3 files changed, 72 insertions, 2 deletions
diff --git a/2018/src/day21.rs b/2018/src/day21.rs
new file mode 100644
index 0000000..e0983a3
--- /dev/null
+++ b/2018/src/day21.rs
@@ -0,0 +1,68 @@
+use std::io;
+use std::io::BufRead;
+use std::collections::HashSet;
+use crate::device::*;
+
+#[allow(dead_code)]
+fn optimise(mut instrs: Vec<Instr>, ip_reg: usize) -> Vec<Instr> {
+    for i in 0..instrs.len() {
+        instrs[i].inline_reg(ip_reg, Oper::Num(i));
+    }
+
+    instrs
+}
+
+#[allow(dead_code)]
+fn run_program(instrs: &[Instr], r0: usize, ip_reg: usize) -> usize {
+    let mut m = Machine { regs: [r0, 0, 0, 0, 0, 0] };
+    m.exec_program(&instrs, ip_reg);
+    m.regs[0]
+}
+
+pub fn main<T: BufRead>(reader: T) -> io::Result<(String, String)> {
+    let (ip_reg, instrs) = parse_program(reader)?;
+
+    let instrs = optimise(instrs, ip_reg);
+
+    // for instr in &instrs { println!("{}", instr); }
+
+    let part1;
+
+    let mut ip = 0;
+    let mut m = Machine { regs: [0, 0, 0, 0, 0, 0] };
+    loop {
+        assert!(ip < instrs.len());
+        if ip == 28 {
+            part1 = m.regs[4];
+            break;
+        }
+        m.regs[ip_reg] = ip;
+        m.exec(&instrs[ip]);
+        ip = m.regs[ip_reg] + 1;
+    }
+
+    let mut part2 = None;
+
+    let mut seen_machines = HashSet::new();
+    let mut seen_values = HashSet::new();
+
+    loop {
+        assert!(ip < instrs.len());
+        if ip == 28 {
+            // println!("{:?}", m);
+            if seen_machines.contains(&m) {
+                break;
+            }
+            seen_machines.insert(m.clone());
+            if !seen_values.contains(&m.regs[4]) {
+                part2 = Some(m.regs[4]);
+                seen_values.insert(m.regs[4]);
+            }
+        }
+        m.regs[ip_reg] = ip;
+        m.exec(&instrs[ip]);
+        ip = m.regs[ip_reg] + 1;
+    }
+
+    Ok((part1.to_string(), part2.unwrap().to_string()))
+}
diff --git a/2018/src/device.rs b/2018/src/device.rs
index 7eaf8d3..cf6d94e 100644
--- a/2018/src/device.rs
+++ b/2018/src/device.rs
@@ -181,7 +181,7 @@ impl fmt::Display for Instr {
     }
 }
 
-#[derive(PartialEq, Eq, Clone)]
+#[derive(PartialEq, Eq, Clone, Hash, Debug)]
 pub struct Machine {
     pub regs: [usize; 6],
 }
diff --git a/2018/src/main.rs b/2018/src/main.rs
index 34c3964..fa1cab8 100644
--- a/2018/src/main.rs
+++ b/2018/src/main.rs
@@ -26,8 +26,9 @@ mod day17;
 mod day18;
 mod day19;
 mod day20;
+mod day21;
 
-static NUM_DAYS: i32 = 20;
+static NUM_DAYS: i32 = 21;
 
 fn day_switch<T: BufRead>(day: i32, reader: T) -> io::Result<(String, String)> {
     match day {
@@ -51,6 +52,7 @@ fn day_switch<T: BufRead>(day: i32, reader: T) -> io::Result<(String, String)> {
         18 => day18::main(reader),
         19 => day19::main(reader),
         20 => day20::main(reader),
+        21 => day21::main(reader),
         _ => Err(Error::new(ErrorKind::Other, "Invalid day"))
     }
 }