<?php

declare(strict_types=1);

echo "=== Padding Sequence Analysis ===\n\n";

$testData = 'HELLO WORLD';

// Build bit string
$bits = '';

// 1. Mode indicator (4 bits)
$bits .= '0100';

// 2. Character count (8 bits)
$bits .= str_pad(decbin(11), 8, '0', STR_PAD_LEFT);

// 3. Data bytes
for ($i = 0; $i < 11; $i++) {
    $byte = ord($testData[$i]);
    $bits .= str_pad(decbin($byte), 8, '0', STR_PAD_LEFT);
}

echo "After mode + count + data: " . strlen($bits) . " bits\n";

// 4. Terminator
$requiredBits = 16 * 8; // 128 bits
$terminatorLength = min(4, max(0, $requiredBits - strlen($bits)));
$bits .= str_repeat('0', $terminatorLength);
echo "After terminator: " . strlen($bits) . " bits\n";

// 5. Pad to multiple of 8
$remainder = strlen($bits) % 8;
if ($remainder !== 0) {
    $bits .= str_repeat('0', 8 - $remainder);
}
echo "After padding to byte: " . strlen($bits) . " bits\n";
echo "Padding bits added: " . (8 - $remainder) . "\n\n";

// 6. Add pad codewords
$padBytes = ['11101100', '00010001'];
$padIndex = 0;
$padSequence = '';
while (strlen($bits) < $requiredBits) {
    $padByte = $padBytes[$padIndex % 2];
    $padSequence .= $padByte . ' ';
    $bits .= $padByte;
    $padIndex++;
}

echo "Pad codewords needed: " . (($requiredBits - strlen($bits) + strlen($padSequence)) / 8) . "\n";
echo "Pad sequence: {$padSequence}\n\n";

// Convert to codewords
$codewords = [];
for ($i = 0; $i < strlen($bits); $i += 8) {
    $byte = substr($bits, $i, 8);
    $codewords[] = bindec($byte);
}

// Expected codewords
$expected = [64, 180, 132, 84, 196, 196, 242, 5, 116, 245, 174, 59, 64, 109, 236, 233];

echo "=== Codeword Comparison ===\n";
for ($i = 0; $i < min(count($codewords), count($expected)); $i++) {
    $match = $codewords[$i] === $expected[$i] ? '✅' : '❌';
    $ourBits = str_pad(decbin($codewords[$i]), 8, '0', STR_PAD_LEFT);
    $expBits = str_pad(decbin($expected[$i]), 8, '0', STR_PAD_LEFT);
    echo "Codeword {$i}: ours={$codewords[$i]} ({$ourBits}) exp={$expected[$i]} ({$expBits}) {$match}\n";
    
    if ($codewords[$i] !== $expected[$i] && $i >= 9) {
        // Show bit context
        $offset = $i * 8;
        echo "  Context (bits " . ($offset - 8) . "-" . ($offset + 15) . "):\n";
        echo "    Our:      " . substr($bits, max(0, $offset - 8), 24) . "\n";
        $expectedBits = '';
        foreach ($expected as $cw) {
            $expectedBits .= str_pad(decbin($cw), 8, '0', STR_PAD_LEFT);
        }
        echo "    Expected: " . substr($expectedBits, max(0, $offset - 8), 24) . "\n";
    }
}

// Show what the expected padding should be
echo "\n=== Expected Padding Analysis ===\n";
$expectedBits = '';
foreach ($expected as $cw) {
    $expectedBits .= str_pad(decbin($cw), 8, '0', STR_PAD_LEFT);
}

// Find where our data ends
$dataEnd = 4 + 8 + (11 * 8); // mode + count + data
echo "Data ends at bit: {$dataEnd}\n";
echo "Expected bits 0-{$dataEnd}: " . substr($expectedBits, 0, $dataEnd) . "\n";
echo "Our bits 0-{$dataEnd}:      " . substr($bits, 0, $dataEnd) . "\n";

$expectedPadding = substr($expectedBits, $dataEnd);
$ourPadding = substr($bits, $dataEnd);

echo "\nExpected padding: {$expectedPadding}\n";
echo "Our padding:      {$ourPadding}\n";

if ($expectedPadding === $ourPadding) {
    echo "\n✅ Padding matches!\n";
} else {
    echo "\n❌ Padding doesn't match!\n";
    echo "First difference at bit " . ($dataEnd + strspn($expectedPadding ^ $ourPadding, "\0")) . "\n";
}