Add updated solutions, including day 20.

day19.cr

@@ -0,0 +1,68 @@
+require "advent"
+
+rlines, _, strings = input(2020, 19).partition("\n\n")
+strings = strings.lines
+rules = {} of String => String
+rlines.lines.map do |l|
+  rule, _, text = l.partition(": ")
+  rules[rule] = text
+end
+
+alias Matcher = Proc(String,Int32,Array(Int32))
+
+def char(c)
+  ->(str : String, i : Int32) { str[i]? == c ? [i+1] : [] of Int32 }
+end
+
+def any(ps)
+  ->(str : String, i : Int32) { ps.flat_map { |p| p.call(str, i) } }
+end
+
+def seq(ps)
+  ->(str : String, i : Int32) {
+    base = [i]
+    ps.each do |p|
+      base = base.flat_map { |i| p.call(str, i) }
+    end
+    base
+  }
+end
+
+def to_regex(rules, rule)
+end
+
+def build_rule(rules, built, rule) : Matcher
+  if exists = built[rule]?
+    return exists 
+  end
+
+  body = rules[rule]
+  return built[rule] = char body[1] if body.matches? /"."/
+  
+  branches = [] of Matcher
+  top = any(branches)
+  built[rule] = top
+  branches.concat(body.split(" | ").map { |b| seq(b.split(" ").map { |subrule| build_rule(rules, built, subrule) }) })
+  top
+end
+
+def part1(input)
+  rules, lines = input
+  matcher = build_rule(rules, {} of String => Matcher, "0")
+  lines.count do |l|
+    matcher.call(l, 0).includes? l.size
+  end
+end
+
+def part2(input)
+  rules, lines = input
+  rules["8"] = "42 | 42 8"
+  rules["11"] = "42 31 | 42 11 31"
+  matcher = build_rule(rules, {} of String => Matcher, "0")
+  lines.count do |l|
+    matcher.call(l, 0).includes? l.size
+  end
+end
+
+puts part1({rules,strings})
+puts part2({rules,strings})