# Advent of Code 2020: Day 2

Day 2 was about processing lines that contained two numbers, a character, and a string which is referred to as a password. Both parts are about using the numbers and the character to determine if the password is “valid”. How the numbers and character become a rule is different in parts 1 and 2.

## Challenge

The puzzle can be found here. I’m given a text file with many lines, each with three parts - a number range, a character, and a password, in the following format:

`1-3 a: abcde 1-3 b: cdefg 2-9 c: ccccccccc`

For part one, the range represents the number of times that that given characters has to appear in the password, and I need to figure out how many valid passwords are in my list.

For part two the policy is different. The two numbers represent two character positions in the password, where one and only one of those positions must be the given character. Again, I’m to return the number of valid passwords in the list.

## Solution

### Part 1

For part one, I’ll create a function that takes a line, breaks it into the four parts (two numbers, character, and password), and then returns true or false based on the number of times the character is in the password. Then I’ll use that function in a list comprehension to create a list of passwords that return true from that function, and just take the length of it:

```
#!/usr/bin/env python3
import sys
def line_valid(line):
r, c, p = line.split(" ")
c = c.rstrip(":")
r_low, r_high = map(int, r.split("-"))
return r_low <= p.count(c) <= r_high
with open(sys.argv[1], "r") as f:
lines = f.readlines()
num_valid1 = len([x for x in lines if line_valid(x)])
print(f"Part 1: {num_valid1}")
```

### Part 2

In part two, same idea, just different interpretation of the numbers. I thought about creating a new function to parse the line into its four components, but then I realized I could just use the same function with another arg. I’ll add an arg `part`

with a default value of 1, and then just branch at the end based on that arg as to which criteria to apply:

```
#!/usr/bin/env python3
import sys
def line_valid(line, part=1):
r, c, p = line.split(" ")
c = c.rstrip(":")
r_low, r_high = map(int, r.split("-"))
if part==1:
return r_low <= p.count(c) <= r_high
return (p[r_low - 1] == c) ^ (p[r_high - 1] == c)
with open(sys.argv[1], "r") as f:
lines = f.readlines()
num_valid1 = len([x for x in lines if line_valid(x)])
num_valid2 = len([x for x in lines if line_valid(x, part=2)])
print(f"Part 1: {num_valid1}")
print(f"Part 2: {num_valid2}")
```

It is important to note the question’s hint about their numbering starting at 1, not 0, which is why I subtract 1 from the number before finding that value in the string, as Python is 0 based.

Running this gives both answers:

```
$ time python3 day02.py 02-puzzle_input.txt
Part 1: 454
Part 2: 649
real 0m0.040s
user 0m0.023s
sys 0m0.012s
```