1 files changed, 41 insertions, 0 deletions
diff --git a/2020/nineteen.nim b/2020/nineteen.nim
new file mode 100644
index 0000000..1f815b4
--- /dev/null
+++ b/2020/nineteen.nim
@@ -0,0 +1,41 @@
+# Day Nineteen: Monster Messages
+import std/[os, strutils, sequtils, sugar, algorithm]
+
+let input = paramStr(1).readFile().strip().split("\n\n")
+let rules: seq[string] = input[0].strip().split("\n")
+  .sortedByIt(it.split(": ")[0].parseInt()).mapIt(it.split(": ")[1])
+
+let messages: seq[string] = input[1].strip().split("\n")
+
+func replace(a: string, target: int, with: string): string =
+  var replaced = false
+  for chunk in a.split(" "):
+    if chunk[0] != '"' and chunk.parseInt() == target and not replaced:
+      result &= with # probably the bug
+      replaced = true
+    else:
+      result &= chunk
+    result &= " "
+  result.strip()
+
+var stack: seq[string]
+var generated: seq[string]
+stack.add(rules[0])
+while stack.len > 0: # slow as all hell... but works!
+  var current = stack.pop
+  if not current.contains({'0'..'9'}):
+    generated.add(current)
+  else:
+    var nextindex = current.split(" ")
+      .filterIt(it[0] != '"')[0].parseInt()
+    var nextrule = rules[nextindex]
+    for chunk in nextrule.split(" | "):
+      stack.add(current.replace(nextindex, chunk))
+
+generated.applyIt(it.replace("\"", "").replace(" ", ""))
+
+var sum = 0
+for line in messages:
+  if line in generated:
+    inc sum
+echo sum