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// SPDX-FileCopyrightText: Peter Pentchev <roam@ringlet.net>
// SPDX-License-Identifier: BSD-2-Clause
//! Solve the first task for week 285, "No Connection".
use std::collections::HashSet;
use tracing::{debug, trace};
use crate::defs::Error;
/// Week 285 task 1: No Connection.
///
/// # Errors
///
/// [`Error::NotImplemented`] so far.
#[tracing::instrument(skip(routes))]
#[inline]
pub fn solve_no_connection(routes: &[(String, String)]) -> Result<String, Error> {
debug!(
"About to look for a leaf destination in {count} routes",
count = routes.len()
);
let (set_from, set_to) = routes.iter().fold(
(HashSet::new(), HashSet::new()),
|(mut set_from, mut set_to): (HashSet<&str>, HashSet<&str>),
&(ref route_from, ref route_to)| {
set_from.insert(route_from.as_ref());
set_to.insert(route_to.as_ref());
(set_from, set_to)
},
);
trace!("from: {count}", count = set_from.len());
trace!("to: {count}", count = set_to.len());
let mut diff = set_to.difference(&set_from);
let first = diff
.next()
.ok_or_else(|| Error::NoSolution("Not even a single leaf destination".to_owned()))?;
trace!(first);
diff.next().map_or_else(
|| Ok((*first).to_owned()),
|second| {
trace!(second);
Err(Error::NoSolution(
"More than one leaf destination".to_owned(),
))
},
)
}
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