Day 2. Password Philosophy

Problem statement

https://adventofcode.com/2020/day/2 (mirror)

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My solution

Nothing exciting here.

We analyze input line by line, parse each line with a regular expression.

Example line: 5-11 t: glhbttzvzttkdx

Regular expression: ^(\d+)-(\d+) (\w): (.*?)$, which in readable terms means: (first number)-(second number) (single policy character): (password till the end of line).

Then, depending on the part:

• in Part A, we check if frequency of the policy character in the password is the range specified by first and second numbers; there is a silent assumption that first number is smaller than the second one.

• in Part B, we check the characters at indexes specified by the first and second numbers (remembering that in C# indexes are zero-based, and in our input indexes are one-based), and then flip the bool variable if there is a match with the policy character – we start with false; if exactly one character is a match, then we flip once and end with true; if both characters are matches, we flip twice and end with false.

bool passwordIsValid = false;

if (policyPassword[firstIndex - 1] == policyCharacter)
{
    passwordIsValid = !passwordIsValid;
}

if (policyPassword[secondIndex - 1] == policyCharacter)
{
    passwordIsValid = !passwordIsValid;
}