update README (day 19 part 2)

README

@@ -7,4 +7,4 @@ Status:
 Done:
    Day  1:     Bash, C, and Cobol
    Days 2-17:  Bash and C
-   Days 18:    Bash
+   Days 18-19: Bash

day19/README

@@ -52,3 +52,94 @@ aaaabbb
 Your goal is to determine the number of messages that completely match rule 0. In the above example, ababbb and abbbab match, but bababa, aaabbb, and aaaabbb do not, producing the answer 2. The whole message must match all of rule 0; there can't be extra unmatched characters in the message. (For example, aaaabbb might appear to match rule 0 above, but it has an extra unmatched b on the end.)
 
 How many messages completely match rule 0?
+
+Your puzzle answer was 285.
+
+The first half of this puzzle is complete! It provides one gold star: *
+--- Part Two ---
+
+As you look over the list of messages, you realize your matching rules aren't quite right. To fix them, completely replace rules 8: 42 and 11: 42 31 with the following:
+
+8: 42 | 42 8
+11: 42 31 | 42 11 31
+
+This small change has a big impact: now, the rules do contain loops, and the list of messages they could hypothetically match is infinite. You'll need to determine how these changes affect which messages are valid.
+
+Fortunately, many of the rules are unaffected by this change; it might help to start by looking at which rules always match the same set of values and how those rules (especially rules 42 and 31) are used by the new versions of rules 8 and 11.
+
+(Remember, you only need to handle the rules you have; building a solution that could handle any hypothetical combination of rules would be significantly more difficult.)
+
+For example:
+
+42: 9 14 | 10 1
+9: 14 27 | 1 26
+10: 23 14 | 28 1
+1: "a"
+11: 42 31
+5: 1 14 | 15 1
+19: 14 1 | 14 14
+12: 24 14 | 19 1
+16: 15 1 | 14 14
+31: 14 17 | 1 13
+6: 14 14 | 1 14
+2: 1 24 | 14 4
+0: 8 11
+13: 14 3 | 1 12
+15: 1 | 14
+17: 14 2 | 1 7
+23: 25 1 | 22 14
+28: 16 1
+4: 1 1
+20: 14 14 | 1 15
+3: 5 14 | 16 1
+27: 1 6 | 14 18
+14: "b"
+21: 14 1 | 1 14
+25: 1 1 | 1 14
+22: 14 14
+8: 42
+26: 14 22 | 1 20
+18: 15 15
+7: 14 5 | 1 21
+24: 14 1
+
+abbbbbabbbaaaababbaabbbbabababbbabbbbbbabaaaa
+bbabbbbaabaabba
+babbbbaabbbbbabbbbbbaabaaabaaa
+aaabbbbbbaaaabaababaabababbabaaabbababababaaa
+bbbbbbbaaaabbbbaaabbabaaa
+bbbababbbbaaaaaaaabbababaaababaabab
+ababaaaaaabaaab
+ababaaaaabbbaba
+baabbaaaabbaaaababbaababb
+abbbbabbbbaaaababbbbbbaaaababb
+aaaaabbaabaaaaababaa
+aaaabbaaaabbaaa
+aaaabbaabbaaaaaaabbbabbbaaabbaabaaa
+babaaabbbaaabaababbaabababaaab
+aabbbbbaabbbaaaaaabbbbbababaaaaabbaaabba
+
+Without updating rules 8 and 11, these rules only match three messages: bbabbbbaabaabba, ababaaaaaabaaab, and ababaaaaabbbaba.
+
+However, after updating rules 8 and 11, a total of 12 messages match:
+
+    bbabbbbaabaabba
+    babbbbaabbbbbabbbbbbaabaaabaaa
+    aaabbbbbbaaaabaababaabababbabaaabbababababaaa
+    bbbbbbbaaaabbbbaaabbabaaa
+    bbbababbbbaaaaaaaabbababaaababaabab
+    ababaaaaaabaaab
+    ababaaaaabbbaba
+    baabbaaaabbaaaababbaababb
+    abbbbabbbbaaaababbbbbbaaaababb
+    aaaaabbaabaaaaababaa
+    aaaabbaabbaaaaaaabbbabbbaaabbaabaaa
+    aabbbbbaabbbaaaaaabbbbbababaaaaabbaaabba
+
+After updating rules 8 and 11, how many messages completely match rule 0?
+
+Answer:
+
+Although it hasn't changed, you can still get your puzzle input.
+
+You can also [Share] this puzzle.