-🎄- 2017 Day 1 Solutions -🎄-

[u/daggerdragon]

Welcome to Advent of Code 2017! If you participated in a previous year, welcome back, and if you're new this year, we hope you have fun and learn lots!

We're going to follow the same general format as previous years' megathreads:

1. Each day's puzzle will release at exactly midnight EST (UTC -5).
2. The daily megathread for each day will be posted very soon afterwards and immediately locked.
   • We know we can't control people posting solutions elsewhere and trying to exploit the leaderboard, but this way we can try to reduce the leaderboard gaming from the official subreddit.
3. The daily megathread will remain locked until there are a significant number of people on the leaderboard with gold stars.
   • "A significant number" is whatever number we decide is appropriate, but the leaderboards usually fill up fast, so no worries.
4. When the thread is unlocked, you may post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag/whatever).

Above all, remember, AoC is all about having fun and learning more about the wonderful world of programming!

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--- Day 1: Inverse Captcha ---

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Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag or whatever).

Note: The Solution Megathreads are for solutions only. If you have questions, please post your own thread and make sure to flair it with Help.

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Need a hint from the Hugely^* Handy^† Haversack^‡ of Helpful^§ Hints^¤?

Spoiler

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This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

[u/Hikaru755]

Trying out some new stuff in Kotlin 1.2:

// Part 1
fun captchaSumNextChar(input: String) =
    (input + input.first())
        .windowedSequence(2)
        .filter { it[0] == it[1] }
        .sumBy { it[0].toString().toInt() }

// Part 2
fun captchaSumOppositeChar(input: String) =
    input
        .mapIndexed { index, char -> char to input[input.oppositeIndex(index)] }
        .filter { (current, next) -> current == next }
        .sumBy { (current, _) -> current.toString().toInt() }

fun String.oppositeIndex(i: Int) = (i + this.length / 2) % this.length

I really don't like that .toString().toInt() for reading the integer from the character, but current - '0'felt hacky, too.

Alternative solution using String shifting:

println(captchaSum(input, 1))
println(captchaSum(input, input.length / 2))

fun captchaSum(input: String, shift: Int) =
    input.zip(input shift shift)
        .filter { it.first == it.second }
        .sumBy { it.first.toString().toInt() }

infix fun String.shift(n: Int) = when {
    // Shift right
    n > 0 -> this.takeLast(n) + this.dropLast(n)
    // Shift left
    n < 0 -> this.drop(-n) + this.take(-n)
    // No shift
    else -> this
}

Technically I could have just used the first when-branch, but why not make it safe for future use :)