Solution to problem 9 in Python

src/P9.py

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+# --- Day 9: Encoding Error ---
+
+# With your neighbor happily enjoying their video game, you turn your attention
+# to an open data port on the little screen in the seat in front of you.
+
+# Though the port is non-standard, you manage to connect it to your computer
+# through the clever use of several paperclips. Upon connection, the port
+# outputs a series of numbers (your puzzle input).
+
+# The data appears to be encrypted with the eXchange-Masking Addition System
+# (XMAS) which, conveniently for you, is an old cypher with an important
+# weakness.
+
+# XMAS starts by transmitting a preamble of 25 numbers. After that, each number
+# you receive should be the sum of any two of the 25 immediately previous
+# numbers. The two numbers will have different values, and there might be more
+# than one such pair.
+
+# For example, suppose your preamble consists of the numbers 1 through 25 in a
+# random order. To be valid, the next number must be the sum of two of those
+# numbers:
+
+#     26 would be a valid next number, as it could be 1 plus 25 (or many other
+# pairs, like 2 and 24).
+#     49 would be a valid next number, as it is the sum of 24 and 25.
+#     100 would not be valid; no two of the previous 25 numbers sum to 100.
+#     50 would also not be valid; although 25 appears in the previous 25
+# numbers, the two numbers in the pair must be different.
+
+# Suppose the 26th number is 45, and the first number (no longer an option, as
+# it is more than 25 numbers ago) was 20. Now, for the next number to be valid,
+# there needs to be some pair of numbers among 1-19, 21-25, or 45 that add up
+# to it:
+
+#     26 would still be a valid next number, as 1 and 25 are still within the
+# previous 25 numbers.
+#     65 would not be valid, as no two of the available numbers sum to it.
+#     64 and 66 would both be valid, as they are the result of 19+45 and 21+45
+# respectively.
+
+# Here is a larger example which only considers the previous 5 numbers (and has
+# a preamble of length 5):
+
+# 35
+# 20
+# 15
+# 25
+# 47
+# 40
+# 62
+# 55
+# 65
+# 95
+# 102
+# 117
+# 150
+# 182
+# 127
+# 219
+# 299
+# 277
+# 309
+# 576
+
+# In this example, after the 5-number preamble, almost every number is the sum
+# of two of the previous 5 numbers; the only number that does not follow this
+# rule is 127.
+
+# The first step of attacking the weakness in the XMAS data is to find the
+# first number in the list (after the preamble) which is not the sum of two of
+# the 25 numbers before it. What is the first number that does not have this
+# property?
+
+
+with open("files/P9.txt", "r") as f:
+    numbers = [code for code in f.read().strip().split("\n")]
+
+from itertools import combinations
+
+
+def part_1() -> None:
+    window = 25
+    for i in range(window, len(numbers)):
+        if all(
+            [
+                int(x) + int(y) != int(numbers[i])
+                for x, y in combinations(numbers[i - window : i], 2)
+            ]
+        ):
+            break
+
+    print(f"The first number without the property is {numbers[i]}")
+
+
+if __name__ == "__main__":
+    part_1()