Solution to problem 15 in Python

src/P15.py

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+# --- Day 15: Rambunctious Recitation ---
+
+# You catch the airport shuttle and try to book a new flight to your vacation
+# island. Due to the storm, all direct flights have been cancelled, but a route
+# is available to get around the storm. You take it.
+
+# While you wait for your flight, you decide to check in with the Elves back at
+# the North Pole. They're playing a memory game and are ever so excited to
+# explain the rules!
+
+# In this game, the players take turns saying numbers. They begin by taking
+# turns reading from a list of starting numbers (your puzzle input). Then, each
+# turn consists of considering the most recently spoken number:
+
+#     If that was the first time the number has been spoken, the current player
+# says 0.
+#     Otherwise, the number had been spoken before; the current player
+# announces how many turns apart the number is from when it was previously
+# spoken.
+
+# So, after the starting numbers, each turn results in that player speaking
+# aloud either 0 (if the last number is new) or an age (if the last number is a
+# repeat).
+
+# For example, suppose the starting numbers are 0,3,6:
+
+#     Turn 1: The 1st number spoken is a starting number, 0.
+#     Turn 2: The 2nd number spoken is a starting number, 3.
+#     Turn 3: The 3rd number spoken is a starting number, 6.
+#     Turn 4: Now, consider the last number spoken, 6. Since that was the first
+# time the number had been spoken, the 4th number spoken is 0.
+#     Turn 5: Next, again consider the last number spoken, 0. Since it had been
+# spoken before, the next number to speak is the difference between the turn
+# number when it was last spoken (the previous turn, 4) and the turn number of
+# the time it was most recently spoken before then (turn 1). Thus, the 5th
+# number spoken is 4 - 1, 3.
+#     Turn 6: The last number spoken, 3 had also been spoken before, most
+# recently on turns 5 and 2. So, the 6th number spoken is 5 - 2, 3.
+#     Turn 7: Since 3 was just spoken twice in a row, and the last two turns
+# are 1 turn apart, the 7th number spoken is 1.
+#     Turn 8: Since 1 is new, the 8th number spoken is 0.
+#     Turn 9: 0 was last spoken on turns 8 and 4, so the 9th number spoken is
+# the difference between them, 4.
+#     Turn 10: 4 is new, so the 10th number spoken is 0.
+
+# (The game ends when the Elves get sick of playing or dinner is ready,
+# whichever comes first.)
+
+# Their question for you is: what will be the 2020th number spoken? In the
+# example above, the 2020th number spoken will be 436.
+
+# Here are a few more examples:
+
+#     Given the starting numbers 1,3,2, the 2020th number spoken is 1.
+#     Given the starting numbers 2,1,3, the 2020th number spoken is 10.
+#     Given the starting numbers 1,2,3, the 2020th number spoken is 27.
+#     Given the starting numbers 2,3,1, the 2020th number spoken is 78.
+#     Given the starting numbers 3,2,1, the 2020th number spoken is 438.
+#     Given the starting numbers 3,1,2, the 2020th number spoken is 1836.
+
+# Given your starting numbers, what will be the 2020th number spoken?
+
+from collections import defaultdict
+
+with open("files/P15.txt", "r") as f:
+    sequence = [int(val) for val in f.read().strip().split(",")]
+
+
+numbers = defaultdict(list)
+for idx, number in enumerate(sequence):
+    numbers[number].append(idx)
+
+
+def part_1() -> None:
+    last = list(numbers.keys())[-1]
+    for i in range(len(sequence), 2020):
+        if len(numbers[last]) < 2:
+            numbers[0].append(i)
+            last = 0
+        else:
+            last = numbers[last][-1] - numbers[last][-2]
+            numbers[last].append(i)
+
+    print(f"The 2020th number spoken is {last}")
+
+
+if __name__ == "__main__":
+    part_1()