2017 Advent of Code - day 1

gistfile1.txt

$in = '1212'

# Just regex out the characters which are the same as the following character, with a backreference
# convert to int, and sum them. Handle the wraparound condition by adding the first character
# to the end of the string

function day1 {
    param($s)

    $s = $s + $s[0]
    [regex]::Matches($s, '(.)(?=\1)') |% { [int]($_.Groups[1].Value) } | measure -Sum | % sum
}

day1 $in


# Can't do part2 with regex as easily;
# loop through the indexes, use remainder calculation to wrap around.
# careful to convert strings to digit-value-ints, not chars to ASCII-character-code-ints

function day1-part2 {
    param([string]$s)

    $halfway = $s.Length / 2

    $nums = for ($i = 0; $i -lt $s.Length; $i++)
    {
        $nextidx = ($i + $halfway) % $s.Length
        if ($s[$i] -eq $s[$nextidx])
        {
            [int][string]$s[$i]
        }
    }
    $nums | Measure-Object -Sum | Select-Object -ExpandProperty Sum
}

day1-part2 $in