From e822334261b74295a5788f02da1bf25cea0c61a7 Mon Sep 17 00:00:00 2001
From: Xiretza <xiretza@xiretza.xyz>
Date: Tue, 15 Dec 2020 11:27:54 +0100
Subject: [PATCH] 2020 day15/python: add solution

---
 2020/README.md           |  1 +
 2020/data/day15.expected |  2 ++
 2020/data/day15.input    |  1 +
 2020/day15/day15.py      | 27 +++++++++++++++++++++++++++
 4 files changed, 31 insertions(+)
 create mode 100644 2020/data/day15.expected
 create mode 100644 2020/data/day15.input
 create mode 100644 2020/day15/day15.py

diff --git a/2020/README.md b/2020/README.md
index 70d023d..3074ff9 100644
--- a/2020/README.md
+++ b/2020/README.md
@@ -19,5 +19,6 @@ https://adventofcode.com/2020/
 |11 |  `**`  |         |      |
 |13 |        |  `**`   |      |
 |14 |        |  `**`   |      |
+|15 |  `**`  |         |      |
 
 `test.sh` can be used to run all solutions and automatically compares them to (my) puzzle inputs and the expected outputs.
diff --git a/2020/data/day15.expected b/2020/data/day15.expected
new file mode 100644
index 0000000..6e90eaa
--- /dev/null
+++ b/2020/data/day15.expected
@@ -0,0 +1,2 @@
+496
+883
diff --git a/2020/data/day15.input b/2020/data/day15.input
new file mode 100644
index 0000000..1f8b084
--- /dev/null
+++ b/2020/data/day15.input
@@ -0,0 +1 @@
+2,0,1,7,4,14,18
diff --git a/2020/day15/day15.py b/2020/day15/day15.py
new file mode 100644
index 0000000..eb63221
--- /dev/null
+++ b/2020/day15/day15.py
@@ -0,0 +1,27 @@
+#!/usr/bin/env python
+
+import sys
+
+def run_game(init, iterations):
+    last_seen = {}
+    previous = None
+
+    for i, n in enumerate(init):
+        if previous is not None:
+            last_seen[previous] = i-1
+        previous = n
+
+    for i in range(i+1, iterations):
+        if previous in last_seen:
+            x = i - last_seen[previous] - 1
+        else:
+            x = 0
+        last_seen[previous] = i-1
+        previous = x
+
+    return previous
+
+if __name__ == '__main__':
+    init = [int(n) for n in sys.stdin.read().split(',')]
+    print(run_game(init, 2020))
+    print(run_game(init, 30000000))