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//! Check whether an edge exists in a digraph.
//!
//! To determine whether an arc exists from `u` to `v`, see [`HasArc`].
//!
//! # Examples
//!
//! ```
//! use graaf::{
//! AddArc,
//! AdjacencyList,
//! Circuit,
//! Empty,
//! HasEdge,
//! };
//!
//! let digraph = AdjacencyList::circuit(2);
//!
//! assert!(digraph.has_edge(0, 1));
//! assert!(digraph.has_edge(1, 0));
//!
//! let mut digraph = AdjacencyList::empty(2);
//!
//! digraph.add_arc(0, 1);
//!
//! assert!(!digraph.has_edge(0, 1));
//! assert!(!digraph.has_edge(1, 0));
//!
//! let mut digraph = AdjacencyList::empty(3);
//!
//! digraph.add_arc(0, 1);
//! digraph.add_arc(0, 2);
//! digraph.add_arc(1, 2);
//! digraph.add_arc(2, 0);
//!
//! assert!(!digraph.has_edge(0, 1));
//! assert!(digraph.has_edge(0, 2));
//! assert!(!digraph.has_edge(1, 0));
//! assert!(!digraph.has_edge(1, 2));
//! assert!(digraph.has_edge(2, 0));
//! assert!(!digraph.has_edge(2, 1));
//! ```
//!
//! [`HasArc`]: crate::HasArc
/// Check whether an edge exists in a digraph.
pub trait HasEdge {
/// Check whether an edge exists in the digraph.
///
/// # Arguments
///
/// * `u`: The tail vertex.
/// * `v`: The head vertex.
///
/// # Examples
///
/// ```
/// use graaf::{
/// AddArc,
/// AdjacencyList,
/// Circuit,
/// Empty,
/// HasEdge,
/// };
///
/// let digraph = AdjacencyList::circuit(2);
///
/// assert!(digraph.has_edge(0, 1));
/// assert!(digraph.has_edge(1, 0));
///
/// let mut digraph = AdjacencyList::empty(2);
///
/// digraph.add_arc(0, 1);
///
/// assert!(!digraph.has_edge(0, 1));
/// assert!(!digraph.has_edge(1, 0));
///
/// let mut digraph = AdjacencyList::empty(3);
///
/// digraph.add_arc(0, 1);
/// digraph.add_arc(0, 2);
/// digraph.add_arc(1, 2);
/// digraph.add_arc(2, 0);
///
/// assert!(!digraph.has_edge(0, 1));
/// assert!(digraph.has_edge(0, 2));
/// assert!(!digraph.has_edge(1, 0));
/// assert!(!digraph.has_edge(1, 2));
/// assert!(digraph.has_edge(2, 0));
/// assert!(!digraph.has_edge(2, 1));
/// ```
#[must_use]
fn has_edge(&self, u: usize, v: usize) -> bool;
}