Add, complete day 12.

I'm pretty OK with this one; finding the graph groups of 2000 lines of
connection input in less than half a second seems like a decent job
to me.

I hadn't expected to need to factor out map_connections_generic for part 2,
but I can't complain about the result; I kind of like how it came out.

I do wish that closures implemented Copy/Clone appropriately, though:
rust-lang/rfcs#2132

$ cargo build --release && time target/release/day12 && wc -l input.txt
   Compiling day12 v0.1.0 (file:///mnt/d/Users/coriolinus/Documents/Projects/adventofcode.com/adventofcode-2017/day12)
    Finished release [optimized] target(s) in 4.92 secs

real    0m0.397s
user    0m0.375s
sys     0m0.000s
2000 input.txt

day12/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "day12"
+version = "0.1.0"
+authors = ["Peter Goodspeed-Niklaus <peter.r.goodspeedniklaus@gmail.com>"]
+build = "build.rs" # LALRPOP preprocessing
+
+[dependencies]
+util = { path = "../util" }
+lalrpop-util = "0.13.1"
+regex = "0.2.0"
+
+[build-dependencies]
+lalrpop = "0.13.1"

day12/build.rs

@@ -0,0 +1,5 @@
+extern crate lalrpop;
+
+fn main() {
+    lalrpop::process_root().unwrap();
+}

day12/src/.gitignore

@@ -0,0 +1,6 @@
+# parser.rs is generated at compile time by parser.lalrpop,
+# so is not necessary.
+# Ideally, we could express the rule "<n:*>.rs iff <n>.lalrpop",
+# but if that's possible in a standard .gitignore file,
+# I don't know how to do it.
+parser.rs

day12/src/lib.rs

@@ -0,0 +1,81 @@
+pub mod parser;
+pub use parser::parse_connections;
+
+type Connections = (usize, Vec<usize>);
+type Graph = Vec<bool>;
+
+fn map_connections(graph: &mut Graph, connections: &[Connections], node: usize) {
+    map_connections_generic(graph, connections, node, true, &|b: bool| !b);
+}
+
+fn map_connections_node(
+    graph: &mut Vec<Node>,
+    connections: &[Connections],
+    node: usize,
+    group: usize,
+) {
+    map_connections_generic(graph, connections, node, Some(group), &|n| n.is_none());
+}
+
+fn map_connections_generic<N, IsFalse>(
+    graph: &mut Vec<N>,
+    connections: &[Connections],
+    node: usize,
+    true_value: N,
+    is_false: &IsFalse,
+) where
+    N: Copy,
+    IsFalse: Fn(N) -> bool,
+{
+    graph[node] = true_value;
+    for &connection in connections[node].1.iter() {
+        if is_false(graph[connection]) {
+            map_connections_generic(graph, connections, connection, true_value, is_false);
+        }
+    }
+}
+
+pub fn connected_to_zero(connections: &[Connections]) -> Vec<usize> {
+    assert!(
+        connections.iter().enumerate().all(
+            |(idx, cxn)| idx == cxn.0,
+        ),
+        "Input connections must be sorted by node index"
+    );
+    let mut graph = vec![false; connections.len()];
+    if graph.len() > 0 {
+        map_connections(&mut graph, connections, 0)
+    }
+    graph
+        .iter()
+        .enumerate()
+        .filter(|&(_, node)| *node)
+        .map(|(index, _)| index)
+        .collect()
+}
+
+type Node = Option<usize>;
+
+pub fn count_groups(connections: &[Connections]) -> usize {
+    assert!(
+        connections.iter().enumerate().all(
+            |(idx, cxn)| idx == cxn.0,
+        ),
+        "Input connections must be sorted by node index"
+    );
+    let mut graph: Vec<Node> = vec![None; connections.len()];
+    let mut group = 0;
+
+    while let Some(idx) = graph
+        .iter()
+        .enumerate()
+        .filter(|&(_, g)| g.is_none())
+        .map(|(idx, _)| idx)
+        .next()
+    {
+        group += 1;
+        map_connections_node(&mut graph, connections, idx, group);
+    }
+
+    group
+}

day12/src/main.rs

@@ -0,0 +1,17 @@
+extern crate day12;
+use day12::{parse_connections, connected_to_zero, count_groups};
+
+extern crate util;
+use util::read_file;
+
+fn main() {
+    let input = read_file("input.txt");
+    let connections = input
+        .trim()
+        .lines()
+        .map(|line| parse_connections(line).expect("Parse error"))
+        .collect::<Vec<_>>();
+    let zero_graph = connected_to_zero(&connections);
+    println!("# nodes connected to 0: {}", zero_graph.len());
+    println!("# groups: {}", count_groups(&connections));
+}

day12/src/parser.lalrpop

@@ -0,0 +1,24 @@
+use std::str::FromStr;
+
+grammar;
+
+pub connections: (usize, Vec<usize>) = {
+   <id:NodeId> "<->" <l:NodeIdList> => (id, l),
+};
+
+NodeId: usize = {
+   r"\d+" => usize::from_str(<>).unwrap(),
+};
+
+NodeIdList = CommaSeparatedList<NodeId>;
+
+CommaSeparatedList<T>: Vec<T> = { // (1)
+    <v:(<T> ",")*> <e:T?> => match e { // (2)
+        None => v,
+        Some(e) => {
+            let mut v = v;
+            v.push(e);
+            v
+        }
+    }
+};