1 files changed, 129 insertions, 0 deletions
diff --git a/d12/src/main.rs b/d12/src/main.rs
new file mode 100644
index 0000000..d49ec45
--- /dev/null
+++ b/d12/src/main.rs
@@ -0,0 +1,129 @@
+use std::fs;
+use std::collections::HashMap;
+
+enum Oper<'a> {
+    IncReg,
+    DecReg,
+    CpyReg(&'a str),
+    CpyLit(isize),
+    JnzReg(&'a str, isize),
+    JnzLit(isize,   isize),
+}
+
+fn parse(lines: Vec<&str>) -> Vec<(Oper, &str)> {
+    // We emit tuples of type `(Oper, &str)', where the second field
+    // is the destination register. Storing the destination this way,
+    // instead of inside the `Oper' enum, helps keep `execute()' DRY.
+    let mut insts = Vec::new();
+
+    for line in lines {
+        let words: Vec<&str> = line.split_ascii_whitespace().collect();
+
+        if words[0] == "inc" {
+            // words[1] is register name
+            insts.push((Oper::IncReg, words[1]));
+        } else if words[0] == "dec" {
+            // words[1] is register name
+            insts.push((Oper::DecReg, words[1]));
+        } else if words[0] == "cpy" {
+            // words[1] is source, can be literal number or register name
+            // words[2] is destination register name
+            if words[1].chars().all(|c| c.is_ascii_digit() || c == '-') { // yes I know
+                let val = words[1].parse().unwrap();
+                insts.push((Oper::CpyLit(val), words[2]));
+            } else {
+                insts.push((Oper::CpyReg(words[1]), words[2]));
+            }
+        } else if words[0] == "jnz" {
+            // words[1] is value, can be literal number or register name
+            // words[2] is offset to jump by if branch is taken
+            let off = words[2].parse().unwrap();
+            if words[1].chars().all(|c| c.is_ascii_digit() || c == '-') { // I know
+                let val = words[1].parse().unwrap();
+                insts.push((Oper::JnzLit(val, off), "_"));
+            } else {
+                insts.push((Oper::JnzReg(words[1], off), "_"));
+            }
+        }
+    }
+
+    insts
+}
+
+fn execute(insts: &Vec<(Oper, &str)>, init: Vec<(&str, isize)>) -> isize {
+    // `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() {
+        // Get destination register now, because we'll mess with `ip' later
+        let dst = insts[ip].1;
+
+        // Too early for mutable borrow of `state', so write to `tmp' instead
+        let mut tmp = state.get(dst).copied().unwrap_or(0);
+        match insts[ip].0 {
+            Oper::IncReg => {
+                tmp += 1;
+            },
+            Oper::DecReg => {
+                tmp -= 1;
+            },
+            Oper::CpyLit(val) => {
+                tmp = val;
+            },
+            Oper::CpyReg(src) => {
+                let val = state.get(&src).copied().unwrap_or(0);
+                tmp = val;
+            },
+            Oper::JnzLit(val, off) => {
+                if val != 0 {
+                    // Subtract 1 to compensate for `ip += 1;' below
+                    ip = ((ip as isize) + off - 1) as usize;
+                }
+            },
+            Oper::JnzReg(src, off) => {
+                let val = state.get(&src).copied().unwrap_or(0);
+                if val != 0 {
+                    // Subtract 1 to compensate for `ip += 1;' below
+                    ip = ((ip as isize) + off - 1) as usize;
+                }
+            },
+        };
+
+        // "_" is a dummy destination register used by the "jnz" instructions
+        if dst != "_" {
+            *state.entry(dst).or_insert(0) = tmp;
+        }
+
+        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 318117 for my input
+    println!("Part 1 solution: {}", execute(&insts, vec![]));
+
+    // Part 2 gives 9227771 for my input
+    println!("Part 2 solution: {}", execute(&insts, vec![("c", 1)]));
+}
+
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn part1_example1() {
+        let lines = vec!["cpy 41 a", "inc a", "inc a", "dec a", "jnz a 2", "dec a"];
+        let insts = parse(lines);
+        assert_eq!(execute(&insts, vec![]), 42);
+    }
+}