3.php

<?php

$input = "v>v<vvv<<vv^v<v>vv>v<<<^^^^^<<^<vv>^>v^>^>^>^>^><vvvv<^>^<<^><<<^vvvv>^>^><^v^><^<>^^>^vvv^<vv>>^>^^<>><>^>vvv>>^vv>^<><>^<v^>^>^><vv^vv^>><<^><<v>><>^<^>>vvv>v>>>v<<^<><^<v<>v>^^v^^^<^v^^>>><^>^>v<>^<>>^>^^v^><v<v>>><>v<v^v>^v<>>^><v>^<>v^>^<>^v^^^v^^>>vv<<^^><^<vvv>^>^^<^>>^^^^^v^<v>vv<>>v^v<^v^^<><^<^vv^><>><><>v>vvv^vv^^<<><<vvv><<^v^><v<>vvv^<^>vvvv^>^>>^v^<v^vv<^^v<>v>vv^<>><v<<<^v^<<><v<^<^<><^^^>^>>v>^>v^<>v><^<^<v^>^^vv<^^<>v^v^vv<>>>>v^v<>><^^v>vv^^>v^v>v<vv>>v>><v^v^v>vv>^^>^v><<vv^v^^vv<^v><^<<v<v^>vv^^^<v^>v>v^^^>><^^<v^<^>>v><vv<v^^>^^v>>v^^^<^^v>^v>><^<^<>>v<<^^vv>^^^v<^<^<v<v^^vv>^vv^>>v^><v>><<<>^vv^<^<>v^^<<<v<^>^><><v^^>>^^^<^vv<^^^>><^^v>^^v^<v^v^>^^<v>^<^v<^<<<<^<v^>v^<^^<>^^>^><<>>^v><>><^<v><^^^>>vv>^><vv>^^^^^v^vvv><><^<^>v>v^v^>^><><^<^><>v<<vv<^>><>^v^^v>^<<<>^v^>^<<v^vv<>v^<v^^vv><<v^<>>>^<v>vv>v>>>^<^>><vv<>>>>v<v>>>^v>v><>>vvv<^^><<^>^>v<^vvvv<v><vv<><^^^v^^^>v^v<>v<^^v>>><>v<v^>>v><v^>>^^<v<<<^<v<><^^v><<v^><<<<^vv<^<>^><vv<<<<^>>>^v>^v>vv>^v<>v>v<v><^>>v>>^>^><^<v^v^>^v<><><^^>^<vvvv^^<>^^^>vv^v^v>^v^^v^^v><v^<^<>><^<v>v>>vv<<v>>vvvv<vv><>>^v^>^>>v^v^<<<vv<><v<<>>>^v<<v>^^vv^><>v>^>v><<<<<<<^>^^v^<<^^>>vvv^<><>><>^^v<<vv><^^v<^^><vv>v^>>>v^v><v^v<^>v^><>v<<>v>^^v><<<<><^v^v>>^<>^<<>^<v<<>>v<<>><^<<<<^v>^<^v>v>vv<v<v<<>^>v<^<<>v^<vvvv^>v>><<v><v<>v>v>>v^vvv^^>>>v^<^<<^^<<<><v>v^<<v<<<>v<^^<><v<v^^<v>^>v>>v<>^>^^>>^v<<>v^^^>>>^vv<^v<v>^>v>^><>v^^<>^^v^^vv^<^>^<<>><<^>^v>>><<<vvvv><<><v<^v^v<vvv^<><<^<vv><v^v^v^>v>v^<vvv^><^><^<vv><>>v^>^^^<>><v^<^^^<>v<<v<^v>>>^>>v^><<>vvv><^>>v><v><>v>>^>v><<><<>^<>^^^vv><v^>v^^>>^>^<^v<v<^^<^vvvv>v<v>^>v^>^><^<vvvv><^><><<v<>v<v^><^<v^>^v^^<<<<^><^^<^><>>^v<<^<<^vv>v>>v<^<^vv>><v<vv>v<v<v>^v<>^>v<>^v<<<v>>^^v>>><vvv>v^>^v^v>^^^v<vvvv>><^>vvv^<vv^^vv><<<>v<>v>^<vvv^<^<v<v<^vv^^>>vv^<^^v^><^^^^^v<^<v<^>>>vv^v^>^<v>^<><v^<^v>>><^v^<<v<<v<>v>^v<v^v>>^^v<<v<v<<>>>vv>>^v>>^<<<<^><<<><^^>>v<>^vvvv>v^^^>^^^>^<vvvv><^^v<v<>v<^v^v<<v^^^v^<v<^v>v^^<>^>^<^v>vv<v^vv<^<<>v><<^><><^^v<<><^^><>^v>^<><<^<^^<<>vv<>^^<<^>><<<>>vvv>^>v^^v^><<^>v>^>^<^<<>v<^>vv^v^v<>vv<<v>vv<vv><^>v^<>^vv^v^<v<^>>>>v^v><^<><<>vv^<vvv^>>vvv^>v>>><^^vv<vvvv>v<^<^>>^^>^^vv>>><^v<>^v^<<>v^^^<v>^>>^<^<v>>^v<^^^<v>^v>^>>v<vv>>^<v^<<>>^>>><v>v^<<^<v>>^<<^^<>v<^v<^<>v^v>^^v<vvvv>^vv>vvv>v^<^>><v^^vv<<<^>vvvv<>>^^<>v^<><>v<^<>v<>^>v<>vv<v^v>>v<v<^<v^^v^vv^vvv><^^>v>><>>^<^^<>>^>^<v^>>vvv^v><v>>^>^>v><><<><vv^v>v<>^v<^vv^^^<>^^<<^^^v<>><v<^<^<^<^^><v^v<^>v^>vvvv>^^v^>^<v<^^^>>^<<vv^<><><^^^^<<>^<><v>vv^<><^>^^<>v^<>>>v><>vvvvv>v>v^^>^<<vvvv<>vv>>v<<^<>^^^v^<><>>^<<<v<v<>>>><><v>v<v<>>^>^^^^vv^^<<><^^<<vv<^<>v>vv<v<><<<^<<v<<<<>v<>>^<^>^>><v>v>><^^<>><<<><<><v^^v<<><^<^v<v^><^^v<<>><<<<^>v^<v>><v^><v<vvv>v^v^<v><<>>v<><<v>^<>><>>^><>v^v>v<<>v<>v^^><<>>>v<<>>>>^>v>><v<<>>>vv>v>^<^^^<>v<v>^<^^v^vvv^>vv>^<v><vvvv>^<<>vvv<<<vv>^^<^>^>>v>v<<<<<>^^vv^>>v>^<^<v^v^>^v>>v>^v<><>^<^>v>v<<<^^^v>^<<<>vvv^v^^>^>>^>v>v<>^^><>>v>^>v<<<^^^v^<v^vv>><><^<^<><vvv<v^>>^v>vv<^v<<^vv>v^<<v>v>v>^v^>^v<<^v^vv>v<v>^<<><v^>>v<>><v<<<^v<<>vvv^<vv<vvv<<>^vv^^v><^>v^vv<<v^<<^^^<^<>^^<<>v<><<v>^><>^<><<v<v^^>vv<>^<v<^<vvv>vv>v><^^v<>><^v^v><><>><v<v>vv<>>><v^^v<>><<^>>><^^^vvv<<<vv<<^v<<<>><<vv>>>>v<<<<<vv><><v>v^^<<^vv^<vv<>>vv>^<>^v^^<>^^^vv>v^^<v<><v>v<v>>^v<v<>>^<v^^><>v^^^>v^^v<vv><^>v^v^<>v>v<v<^^>>v<^^vv^v<^^^^vv<<><<^>>^^<<v^^<<^>v^>>^^^><^^>^v^v>^<<v<vv<<<v<^^^>^>>^v<>^<^>v>^>^v^<^^^<^vv<v><^^>>v<v>^>^v^>>>>^v>^^<<^<v^v<^<<v<<^><^^<v^<><v>v^<<v^^<><<>>><vv<<><>^<>>>v<<v^^^v^^<<<vv<<^<^<^vv^<><><<^^<^^>v^>^<v<>>v^v<><<v>^^v>^<^<vvv<v>v^v>>>^^<^<v^>^vv<<<v<<>^><><^<>v>>>v<v^<>v>><^^^v^^^v<^^<vv^^^>v>v<>>^^<><>v>^<v<>^>>>><>v>^v>^vv^v<vv<<^^>><v<>^>^^<v<^>^<vvv>><>^<<>>><<<><>^^<<<v<>v^>v>v<v>^^^>^>^v<<>v>vv>><<<v>^^<v><vv<<v^^>^>>^><^>v<^<^v>><^^>v<vv^^><><>^><<><>v^>v<><^^>><>^<^^v<^<<v>><v><<<^^<<v<^vv^v<>><>>>^>v<vvv^>^<><v^><^<<^vv<^v^v^v<>v^^v>v^<^>^vv^>>><<>v^vv^<>^v^><<v^v<v>v^<><>>v^v^><>v^vvv^^^<<^<<v<<v<^vv^>>v^v>^^<v<>><>v>>v^<>^>v>^>><<>v^v><^v>v>>><v<v><^<^^>vv<v><^>^<^>^^v><><v<^^v<<><^<<v^<v<<><^^vvv^v>^>^<>>vv>v^^v^^vv<^^>><v^^vv><^v>v^<<v<^v>vvv<>>^v><<>^v<<<>^><^vv><<^^<v^>v<<v>^vv<>^v>>>><<<<^^<^v>^<^^<^<^^>>^^v>^^^^v^^^<<>^^vv<<v^^><v>><^<<><>^>v<>>v^^^>^v^^v^<v^v>v>>>>>^v>^>^^<vvv^^<v^<<<v<<>v>><^^^v<<^^<v>>^<^<^><^<<v^v><<vv<^<>>v>v>^v<><<v>^>vv^v<v>v><^<v>><>^<vv<v^^^^v<^^>><<^^>v>v>^^^<>v>^v^^>vv^vv<^^>><>^>^<>v>><>^v<<v>v>^><^^^v^<vv><<^v^>v^>vv>v^<>v><vv><^v>v<><v^v^v<^v<>^v<v^<<><<v>>^v><v>^^<>vvv^>^<<v^>><^>><^<>^v<v<v<^vvv<><<^v^<v>><<<v>^<^<v>v>^vv^v>v<^^vv<<vvv^<v>><>vv^>v<<>v<vvvv>>v>^^>>><<<^>^vv>><v>^^^>v<^vv<>v<<<v<<<<v>>>>^<^^^^>v<^^<><v>v>v<v^>vv^>v>v<^>^v^<>v>>vvv>^^><^vvv>><>>>^<<^<v<>>>v^^><v<v>>^><>v<^^v^<<v><>^<>>><^v^v>>>^vvvv^<><<<v<^>>v>^v^<v<v<<^<<v^vv^v>v<v<>>v<v^<<<><v^>><^<<^>^^><v>v<^v^<^>v>^<<v>v^<>v^<>vv^<>^>^>v^>^vv<>^^<<>>v<>^v<><v^><><<<vv>v>v^>vv^><<<<v>^v<><>^^<vv>v^^v^^^<v<^^><v^v<>><v<vv>^<>>><vv<^v<<>>^><>>v<v^v^>>>v<<>v<<<<<<<^v<<^^^v<^v<>v^^<<<^<>>v^vv<v>^<^^<^^<<^>vv><^<^^v<<<^><^v<^><>v<vv^>^v^^>>><<vv^^v><^<<^<>>^>>^<<<<v^vv<>>>v>^v>><>v>>v>><>v>><^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// Split the input into an easily malleable array.
$arrayInput = str_split($input);

// Santa (and robo-Santa) start(s) at 0, 0.
$santaPos = $santa2Pos = $roboPos = [];
$santaX = $santaY = $santa2X = $santa2Y = $roboX = $roboY = 0;

foreach ($arrayInput as $turn => $direction) {
    // What direction should (robo-) Santa go in?
    switch ($direction) {
        // Move north.
        case '^':
            // Should Santa or robo-Santa move?
            $turn % 2 == 0 ? $santaY++ : $roboY++;

            // Move the Santa from part 1.
            $santa2Y++;
            break;

        // Move east.
        case '>':
            // Should Santa or robo-Santa move?
            $turn % 2 == 0 ? $santaX++ : $roboX++;

            // Move the Santa from part 1.
            $santa2X++;
            break;

        // Move south.
        case 'v':
            // Should Santa or robo-Santa move?
            $turn % 2 == 0 ? $santaY-- : $roboY--;

            // Move the Santa from part 1.
            $santa2Y--;
            break;

        // Move west.
        case '<':
            // Should Santa or robo-Santa move?
            $turn % 2 == 0 ? $santaX-- : $roboX--;

            // Move the Santa from part 1.
            $santa2X--;
            break;
    }

    // Save their new positions.
    $santaPos[]  = $santaX . "," . $santaY;
    $roboPos[]   = $roboX . "," . $roboY;
    $santa2Pos[] = $santa2X . "," . $santa2Y;
}

// What houses have the Santas visited tonight?
$santaVisited   = array_count_values($santaPos);
$roboVisited    = array_count_values($roboPos);
$totalOneSanta  = array_count_values($santa2Pos);

// Add robo-Santa's and real Santa's houses together.
$totalTwoSantas = array_merge($santaVisited, $roboVisited);

// How many houses receive at least one present?
echo "Part 1: " . count($totalOneSanta) . ". Part 2: " . count($totalTwoSantas);