day 5

5/a.py

@@ -0,0 +1,36 @@
+#!/usr/bin/env python3
+
+
+def is_react(A, B):
+    """
+    Return true if A and B's case differs but letter matches
+    """
+    return A.lower() == B.lower() and B != A
+
+
+def react_poly(poly):
+    lasti = 0
+    while True:
+        reacted = False
+        for i in range(lasti, len(poly) - 1):
+            if is_react(poly[i], poly[i + 1]):
+                poly = poly[0:i] + poly[i + 2:]
+                reacted = True
+                lasti = i - 1 if i > 0 else i
+                break
+        if not reacted:
+            break
+    return poly
+
+
+def main():
+    with open("input.txt") as f:
+        poly = f.read().strip()
+
+    poly = react_poly(poly)
+
+    print(len(poly))
+
+
+if __name__ == '__main__':
+    main()

5/b.py

@@ -0,0 +1,45 @@
+#!/usr/bin/env python3
+
+
+from a import is_react
+
+
+def react_poly2(poly, ignore=[]):
+    lasti = 0
+    while True:
+        reacted = False
+        for i in range(lasti, len(poly) - 1):
+            if poly[i] in ignore:
+                reacted = True
+                poly = poly[0:i] + poly[i + 1:]
+                lasti = i - 1 if i > 0 else i
+                break
+            if is_react(poly[i], poly[i + 1]):
+                poly = poly[0:i] + poly[i + 2:]
+                reacted = True
+                lasti = i - 1 if i > 0 else i
+                break
+        if not reacted:
+            break
+    return poly
+
+
+def main():
+    with open("input.txt") as f:
+        poly = f.read().strip()
+
+    unique_chars = set(poly.lower())
+    bestlen = 99999999
+    bestchar = None
+
+    for char in unique_chars:
+        chainlen = len(react_poly2(poly, [char, char.upper()]))
+        if chainlen < bestlen:
+            bestlen = chainlen
+            bestchar = char
+
+    print(bestlen, "by removing", bestchar)
+
+
+if __name__ == '__main__':
+    main()