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Advent of Code 2017 Day 1

Part 1

Circular lists from the beginning!
Writing my working algorithm

Circular lists from the beginning!

I knew it was a recurring theme throughout the year
But I didn't know it started with Day 1

Writing my working algorithm

An outline of my algorithm

Split the input into an array of numbers
For each number
  Accumulate a sum, starting a 0
    If the next number (wrapping if necessary) is the same number
      Increment the sum by the number at the current index
    Otherwise, increment the sum by 0...leaving it unchanged

A reduce() that leverages all of the possible parameters:

return input
  .split('')
  .map(Number)
  .reduce((acc, curr, index, RA) =>
    acc += RA[index] == RA[(index + 1) % RA.length]
        ? curr : 0
  , 0)

Part 2

Adjusting my working algorithm

Instead of checking the next index, I'll check the index half a list away.

An outline of my algorithm

Split the input into an array of numbers
For each number
  Accumulate a sum, starting a 0
    If the number half a list length away (wrapping if necessary) is the same number
      Increment the sum by the number at the current index
    Otherwise, increment the sum by 0...leaving it unchanged

A small tweak to the number being added to index, and Voila!

return input
  .split('')
  .map(Number)
  .reduce((acc, curr, index, RA) =>
    acc += RA[index] == RA[(index + (RA.length / 2)) % RA.length]
        ? curr : 0
  , 0)