add day 7 solution

solved by claude.ai

day07/Cargo.toml

@@ -2,5 +2,3 @@
 name = "day07"
 version = "0.1.0"
 edition = "2024"
-
-[dependencies]

day07/src/main.rs

@@ -1,5 +1,143 @@
+use std::collections::HashMap;
+
+// An operand is either a hard-coded 16-bit value or a reference to another wire.
+#[derive(Debug, Clone)]
+enum Operand {
+    Literal(u16),
+    Wire(String),
+}
+
+// Each gate variant carries its operand(s) as parsed from the instruction.
+#[derive(Debug, Clone)]
+enum Gate {
+    Signal(Operand),
+    Not(Operand),
+    And(Operand, Operand),
+    Or(Operand, Operand),
+    LShift(Operand, u16),
+    RShift(Operand, u16),
+}
+
+type Circuit = HashMap<String, Gate>;
+
+// ── Parsing ────────────────────────────────────────────────────────────────────
+
+fn parse_operand(s: &str) -> Operand {
+    match s.parse::<u16>() {
+        Ok(n) => Operand::Literal(n),
+        Err(_) => Operand::Wire(s.to_string()),
+    }
+}
+
+fn parse_line(line: &str) -> (String, Gate) {
+    let (expr, target) = line
+        .split_once(" -> ")
+        .expect("every line must contain ' -> '");
+
+    let tokens: Vec<&str> = expr.split_whitespace().collect();
+
+    let gate = match tokens.as_slice() {
+        [a] => Gate::Signal(parse_operand(a)),
+        ["NOT", a] => Gate::Not(parse_operand(a)),
+        [a, "AND", b] => Gate::And(parse_operand(a), parse_operand(b)),
+        [a, "OR", b] => Gate::Or(parse_operand(a), parse_operand(b)),
+        [a, "LSHIFT", n] => Gate::LShift(parse_operand(a), n.parse().expect("shift amount")),
+        [a, "RSHIFT", n] => Gate::RShift(parse_operand(a), n.parse().expect("shift amount")),
+        _ => panic!("unrecognised expression: {expr}"),
+    };
+
+    (target.to_string(), gate)
+}
+
+// ── Evaluation ─────────────────────────────────────────────────────────────────
+
+// Resolves a single operand: literals are returned as-is, wire names recurse
+// into `evaluate`.
+fn resolve(op: &Operand, circuit: &Circuit, cache: &mut HashMap<String, u16>) -> u16 {
+    match op {
+        Operand::Literal(n) => *n,
+        Operand::Wire(name) => evaluate(name, circuit, cache),
+    }
+}
+
+// Returns the 16-bit signal on `wire`, computing it recursively if not yet
+// cached.  The gate is cloned before recursing so that the immutable borrow
+// of `circuit` is released before we mutably borrow `cache` deeper in the
+// call stack — keeping the borrow checker happy without needing unsafe code.
+fn evaluate(wire: &str, circuit: &Circuit, cache: &mut HashMap<String, u16>) -> u16 {
+    if let Some(&val) = cache.get(wire) {
+        return val;
+    }
+
+    let gate = circuit
+        .get(wire)
+        .unwrap_or_else(|| panic!("unknown wire: {wire}"))
+        .clone();
+
+    let val = match &gate {
+        Gate::Signal(a) => resolve(a, circuit, cache),
+        Gate::Not(a) => !resolve(a, circuit, cache),
+        Gate::And(a, b) => resolve(a, circuit, cache) & resolve(b, circuit, cache),
+        Gate::Or(a, b) => resolve(a, circuit, cache) | resolve(b, circuit, cache),
+        Gate::LShift(a, n) => resolve(a, circuit, cache) << n,
+        Gate::RShift(a, n) => resolve(a, circuit, cache) >> n,
+    };
+
+    cache.insert(wire.to_string(), val);
+    val
+}
+
+// ── Entry point ────────────────────────────────────────────────────────────────
+
 fn main() {
-    println!("Hello, world!");
+    let input = std::fs::read_to_string("input.txt").expect("could not read input.txt");
+
+    let circuit: Circuit = input
+        .lines()
+        .filter(|l| !l.is_empty())
+        .map(parse_line)
+        .collect();
+
+    let mut cache: HashMap<String, u16> = HashMap::new();
+    println!("{}", evaluate("a", &circuit, &mut cache));
+}
+
+// ── Tests ──────────────────────────────────────────────────────────────────────
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn build_example_circuit() -> Circuit {
+        let src = "\
+123 -> x
+456 -> y
+x AND y -> d
+x OR y -> e
+x LSHIFT 2 -> f
+y RSHIFT 2 -> g
+NOT x -> h
+NOT y -> i";
+        src.lines().map(parse_line).collect()
+    }
+
+    #[test]
+    fn test_example_signals() {
+        let circuit = build_example_circuit();
+        let mut cache = HashMap::new();
+
+        // Evaluate every wire once; the cache is shared across all calls.
+        let get = |w: &str, cache: &mut HashMap<String, u16>| evaluate(w, &circuit, cache);
+
+        assert_eq!(get("d", &mut cache), 72);
+        assert_eq!(get("e", &mut cache), 507);
+        assert_eq!(get("f", &mut cache), 492);
+        assert_eq!(get("g", &mut cache), 114);
+        assert_eq!(get("h", &mut cache), 65412);
+        assert_eq!(get("i", &mut cache), 65079);
+        assert_eq!(get("x", &mut cache), 123);
+        assert_eq!(get("y", &mut cache), 456);
+    }
 }
 
 // https://adventofcode.com/2015/day/7