereo = if prev_bond_stereo != BondStereo::None {
                                    prev_bond_stereo
                                } else {
                                    stereo
                                };
                                self.mol.add_bond(
                                    prev,
                                    target,
                                    Bond {
                                        order: final_order,
                                        stereo: final_stereo,
                                    },
                                );
                            } else {
                                self.rings
                                    .insert(ring_num, (prev, prev_bond_order, prev_bond_stereo));
                            }
                            // Reset bond state after closing/opening ring
                            prev_bond_order = BondOrder::Single;
                            prev_bond_stereo = BondStereo::None;
                        }
                    }
                }
                b'[' => {
                    self.advance();
                    let node = self.parse_bracket_atom()?;
                    if let Some(prev) = prev_node {
                        let order = get_bond_order(self.mol, prev, node, prev_bond_order);
                        self.mol.add_bond(
                            prev,
                            node,
                            Bond {
                                order,
                                stereo: prev_bond_stereo,
                            },
                        );
                    }
                    prev_node = Some(node);
                    prev_bond_order = BondOrder::Single;
                    prev_bond_stereo = BondStereo::None;
                }
                _ if c.is_ascii_alphabetic() || c == b'*' => {
                    let node = self.parse_organic_subset()?;
                    if let Some(prev) = prev_node {
                        let order = get_bond_order(self.mol, prev, node, prev_bond_order);
                        self.mol.add_bond(
                            prev,
                            node,
                            Bond {
                                order,
                                stereo: prev_bond_stereo,
                            },
                        );
                    }
                    prev_node = Some(node);
                    prev_bond_order = BondOrder::Single;
                    prev_bond_stereo = BondStereo::None;
                }
                _ => return Err(format!("Unexpected character: {}", c as char)),
            }
        }
        Ok(())
    }

    fn parse_organic_subset(&mut self) -> Result<NodeIndex, String> {
        let mut elem_str = String::new();
        let c = self.peek().unwrap();
        self.advance();
        elem_str.push(c as char);
        if let Some(nc) = self.peek() {
            if nc.is_ascii_lowercase()
                && (c == b'C' && (nc == b'l' || nc == b'u' || nc == b'o' || nc == b'r')
                    || c == b'B' && nc == b'r')
            {
                elem_str.push(nc as char);
                self.advance();
            }
        }

        // Handle aromatic
        let mut aromatic = false;
        if elem_str.chars().next().unwrap().is_lowercase() {
            aromatic = true;
            elem_str = elem_str.to_uppercase();
        }

        let element = match elem_str.as_str() {
            "C" => 6,
            "N" => 7,
            "O" => 8,
            "P" => 15,
            "S" => 16,
            "F" => 9,
            "CL" | "Cl" => 17,
            "BR" | "Br" => 35,
            "I" => 53,
            "B" => 5,
            "*" => 0,
            &_ => return Err(format!("Unknown element: {}", elem_str)),
        };

        let atom = Atom {
            element,
            position: Vector3::zeros(),
            charge: 0.0,
            formal_charge: 0,
            hybridization: if aromatic {
                Hybridization::SP2
            } else {
                Hybridization::SP3
            },
            chiral_tag: ChiralType::Unspecified,
            explicit_h: 0,
        };

        Ok(self.mol.add_atom(atom))
    }

    fn parse_bracket_atom(&mut self) -> Result<NodeIndex, String> {
        let mut isotope = 0u32;
        let mut aromatic = false;
        let mut chiral = ChiralType::Unspecified;
        let mut h_count = 0;
        let mut charge = 0;

        // Isotope
        while let Some(c) = self.peek() {
            if c.is_ascii_digit() {
                isotope = isotope * 10 + (c - b'0') as u32;
                self.advance();
            } else {
                break;
            }
        }

        // Element
        let mut elem_str = String::new();
        if let Some(c) = self.peek() {
            if c.is_ascii_alphabetic() {
                elem_str.push(c as char);
                self.advance();
                if let Some(nc) = self.peek() {
                    if nc.is_ascii_lowercase() && c.is_ascii_uppercase() {
                        elem_str.push(nc as char);
                        self.advance();
                    }
                }
            } else if c == b'*' {
                elem_str.push('*');
                self.advance();
            }
        }

        if elem_str.chars().next().unwrap().is_lowercase() {
            aromatic = true;
            elem_str = elem_str.to_uppercase();
        }

        let element: u8 = match elem_str.as_str() {
            "H" => 1,
            "HE" | "He" => 2,
            "LI" | "Li" => 3,
            "BE" | "Be" => 4,
            "B" => 5,
            "C" => 6,
            "N" => 7,
            "O" => 8,
            "F" => 9,
            "NE" | "Ne" => 10,
            "NA" | "Na" => 11,
            "MG" | "Mg" => 12,
            "AL" | "Al" => 13,
            "SI" | "Si" => 14,
            "P" => 15,
            "S" => 16,
            "CL" | "Cl" => 17,
            "AR" | "Ar" => 18,
            "K" => 19,
            "CA" | "Ca" => 20,
            "SC" | "Sc" => 21,
            "TI" | "Ti" => 22,
            "V" => 23,
            "CR" | "Cr" => 24,
            "MN" | "Mn" => 25,
            "FE" | "Fe" => 26,
            "CO" | "Co" => 27,
            "NI" | "Ni" => 28,
            "CU" | "Cu" => 29,
            "ZN" | "Zn" => 30,
            "GA" | "Ga" => 31,
            "GE" | "Ge" => 32,
            "AS" | "As" => 33,
            "SE" | "Se" => 34,
            "BR" | "Br" => 35,
            "KR" | "Kr" => 36,
            "RB" | "Rb" => 37,
            "SR" | "Sr" => 38,
            "Y" => 39,
            "ZR" | "Zr" => 40,
            "MO" | "Mo" => 42,
            "RU" | "Ru" => 44,
            "RH" | "Rh" => 45,
            "PD" | "Pd" => 46,
            "AG" | "Ag" => 47,
            "CD" | "Cd" => 48,
            "IN" | "In" => 49,
            "SN" | "Sn" => 50,
            "SB" | "Sb" => 51,
            "TE" | "Te" => 52,
            "I" => 53,
            "XE" | "Xe" => 54,
            "CS" | "Cs" => 55,
            "BA" | "Ba" => 56,
            "LA" | "La" => 57,
            "W" => 74,
            "RE" | "Re" => 75,
            "OS" | "Os" => 76,
            "IR" | "Ir" => 77,
            "PT" | "Pt" => 78,
            "AU" | "Au" => 79,
            "HG" | "Hg" => 80,
            "TL" | "Tl" => 81,
            "PB" | "Pb" => 82,
            "BI" | "Bi" => 83,
            "*" => 0,
            _ => return Err(format!("Unknown bracket element: {}", elem_str)),
        };

        // Chirality
        if let Some(c) = self.peek() {
            if c == b'@' {
                self.advance();
                chiral = ChiralType::TetrahedralCCW;
                if self.peek() == Some(b'@') {
                    self.advance();
                    chiral = ChiralType::TetrahedralCW;
                }
            }
        }

        // Hydrogens
        if let Some(c) = self.peek() {
            if c == b'H' {
                self.advance();
                h_count = 1;
                if let Some(hc) = self.peek() {
                    if hc.is_ascii_digit() {
                        h_count = hc - b'0';
                        self.advance();
                    }
                }
            }
        }

        // Charge
        if let Some(c) = self.peek() {
            if c == b'+' {
                self.advance();
                charge = 1;
                if let Some(cc) = self.peek() {
                    if cc.is_ascii_digit() {
                        charge = (cc - b'0') as i8;
                        self.advance();
                    } else if cc == b'+' {
                        charge = 2;
                        self.advance();
                        if self.peek() == Some(b'+') {
                            charge = 3;
                            self.advance();
                        }
                    }
                }
            } else if c == b'-' {
                self.advance();
                charge = -1;
                if let Some(cc) = self.peek() {
                    if cc.is_ascii_digit() {
                        charge = -((cc - b'0') as i8);
                        self.advance();
                    } else if cc == b'-' {
                        charge = -2;
                        self.advance();
                        if self.peek() == Some(b'-') {
                            charge = -3;
                            self.advance();
                        }
                    }
                }
            }
        }

        if self.peek() == Some(b']') {
            self.advance();
        } else {
            return Err("Expected ']'".to_string());
        }

        let atom = Atom {
            element,
            position: Vector3::zeros(),
            charge: charge as f32,
            formal_charge: charge,
            hybridization: if aromatic {
                Hybridization::SP2
            } else {
                Hybridization::SP3
            }, // Simplification
            chiral_tag: chiral,
            explicit_h: h_count,
        };

        let node = self.mol.add_atom(atom);

        Ok(node)
    }

    fn add_implicit_hydrogens(&mut self) {
        let n = self.mol.graph.node_count();
        let mut to_add = Vec::new();

        for i in 0..n {
            let ni = NodeIndex::new(i);
            let atom = &self.mol.graph[ni];
            if atom.element == 1 || atom.element == 0 {
                continue;
            } // H or dummy

            let mut current_valents = 0;
            let mut double_bonds = 0;
            let mut triple_bonds = 0;
            let mut aromatic_bonds = 0;

            for edge in self.mol.graph.edges(ni) {
                match edge.weight().order {
                    BondOrder::Single => {
                        current_valents += 1;
                    }
                    BondOrder::Double => {
                        current_valents += 2;
                        double_bonds += 1;
                    }
                    BondOrder::Triple => {
                        current_valents += 3;
                        triple_bonds += 1;
                    }
                    BondOrder::Aromatic => {
                        current_valents += 1;
                        aromatic_bonds += 1;
                    }
                    _ => {
                        current_valents += 1;
                    }
                };
            }

            // Fix hybridization for non-aromatics based on exact bond types
            if aromatic_bonds == 0 {
                let actual_hybridization = if triple_bonds > 0 || double_bonds > 1 {
                    Hybridization::SP
                } else if double_bonds == 1 {
                    Hybridization::SP2
                } else {
                    // Check for conjugation: N or O adjacent to double/aromatic bond → SP2
                    // Matches RDKit: norbs==4, degree<=3, atomHasConjugatedBond → SP2
                    let elem = atom.element;
                    let is_conjugated = if (elem == 7 || elem == 8) && double_bonds == 0 {
                        // Check if any neighbor has a double or aromatic bond (conjugation)
                        self.mol.graph.neighbors(ni).any(|nb| {
                            self.mol.graph.edges(nb).any(|e| {
                                matches!(e.weight().order, BondOrder::Double | BondOrder::Aromatic)
                            })
                        })
                    } else {
                        false
                    };
                    if is_conjugated {
                        Hybridization::SP2
                    } else {
                        Hybridization::SP3
                    }
                };

                if let Some(atom_mut) = self.mol.graph.node_weight_mut(ni) {
                    atom_mut.hybridization = actual_hybridization;
                }
            } else {
                if let Some(atom_mut) = self.mol.graph.node_weight_mut(ni) {
                    atom_mut.hybridization = Hybridization::SP2;
                }
            }

            let atom = &self.mol.graph[ni];
            // If it's aromatic, an uncharged Carbon expects 3 bonds normally (2 from ring, 1 external or 1 H).
            let target_val = match atom.element {
                5 => 3, // B
                6 => 4, // C
                7 => {
                    if atom.formal_charge == 1 {
                        4
                    } else {
                        3
                    }
                } // N
                8 => {
                    if atom.formal_charge == 1 {
                        3
                    } else {
                        2
                    }
                } // O
                9 | 17 | 35 | 53 => 1, // Halogens
                14 => 4, // Si
                15 => 3, // P
                16 => 2, // S
                32 => 4, // Ge
                33 => 3, // As
                34 => 2, // Se
                50 => 4, // Sn
                51 => 3, // Sb
                52 => 2, // Te
                _ => 0,
            };

            let mut h_needed = target_val - current_valents - atom.formal_charge.abs() as i32;

            if atom.hybridization == Hybridization::SP2 && atom.element == 6 {
                // simple aromatic C checks
                let aromatic_bonds = self
                    .mol
                    .graph
                    .edges(ni)
                    .filter(|e| e.weight().order == BondOrder::Aromatic)
                    .count();
                if aromatic_bonds == 2 {
                    h_needed = 1 - (current_valents - 2); // 1 H max on a standard aromatic carbon in a ring
                } else if aromatic_bonds == 3 {
                    h_needed = 0; // Bridgehead aromatic carbon
                }
            }

            // Bare aromatic nitrogen (lowercase 'n', no bracket/explicit H) is pyridine-type:
            // lone pair NOT in pi system, only 2 bonds needed → 0 implicit H.
            // [nH] form has explicit_h > 0 and keeps its H (pyrrole-type).
            if aromatic_bonds > 0 && atom.element == 7 && atom.explicit_h == 0 {
                h_needed = 0;
            }

            // Calculate total hydrogens to add for this atom in THIS iteration
            // to maintain exact topological index order!
            let total_h_for_this_atom = atom.explicit_h as i32
                + if h_needed - (atom.explicit_h as i32) > 0 {
                    h_needed - (atom.explicit_h as i32)
                } else {
                    0
                };

            if total_h_for_this_atom > 0 {
                to_add.push((ni, total_h_for_this_atom as u8));
            }
        }

        // Add all hydrogens sequentially
        for (parent, count) in to_add {
            for _ in 0..count {
                let h_node = self.mol.add_atom(Atom {
                    element: 1,
                    position: Vector3::zeros(),
                    charge: 0.0,
                    formal_charge: 0,
                    hybridization: Hybridization::Unknown,
                    chiral_tag: ChiralType::Unspecified,
                    explicit_h: 0,
                });
                self.mol.add_bond(
                    parent,
                    h_node,
                    Bond {
                        order: BondOrder::Single,
                        stereo: BondStereo::None,
                    },
                );
            }
        }
    }
}

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

    #[test]
    fn test_parse_benzene_aliphatic() {
        let mol = Molecule::from_smiles("C1=CC=CC=C1").unwrap();
        assert_eq!(mol.graph.node_count(), 12); // 6 C, 6 implicit H
        assert_eq!(mol.graph.edge_count(), 12); // 6 ring bonds, 6 C-H bonds
    }

    #[test]
    fn test_parse_benzene_aromatic() {
        let mol = Molecule::from_smiles("c1ccccc1").unwrap();
        assert_eq!(mol.graph.node_count(), 12); // 6 C, 6 implicit H
        assert_eq!(mol.graph.edge_count(), 12); // 6 ring bonds, 6 C-H bonds
    }

    #[test]
    fn test_parse_brackets_and_branches() {
        let mol = Molecule::from_smiles("C(C)(C)C").unwrap(); // Isobutane
        assert_eq!(mol.graph.node_count(), 14); // 4 C, 10 H
    }
}