solution for day 21 part 1

Author: Adrian Reimann

day_21/21-1.php

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+<?php
+
+//define constants for some fixed values
+const WIN_SCORE = 1000;
+const MAX_DIE = 100;
+const MAX_FIELD = 10;
+
+//read input (just need number after last space of each line for the actual position)
+$player_pos = [];
+$input = fopen("input.txt", "r");
+while (($input_line = fgets($input)) !== false) {
+    $line = trim($input_line);
+
+    $split_str = explode(' ', $line);
+    $player_pos[] = array_pop($split_str);
+}
+fclose($input);
+
+//initialise player scores
+$player_scores = [];
+foreach ($player_pos as $no => $pos) {
+    $player_scores[$no] = 0;
+}
+
+//count die rolls and increase the die value every roll
+$die_rolls = 0;
+$die_val = 1;
+while (max($player_scores) < WIN_SCORE) {
+    foreach ($player_pos as $no => $pos) {
+
+        //increase move with 3 rolls, if the die reaches it max it resets to 1
+        $move = 0;
+        for($i = 0; $i < 3; $i++) {
+            if($die_val > MAX_DIE) {
+                $die_val = 1;
+            }
+            $move += $die_val++;
+        }
+
+        //add the 3 rolls to the counter and move the player
+        $die_rolls += 3;
+        $new_pos = $pos + $move;
+
+        //decrement new position to 10 or below
+        while($new_pos > MAX_FIELD) {
+            $new_pos -= MAX_FIELD;
+        }
+
+        //update player position & score
+        $player_pos[$no] = $new_pos;
+        $player_scores[$no] += $new_pos;
+
+        //game ends immmediately when a player wins, do not do next player's turn
+        if($player_scores[$no] >= WIN_SCORE) {
+            break;
+        }
+    }
+}
+
+//result
+$losing_score = min($player_scores);
+$result = $losing_score * $die_rolls;
+
+echo "Losing player's score times dice rolls equals $result." . PHP_EOL;
+

day_21/input.txt

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+Player 1 starting position: 10
+Player 2 starting position: 1

day_21/testinput.txt

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+Player 1 starting position: 4
+Player 2 starting position: 8