From 5d09932b11b9eb1ae79b68d4fa8a83a3f782ce91 Mon Sep 17 00:00:00 2001
From: dece <shgck@pistache.land>
Date: Tue, 8 Dec 2020 19:45:26 +0100
Subject: [PATCH] Day 8 in Rust

---
 2020/day8.rs | 78 ++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 78 insertions(+)
 create mode 100644 2020/day8.rs

diff --git a/2020/day8.rs b/2020/day8.rs
new file mode 100644
index 0000000..b32b015
--- /dev/null
+++ b/2020/day8.rs
@@ -0,0 +1,78 @@
+use std::collections;
+use std::fs;
+use std::io::{self, BufRead};
+
+#[derive(Clone)]
+enum Op {
+    Nop(i64),
+    Jmp(i64),
+    Acc(i64),
+}
+
+struct Cpu {
+    ip: u64,
+    acc: i64,
+}
+
+impl Cpu {
+    fn step(&mut self, code: &Vec<Op>) {
+        match code[self.ip as usize] {
+            Op::Nop(_) => {},
+            Op::Jmp(i) => { self.ip = (self.ip as i64 + i) as u64; return }
+            Op::Acc(i) => { self.acc += i }
+        }
+        self.ip += 1;
+    }
+}
+
+fn main() {
+    let file = fs::File::open("day8.txt").unwrap();
+    let lines: Vec<String> = io::BufReader::new(file).lines().map(|r| r.unwrap()).collect();
+    let mut code: Vec<Op> = lines.iter().map(|line| {
+        let inst: Vec<&str> = line.splitn(2, " ").collect();
+        match inst[0] {
+            "nop" => Op::Nop(inst[1].parse::<i64>().unwrap()),
+            "jmp" => Op::Jmp(inst[1].parse::<i64>().unwrap()),
+            "acc" => Op::Acc(inst[1].parse::<i64>().unwrap()),
+            unk => panic!("Unknown op '{}'", unk),
+        }
+    }).collect();
+
+    // Part 1.
+    let mut visited: collections::HashSet<u64> = collections::HashSet::new();
+    let mut cpu = Cpu { ip: 0, acc: 0 };
+    while !visited.contains(&cpu.ip) {
+        visited.insert(cpu.ip);
+        cpu.step(&code);
+    }
+    println!("Acc after first revisit: {}.", cpu.acc);
+
+    // Part 2.
+    for i in 0..code.len() {
+        match &code[i] {
+            Op::Jmp(val) => {
+                let op_backup = code[i].clone();
+                code[i] = Op::Nop(*val);
+                if let Some(acc) = run(&code) {
+                    println!("Acc after non-looping execution: {}.", acc);
+                    break
+                }
+                code[i] = op_backup;
+            }
+            _ => {}
+        }
+    }
+}
+
+fn run(code: &Vec<Op>) -> Option<i64> {
+    let mut visited: collections::HashSet<u64> = collections::HashSet::new();
+    let mut cpu = Cpu { ip: 0, acc: 0 };
+    while cpu.ip < code.len() as u64 {
+        visited.insert(cpu.ip);
+        cpu.step(&code);
+        if visited.contains(&cpu.ip) {
+            return None
+        }
+    }
+    Some(cpu.acc)
+}