From 6788ee82183ec33bd7cccb9cf9a74aa0579a9b69 Mon Sep 17 00:00:00 2001
From: Ettore Dreucci <ettore.dreucci@gmail.com>
Date: Sun, 13 Dec 2020 22:26:14 +0100
Subject: [PATCH] AoC 2020: day13

Signed-off-by: Ettore Dreucci <ettore.dreucci@gmail.com>
---
 2020-python/inputs/day_13       |  2 +
 2020-python/solutions/day_13.py | 87 +++++++++++++++++++++++++++++++++
 2 files changed, 89 insertions(+)
 create mode 100644 2020-python/inputs/day_13
 create mode 100644 2020-python/solutions/day_13.py

diff --git a/2020-python/inputs/day_13 b/2020-python/inputs/day_13
new file mode 100644
index 0000000..0a5136f
--- /dev/null
+++ b/2020-python/inputs/day_13
@@ -0,0 +1,2 @@
+1000507
+29,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,37,x,x,x,x,x,631,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,x,13,19,x,x,x,23,x,x,x,x,x,x,x,383,x,x,x,x,x,x,x,x,x,41,x,x,x,x,x,x,17
diff --git a/2020-python/solutions/day_13.py b/2020-python/solutions/day_13.py
new file mode 100644
index 0000000..cd22d38
--- /dev/null
+++ b/2020-python/solutions/day_13.py
@@ -0,0 +1,87 @@
+"""AOC 2020 Day 13"""
+
+import pathlib
+import time
+
+TEST_INPUT = """939
+7,13,x,x,59,x,31,19"""
+
+def read_input(input_path: str) -> str:
+    """take input file path and return a str with the file's content"""
+    with open(input_path, 'r') as input_file:
+        input_data = input_file.read().strip()
+        return input_data
+
+def extract(input_data: str) -> tuple:
+    """take input data and return the appropriate data structure"""
+    entries = input_data.split('\n')
+    departure_ts = int(entries[0])
+    timetable = entries[1].split(',')
+    ids = [(-index, int(bus_id)) for index, bus_id in enumerate(timetable) if bus_id != 'x']
+    return (departure_ts, ids)
+
+def egcd(a, b):
+    """"extended euclidean algorithm"""
+    if a == 0:
+        return (b, 0, 1)
+
+    g, y, x = egcd(b % a, a)
+    return (g, x - (b // a) * y, y)
+
+def modinv(x, m):
+    """calculate the modular multiplicative inverse"""
+    g, inv, _ = egcd(x, m)
+    assert g == 1
+    return inv % m
+
+def part1(entries: tuple) -> int:
+    """part1 solver"""
+    wait_time = float('inf')
+    best_bus_id = -1
+    min_dep_time = entries[0]
+    for _, bus_id in entries[1]:
+        if min_dep_time % bus_id == 0:
+            multiple = min_dep_time // bus_id
+        else:
+            multiple = (min_dep_time // bus_id) + 1
+        time_diff = (bus_id * multiple) - min_dep_time
+        if time_diff < wait_time:
+            wait_time = time_diff
+            best_bus_id = bus_id
+    return wait_time * best_bus_id
+
+def part2(entries: tuple) -> int:
+    """part2 solver"""
+    min_timestamp = 0
+    moduli_product = 1
+    for _, modulo in entries[1]:
+        moduli_product *= modulo
+    for remainder, modulo in entries[1]:
+        ni = moduli_product // modulo
+        modulo_inv = modinv(ni, modulo)
+        min_timestamp += (remainder * ni * modulo_inv)
+    return min_timestamp % moduli_product
+
+def test_input_day_13():
+    """pytest testing function"""
+    entries = extract(TEST_INPUT)
+    assert part1(entries) == 295
+    assert part2(entries) == 1068781
+
+def test_bench_day_13(benchmark):
+    """pytest-benchmark function"""
+    benchmark(main)
+
+def main():
+    """main function"""
+    input_path = str(pathlib.Path(__file__).resolve().parent.parent) + "/inputs/" + str(pathlib.Path(__file__).stem)
+    start_time = time.time()
+    input_data = read_input(input_path)
+    entries = extract(input_data)
+    print("Part 1: %d" % part1(entries))
+    print("Part 2: %d" % part2(entries))
+    end_time = time.time()
+    print("Execution time: %f" % (end_time-start_time))
+
+if __name__ == "__main__":
+    main()