2020 Day 15

2020/15/README.md

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+# 2020 Day 15: Rambunctious Recitation
+#### [Direct Link](https://adventofcode.com/2020/day/15)
+
+## Part 1
+
+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 `1`st number spoken is a starting number, **`0`**.
+- **Turn 2**: The `2`nd number spoken is a starting number, **`3`**.
+- **Turn 3**: The `3`rd 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 `4`th 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 `6`th 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 `7`th number spoken is **`1`**.
+- **Turn 8**: Since `1` is new, the `8`th number spoken is **`0`**.
+- **Turn 9**: `0` was last spoken on turns `8` and `4`, so the `9`th number spoken is the difference between them, **`4`**.
+- **Turn 10**: `4` is new, so the `10`th 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 **`2020`th** number spoken? In the example above, the `2020`th number spoken will be `436`.
+
+Here are a few more examples:
+
+- Given the starting numbers `1,3,2`, the `2020`th number spoken is `1`.
+- Given the starting numbers `2,1,3`, the `2020`th number spoken is `10`.
+- Given the starting numbers `1,2,3`, the `2020`th number spoken is `27`.
+- Given the starting numbers `2,3,1`, the `2020`th number spoken is `78`.
+- Given the starting numbers `3,2,1`, the `2020`th number spoken is `438`.
+- Given the starting numbers `3,1,2`, the `2020`th number spoken is `1836`.
+
+Given your starting numbers, **what will be the 2020th number spoken?**
+
+## Part 2
+
+Impressed, the Elves issue you a challenge: determine the 30000000th number spoken. For example, given the same starting numbers as above:
+
+- Given `0,3,6`, the `30000000th` number spoken is `175594`.
+- Given `1,3,2`, the `30000000th` number spoken is `2578`.
+- Given `2,1,3`, the `30000000th` number spoken is `3544142`.
+- Given `1,2,3`, the `30000000th` number spoken is `261214`.
+- Given `2,3,1`, the `30000000th` number spoken is `6895259`.
+- Given `3,2,1`, the `30000000th` number spoken is `18`.
+- Given `3,1,2`, the `30000000th` number spoken is `362`.
+
+Given your starting numbers, **what will be the 30000000th number spoken?**

2020/15/code.py

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+# SPDX-License-Identifier: MIT
+# Copyright (c) 2020 Akumatic
+#
+#https://adventofcode.com/2020/day/15
+
+def readFile() -> tuple:
+    with open(f"{__file__.rstrip('code.py')}input.txt", "r") as f:
+        return [int(num) for num in f.readline().split(",")]
+
+def play(input: list, turns: int) -> int:
+    mem = {val: [idx + 1] for idx, val in enumerate(input)}
+    prev = input[-1]
+    for turn in range(len(input) + 1, turns + 1):
+        # set last spoken number
+        if prev not in mem or len(mem[prev]) == 1:
+            prev = 0
+        else: 
+            prev = mem[prev][-1] - mem[prev][-2]
+        # store the counter
+        if prev in mem:
+            mem[prev].append(turn)
+        else:
+            mem[prev] = [turn]
+
+    return prev
+
+def part1(input: list) -> int:
+    return play(input, 2020)
+
+def part2(input: list) -> int:
+    return play(input, 30000000)
+
+if __name__ == "__main__":
+    input = readFile()
+    print(f"Part 1: {part1(input)}")
+    print(f"Part 2: {part2(input)}")

2020/15/input.txt

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+5,2,8,16,18,0,1

2020/15/solution.txt

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+Part 1: 517
+Part 2: 1047739

2020/15/test_code.py

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+# SPDX-License-Identifier: MIT
+# Copyright (c) 2020 Akumatic
+
+from code import part1, part2
+
+def test():
+    assert part1([0, 3, 6]) == 436
+    assert part1([1, 3, 2]) == 1
+    assert part1([2, 1, 3]) == 10
+    assert part1([1, 2, 3]) == 27
+    assert part1([2, 3, 1]) == 78
+    assert part1([3, 2, 1]) == 438
+    assert part1([3, 1, 2]) == 1836
+    print(f"Passed part 1")
+
+    assert part2([0,3,6]) == 175594
+    assert part2([1,3,2]) == 2578
+    assert part2([2,1,3]) == 3544142
+    assert part2([1,2,3]) == 261214
+    assert part2([2,3,1]) == 6895259
+    assert part2([3,2,1]) == 18
+    assert part2([3,1,2]) == 362
+    print(f"Passed part 2")
+
+if __name__ == "__main__":
+    test()

2020/README.md

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