<?php
declare(strict_types=1);

const DATA_FILE = __DIR__ . '/what.json';

function not_found(): never
{
    http_response_code(404);
    header('Content-Type: text/plain; charset=utf-8');
    echo "404 Not Found\n";
    exit;
}

function fail(int $status, string $message): never
{
    http_response_code($status);
    header('Content-Type: text/plain; charset=utf-8');
    echo $message . "\n";
    exit;
}

function h(string $value): string
{
    return htmlspecialchars($value, ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8');
}

function wifi_escape(string $value): string
{
    return strtr($value, [
        '\\' => '\\\\',
        ';' => '\\;',
        ',' => '\\,',
        ':' => '\\:',
        '"' => '\\"',
    ]);
}

function qr_entry_to_payload(string $key, mixed $entry): ?array
{
    if (is_string($entry)) {
        return [
            'payload' => $entry,
            'title' => $key,
            'subtitle' => 'Scan this QR code.',
            'link' => filter_var($entry, FILTER_VALIDATE_URL) ? $entry : null,
        ];
    }

    if (!is_array($entry)) {
        return null;
    }

    $type = strtolower((string)($entry['type'] ?? ''));
    $label = (string)($entry['label'] ?? $key);

    if ($type === 'wifi' || isset($entry['ssid'])) {
        $ssid = (string)($entry['ssid'] ?? '');
        if ($ssid === '') {
            return null;
        }

        $security = strtoupper((string)($entry['security'] ?? $entry['encryption'] ?? 'WPA'));
        if (in_array($security, ['', 'NONE', 'OPEN', 'NOPASS'], true)) {
            $security = 'nopass';
        }

        $hidden = !empty($entry['hidden']) ? 'true' : 'false';
        $payload = 'WIFI:T:' . wifi_escape($security) . ';S:' . wifi_escape($ssid) . ';';
        if ($security !== 'nopass') {
            $payload .= 'P:' . wifi_escape((string)($entry['password'] ?? '')) . ';';
        }
        $payload .= 'H:' . $hidden . ';;';

        return [
            'payload' => $payload,
            'title' => $label,
            'subtitle' => 'Scan to connect to ' . $ssid,
            'link' => null,
        ];
    }

    $payload = $entry['url'] ?? $entry['text'] ?? $entry['payload'] ?? null;
    if (!is_string($payload) || $payload === '') {
        return null;
    }

    return [
        'payload' => $payload,
        'title' => $label,
        'subtitle' => filter_var($payload, FILTER_VALIDATE_URL) ? $payload : 'Scan this QR code.',
        'link' => filter_var($payload, FILTER_VALIDATE_URL) ? $payload : null,
    ];
}

final class QrCode
{
    private const ECC_CODEWORDS_PER_BLOCK = [1 => 10, 2 => 16, 3 => 26, 4 => 18, 5 => 24, 6 => 16, 7 => 18, 8 => 22, 9 => 22, 10 => 26];
    private const DATA_CODEWORDS_PER_BLOCK = [
        1 => [16],
        2 => [28],
        3 => [44],
        4 => [32, 32],
        5 => [43, 43],
        6 => [27, 27, 27, 27],
        7 => [31, 31, 31, 31],
        8 => [38, 38, 39, 39],
        9 => [36, 36, 36, 37, 37],
        10 => [43, 43, 43, 43, 44],
    ];
    private const ALIGNMENT_POSITIONS = [
        1 => [],
        2 => [6, 18],
        3 => [6, 22],
        4 => [6, 26],
        5 => [6, 30],
        6 => [6, 34],
        7 => [6, 22, 38],
        8 => [6, 24, 42],
        9 => [6, 26, 46],
        10 => [6, 28, 50],
    ];

    /** @var array<int,int> */
    private static array $gfExp = [];
    /** @var array<int,int> */
    private static array $gfLog = [];

    private int $version;
    private int $size;
    /** @var array<int,array<int,bool>> */
    private array $modules;
    /** @var array<int,array<int,bool>> */
    private array $isFunction;

    private function __construct(int $version)
    {
        $this->version = $version;
        $this->size = 21 + 4 * ($version - 1);
        $row = array_fill(0, $this->size, false);
        $this->modules = array_fill(0, $this->size, $row);
        $this->isFunction = array_fill(0, $this->size, $row);
    }

    public static function fromText(string $text): self
    {
        $bytes = array_values(unpack('C*', $text) ?: []);
        $version = self::chooseVersion(count($bytes));
        $qr = new self($version);
        $qr->drawFunctionPatterns();
        $dataCodewords = $qr->makeDataCodewords($bytes);
        $allCodewords = self::addEccAndInterleave($version, $dataCodewords);
        $qr->drawCodewords($allCodewords, 0);
        $qr->drawFormatBits(0);
        return $qr;
    }

    public function toSvg(string $centerText = '', int $border = 4): string
    {
        $fullSize = $this->size + $border * 2;
        $svg = '<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 ' . $fullSize . ' ' . $fullSize . '" role="img" aria-label="QR code">';
        $svg .= '<rect width="100%" height="100%" rx="2" fill="#fff"/>';
        $svg .= '<g fill="#111">';
        for ($y = 0; $y < $this->size; $y++) {
            for ($x = 0; $x < $this->size; $x++) {
                if ($this->modules[$y][$x]) {
                    $svg .= '<rect x="' . ($x + $border) . '" y="' . ($y + $border) . '" width="1" height="1" rx="0.18"/>';
                }
            }
        }
        $svg .= '</g>';

        if ($centerText !== '') {
            $safeText = htmlspecialchars($centerText, ENT_QUOTES | ENT_SUBSTITUTE, 'UTF-8');
            $chars = max(1, strlen($centerText));
            $badgeWidth = min($this->size * 0.48, max($this->size * 0.25, $chars * 1.15 + 4));
            $badgeHeight = max(4.8, min(6.2, $this->size * 0.18));
            $x = ($fullSize - $badgeWidth) / 2;
            $y = ($fullSize - $badgeHeight) / 2;
            $fontSize = min(3.2, max(2.1, ($badgeWidth - 2) / ($chars * 0.62)));
            $svg .= '<rect x="' . $x . '" y="' . $y . '" width="' . $badgeWidth . '" height="' . $badgeHeight . '" rx="' . ($badgeHeight / 2) . '" fill="#fff"/>';
            $svg .= '<text x="' . ($fullSize / 2) . '" y="' . ($fullSize / 2) . '" fill="#111" font-family="system-ui, -apple-system, Segoe UI, sans-serif" font-size="' . $fontSize . '" font-weight="800" text-anchor="middle" dominant-baseline="central">' . $safeText . '</text>';
        }

        $svg .= '</svg>';
        return $svg;
    }

    private static function chooseVersion(int $byteLength): int
    {
        for ($version = 1; $version <= 10; $version++) {
            $countBits = $version <= 9 ? 8 : 16;
            $capacityBits = array_sum(self::DATA_CODEWORDS_PER_BLOCK[$version]) * 8;
            if (4 + $countBits + $byteLength * 8 <= $capacityBits) {
                return $version;
            }
        }
        fail(413, 'QR payload is too long.');
    }

    /** @param array<int,int> $bytes @return array<int,int> */
    private function makeDataCodewords(array $bytes): array
    {
        $capacityBits = array_sum(self::DATA_CODEWORDS_PER_BLOCK[$this->version]) * 8;
        $bits = [];
        self::appendBits($bits, 0b0100, 4); // Byte mode
        self::appendBits($bits, count($bytes), $this->version <= 9 ? 8 : 16);
        foreach ($bytes as $byte) {
            self::appendBits($bits, $byte, 8);
        }

        $terminator = min(4, $capacityBits - count($bits));
        for ($i = 0; $i < $terminator; $i++) {
            $bits[] = 0;
        }
        while (count($bits) % 8 !== 0) {
            $bits[] = 0;
        }

        $result = [];
        for ($i = 0; $i < count($bits); $i += 8) {
            $byte = 0;
            for ($j = 0; $j < 8; $j++) {
                $byte = ($byte << 1) | $bits[$i + $j];
            }
            $result[] = $byte;
        }

        $pad = [0xEC, 0x11];
        for ($i = 0; count($result) < intdiv($capacityBits, 8); $i++) {
            $result[] = $pad[$i % 2];
        }
        return $result;
    }

    /** @param array<int,int> $bits */
    private static function appendBits(array &$bits, int $value, int $length): void
    {
        for ($i = $length - 1; $i >= 0; $i--) {
            $bits[] = ($value >> $i) & 1;
        }
    }

    /** @param array<int,int> $data @return array<int,int> */
    private static function addEccAndInterleave(int $version, array $data): array
    {
        $blockLengths = self::DATA_CODEWORDS_PER_BLOCK[$version];
        $eccLength = self::ECC_CODEWORDS_PER_BLOCK[$version];
        $divisor = self::reedSolomonDivisor($eccLength);
        $blocks = [];
        $offset = 0;
        foreach ($blockLengths as $length) {
            $blockData = array_slice($data, $offset, $length);
            $offset += $length;
            $blocks[] = ['data' => $blockData, 'ecc' => self::reedSolomonRemainder($blockData, $divisor)];
        }

        $result = [];
        $maxData = max($blockLengths);
        for ($i = 0; $i < $maxData; $i++) {
            foreach ($blocks as $block) {
                if ($i < count($block['data'])) {
                    $result[] = $block['data'][$i];
                }
            }
        }
        for ($i = 0; $i < $eccLength; $i++) {
            foreach ($blocks as $block) {
                $result[] = $block['ecc'][$i];
            }
        }
        return $result;
    }

    /** @return array<int,int> */
    private static function reedSolomonDivisor(int $degree): array
    {
        self::initGalois();
        $result = array_fill(0, $degree, 0);
        $result[$degree - 1] = 1;
        $root = 1;
        for ($i = 0; $i < $degree; $i++) {
            for ($j = 0; $j < $degree; $j++) {
                $result[$j] = self::gfMultiply($result[$j], $root);
                if ($j + 1 < $degree) {
                    $result[$j] ^= $result[$j + 1];
                }
            }
            $root = self::gfMultiply($root, 0x02);
        }
        return $result;
    }

    /** @param array<int,int> $data @param array<int,int> $divisor @return array<int,int> */
    private static function reedSolomonRemainder(array $data, array $divisor): array
    {
        $result = array_fill(0, count($divisor), 0);
        foreach ($data as $byte) {
            $factor = $byte ^ $result[0];
            array_shift($result);
            $result[] = 0;
            foreach ($divisor as $i => $coefficient) {
                $result[$i] ^= self::gfMultiply($coefficient, $factor);
            }
        }
        return $result;
    }

    private static function initGalois(): void
    {
        if (self::$gfExp !== []) {
            return;
        }
        $x = 1;
        for ($i = 0; $i < 255; $i++) {
            self::$gfExp[$i] = $x;
            self::$gfLog[$x] = $i;
            $x <<= 1;
            if (($x & 0x100) !== 0) {
                $x ^= 0x11D;
            }
        }
        for ($i = 255; $i < 512; $i++) {
            self::$gfExp[$i] = self::$gfExp[$i - 255];
        }
    }

    private static function gfMultiply(int $x, int $y): int
    {
        if ($x === 0 || $y === 0) {
            return 0;
        }
        return self::$gfExp[self::$gfLog[$x] + self::$gfLog[$y]];
    }

    private function drawFunctionPatterns(): void
    {
        $this->drawFinderPattern(3, 3);
        $this->drawFinderPattern($this->size - 4, 3);
        $this->drawFinderPattern(3, $this->size - 4);

        for ($i = 0; $i < $this->size; $i++) {
            if (!$this->isFunction[6][$i]) {
                $this->setFunctionModule($i, 6, $i % 2 === 0);
            }
            if (!$this->isFunction[$i][6]) {
                $this->setFunctionModule(6, $i, $i % 2 === 0);
            }
        }

        foreach (self::ALIGNMENT_POSITIONS[$this->version] as $x) {
            foreach (self::ALIGNMENT_POSITIONS[$this->version] as $y) {
                if (!$this->isFunction[$y][$x]) {
                    $this->drawAlignmentPattern($x, $y);
                }
            }
        }

        $this->drawFormatBits(0);
        if ($this->version >= 7) {
            $this->drawVersionBits();
        }
    }

    private function drawFinderPattern(int $cx, int $cy): void
    {
        for ($dy = -4; $dy <= 4; $dy++) {
            for ($dx = -4; $dx <= 4; $dx++) {
                $x = $cx + $dx;
                $y = $cy + $dy;
                if ($x < 0 || $x >= $this->size || $y < 0 || $y >= $this->size) {
                    continue;
                }
                $dist = max(abs($dx), abs($dy));
                $this->setFunctionModule($x, $y, $dist !== 2 && $dist !== 4);
            }
        }
    }

    private function drawAlignmentPattern(int $cx, int $cy): void
    {
        for ($dy = -2; $dy <= 2; $dy++) {
            for ($dx = -2; $dx <= 2; $dx++) {
                $this->setFunctionModule($cx + $dx, $cy + $dy, max(abs($dx), abs($dy)) !== 1);
            }
        }
    }

    private function drawFormatBits(int $mask): void
    {
        $data = $mask; // Medium ECC has format bits 00.
        $rem = $data;
        for ($i = 0; $i < 10; $i++) {
            $rem = ($rem << 1) ^ ((($rem >> 9) & 1) * 0x537);
        }
        $bits = (($data << 10) | ($rem & 0x3FF)) ^ 0x5412;

        for ($i = 0; $i <= 5; $i++) {
            $this->setFunctionModule(8, $i, (($bits >> $i) & 1) !== 0);
        }
        $this->setFunctionModule(8, 7, (($bits >> 6) & 1) !== 0);
        $this->setFunctionModule(8, 8, (($bits >> 7) & 1) !== 0);
        $this->setFunctionModule(7, 8, (($bits >> 8) & 1) !== 0);
        for ($i = 9; $i < 15; $i++) {
            $this->setFunctionModule(14 - $i, 8, (($bits >> $i) & 1) !== 0);
        }

        for ($i = 0; $i < 8; $i++) {
            $this->setFunctionModule($this->size - 1 - $i, 8, (($bits >> $i) & 1) !== 0);
        }
        for ($i = 8; $i < 15; $i++) {
            $this->setFunctionModule(8, $this->size - 15 + $i, (($bits >> $i) & 1) !== 0);
        }
        $this->setFunctionModule(8, $this->size - 8, true);
    }

    private function drawVersionBits(): void
    {
        $rem = $this->version;
        for ($i = 0; $i < 12; $i++) {
            $rem = ($rem << 1) ^ ((($rem >> 11) & 1) * 0x1F25);
        }
        $bits = ($this->version << 12) | ($rem & 0xFFF);
        for ($i = 0; $i < 18; $i++) {
            $bit = (($bits >> $i) & 1) !== 0;
            $a = $this->size - 11 + ($i % 3);
            $b = intdiv($i, 3);
            $this->setFunctionModule($a, $b, $bit);
            $this->setFunctionModule($b, $a, $bit);
        }
    }

    /** @param array<int,int> $codewords */
    private function drawCodewords(array $codewords, int $mask): void
    {
        $bits = [];
        foreach ($codewords as $byte) {
            self::appendBits($bits, $byte, 8);
        }

        $i = 0;
        $upward = true;
        for ($right = $this->size - 1; $right >= 1; $right -= 2) {
            if ($right === 6) {
                $right = 5;
            }
            for ($vert = 0; $vert < $this->size; $vert++) {
                $y = $upward ? $this->size - 1 - $vert : $vert;
                for ($j = 0; $j < 2; $j++) {
                    $x = $right - $j;
                    if ($this->isFunction[$y][$x]) {
                        continue;
                    }
                    $bit = $i < count($bits) && $bits[$i] === 1;
                    $i++;
                    if ($this->maskBit($mask, $x, $y)) {
                        $bit = !$bit;
                    }
                    $this->modules[$y][$x] = $bit;
                }
            }
            $upward = !$upward;
        }
    }

    private function maskBit(int $mask, int $x, int $y): bool
    {
        return match ($mask) {
            0 => (($x + $y) % 2) === 0,
            default => false,
        };
    }

    private function setFunctionModule(int $x, int $y, bool $dark): void
    {
        $this->modules[$y][$x] = $dark;
        $this->isFunction[$y][$x] = true;
    }
}

$what = (string)($_GET['what'] ?? '');
if ($what === '') {
    fail(400, 'Missing required query parameter: what');
}
if (!preg_match('/^[A-Za-z0-9_.-]+$/', $what)) {
    not_found();
}
if (!is_readable(DATA_FILE)) {
    not_found();
}

$config = json_decode((string)file_get_contents(DATA_FILE), true);
if (!is_array($config)) {
    fail(500, 'Invalid what.json');
}
if (!array_key_exists($what, $config)) {
    not_found();
}

$entry = qr_entry_to_payload($what, $config[$what]);
if ($entry === null) {
    not_found();
}

$qr = QrCode::fromText($entry['payload']);
$title = (string)$entry['title'];
$subtitle = (string)$entry['subtitle'];
$link = is_string($entry['link']) ? $entry['link'] : null;

header('Content-Type: text/html; charset=utf-8');
?>
<!doctype html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title><?= h($title) ?> QR</title>
  <link rel="icon" href="favicon.svg" type="image/svg+xml">
  <style>
    :root { color-scheme: light dark; font-family: system-ui, sans-serif; }
    body { margin: 0; min-height: 100vh; display: grid; place-items: center; background: #f5f5f5; color: #111; }
    main { width: min(34rem, calc(100vw - 2rem)); text-align: center; padding: 1rem; }
    .card { background: #fff; border-radius: 1.5rem; box-shadow: 0 1rem 3rem rgb(0 0 0 / 12%); padding: clamp(1.25rem, 5vw, 2.5rem); }
    h1 { margin: 0 0 .5rem; font-size: clamp(1.75rem, 8vw, 3rem); }
    p { margin: .5rem 0 1.5rem; overflow-wrap: anywhere; }
    a { color: inherit; }
    .qr { margin: auto; max-width: 24rem; }
    .qr svg { display: block; width: 100%; height: auto; }
    @media (prefers-color-scheme: dark) {
      body { background: #111; color: #f5f5f5; }
      .card { background: #1d1d1d; }
    }
  </style>
</head>
<body>
  <main>
    <section class="card">
      <h1><?= h($title) ?></h1>
      <p><?php if ($link !== null): ?><a href="<?= h($link) ?>"><?= h($subtitle) ?></a><?php else: ?><?= h($subtitle) ?><?php endif; ?></p>
      <div class="qr"><?= $qr->toSvg($what) ?></div>
    </section>
  </main>
</body>
</html>