Some kind of mess that somehow solves day 19.

19.py

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+#!/usr/bin/python
+
+from collections import defaultdict
+import re
+
+subs = defaultdict(list)
+
+with open('19.txt', 'r') as f:
+    lines = f.read().splitlines()
+    gen = set()
+    long = lines[-1]
+    for search, replace in [line.split(' => ') for line in lines[0:-2]]:
+        for start, end in [(i.start(), i.end()) for i in re.finditer(search, long)]:
+            gen.add(long[0:start] + replace + long[end:])
+    print(len(gen))
+
+# --- This is ugly as sin, and I've no idea why it seems to work. ¯\_(ツ)_/¯
+
+with open('19.txt', 'r') as f:
+    lines = f.read().splitlines()
+    long = lines[-1]
+    for search, replace in [line.split(' => ') for line in lines[0:-2]]:
+        subs[search].append(replace)
+    results = set([long])
+    previous = set([long])
+    step = 0
+    while True:
+        step += 1
+        new_results = set()
+        for candidate in results:
+            for search, replacements in subs.iteritems():
+                for replacement in replacements:
+                    for start, end in [(i.start(), i.end()) for i in re.finditer(replacement, candidate)]:
+                        r = candidate[0:start] + search + candidate[end:]
+                        if r not in previous:
+                            new_results.add(r)
+                            previous.add(r)
+        shortest = min(len(x) for x in new_results)
+        new_results = set(list({r for r in new_results if len(r) == shortest})[:1])
+        print("%s (%s)" % (step, len(new_results)))
+        if 'e' in new_results:
+            print("*** %s" % step)
+            break
+        results = new_results

19.txt

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+Al => ThF
+Al => ThRnFAr
+B => BCa
+B => TiB
+B => TiRnFAr
+Ca => CaCa
+Ca => PB
+Ca => PRnFAr
+Ca => SiRnFYFAr
+Ca => SiRnMgAr
+Ca => SiTh
+F => CaF
+F => PMg
+F => SiAl
+H => CRnAlAr
+H => CRnFYFYFAr
+H => CRnFYMgAr
+H => CRnMgYFAr
+H => HCa
+H => NRnFYFAr
+H => NRnMgAr
+H => NTh
+H => OB
+H => ORnFAr
+Mg => BF
+Mg => TiMg
+N => CRnFAr
+N => HSi
+O => CRnFYFAr
+O => CRnMgAr
+O => HP
+O => NRnFAr
+O => OTi
+P => CaP
+P => PTi
+P => SiRnFAr
+Si => CaSi
+Th => ThCa
+Ti => BP
+Ti => TiTi
+e => HF
+e => NAl
+e => OMg
+
+CRnSiRnCaPTiMgYCaPTiRnFArSiThFArCaSiThSiThPBCaCaSiRnSiRnTiTiMgArPBCaPMgYPTiRnFArFArCaSiRnBPMgArPRnCaPTiRnFArCaSiThCaCaFArPBCaCaPTiTiRnFArCaSiRnSiAlYSiThRnFArArCaSiRnBFArCaCaSiRnSiThCaCaCaFYCaPTiBCaSiThCaSiThPMgArSiRnCaPBFYCaCaFArCaCaCaCaSiThCaSiRnPRnFArPBSiThPRnFArSiRnMgArCaFYFArCaSiRnSiAlArTiTiTiTiTiTiTiRnPMgArPTiTiTiBSiRnSiAlArTiTiRnPMgArCaFYBPBPTiRnSiRnMgArSiThCaFArCaSiThFArPRnFArCaSiRnTiBSiThSiRnSiAlYCaFArPRnFArSiThCaFArCaCaSiThCaCaCaSiRnPRnCaFArFYPMgArCaPBCaPBSiRnFYPBCaFArCaSiAl