56 lines
1.7 KiB
PHP
56 lines
1.7 KiB
PHP
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<?php
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[$template, $rules] = explode("\n\n", file_get_contents('./input.txt'));
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$template = str_split($template);
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$pair_insertion_rules = [];
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foreach (explode("\n", $rules) as $rule) {
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[$key, $value] = explode(' -> ', $rule);
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$pair_insertion_rules[$key] = $value;
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}
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$pair_counts = count_pairs($template);
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for ($i = 0; $i < 40; $i++) {
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$new_pair_counts = $pair_counts;
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foreach ($pair_counts as $key => $value) {
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$substitute = $pair_insertion_rules[$key];
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$pair1 = $key[0] . $substitute;
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$pair2 = $substitute . $key[1];
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$new_pair_counts[$pair1] = array_key_exists($pair1, $new_pair_counts) ? $new_pair_counts[$pair1] + $value : $value;
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$new_pair_counts[$pair2] = array_key_exists($pair2, $new_pair_counts) ? $new_pair_counts[$pair2] + $value : $value;
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$new_pair_counts[$key] = $new_pair_counts[$key] - $value;
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if ($new_pair_counts[$key] < 1) {
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unset($new_pair_counts[$key]);
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}
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}
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$pair_counts = $new_pair_counts;
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}
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$count_elements = [];
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foreach ($pair_counts as $pair => $pair_count) {
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foreach (str_split($pair) as $element) {
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$count_elements[$element] = array_key_exists($element, $count_elements) ? $count_elements[$element] + $pair_count : $pair_count;
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}
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}
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array_walk($count_elements, static function (&$count_element) {
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$count_element = ceil($count_element / 2);
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});
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function count_pairs($template): array
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{
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$counts = [];
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for ($i = 0; $i < count($template) - 1; $i++) {
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$pair = $template[$i] . $template[$i + 1];
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$counts[$pair] = array_key_exists($pair, $counts) ? $counts[$pair]+1 : 1;
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}
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return $counts;
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}
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echo max($count_elements) - min($count_elements);
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