From 7d0de7aa2b8fdcb69cdae7e494dc89d25bc43db6 Mon Sep 17 00:00:00 2001
From: Alpha Chen <alpha.chen@gmail.com>
Date: Fri, 7 Dec 2018 07:37:16 -0800
Subject: [PATCH] [2018][rust][07.1]

---
 2018/rust/src/bin/day_07.rs | 73 +++++++++++++++++++++++++++++++++++++
 1 file changed, 73 insertions(+)
 create mode 100644 2018/rust/src/bin/day_07.rs

diff --git a/2018/rust/src/bin/day_07.rs b/2018/rust/src/bin/day_07.rs
new file mode 100644
index 0000000..895b60b
--- /dev/null
+++ b/2018/rust/src/bin/day_07.rs
@@ -0,0 +1,73 @@
+use std::collections::{HashMap, HashSet};
+use std::error::Error;
+use std::io::{self, Read};
+
+use regex::Regex;
+
+fn main() -> Result<(), Box<Error>> {
+    let mut input = String::new();
+    io::stdin().read_to_string(&mut input)?;
+
+    let output = solve(&input)?;
+    println!("{}", output);
+
+    Ok(())
+}
+
+fn solve(input: &str) -> Result<String, Box<Error>> {
+    let re = Regex::new(r"^Step (\w).*step (\w)").unwrap();
+
+    let mut steps = HashMap::new();
+    input
+        .trim()
+        .lines()
+        .flat_map(|x| re.captures(x))
+        .map(|x| {
+            x.iter()
+                .flat_map(|x| x)
+                .map(|x| x.as_str())
+                .collect::<Vec<_>>()
+        })
+        .map(|x| (x[1], x[2]))
+        .for_each(|(x, y)| {
+            steps.entry(x).or_insert_with(HashSet::new);
+            let entry = steps.entry(y).or_insert_with(HashSet::new);
+            entry.insert(x);
+        });
+
+    let mut output = String::new();
+    while !steps.is_empty() {
+        let mut ready: Vec<_> = steps
+            .iter()
+            .filter(|(_, v)| v.is_empty())
+            .map(|(k, _)| k)
+            .collect();
+        ready.sort();
+        let done = ready[0].clone();
+
+        steps.remove(done);
+        for dependencies in steps.values_mut() {
+            dependencies.remove(done);
+        }
+
+        output.push_str(done);
+    }
+
+    Ok(output)
+}
+
+#[test]
+fn test_solve() {
+    let input = r"
+Step C must be finished before step A can begin.
+Step C must be finished before step F can begin.
+Step A must be finished before step B can begin.
+Step A must be finished before step D can begin.
+Step B must be finished before step E can begin.
+Step D must be finished before step E can begin.
+Step F must be finished before step E can begin.
+    ";
+
+    let output = solve(input).unwrap();
+    assert_eq!(output, "CABDFE".to_string());
+}