Advent of Code 2020 Day 2
Task: Solve for X where...
X = the number of valid passwords
Example input
1-3 a: abcde
1-3 b: cdefg
2-9 c: ccccccccc
It represents:
- A password policy, followed by
- A password
Part 1
- Comparing
split()withRegEx - Leveraging a helper function from a prior puzzle
- Writing a working algorithm
Comparing split() with RegEx
- I completed both parts of this puzzle when I first discovered it a year ago
- But my solution used
split()because I was far less adept at using regular expressions
Given this line:
1-3 a: abcde
Using split required me to:
Separate at ': '
['1-3 a', 'abcde']
Separate the first item at ' ' and flatten
['1-3','a', 'abcde']
Separate the first item in the first item at '-' and flatten
['1','3','a','abcde']
Using regex offered a simpler approach:
Match each capture group in the following RegEx
/(\d+)-(\d+) (\w): (\w+)/g
Leveraging a helper function from a prior puzzle
Part 1's password policy requires that the specified character be at a location in the string designated by boundaries of a range:
For this line:
1-3 a: abcde
The range is 1,2,3
For this line:
2-9 c: ccccccccc
The range is 2,3,4,5,6,7,8,9
In a puzzle featured earlier in this series - meaning one of the later 2020 puzzles, I wrote an algorithm to generate such a list:
const createRange = (lower, upper) => {
return new Array(upper - (lower - 1)).fill(0).map((item, index) => index + lower)
}
Writing a working algorithm
Generate a list of matched capture groups for each line using the regular expression above
Filter that list to only include items where the following operations return 'true'
Create a range of numbers that span the lower and upper designated boundaries from the first and second captured groups
Check that list of numbers for the inclusion of a number that is equivalent to the number returned from the following operations:
Split the password string into an array of characters
Filter the list of characters to only include characters that match the character specified by this item's policy
Return the length of the filtered list
Return the length of the filtered list
Here's a visualization of my algorithm:
Part 2
Writing a slightly modified, working algorithm
Generate a list of matched capture groups for each line using the regular expression above
Filter that list to only include items where the following operations return 'true'
Create a 2-element list containing the lower and upper designated boundaries from the first and second captured groups
Filter that list to include either 0, 1 or both numbers based on the following operations:
For each of the two numbers:
Split the password string into an array of characters
Remove N characters from the beginning of the list where N = the current number minus 1
Return true if the first character is the one specified in the policy
Return true if the filtered list only contains one number
Return the length of the filtered list
Here's a visualization of my algorithm:
Accomplishments
- Feeling confident enough to use
RegExinstead ofsplit()to solve this puzzle more eloquently - Re-using a helper function from a previous puzzle's solution
- Writing both parts each as one long chained statement
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