Solutions to day 19, finally.

day-19/input.txt

@@ -0,0 +1,45 @@
+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

day-19/solution.py

@@ -0,0 +1,57 @@
+from __future__ import print_function
+import fileinput
+import re
+
+def doReplace(replacement, medicine):
+    replacements = []
+    for match in re.finditer(replacement[0], medicine):
+        pos = match.start()
+        sub = medicine[:pos] + replacement[1] + medicine[pos + len(replacement[0]):]
+        replacements.append(sub)
+
+    return replacements
+
+def allReplacements(replacements, medicine):
+    options = set()
+    # Simply attempt all replacements
+    for replacement in replacements:
+        for option in doReplace(replacement, medicine):
+            options.add(option)
+
+    return options
+
+
+# Exhaustive search trying to work back from the medicine.
+#
+# This is minimal because we try the larger substitutions first.
+def solve(replacements, target, current):
+    if current == target:
+        return 0
+
+    for org, rep in replacements:
+        for option in doReplace((rep, org), current):
+            result = solve(replacements, target, option)
+            if result is not None:
+                return result + 1
+
+    return None
+
+replacements = []
+medicine = None
+
+for line in fileinput.input():
+    if len(line.strip()) == 0:
+        continue
+
+    match = re.match(r"(\w+) => (\w+)", line)
+    if match:
+        replacements.append(match.group(1, 2))
+    else:
+        medicine = line.strip()
+
+replacements.sort(key=lambda x: -len(x[1]))
+
+
+print("Options:", len(allReplacements(replacements, medicine)))
+
+print("Steps:", solve(replacements, 'e', medicine))