<?php
include_once $_SERVER['DOCUMENT_ROOT'] . '/config.php';

$question = $_POST['question'] ?? null;
$max_length = min(max(intval($_POST['max_length'] ?? '100'), 20), 200);

$db = new PDO(DB_URL, DB_USER, DB_PASS);

// choosing a random word from the question
$question_parts = $question ? explode(' ', $question ?? '') : [];
$start_word = null;

while ($start_word == null && !empty($question_parts)) {
  $word_index = random_int(0, count($question_parts) - 1);
  $start_word = $question_parts[$word_index];

  $stmt = $db->prepare('SELECT id FROM chains WHERE from_word = ?');
  $stmt->execute([$start_word]);
  $row = $stmt->fetch(PDO::FETCH_ASSOC);

  if (!$row) {
    $start_word = null;
    array_splice($question_parts, $word_index, 1);
  }
}

// selecting a random word from db if there is no question
if (!$question) {
  $stmt = $db->query('SELECT from_word FROM chains ORDER BY rand() LIMIT 1');
  $stmt->execute();

  if ($row = $stmt->fetch(PDO::FETCH_ASSOC)) {
    $start_word = $row['from_word'];
  }
}

if (!$start_word && !$question) {
  http_response_code(400);
  header('Content-Type: application/json');
  echo json_encode([
    'status_code' => 400,
    'message' => 'No question.',
    'data' => null
  ], JSON_UNESCAPED_SLASHES);
  exit;
}

// generating markov chain
$output = [];

while (strlen(implode(' ', $output)) < $max_length) {
  array_push($output, $start_word);
  $stmt = $db->prepare('SELECT to_word FROM chains WHERE from_word = ? ORDER BY rand() LIMIT 1');
  $stmt->execute([$start_word]);
  $row = $stmt->fetch(PDO::FETCH_ASSOC);
  if (!$row) {
    break;
  }
  $start_word = $row['to_word'];
}

header('Content-Type: application/json');

echo json_encode([
  'status_code' => 200,
  'message' => null,
  'data' => [
    'answer' => implode(' ', $output) ?: '...'
  ]
], JSON_UNESCAPED_SLASHES);
?>