1 files changed, 137 insertions, 0 deletions
diff --git a/2018/src/bin/day_21.rs b/2018/src/bin/day_21.rs
new file mode 100644
index 0000000..0d9a97c
--- /dev/null
+++ b/2018/src/bin/day_21.rs
@@ -0,0 +1,137 @@
+extern crate advent_of_code_2018;
+use advent_of_code_2018::*;
+
+use std::error::Error;
+use std::path::PathBuf;
+use std::u32;
+
+use std::collections::HashMap;
+// cargo watch -cs "cargo run --release --bin day_21"
+
+use std::i32;
+
+#[derive(Debug)]
+struct Instruction {
+    op: Op,
+    a: i32,
+    b: i32,
+    c: i32
+}
+
+impl Instruction {
+    fn execute(&self, counter_register: usize, registers: &mut [i32; 6]) -> i32{
+        use Op::*;
+        
+        registers[self.c as usize] = match self.op {
+            Addr => registers[self.a as usize].wrapping_add(registers[self.b as usize]),
+            Addi => registers[self.a as usize].wrapping_add(self.b),
+            Mulr => registers[self.a as usize].wrapping_mul(registers[self.b as usize]),
+            Muli => registers[self.a as usize].wrapping_mul(self.b),
+            Banr => registers[self.a as usize] & registers[self.b as usize],
+            Bani => registers[self.a as usize] & self.b,
+            Borr => registers[self.a as usize] | registers[self.b as usize],
+            Bori => registers[self.a as usize] | self.b,
+            Setr => registers[self.a as usize],
+            Seti => self.a,
+            Gtir => if self.a > registers[self.b as usize] { 1 } else { 0 },
+            Gtri => if registers[self.a as usize] > self.b { 1 } else { 0 },
+            Gtrr => if registers[self.a as usize] > registers[self.b as usize] { 1 } else { 0 },
+            Eqir => if self.a == registers[self.b as usize] { 1 } else { 0 },
+            Eqri => if registers[self.a as usize] == self.b { 1 } else { 0 },
+            Eqrr => if registers[self.a as usize] == registers[self.b as usize] { 1 } else { 0 },
+        };
+
+        registers[counter_register]+1
+    }
+}
+
+#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
+enum Op {
+    Addr,
+    Addi,
+    Mulr,
+    Muli,
+    Banr,
+    Bani,
+    Borr,
+    Bori,
+    Setr,
+    Seti,
+    Gtir,
+    Gtri,
+    Gtrr,
+    Eqir,
+    Eqri,
+    Eqrr
+}
+
+
+fn main() -> Result<(), Box<Error>> {
+    let input = read_file(&PathBuf::from("inputs/21.txt"))?;
+
+    let mut input_iter = input.iter();
+    let counter_register: usize = input_iter.next()
+        .map(|line| line.split_whitespace().nth(1).unwrap().parse().unwrap())
+        .unwrap();
+
+    let instructions: Vec<Instruction> = input_iter
+        .map(|line| {
+            let mut instruction_iter = line.split_whitespace();
+            Instruction {
+                op: match instruction_iter.next().unwrap() {
+                    "addr" => Op::Addr,
+                    "addi" => Op::Addi,
+                    "mulr" => Op::Mulr,
+                    "muli" => Op::Muli,
+                    "banr" => Op::Banr,
+                    "bani" => Op::Bani,
+                    "borr" => Op::Borr,
+                    "bori" => Op::Bori,
+                    "setr" => Op::Setr,
+                    "seti" => Op::Seti,
+                    "gtir" => Op::Gtir,
+                    "gtri" => Op::Gtri,
+                    "gtrr" => Op::Gtrr,
+                    "eqir" => Op::Eqir,
+                    "eqri" => Op::Eqri,
+                    "eqrr" => Op::Eqrr,
+                    _ => panic!("unknown instruction")
+                },
+                a: instruction_iter.next().unwrap().parse().unwrap(),
+                b: instruction_iter.next().unwrap().parse().unwrap(),
+                c: instruction_iter.next().unwrap().parse().unwrap(),
+            }
+        })
+        .collect();
+
+    execute_program(&instructions, counter_register, [0, 0, 0, 0, 0, 0]);
+
+    Ok(())
+}
+
+fn execute_program(instructions: &Vec<Instruction>, counter_register: usize, registers: [i32; 6]) {
+    let mut instructions_executed: u64 = 0;
+    let mut counter_val: i32 = 0;
+    let mut registers = registers.clone();
+    let mut halting_values = HashMap::new();
+    
+    while counter_val >= 0 && counter_val < instructions.len() as i32 {
+        if counter_val == 28 {
+            if halting_values.contains_key(&registers[4]) {
+                break;
+            }
+            halting_values.entry(registers[4]).or_insert(instructions_executed+2);
+        }
+        registers[counter_register] = counter_val;
+        counter_val = instructions[counter_val as usize].execute(counter_register, &mut registers);
+        instructions_executed += 1;
+    }
+
+    let (min_halting, min_halting_time) = halting_values.iter().min_by_key(|&(_, v)| v).unwrap();
+    debug!(min_halting);
+    debug!(min_halting_time);
+    let (max_halting, max_halting_time) = halting_values.iter().max_by_key(|&(_, v)| v).unwrap();
+    debug!(max_halting);
+    debug!(max_halting_time);
+    
+}