From 37c578d7cc12fa70a4e6ca2bd4768c2a7629b140 Mon Sep 17 00:00:00 2001
From: Bert Peters <bert@bertptrs.nl>
Date: Wed, 20 Dec 2023 23:00:12 +0100
Subject: [PATCH] Implement 2023 day 20 part 1

---
 2023/src/day20.rs         | 157 +++++++++++++++++++++++++++++++++++++-
 2023/src/samples/20.1.txt |   5 ++
 2023/src/samples/20.2.txt |   5 ++
 3 files changed, 165 insertions(+), 2 deletions(-)
 create mode 100644 2023/src/samples/20.1.txt
 create mode 100644 2023/src/samples/20.2.txt

diff --git a/2023/src/day20.rs b/2023/src/day20.rs
index 7c1760f..2cfec46 100644
--- a/2023/src/day20.rs
+++ b/2023/src/day20.rs
@@ -1,7 +1,160 @@
-pub fn part1(_input: &[u8]) -> anyhow::Result<String> {
-    anyhow::bail!("Not implemented")
+use std::collections::HashMap;
+use std::collections::VecDeque;
+
+use nom::branch::alt;
+use nom::bytes::complete::tag;
+use nom::bytes::complete::take_until;
+use nom::bytes::complete::take_while1;
+use nom::combinator::map;
+use nom::combinator::value;
+use nom::multi::fold_many1;
+use nom::multi::separated_list1;
+use nom::sequence::delimited;
+use nom::IResult;
+
+use crate::common::parse_input;
+
+#[derive(Clone)]
+enum Node {
+    FlipFlop(bool),
+    Conjunction(Vec<u32>),
+    Broadcaster,
+}
+
+#[derive(Clone)]
+struct Entry {
+    node: Node,
+    dest: Vec<u32>,
+}
+
+// Wouldn't ya know it, big boy base 26 at it again
+fn convert_name(name: &[u8]) -> u32 {
+    name.iter()
+        // The word "broadcaster" overflows but nothing else is nearly as long so it's fine to cut
+        // it short.
+        .take(5)
+        .fold(0, |cur, &c| cur * 26 + u32::from(c - b'a'))
+}
+
+fn parse_cables(i: &[u8]) -> IResult<&[u8], HashMap<u32, Entry>> {
+    fn parse_cable(i: &[u8]) -> IResult<&[u8], (u32, Entry)> {
+        let (i, node) = alt((
+            value(Node::FlipFlop(false), tag("%")),
+            value(Node::Conjunction(Vec::new()), tag("&")),
+            value(Node::Broadcaster, tag("broadcaster")),
+        ))(i)?;
+
+        let (i, id) = if let Node::Broadcaster = node {
+            (i, convert_name(b"broadcaster"))
+        } else {
+            map(take_until(" "), convert_name)(i)?
+        };
+
+        let (i, dest) = delimited(
+            tag(" -> "),
+            separated_list1(
+                tag(", "),
+                map(take_while1(|c: u8| c.is_ascii_alphabetic()), convert_name),
+            ),
+            tag("\n"),
+        )(i)?;
+
+        Ok((i, (id, Entry { node, dest })))
+    }
+
+    let (i, mut cables) = fold_many1(parse_cable, HashMap::new, |mut map, (id, entry)| {
+        map.insert(id, entry);
+        map
+    })(i)?;
+
+    let cable_ids: Vec<_> = cables.keys().copied().collect();
+    let mut buffer = Vec::new();
+
+    for cable_id in &cable_ids {
+        buffer.extend_from_slice(&cables[cable_id].dest);
+
+        for dest_id in &buffer {
+            let Some(cable) = cables.get_mut(dest_id) else {
+                continue;
+            };
+
+            if let Node::Conjunction(sources) = &mut cable.node {
+                sources.push(*cable_id);
+            }
+        }
+        buffer.clear();
+    }
+
+    Ok((i, cables))
+}
+
+// 604293120 too low
+pub fn part1(input: &[u8]) -> anyhow::Result<String> {
+    let mut cables = parse_input(input, parse_cables)?;
+    let mut low_pulses = 0;
+    let mut high_pulses = 0;
+
+    let mut todo = VecDeque::new();
+    let mut last_pulse: HashMap<u32, bool> = HashMap::new();
+
+    for _ in 0..1000 {
+        low_pulses += 1;
+        todo.push_back((false, convert_name(b"broadcaster")));
+
+        while let Some((pulse, pos)) = todo.pop_front() {
+            let Some(cable) = cables.get_mut(&pos) else {
+                // Sometimes cables aren't real, and that's okay
+                continue;
+            };
+
+            let next_pulse = match &mut cable.node {
+                Node::FlipFlop(state) => {
+                    if pulse {
+                        // Ignore, nothing to be done since it's a high pulse
+                        continue;
+                    } else {
+                        *state = !*state;
+                        *state
+                    }
+                }
+                Node::Conjunction(inwards) => {
+                    // Need to deal with the check outside the match otherwise lifetime issues :(
+                    !inwards
+                        .iter()
+                        .all(|source| *last_pulse.get(source).unwrap_or(&false))
+                }
+                Node::Broadcaster => pulse,
+            };
+
+            last_pulse.insert(pos, next_pulse);
+            if next_pulse {
+                high_pulses += cable.dest.len();
+            } else {
+                low_pulses += cable.dest.len();
+            }
+            for &other in &cable.dest {
+                todo.push_back((next_pulse, other));
+            }
+        }
+    }
+
+    Ok((low_pulses * high_pulses).to_string())
 }
 
 pub fn part2(_input: &[u8]) -> anyhow::Result<String> {
     anyhow::bail!("Not implemented")
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const SAMPLE1: &[u8] = include_bytes!("samples/20.1.txt");
+    const SAMPLE2: &[u8] = include_bytes!("samples/20.2.txt");
+
+    #[test]
+    fn sample_part1() {
+        assert_eq!("32000000", part1(SAMPLE1).unwrap());
+        assert_eq!("11687500", part1(SAMPLE2).unwrap());
+    }
+}
diff --git a/2023/src/samples/20.1.txt b/2023/src/samples/20.1.txt
new file mode 100644
index 0000000..2dc1bab
--- /dev/null
+++ b/2023/src/samples/20.1.txt
@@ -0,0 +1,5 @@
+broadcaster -> a, b, c
+%a -> b
+%b -> c
+%c -> inv
+&inv -> a
diff --git a/2023/src/samples/20.2.txt b/2023/src/samples/20.2.txt
new file mode 100644
index 0000000..2738ceb
--- /dev/null
+++ b/2023/src/samples/20.2.txt
@@ -0,0 +1,5 @@
+broadcaster -> a
+%a -> inv, con
+&inv -> b
+%b -> con
+&con -> output