1 files changed, 149 insertions, 0 deletions
diff --git a/d23/src/main.rs b/d23/src/main.rs
new file mode 100644
index 0000000..234ebb6
--- /dev/null
+++ b/d23/src/main.rs
@@ -0,0 +1,149 @@
+use std::collections::HashMap;
+use std::fs;
+
+// Compared to day 12, for day 23 it makes more sense to abstract
+// over operands in this way; they could be registers or literals.
+#[derive(Copy, Clone, Debug)]
+enum Arg<'a> {
+    Reg(&'a str),
+    Lit(isize),
+}
+
+// It's convenient to use the `From' trait for parsing operands
+impl<'a> From<&'a str> for Arg<'a> {
+    fn from(s: &'a str) -> Self {
+        if s.chars().all(|c| c.is_ascii_digit() || c == '-') { // yes I know
+            Self::Lit(s.parse().unwrap())
+        } else {
+            Self::Reg(s)
+        }
+    }
+}
+
+#[derive(Copy, Clone, Debug)]
+enum Oper<'a> {
+    Inc(Arg<'a>),
+    Dec(Arg<'a>),
+    Tgl(Arg<'a>),
+    Cpy(Arg<'a>, Arg<'a>),
+    Jnz(Arg<'a>, Arg<'a>),
+}
+
+fn parse(lines: Vec<&str>) -> Vec<Oper> {
+    let mut insts = Vec::new();
+
+    for line in lines {
+        let words: Vec<&str> = line.split_ascii_whitespace().collect();
+
+        if words[0] == "inc" {
+            insts.push(Oper::Inc(Arg::from(words[1])));
+        } else if words[0] == "dec" {
+            insts.push(Oper::Dec(Arg::from(words[1])));
+        } else if words[0] == "tgl" {
+            insts.push(Oper::Tgl(Arg::from(words[1])));
+        } else if words[0] == "cpy" {
+            insts.push(Oper::Cpy(Arg::from(words[1]), Arg::from(words[2])));
+        } else if words[0] == "jnz" {
+            insts.push(Oper::Jnz(Arg::from(words[1]), Arg::from(words[2])));
+        }
+    }
+
+    insts
+}
+
+fn execute(program: &Vec<Oper>, init: Vec<(&str, isize)>) -> isize {
+    // Make a local mutable copy of the program
+    let mut insts = program.clone();
+
+    // `init' contains initial register values
+    let mut state: HashMap<&str, isize> = init.into_iter().collect();
+
+    let mut ip = 0; // instruction pointer
+    while ip < insts.len() {
+        match insts[ip] {
+            Oper::Inc(arg) => {
+                if let Arg::Reg(dst) = arg {
+                    *state.entry(dst).or_insert(0) += 1;
+                }
+            },
+            Oper::Dec(arg) => {
+                if let Arg::Reg(dst) = arg {
+                    *state.entry(dst).or_insert(0) -= 1;
+                }
+            },
+            Oper::Tgl(arg) => {
+                let off = match arg {
+                    Arg::Reg(reg) => state.get(&reg).copied().unwrap_or(0),
+                    Arg::Lit(lit) => lit,
+                };
+                // Calculate index of target instruction
+                let t = ((ip as isize) + off) as usize;
+                if t < insts.len() {
+                    insts[t] = match insts[t] {
+                        Oper::Inc(a)      => Oper::Dec(a),
+                        Oper::Dec(a)      => Oper::Inc(a),
+                        Oper::Tgl(a)      => Oper::Inc(a),
+                        Oper::Cpy(a1, a2) => Oper::Jnz(a1, a2),
+                        Oper::Jnz(a1, a2) => Oper::Cpy(a1, a2),
+                    };
+                }
+            },
+            Oper::Cpy(arg1, arg2) => {
+                // If destination is a literal (due to `Tgl'), ignore this `Cpy'
+                if let Arg::Reg(dst) = arg2 {
+                    let val = match arg1 {
+                        Arg::Reg(src) => state.get(&src).copied().unwrap_or(0),
+                        Arg::Lit(lit) => lit,
+                    };
+                    *state.entry(dst).or_insert(0) = val;
+                }
+            },
+            Oper::Jnz(arg1, arg2) => {
+                let val = match arg1 {
+                    Arg::Reg(reg) => state.get(&reg).copied().unwrap_or(0),
+                    Arg::Lit(lit) => lit,
+                };
+                // Should the branch be taken? If yes:
+                if val != 0 {
+                    let off = match arg2 {
+                        Arg::Reg(reg) => state.get(&reg).copied().unwrap_or(0),
+                        Arg::Lit(lit) => lit,
+                    };
+                    // Subtract 1 to compensate for `ip += 1;' below
+                    ip = ((ip as isize) + off - 1) as usize;
+                }
+            },
+        };
+
+        ip += 1;
+    }
+
+    state.get("a").copied().unwrap()
+}
+
+fn main() {
+    // Read assembly from input text file and parse it
+    let input = fs::read_to_string("input.txt").unwrap();
+    let lines = input.lines().collect();
+    let insts = parse(lines);
+
+    // Part 1 gives 14065 for my input
+    println!("Part 1 solution: {}", execute(&insts, vec![("a", 7)]));
+
+    // Part 2 gives 479010625 for my input
+    println!("Part 2 solution: {}", execute(&insts, vec![("a", 12)]));
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn part1_example1() {
+        let lines = vec![
+            "cpy 2 a", "tgl a", "tgl a", "tgl a", "cpy 1 a", "dec a", "dec a"
+        ];
+        let insts = parse(lines);
+        assert_eq!(execute(&insts, vec![]), 3);
+    }
+}