2015 day 19.

src/holt59/aoc/2015/day19.py

@@ -0,0 +1,56 @@
+import sys
+from collections import defaultdict
+
+replacements_s, molecule = sys.stdin.read().split("\n\n")
+
+REPLACEMENTS: dict[str, list[str]] = defaultdict(list)
+for replacement_s in replacements_s.splitlines():
+    p = replacement_s.split(" => ")
+    REPLACEMENTS[p[0]].append(p[1])
+molecule = molecule.strip()
+
+generated = [
+    molecule[:i] + replacement + molecule[i + len(symbol) :]
+    for symbol, replacements in REPLACEMENTS.items()
+    for replacement in replacements
+    for i in range(len(molecule))
+    if molecule[i:].startswith(symbol)
+]
+
+answer_1 = len(set(generated))
+print(f"answer 1 is {answer_1}")
+
+inversion: dict[str, str] = {
+    replacement: symbol
+    for symbol, replacements in REPLACEMENTS.items()
+    for replacement in replacements
+}
+
+# there is actually only one way to create the molecule, and we can greedily replace
+# tokens with their replacements, e.g., if H => OH then we can replace OH by H directly
+# without thinking
+
+count = 0
+while molecule != "e":
+    i = 0
+    m2 = ""
+    while i < len(molecule):
+        found = False
+        for replacement in inversion:
+            if molecule[i:].startswith(replacement):
+                m2 += inversion[replacement]
+                i += len(replacement)
+                count += 1
+                found = True
+                break
+
+        if not found:
+            m2 += molecule[i]
+            i += 1
+
+    # print(m2)
+    molecule = m2
+
+
+answer_2 = count
+print(f"answer 2 is {count}")