day19 working part1, not working part2.

I don't really understand what's expected of part2 and the example is
not simple enough for me to step through in my brain so I'm probably not
going to get it.

src/day19.rs

@@ -0,0 +1,103 @@
+use crate::split_once;
+use aoc_runner_derive::aoc;
+use std::collections::HashMap;
+
+#[derive(Debug)]
+enum Parser<'a> {
+    Lit(&'a str),
+    Seq(Vec<usize>),
+    Alt(Vec<usize>, Vec<usize>),
+}
+
+impl<'a> Parser<'a> {
+    fn new(parser_def: &'a str) -> Option<(usize, Self)> {
+        let (idx, def) = split_once(parser_def, ": ")?;
+        let idx = idx.parse().ok()?;
+        if let Some((a, b)) = split_once(def, " | ") {
+            let a = a
+                .split(' ')
+                .map(|x| x.parse().ok())
+                .collect::<Option<Vec<usize>>>()?;
+            let b = b
+                .split(' ')
+                .map(|x| x.parse().ok())
+                .collect::<Option<Vec<usize>>>()?;
+            Some((idx, Parser::Alt(a, b)))
+        } else if let Some(_) = def.find('"') {
+            Some((idx, Parser::Lit(&def[1..def.len() - 1])))
+        } else {
+            let seq = def
+                .split(' ')
+                .map(|x| x.parse().ok())
+                .collect::<Option<Vec<usize>>>()?;
+            Some((idx, Parser::Seq(seq)))
+        }
+    }
+
+    // returns Some(remaining_input) when valid, None when doesn't match.
+    fn parse<'b>(&self, parsers: &HashMap<usize, Parser>, input: &'b str) -> Option<&'b str> {
+        match self {
+            &Parser::Lit(s) => {
+                if input.is_empty() || &input[..s.len()] != s {
+                    None
+                } else {
+                    Some(&input[s.len()..])
+                }
+            }
+            Parser::Seq(a) => {
+                let mut input = input;
+                let mut iter = a.iter().copied();
+                while let Some(parser_idx) = iter.next() {
+                    input = parsers
+                        .get(&parser_idx)
+                        .and_then(|p| p.parse(parsers, input))?;
+                }
+                Some(input)
+            }
+            Parser::Alt(a, b) => {
+                let parser_a = Parser::Seq(a.clone());
+                match parser_a.parse(parsers, input) {
+                    Some(input) => Some(input),
+                    None => {
+                        let parser_b = Parser::Seq(b.clone());
+                        parser_b.parse(parsers, input)
+                    }
+                }
+            }
+        }
+    }
+}
+
+#[aoc(day19, part1)]
+fn solve_d19_p1(input: &str) -> usize {
+    let (parser_input, pattern_input) = split_once(input, "\n\n").unwrap();
+    let parsers: HashMap<usize, Parser> = parser_input
+        .split('\n')
+        .map(|parser_def| Parser::new(parser_def).unwrap())
+        .collect();
+
+    let rule0 = parsers.get(&0).unwrap();
+    pattern_input
+        .split('\n')
+        .filter(|pattern| rule0.parse(&parsers, pattern) == Some(""))
+        .count()
+}
+
+#[aoc(day19, part2)]
+fn solve_d19_p2(input: &str) -> usize {
+    let (parser_input, pattern_input) = split_once(input, "\n\n").unwrap();
+    let mut parsers: HashMap<usize, Parser> = parser_input
+        .split('\n')
+        .map(|parser_def| Parser::new(parser_def).unwrap())
+        .collect();
+
+    parsers.insert(8, Parser::Alt(vec![42], vec![42, 8]));
+    parsers.insert(11, Parser::Alt(vec![42, 31], vec![42, 11, 31]));
+
+    let rule0 = parsers.get(&0).unwrap();
+
+    pattern_input
+        .split('\n')
+        .filter(|pattern| rule0.parse(&parsers, pattern) == Some(""))
+        .count()
+}