<?php
// The one place CourseHub talks to an AI provider: OpenAI-format chat
// completions over curl — DeepSeek by default, any compatible endpoint via
// AI_BASE_URL / AI_MODEL in config.php. Nothing here runs unless aiEnabled(),
// and nothing else in the app makes outbound requests.
require_once __DIR__ . '/db.php';   // config constants

class AiUnavailable extends RuntimeException {}   // network, timeout, 5xx — try later
class AiRejected extends RuntimeException {}      // 4xx — the request, the key or the quota
class AiBadReply extends RuntimeException {}      // 200, but nothing usable in it

/** The master switch: config says on, a key is set, and curl is there to use it. */
function aiEnabled(): bool
{
    return defined('AI_ENABLED') && AI_ENABLED === true
        && defined('AI_API_KEY') && is_string(AI_API_KEY) && AI_API_KEY !== ''
        && function_exists('curl_init');
}

function aiModel(): string
{
    return (defined('AI_MODEL') && is_string(AI_MODEL) && AI_MODEL !== '') ? AI_MODEL : 'deepseek-v4-flash';
}

function aiBaseUrl(): string
{
    $url = (defined('AI_BASE_URL') && is_string(AI_BASE_URL) && AI_BASE_URL !== '')
        ? AI_BASE_URL : 'https://api.deepseek.com';
    return rtrim($url, '/');
}

/**
 * One chat completion, no streaming.
 *
 * $messages: [['role' => 'system'|'user'|'assistant', 'content' => string], ...]
 * $opts:     json       — ask for a JSON object (the prompt must still say so)
 *            max_tokens — cap on the reply (default 4000)
 *            reasoning  — 'off' | 'low' | 'high' | 'max' (default 'low')
 *            timeout    — seconds to wait for the whole reply (default 90)
 *
 * Returns ['content' => string, 'finish_reason' => ?string, 'model' => string,
 *          'usage' => [prompt_tokens, cache_hit_tokens, completion_tokens, reasoning_tokens]].
 */
function aiChat(array $messages, array $opts = []): array
{
    if (!aiEnabled()) {
        throw new AiUnavailable('The AI advisor is switched off.');
    }

    $body = [
        'model'      => aiModel(),
        'messages'   => $messages,
        'stream'     => false,
        'max_tokens' => max(256, (int)($opts['max_tokens'] ?? 4000)),
    ];
    if (!empty($opts['json'])) {
        $body['response_format'] = ['type' => 'json_object'];
    }
    // DeepSeek's thinking switch: `thinking.type` turns it on or off and
    // `reasoning_effort` (top level) says how hard. Other OpenAI-format servers
    // may reject unknown fields — set AI_REASONING_PARAM to false in config.php
    // to leave both out.
    if (!defined('AI_REASONING_PARAM') || AI_REASONING_PARAM !== false) {
        $reasoning = (string)($opts['reasoning'] ?? 'low');
        if ($reasoning === 'off') {
            $body['thinking'] = ['type' => 'disabled'];
        } else {
            $body['thinking']         = ['type' => 'enabled'];
            $body['reasoning_effort'] = $reasoning;
        }
    }

    $ch = curl_init(aiBaseUrl() . '/chat/completions');
    curl_setopt_array($ch, [
        CURLOPT_POST           => true,
        CURLOPT_HTTPHEADER     => [
            'Content-Type: application/json',
            'Accept: application/json',
            'Authorization: Bearer ' . AI_API_KEY,
        ],
        CURLOPT_POSTFIELDS     => json_encode($body, JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES),
        CURLOPT_RETURNTRANSFER => true,
        CURLOPT_CONNECTTIMEOUT => 10,
        CURLOPT_TIMEOUT        => max(10, (int)($opts['timeout'] ?? 90)),
        CURLOPT_SSL_VERIFYPEER => true,
        CURLOPT_SSL_VERIFYHOST => 2,
    ]);
    $raw    = curl_exec($ch);
    $errno  = curl_errno($ch);
    $error  = curl_error($ch);
    $status = (int)curl_getinfo($ch, CURLINFO_RESPONSE_CODE);
    curl_close($ch);

    if ($raw === false || $errno !== 0) {
        throw new AiUnavailable('Could not reach the AI service' . ($error ? " ($error)" : '') . '.');
    }
    $data = json_decode((string)$raw, true);
    if ($status >= 500) {
        throw new AiUnavailable("The AI service is having trouble (HTTP $status) — try again later.");
    }
    if ($status === 429) {
        throw new AiRejected('The AI service is busy — try again in a moment.');
    }
    if ($status >= 400) {
        $msg = is_array($data) ? ($data['error']['message'] ?? null) : null;
        throw new AiRejected('The AI service rejected the request'
            . (is_string($msg) && $msg !== '' ? ': ' . mb_substr($msg, 0, 200) : " (HTTP $status).")); 
    }
    if (!is_array($data)) {
        throw new AiBadReply('The AI service sent something that is not JSON.');
    }

    $choice  = $data['choices'][0] ?? null;
    $content = is_array($choice) ? ($choice['message']['content'] ?? '') : '';
    if (!is_string($content) || trim($content) === '') {
        throw new AiBadReply('The AI service returned an empty reply.');
    }
    $u = is_array($data['usage'] ?? null) ? $data['usage'] : [];
    return [
        'content'       => $content,
        'finish_reason' => is_array($choice) ? ($choice['finish_reason'] ?? null) : null,
        'model'         => is_string($data['model'] ?? null) ? $data['model'] : aiModel(),
        'usage'         => [
            'prompt_tokens'     => (int)($u['prompt_tokens'] ?? 0),
            'cache_hit_tokens'  => (int)($u['prompt_cache_hit_tokens'] ?? 0),
            'completion_tokens' => (int)($u['completion_tokens'] ?? 0),
            'reasoning_tokens'  => (int)($u['completion_tokens_details']['reasoning_tokens'] ?? 0),
        ],
    ];
}