From e2e8223c6bd35483a17fd803fc1622a1f09ef80c Mon Sep 17 00:00:00 2001
From: Marcin Chrzanowski <marcin.j.chrzanowski@gmail.com>
Date: Fri, 25 Dec 2020 15:45:54 +0100
Subject: Add day 19

---
 19/a.lua | 101 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 101 insertions(+)
 create mode 100644 19/a.lua

(limited to '19/a.lua')

diff --git a/19/a.lua b/19/a.lua
new file mode 100644
index 0000000..0f3164c
--- /dev/null
+++ b/19/a.lua
@@ -0,0 +1,101 @@
+rules = {}
+
+function char(c)
+    return function(s, i)
+        if s:sub(i,i) == c then
+            return i+1
+        else
+            return nil
+        end
+    end
+end
+
+function pick(f, g)
+    return function(s, i)
+        j = f(s, i)
+        if j then
+            return j
+        else
+            return g(s, i)
+        end
+    end
+end
+
+function chain2(fs)
+    return function (s, i)
+        for j = 1, #fs do
+            i = rules[fs[j]](s, i)
+            if not i then
+                return nil
+            end
+        end
+        
+        return i
+    end
+end
+
+function chain(f, g)
+    return function (s, i)
+        j = rules[f](s, i)
+        if j then
+            return rules[g](s, j)
+        else
+            return nil
+        end
+    end
+end
+
+function make_chain(rule)
+    space_index = rule:find(' ')
+    if space_index then
+        return chain2({rule:sub(1, space_index - 1), rule:sub(space_index + 1, -1)})
+    else
+        return function (s, i)
+            return rules[rule](s, i)
+        end
+    end
+end
+
+function make_function(rule)
+    if rule:sub(1, 1) == '"' then
+        return char(rule:sub(2, 2))
+    end
+    pipe_index = rule:find('|')
+    if pipe_index then
+        left = make_chain(rule:sub(1, pipe_index - 2))
+        right = make_chain(rule:sub(pipe_index + 2, #rule))
+        return pick(left, right)
+    else
+        return make_chain(rule)
+    end
+end
+
+function parse_rule(rule)
+    colon_index = rule:find(':')
+    if colon_index then
+        rule_number = rule:sub(1, colon_index - 1)
+        rhs = rule:sub(colon_index + 2, -1)
+        rules[rule_number] = make_function(rhs)
+        return true
+    else
+        return false
+    end
+end
+
+lines = io.lines('input.txt')
+
+for line in lines do
+    if not parse_rule(line) then
+        break
+    end
+end
+
+correct = 0
+for line in lines do
+    j = rules['0'](line, 1)
+    if j == #line + 1 then
+        correct = correct + 1
+    end
+end
+
+print(correct)
-- 
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