day4-1.prdc

l]$ε"\d+"El¨Im4>:LÎ+x}]μ2/(›\μ~¨}¨‡ß}¨$‹_{GL_Æ}×‹Þ .. 50 bytes

{
l]$ .. sort lines
ε   .. for each line:
  "\d+"El¨Im .. regex match runs of digits, flatten, convert to integers
  4> .. take the [4:] slice
     .. guard-beginning-shift rows have 2 numbers; the others have 1
  :L .. compute the length
  Î  .. is it 1?
  +x .. concatenate if so
}
.. we now have many [min, id, min, min, ...] lists on the stack
] .. collect it into a list
μ    .. map across each pair:
  2/ .. split into pairs again:
     .. [[useless min, id], [min, min], [min, min]...]
  (  .. uncons so: [[min, min], [min, min]...] [useless min, id]
  ›\ .. extract that ID, swap it under: id, [[min, min], ...]
  μ~¨}¨ .. convert every pair to a half-open range, flatten
  ৠ.. make pairs with the id: [[id, min], ...]
}
¨  .. flatten again
$  .. sort (lexicographically)
‹_ .. the function to take the first element of a list
{GL_Æ} .. "group, take maximum by length"
× .. do that twice, which works out:
  ..
  .. group (by id), take maximum by length: this leaves
  .. [[id, min], ...] for only the ID of the guard most often asleep
  ..
  .. group (by everything), take maximum by length: this leaves [[id, min], ...] over and over
‹Þ .. take an element and compute the product
};