Finish 2015.

2015/d19.py

@@ -1,41 +1,41 @@
-import sys
-from random import shuffle
-
-data = open(0).read().strip()
+with open("i19.txt", "r") as f:
+    data = f.read()
 
 eqs = []
-text = ""
+molecule = ""
 for line in data.splitlines():
     if "=>" in line:
         lhs, rhs = line.split(" => ")
         eqs.append((lhs, rhs))
     elif line:
-        text = line
+        molecule = line.strip()
 
 molecules = set()
 for lhs, rhs in eqs:
-    for i in range(len(text)):
-        f, t = text[:i], text[i:]
+    for i in range(len(molecule)):
+        f, t = molecule[:i], molecule[i:]
         if t.startswith(lhs):
             n = f + t.replace(lhs, rhs, 1)
             molecules.add(n)
 print(len(molecules))
 
-# m = float("inf")
-# for _ in range(100000):
-#     getout = False
-#     c = str(text)
-#     for i in range(10**4):
-#         if i % 1000 == 0:
-#             shuffle(eqs)
-#         for lhs, rhs in eqs:
-#             if rhs in c:
-#                 c = c.replace(rhs, lhs, 1)
-#                 if c == "e":
-#                     if i < m:
-#                         m = i
-#                         print(i)
-#                     getout = True
-#         if getout:
-#             break
-# print(m)
+
+# It seems like the problem input is specifically designed so that reversing
+# the molecule back to 'e' works by simply replacing RHS with LHS continuously.
+# Intuitively, it seems like this should not work in many cases or at least not
+# result in the shortest sequence. Probably I am just salty that it took me so
+# long to find this trivial solution.
+x = molecule
+count = 0
+while len(x) > 1:
+    old_len = len(x)
+    for lhs, rhs in eqs:
+        if rhs in x:
+            x = x.replace(rhs, lhs, 1)
+            count += 1
+            break
+    else:
+        break
+
+assert x == 'e'
+print(count)