2024 day 24

Author: ictrobot

crates/year2024/src/day24.rs

@@ -0,0 +1,337 @@
+use std::collections::HashMap;
+use std::ops::ControlFlow;
+use utils::prelude::*;
+
+/// Finding swapped logic gates in an adder circuit.
+#[derive(Clone, Debug)]
+pub struct Day24 {
+    wires: Vec<Wire>,
+    wire_names: Vec<[u8; 3]>,
+    x_initial: u64,
+    y_initial: u64,
+    z_indexes: Vec<usize>,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+enum Wire {
+    X(usize),
+    Y(usize),
+    And(usize, usize),
+    Or(usize, usize),
+    Xor(usize, usize),
+}
+
+impl Day24 {
+    pub fn new(input: &str, _: InputType) -> Result<Self, InputError> {
+        let Some((initial_str, gate_str)) = input.split_once("\n\n") else {
+            return Err(InputError::new(input, 0, "expected inputs and gates"));
+        };
+
+        let mut wires = Vec::new();
+        let mut wire_names = Vec::new();
+        let mut indexes = HashMap::new();
+        let mut x_initial = 0;
+        let mut y_initial = 0;
+        let mut input_bits = 64;
+
+        let mut next = (b'x', 0);
+        for item in parser::byte_range(b'x'..=b'y')
+            .then(parser::byte_range(b'0'..=b'9'))
+            .then(parser::byte_range(b'0'..=b'9'))
+            .with_suffix(": ")
+            .then(parser::byte_range(b'0'..=b'1'))
+            .with_suffix(parser::eol())
+            .parse_iterator(initial_str)
+        {
+            let (wire, b) = item?;
+            let n = ((wire.1 - b'0') * 10 + (wire.2 - b'0')) as usize;
+
+            if (wire.0, n) != next {
+                if next.0 == b'x' && wire == (b'y', b'0', b'0') {
+                    input_bits = next.1;
+                } else {
+                    return Err(InputError::new(input, 0, "unexpected initial value"));
+                }
+            }
+
+            if wire.0 == b'x' {
+                x_initial |= u64::from(b == b'1') << n;
+                wires.push(Wire::X(n));
+            } else {
+                y_initial |= u64::from(b == b'1') << n;
+                wires.push(Wire::Y(n));
+            }
+            wire_names.push(wire.into());
+            indexes.insert(wire.into(), wires.len() - 1);
+
+            if n == input_bits - 1 {
+                next = (b'?', 0);
+            } else {
+                next = (wire.0, n + 1);
+            }
+        }
+
+        let mut z_indexes = vec![usize::MAX; input_bits + 1];
+        let wire = parser::byte().repeat_n::<3, _>(parser::noop());
+        for item in wire
+            .then(parser::literal_map!(
+                " AND " => Wire::And as fn(usize, usize) -> Wire,
+                " OR " => Wire::Or,
+                " XOR " => Wire::Xor,
+            ))
+            .then(wire.with_suffix(" -> "))
+            .then(wire)
+            .with_suffix(parser::eol())
+            .parse_iterator(gate_str)
+        {
+            let (in1, gate, in2, out) = item?;
+
+            let mut index_of = |n| {
+                *indexes.entry(n).or_insert_with(|| {
+                    wires.push(Wire::X(usize::MAX)); // Placeholder
+                    wire_names.push(n);
+                    wires.len() - 1
+                })
+            };
+
+            let in1_index = index_of(in1);
+            let in2_index = index_of(in2);
+            let out_index = index_of(out);
+
+            if wires[out_index] != Wire::X(usize::MAX) {
+                return Err(InputError::new(input, 0, "duplicate wire definition"));
+            }
+            if out[0] == b'z' {
+                let index = ((out[1] - b'0') * 10 + (out[2] - b'0')) as usize;
+                if index < z_indexes.len() {
+                    z_indexes[index] = out_index;
+                } else {
+                    return Err(InputError::new(input, 0, "too many z outputs"));
+                }
+            }
+
+            wires[out_index] = gate(in1_index, in2_index);
+        }
+
+        if wires.contains(&Wire::X(usize::MAX)) {
+            return Err(InputError::new(input, 0, "undefined wire"));
+        }
+        if z_indexes.contains(&usize::MAX) {
+            return Err(InputError::new(input, 0, "undefined z output"));
+        }
+
+        Ok(Self {
+            wires,
+            wire_names,
+            x_initial,
+            y_initial,
+            z_indexes,
+        })
+    }
+
+    #[must_use]
+    pub fn part1(&self) -> u64 {
+        let mut z = 0;
+        let mut cache = vec![None; self.wires.len()];
+        for (i, &index) in self.z_indexes.iter().enumerate() {
+            z |= u64::from(Self::evaluate(
+                index,
+                &self.wires,
+                self.x_initial,
+                self.y_initial,
+                &mut cache,
+            )) << i;
+        }
+        z
+    }
+
+    fn evaluate(index: usize, wires: &[Wire], x: u64, y: u64, cache: &mut [Option<bool>]) -> bool {
+        if let Some(c) = cache[index] {
+            return c;
+        }
+        let v = match wires[index] {
+            Wire::X(n) => return x & (1 << n) != 0,
+            Wire::Y(n) => return y & (1 << n) != 0,
+            Wire::And(a, b) => {
+                Self::evaluate(a, wires, x, y, cache) && Self::evaluate(b, wires, x, y, cache)
+            }
+            Wire::Or(a, b) => {
+                Self::evaluate(a, wires, x, y, cache) || Self::evaluate(b, wires, x, y, cache)
+            }
+            Wire::Xor(a, b) => {
+                Self::evaluate(a, wires, x, y, cache) ^ Self::evaluate(b, wires, x, y, cache)
+            }
+        };
+        cache[index] = Some(v);
+        v
+    }
+
+    #[must_use]
+    pub fn part2(&self) -> String {
+        let mut test_cases = Vec::new();
+        for i in 0..self.z_indexes.len() - 1 {
+            test_cases.push((i, 1u64 << i, 0u64));
+            test_cases.push((i, 0u64, 1u64 << i));
+            test_cases.push((i + 1, 1u64 << i, 1u64 << i));
+            test_cases.push((i + 1, (1u64 << (i + 1)) - 1, 1u64));
+            test_cases.push((i + 1, 1u64, (1u64 << (i + 1)) - 1));
+        }
+
+        let mut wires = self.wires.clone();
+        if self.find_swaps(&test_cases, &mut wires, 0).is_continue() {
+            panic!("failed to find working combination");
+        }
+
+        let mut changes = Vec::new();
+        for (i, (&wire, &orig)) in wires.iter().zip(&self.wires).enumerate() {
+            if wire != orig {
+                changes.push(self.wire_names[i]);
+            }
+        }
+        assert_eq!(changes.len(), 8, "found incorrect number of changes");
+
+        changes.sort_unstable();
+        changes.into_iter().fold(String::new(), |mut acc, name| {
+            if !acc.is_empty() {
+                acc.push(',');
+            }
+            acc.push(name[0] as char);
+            acc.push(name[1] as char);
+            acc.push(name[2] as char);
+            acc
+        })
+    }
+
+    fn find_swaps(
+        &self,
+        test_cases: &[(usize, u64, u64)],
+        wires: &mut [Wire],
+        assume: usize,
+    ) -> ControlFlow<()> {
+        let mut cache = vec![None; wires.len()];
+        let mut used = vec![false; wires.len()];
+        for &(n, x, y) in test_cases {
+            cache.fill(None);
+            let sum = x + y;
+            for i in 0..n {
+                let b = Self::evaluate(self.z_indexes[i], wires, x, y, &mut cache);
+                if ((sum >> i) & 1 != 0) != b {
+                    // Previous swap broke even earlier bit
+                    return ControlFlow::Continue(());
+                }
+            }
+
+            for i in 0..wires.len() {
+                used[i] = cache[i].is_some();
+            }
+
+            let b = Self::evaluate(self.z_indexes[n], wires, x, y, &mut cache);
+            if ((sum >> n) & 1 != 0) == b {
+                continue;
+            }
+            if n < assume {
+                // Previous swap didn't fix the bit it was trying to fix
+                return ControlFlow::Continue(());
+            }
+
+            // Found bit with a wrong gate
+
+            // Gates which were used for the first time this bit
+            let candidates1 = cache
+                .iter()
+                .zip(&used)
+                .enumerate()
+                .filter(|&(_, (&c, &u))| c.is_some() && !u)
+                .map(|(i, _)| i)
+                .collect::<Vec<_>>();
+            // Gates which weren't used previously and only contain current bits
+            let mask = (1 << (n + 1)) - 1;
+            let candidates2 = Self::used_bits(wires)
+                .iter()
+                .zip(&used)
+                .enumerate()
+                .filter(|(_, (&b, &u))| b != 0 && b & mask == b && !u)
+                .map(|(i, _)| i)
+                .collect::<Vec<_>>();
+
+            // Try swapping each combination of candidates
+            for &c1 in &candidates1 {
+                for &c2 in &candidates2 {
+                    if c1 == c2 {
+                        continue;
+                    }
+
+                    (wires[c1], wires[c2]) = (wires[c2], wires[c1]);
+
+                    if !self.loops(wires) {
+                        if let ControlFlow::Break(()) = self.find_swaps(test_cases, wires, n + 1) {
+                            return ControlFlow::Break(());
+                        }
+                    }
+
+                    // Failed, swap back and try next combination
+                    (wires[c1], wires[c2]) = (wires[c2], wires[c1]);
+                }
+            }
+
+            // No combinations worked, previous swap must be wrong
+            return ControlFlow::Continue(());
+        }
+
+        ControlFlow::Break(())
+    }
+
+    fn used_bits(wires: &[Wire]) -> Vec<u64> {
+        fn eval(index: usize, wires: &[Wire], used_bits: &mut [u64]) -> u64 {
+            if used_bits[index] != 0 {
+                return used_bits[index];
+            }
+            let v = match wires[index] {
+                Wire::X(n) => return 1 << n,
+                Wire::Y(n) => return 1 << n,
+                Wire::And(a, b) | Wire::Or(a, b) | Wire::Xor(a, b) => {
+                    eval(a, wires, used_bits) | eval(b, wires, used_bits)
+                }
+            };
+            used_bits[index] = v;
+            v
+        }
+
+        let mut used = vec![0; wires.len()];
+        for i in 0..wires.len() {
+            eval(i, wires, &mut used);
+        }
+        used
+    }
+
+    fn loops(&self, wires: &[Wire]) -> bool {
+        fn eval(index: usize, wires: &[Wire], checked: &mut [bool], depth: usize) -> bool {
+            if checked[index] {
+                return false;
+            }
+            if depth > wires.len() {
+                return true;
+            }
+            match wires[index] {
+                Wire::X(_) | Wire::Y(_) => {}
+                Wire::And(a, b) | Wire::Or(a, b) | Wire::Xor(a, b) => {
+                    if eval(a, wires, checked, depth + 1) || eval(b, wires, checked, depth + 1) {
+                        return true;
+                    }
+                }
+            }
+            checked[index] = true;
+            false
+        }
+
+        let mut checked = vec![false; wires.len()];
+        for i in 0..wires.len() {
+            if eval(i, wires, &mut checked, 0) {
+                return true;
+            }
+        }
+        false
+    }
+}
+
+examples!(Day24 -> (u64, &'static str) []);

crates/year2024/src/lib.rs

@@ -25,4 +25,5 @@ utils::year!(2024 => year2024, ${
     21 => day21::Day21,
     22 => day22::Day22,
     23 => day23::Day23,
+    24 => day24::Day24,
 });