PHP - Adventofcode - Day 1

Question

<?php

// From adventofcode.com

// Question 1:
// An opening parenthesis, (, means he should go up one floor, and a closing parenthesis,
// ), means he should go down one floor.
// To what floor do the instructions take Santa?

// Question 2:
// Now, given the same instructions, find the position of the first character that causes him to enter the basement
// (floor -1). The first character in the instructions has position 1, the second character has position 2, and so on.
// For example:
// ) causes him to enter the basement at character position 1.
// ()()) causes him to enter the basement at character position 5.
// What is the position of the character that causes Santa to first enter the basement?

$puzzle = '()(((()))(()()()((((()(((())(()(()((((((()(()(((())))((()(((()))((())(()((()()()()(((())(((((((())))()()(()(()(())(((((()()()((())(((((()()))))()(())(((())(())((((((())())))(()())))()))))()())()())((()()((()()()()(()((((((((()()())((()()(((((()(((())((())(()))()((((()((((((((())()((()())(())((()))())((((()())(((((((((((()()(((((()(()))())(((()(()))())((()(()())())())(()(((())(())())()()(()(()((()))((()))))((((()(((()))))((((()(()(()())())()(((()((((())((((()(((()()(())()()()())((()((((((()((()()))()((()))()(()()((())))(((()(((()))((()((()(()))(((()()(()(()()()))))()()(((()(((())())))))((()(((())()(()(())((()())))((((())))(()(()(()())()((()())))(((()((()(())()()((()((())(()()((())(())()))()))((()(())()))())(((((((()(()()(()(())())))))))(()((((((())((((())((())())(()()))))()(())(()())()())((())(()))))(()))(()((()))()(()((((((()()()()((((((((()(()(())((()()(()()))(())()())()((())))()))()())(((()))(())()(())()))()((()((()(()()())(())()()()((())())))((()()(()()((()(())()()())(((()(()()))))(())))(()(()())()))()()))))))()))))((((((())))())))(()(())())(()())))))(()))()))))))()((()))))()))))(()(()((()())())(()()))))(((())()))())())())(((()(()()))(())()(())(())((((((()()))))((()(()))))))(()))())(((()()(()))()())()()()())))))))))))))(())(()))(()))((()(())(()())(())())(()())(())()()(()())))()()()))(())())()))())())(())((())))))))(())))(())))))()))))((())(()(((()))))(()))()((()(())))(()())(((((()))()())()()))))()))))()))())(()(()()()))()))))))((()))))))))))()((()))((()(())((())()()(()()))()(()))))()()(()))()))(((())))(())()((())(())(()())()())())))))))())))()((())))()))(()))()()))(((((((()))())(()()))(()()(()))()(()((()())()))))))(((()()()())))(())()))()())(()()))()()))))))))(())))()))()()))))))()))()())))()(())(())))))()(())()()(()()))))())((()))))()))))(()(((((()))))))))())))())()(())()()))))(())))())()()())()()())()(()))))()))()))))))))())))((()))()))()))())))()())()()())))())))(()((())()((()))())))))())()(())((())))))))))))())()())(())())())(()))(()))()))())(()(())())()())()()(()))))(()(())))))))(())))())(())))))))())()()(())())())))(())))))()))()(()())()(()))())())))))()()(()))()))))())))))))))()))))()))))))())()())()()))))()())))())))))))))))()()))))()()(((()))()()(())()))))((()))))(()))(())())))(())()))))))(()))()))))(())())))))()))(()())))))))))))))())))))))))()((()())(()())))))))((()))))(())(())))()(()())())))())())(()()()())))()))))))())))))())()()())))))))))))()()(()))))()())()))((()())(()))))()(()))))))))))()())())(((())(()))))())()))()))()))))))()))))))(()))))()))))()(())))(())))(()))())()()(()()))()))(()()))))))))()))(()))())(()()(()(()())()()))()))))))))(())))))((()()(()))())())))))()))())(()())()()))())))()(()()()()))((())())))())()(()()))()))))))))(()))(())))()))))(()(()())(()))))()())())()))()()))())))))))))))())()))))))()))))))))())))))()))))())(()())))(())()))())())))))()()(()()())(()())))()()))(((()))(()()()))))()))))()))))((())))()((((((()()))))))())))))))))))(((()))))))))))))(())())))))())(()))))))(()))((()))())))()(()((()))()))()))))))))))())()))()(()()))))())))())(())()(()))()))())(()))()))))(()()))()()(())))))()))(())(()(()()))(()()())))))(((()))))))()))))))))))))(())(()))))()())())()()((()()))())))))(()))))())))))))()()()))))))))())))()(((()()))(())))))(((())())))))((()))()(()))(()))))(()())))(()))())))))()))))(())(())))()((()))(())())))()()))()))))))))()))(()()()(()()()(()))())(())()())(((()))(())))))))))(((()())))()()))))))))()(())(()))()((((())(())(()())))()))(((())()()()))((()))(()))())())))())))(()))())()())())(()(())())()()()(())))())(())))(())))(())()))()))(()((()))))))))())(()))))))())(()()))()()))()(()(()())))()()(()((()((((((()))(())))()()()))())()))((()()(()))())((()(()(()))(()()))))()())))()))()())))))))()()((()())(())))()))(()))(())(()))())(()(())))()()))))))(((()(((()()))()(()(())())((()()))()))()))()))()(()()()(()))((()())()(())))()()))(((())()()())(())()((()()()()(()(())(()()))()(((((()())))((())))))(()()()))))(((()(())))()))((()((()(())()(()((())))((()())()(()))(((()())()()(()))(())(((()((()())()((())()())(((()()))((()((())(()))(()())(()()()))((()))(())(()((()()())((()))(())))(())(())(())))(()())))(((((()(()(((((()())((((()(()())(())(()()(((())((()(((()()(((()()((((((())))())(()((((((()(()))()))()()((()((()))))()(()()(()((()()))))))(((((()(((((())()()()(())())))))))()))((()()(())))(())(()()()())))))(()((((())))))))()()(((()(()(()(()(()())()()()(((((((((()()())()(()))((()()()()()(((((((()())()((())()))((((((()(()(()(()())(((()(((((((()(((())(((((((((())(())())()))((()(()))(((()()())(())(()(()()(((()(())()))())))(())((((((())(()()())()()(((()(((())(()(((())(((((((()(((((((((()))(())(()(()(()))))((()))()(())())())((()(()((()()))((()()((()(())(())(()((())(((())(((()()()((((((()()(())((((())()))))(())((()(()((())))(((((()(()()())())((())())))((())((()((()()((((((())(((()()(()())())(()(()))(()(()))())())()(((((((()(((()(())()()((())((()(()()((()(()()(((((((((((())((())((((((())((()((((()(()((((()(((((((())()((()))))())()((()((((()(()(((()((()())))(())())(((()(((())((((((()(((((((((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$nbItems = strlen($puzzle);

$floor = 0;
$foundPosition = false;

for ($i=0; $i < $nbItems; $i++)
{
  $char = $puzzle[$i];

  if ($floor === -1 && $foundPosition === false)
  {
    $foundPosition = $i;
  }

  if ($char === '(')
  {
    $floor++;
  }
  else
  {
    $floor--;
  }
}

print $floor . PHP_EOL;
print $foundPosition . PHP_EOL;

?>

The code is correct and returns:

138
1771

I'm looking for the most elegant way / the best time complexity to tackle the problem ;)

Answer 1

To what floor do the instructions take Santa?

There is no need to go character by character to determine that, instead:

$puzzle = '...';

$up = substr_count($puzzle, '(');
$down = substr_count($puzzle, ')');
$floor = $up - $down;
// $floor = 138

The position that causes him to enter the basement

This can be significantly simplified e.g.:

$puzzle = '...';

$position = 0;
$floor = 0;
while ($floor !== -1) {
    $char = $puzzle[$position];
    ($char === '(') ? $floor++ : $floor--;
    $position++;
}
// $position = 1771

Note that there's no need to continue to the end once the answer has been obtained - this alone make determining the answer faster.

This doesn't defend against out-of-bound problems, but it also doesn't need to.