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// SPDX-License-Identifier: BSD-3-Clause
// Copyright (c) 2025 William Bro-Jørgensen
// Copyright (c) 2020 Guillaume Fraux and contributors
//
// See LICENSE at the project root for full text.
use std::ops::Index;
/// Stores the type of bond
#[derive(Clone, Copy, Debug, PartialEq, Eq, Default, Hash)]
pub enum BondOrder {
/// Bond order is unknown or unspecified
#[default]
Unknown,
/// Single bond
Single,
/// Double bond
Double,
/// Triple bond
Triple,
/// Quadruplet bond
Quadruple,
/// Quintuplet bond
Quintuplet,
/// Single bond direction from first atom to second is 'down'. Used for cis-trans isomers
Down,
/// Single bond direction from first atom to second is 'up'. Used for cis-trans isomers
Up,
/// Dative bond where the electrons are localized to the first atom
DativeR,
/// Dative bond where the electrons are localized to the second atom
DativeL,
/// Amide bond (C(=O)-NH)
Amide,
/// Aromatic bond (for example the ring bonds in benzene)
Aromatic,
}
/// Ensure a canonical representation of a bond two atoms
#[derive(PartialEq, Eq, PartialOrd, Ord, Clone, Copy, Debug, Hash)]
pub struct Bond {
data: [usize; 2],
}
impl Index<usize> for Bond {
type Output = usize;
/// Access one of the two atom indices in the bond.
///
/// # Panics
///
/// Panics if `index >= 2`.
fn index(&self, index: usize) -> &Self::Output {
assert!(index < 2, "cannot access atom n° {index} in bond");
&self.data[index]
}
}
impl Bond {
/// Create a new bond between `i` and `j`.
///
/// # Panics
///
/// Panics if `i == j`.
#[must_use]
pub fn new(i: usize, j: usize) -> Self {
assert!(i != j, "can not have a bond between an atom and itself");
let a = std::cmp::min(i, j);
let b = std::cmp::max(i, j);
Bond { data: [a, b] }
}
}