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Day 13 (Python)

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<!-- PARSE START --> <!-- PARSE START -->
| Day | | Python | Go | | Day | | Python | Go |
| --------------------------- | ----------------------------- | ------------------------------------- | --------------------------------- | | --------------------------- | ------------------------------ | ------------------------------------- | --------------------------------- |
| [1](/01-reportRepair) | ![Completed][check] | [Link](/01-reportRepair/python) | [Link](/01-reportRepair/go) | | [1](/01-reportRepair) | ![Completed][check] | [Link](/01-reportRepair/python) | [Link](/01-reportRepair/go) |
| [2](/02-passwordPhilosophy) | ![Completed][check] | [Link](/02-passwordPhilosophy/python) | [Link](/02-passwordPhilosophy/go) | | [2](/02-passwordPhilosophy) | ![Completed][check] | [Link](/02-passwordPhilosophy/python) | [Link](/02-passwordPhilosophy/go) |
| [3](/03-tobogganTrajectory) | ![Completed][check] | [Link](/03-tobogganTrajectory/python) | [Link](/03-tobogganTrajectory/go) | | [3](/03-tobogganTrajectory) | ![Completed][check] | [Link](/03-tobogganTrajectory/python) | [Link](/03-tobogganTrajectory/go) |
| [4](/04-passportProcessing) | ![Completed][check] | [Link](/04-passportProcessing/python) | [Link](/04-passportProcessing/go) | | [4](/04-passportProcessing) | ![Completed][check] | [Link](/04-passportProcessing/python) | [Link](/04-passportProcessing/go) |
| [5](/05-binaryBoarding) | ![Completed][check] | [Link](/05-binaryBoarding/python) | [Link](/05-binaryBoarding/go) | | [5](/05-binaryBoarding) | ![Completed][check] | [Link](/05-binaryBoarding/python) | [Link](/05-binaryBoarding/go) |
| [6](/06-customCustoms) | ![Completed][check] | [Link](/06-customCustoms/python) | [Link](/06-customCustoms/go) | | [6](/06-customCustoms) | ![Completed][check] | [Link](/06-customCustoms/python) | [Link](/06-customCustoms/go) |
| [7](/07-handyHaversacks) | ![Completed][check] | [Link](/07-handyHaversacks/python) | [Link](/07-handyHaversacks/go) | | [7](/07-handyHaversacks) | ![Completed][check] | [Link](/07-handyHaversacks/python) | [Link](/07-handyHaversacks/go) |
| [8](/08-handheldHalting) | ![Completed][check] | [Link](/08-handheldHalting/python) | [Link](/08-handheldHalting/go) | | [8](/08-handheldHalting) | ![Completed][check] | [Link](/08-handheldHalting/python) | [Link](/08-handheldHalting/go) |
| [9](/09-encodingError) | ![Completed][check] | [Link](/09-encodingError/python) | [Link](/09-encodingError/go) | | [9](/09-encodingError) | ![Completed][check] | [Link](/09-encodingError/python) | [Link](/09-encodingError/go) |
| [10](/10-adapterArray) | ![Completed][check] | [Link](/10-adapterArray/python) | [Link](/10-adapterArray/go) | | [10](/10-adapterArray) | ![Completed][check] | [Link](/10-adapterArray/python) | [Link](/10-adapterArray/go) |
| [11](/11-seatingSystem) \* | ![Completed][check] | [Link](/11-seatingSystem/python) | [Link](/11-seatingSystem/python) | | [11](/11-seatingSystem) \* | ![Completed][check] | [Link](/11-seatingSystem/python) | [Link](/11-seatingSystem/python) |
| [12](/12-rainRisk) \* | ![Completed][check] | [Link](/12-rainRisk/python) | [Link](/12-rainRisk/go) | | [12](/12-rainRisk) \* | ![Completed][check] | [Link](/12-rainRisk/python) | [Link](/12-rainRisk/go) |
| 13 | ![Not yet attempted][pending] | | | | [13](/13-shuttleSearch) | ![Partially complete][partial] | [Link](/13-shuttleSearch/python) | |
| 14 | | | | | 14 | | | |
| 15 | | | | | 15 | | | |
| 16 | | | | | 16 | | | |
| 17 | | | | | 17 | | | |
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| 20 | | | | | 20 | | | |
| 21 | | | | | 21 | | | |
| 22 | | | | | 22 | | | |
| 23 | | | | | 23 | | | |
| 24 | | | | | 24 | | | |
| 25 | | | | | 25 | | | |
<!-- PARSE END --> <!-- PARSE END -->

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# [Day 13: Shuttle Search](https://adventofcode.com/2020/day/13)
No, I don't really know why the maths works, but it works!
Here are the notes I made for part two to work this out for a test case before implementing it for the first time to make sure that my method worked.
![test notes](working.jpg)
### Related
* [Maths with Jay - Chinese remainder theorem](https://www.youtube.com/watch?v=zIFehsBHB8o)
* [StackOverflow - Modular multiplicative inverse function in Python](https://stackoverflow.com/a/9758173)
<details><summary>Script output</summary>
```
❯ python .\python\
AoC 2020: day 13 - Shuttle Search
Python 3.8.5
Test cases
1.1 pass
2.1 pass
2.2 pass
2.3 pass
2.4 pass
2.5 pass
2.6 pass
Answers
Part 1: 4782
Part 2: 1118684865113056
```
</details>

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{
"year": "2020",
"day": "13",
"title": "Shuttle Search",
"testCases": {
"one": [
{
"input": "939\n7,13,x,x,59,x,31,19",
"expected": 295
}
],
"two": [
{
"input": "939\n7,13,x,x,59,x,31,19",
"expected": 1068781
},
{
"input": "1\n17,x,13,19",
"expected": 3417
},
{
"input": "1\n67,7,59,61",
"expected": 754018
},
{
"input": "1\n67,x,7,59,61",
"expected": 779210
},
{
"input": "1\n67,7,x,59,61",
"expected": 1261476
},
{
"input": "1\n1789,37,47,1889",
"expected": 1202161486
}
]
}
}

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import json
import platform
import sys
from rich import print
from partOne import partOne
from partTwo import partTwo
def run_tests(test_cases):
do_tests = True
if len(test_cases) == 0:
do_tests = False
elif len(test_cases["one"]) == 0 and len(test_cases["two"]) == 0:
do_tests = False
if not do_tests:
print("Info: no test cases specified. Skipping tests\n")
return
print("Test cases")
def rt(tcs, f, n):
for i, tc in enumerate(tcs):
print(f"{n}.{i+1} ", end="")
expectedInt = tc["expected"]
result = f(str(tc["input"]))
if result == expectedInt:
print("[green]pass[/green]")
else:
print(f"[red]fail[/red] (got {result}, expected {expectedInt})")
rt(test_cases["one"], partOne, 1)
rt(test_cases["two"], partTwo, 2)
print()
if __name__ == "__main__":
try:
info = open("info.json").read()
except FileNotFoundError:
print("Error: could not open info.json")
sys.exit(-1)
info = json.loads(info)
year = info["year"]
day = info["day"]
title = info["title"]
print(f"[yellow]AoC {year}[/yellow]: day {day} - {title}")
print(f"Python {platform.python_version()}\n")
try:
challenge_input = open("input.txt").read()
except FileNotFoundError:
print("Error: could not open input.txt")
sys.exit(-1)
if "vis" in sys.argv:
import visualise
print("[green]Running visualisation....[/green]")
visualise.visualise(challenge_input)
sys.exit()
run_tests(info["testCases"])
if "debug" in sys.argv:
sys.exit()
print("Answers")
print("Part 1:", partOne(challenge_input))
print("Part 2:", partTwo(challenge_input))

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from typing import List
class Timetable:
earliest_departure: int
services: List[int]
def __init__(self, earliest_departure: int, services: str):
self.earliest_departure = earliest_departure
self.services = [int(s) for s in services.split(",") if s != "x"]
def parse(instr: str) -> Timetable:
instr = instr.strip().split("\n")
return Timetable(int(instr[0]), instr[1])
def partOne(instr: str) -> int:
timetable = parse(instr)
earliest_times = []
for service in timetable.services:
earliest_times.append(
(service, (int(timetable.earliest_departure / service) + 1) * service)
)
route, earliest_departure = min(earliest_times, key=lambda x: x[1])
return route * (earliest_departure - timetable.earliest_departure)

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def partTwo(instr: str) -> int:
# This is the parsing section
service_list = instr.strip().split("\n")[-1].split(",")
eqns = []
for i, svc in enumerate(service_list):
if svc == "x":
continue
svc = int(svc)
v = 0
if i != 0:
v = svc - i # This is the only maths stuff in the parsing
eqns.append((v, svc))
# This is the maths section
n = 1
for (_, v) in eqns:
n *= v
sigma_x = 0
for (bi, ni) in eqns:
# this type cast could potentially cause a problem.
# int required for pow function and the division *should* produce a whole number anyway
Ni = int(n / ni)
yi = pow(Ni, -1, ni) # https://stackoverflow.com/a/9758173
print(Ni, ni, yi)
sigma_x += bi * Ni * yi
return sigma_x % n

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