2 files changed, 158 insertions, 0 deletions
diff --git a/input/day12.txt b/input/day12.txt
new file mode 100644
index 0000000..b74bbbc
--- /dev/null
+++ b/input/day12.txt
@@ -0,0 +1,23 @@
+cpy 1 a
+cpy 1 b
+cpy 26 d
+jnz c 2
+jnz 1 5
+cpy 7 c
+inc d
+dec c
+jnz c -2
+cpy a c
+inc a
+dec b
+jnz b -2
+cpy c b
+dec d
+jnz d -6
+cpy 16 c
+cpy 12 d
+inc a
+dec d
+jnz d -2
+dec c
+jnz c -5
diff --git a/src/bin/day12.rs b/src/bin/day12.rs
new file mode 100644
index 0000000..64b09f8
--- /dev/null
+++ b/src/bin/day12.rs
@@ -0,0 +1,135 @@
+use std::io::{self, Read};
+use std::str::FromStr;
+
+#[derive(Clone, Copy)]
+enum Operand {
+    Register(u8),
+    Immediate(i32),
+}
+
+impl FromStr for Operand {
+    type Err = ();
+    fn from_str(s: &str) -> Result<Self, ()> {
+        match s {
+            "a" => Ok(Operand::Register(0)),
+            "b" => Ok(Operand::Register(1)),
+            "c" => Ok(Operand::Register(2)),
+            "d" => Ok(Operand::Register(3)),
+            _ => Ok(Operand::Immediate(s.parse().map_err(|_| ())?)),
+        }
+    }
+}
+
+#[derive(Clone, Copy)]
+enum Operation {
+    Cpy(Operand, Operand),
+    Inc(Operand),
+    Dec(Operand),
+    Jnz(Operand, Operand),
+}
+
+impl FromStr for Operation {
+    type Err = ();
+    fn from_str(s: &str) -> Result<Self, ()> {
+        let mut iter = s.split(' ');
+        match iter.next() {
+            Some("cpy") => {
+                let src = iter.next().ok_or(())?.parse()?;
+                let dest = iter.next().ok_or(())?.parse()?;
+                Ok(Operation::Cpy(src, dest))
+            },
+            Some("inc") => Ok(Operation::Inc(iter.next().ok_or(())?.parse()?)),
+            Some("dec") => Ok(Operation::Dec(iter.next().ok_or(())?.parse()?)),
+            Some("jnz") => {
+                let cond = iter.next().ok_or(())?.parse()?;
+                let jump = iter.next().ok_or(())?.parse()?;
+                Ok(Operation::Jnz(cond, jump))
+            },
+            _ => Err(()),
+        }
+    }
+}
+
+#[derive(Default)]
+struct Vm {
+    registers: [i32; 4],
+    operations: Vec<Operation>,
+    index: i32,
+}
+
+impl FromStr for Vm {
+    type Err = ();
+    fn from_str(s: &str) -> Result<Self, ()> {
+        let mut vm = Self::default();
+        for line in s.lines() {
+            vm.operations.push(line.parse()?);
+        }
+        Ok(vm)
+    }
+}
+
+impl Vm {
+    fn step(&mut self) -> bool {
+        match self.operations[self.index as usize] {
+            Operation::Cpy(Operand::Immediate(imm), Operand::Register(reg)) => {
+                self.registers[reg as usize] = imm;
+                self.index += 1;
+            },
+            Operation::Cpy(Operand::Register(src), Operand::Register(dest)) => {
+                self.registers[dest as usize] = self.registers[src as usize];
+                self.index += 1;
+            },
+            Operation::Inc(Operand::Register(reg)) => {
+                self.registers[reg as usize] += 1;
+                self.index += 1;
+            },
+            Operation::Dec(Operand::Register(reg)) => {
+                self.registers[reg as usize] -= 1;
+                self.index += 1;
+            },
+            Operation::Jnz(Operand::Immediate(cond), Operand::Immediate(jump)) => {
+                if cond != 0 {
+                    self.index += jump;
+                } else {
+                    self.index += 1;
+                }
+            },
+            Operation::Jnz(Operand::Register(reg), Operand::Immediate(jump)) => {
+                if self.registers[reg as usize] != 0 {
+                    self.index += jump;
+                } else {
+                    self.index += 1;
+                }
+            },
+            _ => panic!("invalid operation"),
+        }
+
+        (self.index as usize) < self.operations.len()
+    }
+}
+
+fn solve(input: &str) -> i32 {
+    let mut vm: Vm = input.parse().unwrap();
+    while vm.step() { }
+    vm.registers[0]
+}
+
+fn main() {
+    let mut input = String::new();
+    io::stdin().read_to_string(&mut input).unwrap();
+
+    println!("Part 1: {}", solve(&input));
+}
+
+#[test]
+fn part1() {
+    let input = "
+cpy 41 a
+inc a
+inc a
+dec a
+jnz a 2
+dec a
+";
+    assert_eq!(42, solve(input.trim()));
+}