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.

input/2020/day19.txt

@@ -0,0 +1,562 @@
+72: "b"
+45: 46 52 | 9 72
+85: 9 52 | 9 72
+82: 52 87 | 72 77
+133: 52 30 | 72 56
+118: 7 52 | 70 72
+18: 52 113 | 72 52
+119: 72 46 | 52 18
+25: 19 72 | 103 52
+32: 90 52 | 78 72
+50: 113 113
+71: 72 106 | 52 128
+3: 103 72 | 18 52
+41: 86 72 | 19 52
+96: 86 72 | 108 52
+33: 44 52 | 104 72
+127: 52 36 | 72 50
+51: 72 79 | 52 38
+43: 72 50 | 52 106
+14: 32 72 | 129 52
+6: 2 72 | 33 52
+108: 52 72 | 72 52
+129: 6 52 | 82 72
+34: 127 72 | 3 52
+74: 128 52 | 9 72
+80: 52 103 | 72 19
+2: 134 72 | 13 52
+54: 128 72 | 114 52
+19: 72 72 | 113 52
+0: 8 11
+30: 132 72 | 39 52
+60: 72 64 | 52 5
+4: 18 52 | 114 72
+57: 72 71 | 52 54
+111: 52 27 | 72 102
+76: 122 52 | 75 72
+134: 86 52 | 9 72
+49: 17 52 | 81 72
+124: 103 72 | 86 52
+123: 72 28 | 52 121
+117: 18 52 | 86 72
+26: 51 72 | 58 52
+62: 85 52 | 44 72
+55: 126 52 | 92 72
+115: 72 13 | 52 67
+109: 128 72 | 106 52
+52: "a"
+93: 60 52 | 133 72
+64: 67 72 | 84 52
+102: 52 128 | 72 48
+84: 86 52 | 46 72
+77: 41 72 | 21 52
+27: 52 9 | 72 108
+95: 89 72 | 105 52
+36: 52 72
+110: 72 50 | 52 9
+9: 72 72
+120: 72 19 | 52 18
+67: 52 48
+112: 65 72 | 119 52
+75: 62 52 | 123 72
+23: 15 72 | 119 52
+42: 131 52 | 61 72
+94: 52 91 | 72 18
+66: 52 128 | 72 103
+10: 113 108
+37: 52 18 | 72 50
+98: 72 69 | 52 94
+126: 115 52 | 29 72
+79: 59 52 | 94 72
+104: 19 72 | 36 52
+125: 96 72 | 4 52
+122: 52 73 | 72 112
+130: 52 20 | 72 45
+90: 72 40 | 52 88
+132: 103 52
+121: 91 52 | 46 72
+7: 52 114 | 72 128
+5: 52 66 | 72 41
+97: 52 18 | 72 91
+89: 72 109 | 52 116
+53: 52 108 | 72 128
+114: 52 113 | 72 72
+16: 1 52 | 110 72
+113: 72 | 52
+31: 52 14 | 72 12
+69: 52 103 | 72 48
+128: 72 52
+40: 72 3 | 52 83
+101: 63 52 | 127 72
+1: 72 91 | 52 114
+58: 52 111 | 72 99
+13: 19 72 | 9 52
+35: 22 72 | 125 52
+65: 72 114 | 52 91
+12: 52 24 | 72 93
+46: 72 72 | 52 52
+20: 9 72
+73: 72 37 | 52 43
+15: 52 50 | 72 108
+103: 52 52
+106: 72 52 | 72 72
+21: 36 72
+28: 72 50 | 52 46
+78: 72 16 | 52 23
+70: 36 72 | 19 52
+87: 72 107 | 52 47
+116: 19 52 | 106 72
+29: 72 68 | 52 65
+8: 42
+68: 9 52 | 128 72
+99: 110 72 | 25 52
+17: 52 118 | 72 130
+11: 42 31
+48: 52 52 | 72 52
+83: 48 113
+105: 52 100 | 72 80
+61: 55 52 | 26 72
+100: 48 52 | 108 72
+56: 52 53 | 72 74
+63: 52 106 | 72 9
+44: 72 114 | 52 128
+47: 52 50 | 72 36
+107: 52 36 | 72 91
+39: 114 72 | 86 52
+86: 52 72 | 52 52
+24: 52 95 | 72 35
+81: 98 72 | 34 52
+131: 49 52 | 76 72
+92: 52 101 | 72 57
+59: 52 114 | 72 50
+38: 52 74 | 72 124
+88: 97 72 | 10 52
+22: 117 72 | 120 52
+91: 72 72 | 52 72
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src/day16.rs

@@ -1,10 +1,6 @@
 use aoc_runner_derive::aoc;
 use std::ops::RangeInclusive;
-
+use crate::split_once;
-fn split_once<'a>(input: &'a str, delimeter: &str) -> Option<(&'a str, &'a str)> {
-    let idx = input.find(delimeter)?;
-    Some((&input[..idx], &input[idx + delimeter.len()..]))
-}
 
 struct Rule<'a> {
     name: &'a str,

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()
+}

src/lib.rs

@@ -8,6 +8,7 @@ pub mod day15;
 pub mod day16;
 pub mod day17;
 pub mod day18;
+pub mod day19;
 pub mod day2;
 pub mod day3;
 pub mod day4;
@@ -18,3 +19,8 @@ pub mod day8;
 pub mod day9;
 
 aoc_runner_derive::aoc_lib! { year = 2020 }
+
+fn split_once<'a>(input: &'a str, delimeter: &str) -> Option<(&'a str, &'a str)> {
+    let idx = input.find(delimeter)?;
+    Some((&input[..idx], &input[idx + delimeter.len()..]))
+}