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+# --- Day 5: Alchemical Reduction ---
+
+# You've managed to sneak in to the prototype suit manufacturing lab. The Elves
+# are making decent progress, but are still struggling with the suit's size
+# reduction capabilities.
+
+# While the very latest in 1518 alchemical technology might have solved their
+# problem eventually, you can do better. You scan the chemical composition of
+# the suit's material and discover that it is formed by extremely long polymers
+# (one of which is available as your puzzle input).
+
+# The polymer is formed by smaller units which, when triggered, react with each
+# other such that two adjacent units of the same type and opposite polarity are
+# destroyed. Units' types are represented by letters; units' polarity is
+# represented by capitalization. For instance, r and R are units with the same
+# type but opposite polarity, whereas r and s are entirely different types and
+# do not react.
+
+# For example:
+
+#     In aA, a and A react, leaving nothing behind.
+#     In abBA, bB destroys itself, leaving aA. As above, this then destroys
+# itself, leaving nothing.
+#     In abAB, no two adjacent units are of the same type, and so nothing
+# happens.
+#     In aabAAB, even though aa and AA are of the same type, their polarities
+# match, and so nothing happens.
+
+# Now, consider a larger example, dabAcCaCBAcCcaDA:
+
+# dabAcCaCBAcCcaDA  The first 'cC' is removed.
+# dabAaCBAcCcaDA    This creates 'Aa', which is removed.
+# dabCBAcCcaDA      Either 'cC' or 'Cc' are removed (the result is the same).
+# dabCBAcaDA        No further actions can be taken.
+
+# After all possible reactions, the resulting polymer contains 10 units.
+
+# How many units remain after fully reacting the polymer you scanned? (Note:
+# in this puzzle and others, the input is large; if you copy/paste your input,
+# make sure you get the whole thing.)
+
+import re
+from copy import copy
+from string import ascii_lowercase
+
+with open("files/P5.txt") as f:
+    polymer = [line for line in f.read().split()][0]
+
+
+def react(s: str) -> bool:
+    return abs(ord(s[0]) - ord(s[1])) == 32
+
+
+def remove_substring(substr: str, s: str) -> str:
+    return re.sub(substr, "", s)
+
+
+def part_1() -> None:
+    sequence = copy(polymer)
+    idx = 0
+    while True:
+        try:
+            pair = "".join((sequence[idx], sequence[idx + 1]))
+            if react(pair):
+                sequence = remove_substring(pair, sequence)
+                idx = 0
+            else:
+                idx += 1
+        except IndexError:
+            break
+    print(f"There are {len(sequence)} units remaining")
+
+
+# --- Part Two ---
+
+# Time to improve the polymer.
+
+# One of the unit types is causing problems; it's preventing the polymer from
+# collapsing as much as it should. Your goal is to figure out which unit type
+# is causing the most problems, remove all instances of it (regardless of
+# polarity), fully react the remaining polymer, and measure its length.
+
+# For example, again using the polymer dabAcCaCBAcCcaDA from above:
+
+#     Removing all A/a units produces dbcCCBcCcD. Fully reacting this polymer
+# produces dbCBcD, which has length 6.
+#     Removing all B/b units produces daAcCaCAcCcaDA. Fully reacting this
+# polymer produces daCAcaDA, which has length 8.
+#     Removing all C/c units produces dabAaBAaDA. Fully reacting this polymer
+# produces daDA, which has length 4.
+#     Removing all D/d units produces abAcCaCBAcCcaA. Fully reacting this
+# polymer produces abCBAc, which has length 6.
+
+# In this example, removing all C/c units was best, producing the answer 4.
+
+# What is the length of the shortest polymer you can produce by removing all
+# units of exactly one type and fully reacting the result?
+
+
+def part_2() -> None:
+    lengths = []
+    for char in ascii_lowercase:
+        sequence = copy(polymer)
+        sequence = sequence.replace(char, "").replace(char.upper(), "")
+        idx = 0
+        while True:
+            try:
+                pair = "".join((sequence[idx], sequence[idx + 1]))
+                if react(pair):
+                    sequence = remove_substring(pair, sequence)
+                    idx = 0
+                else:
+                    idx += 1
+            except IndexError:
+                break
+        lengths.append(len(sequence))
+
+    print(f"The length of the shortest polymer is {min(lengths)}")
+
+
+if __name__ == "__main__":
+    part_1()
+    part_2()