Nim

parts 1 and 2 - easy

For part 3 - When I first looked at the example input - it seemed a bit daunting to solve. But then I had a hunch and decided to check the real input and turns out - I was right! The real input is easier to solve because it's longer than 1000 chars.

This means that there is only 3 possible configurations we care about in repeated input: leftmost section, rightmost section and 998 identical sections in the middle. We solve each individually and sum them.

Another trick I used is looking up mentors with modulo to avoid copying the input.

proc solve_part1*(input: string): Solution =
  var mentors: CountTable[char]
  for c in input:
    if c notin {'a','A'}: continue
    if c.isUpperAscii: mentors.inc c
    else:
      result.intVal += mentors.getOrDefault(c.toUpperAscii)

proc solve_part2*(input: string): Solution =
  var mentors: CountTable[char]
  for c in input:
    if c.isUpperAscii: mentors.inc c
    else:
      result.intVal += mentors.getOrDefault(c.toUpperAscii)

proc solve_part3*(input: string): Solution =
  var mentors: Table[char, seq[int]]
  for index in -1000 ..< input.len + 1000:
    let mi = index.euclMod input.len
    if input[mi].isLowerAscii: continue
    let lower = input[mi].toLowerAscii
    if mentors.hasKeyOrPut(lower, @[index]):
      mentors[lower].add index

  var most, first, last = 0
  for ci, ch in input:
    if ch.isUpperAscii: continue
    for mi in mentors[ch]:
      let dist = abs(mi - ci)
      if dist <= 1000:
        inc most
        if mi >= 0: inc first
        if mi <= input.high: inc last

  result := first + (most * 998) + last

Full solution at Codeberg: solution.nim