const SYSTEM_PREFIX = `You are an email-classification assistant.
Read the email below and the classification criterion provided by the user.
`;

const DEFAULT_CUSTOM_SYSTEM_PROMPT = "Determine whether the email satisfies the user's criterion.";

const SYSTEM_SUFFIX = `
Return ONLY a JSON object on a single line of the form:
{"match": true, "reason": "<short explanation>"} - if the email satisfies the criterion
{"match": false, "reason": "<short explanation>"} - otherwise

Do not add any other keys, text, or formatting.`;

function extractLastJsonObject(text) {
  let last = null;
  let start = -1;
  let depth = 0;
  let inString = false;
  let escape = false;

  for (let i = 0; i < text.length; i += 1) {
    const ch = text[i];
    if (inString) {
      if (escape) {
        escape = false;
        continue;
      }
      if (ch === "\\") {
        escape = true;
        continue;
      }
      if (ch === "\"") {
        inString = false;
      }
      continue;
    }
    if (ch === "\"") {
      inString = true;
      continue;
    }
    if (ch === "{") {
      if (depth === 0) {
        start = i;
      }
      depth += 1;
      continue;
    }
    if (ch === "}" && depth > 0) {
      depth -= 1;
      if (depth === 0 && start !== -1) {
        last = text.slice(start, i + 1);
        start = -1;
      }
    }
  }

  return last;
}

export const path = '/v1/chat/completions';
export function buildRequest(text, criterion, config) {
  const payload = {
    messages: [
      { role: 'system', content: SYSTEM_PREFIX + (config.customSystemPrompt || DEFAULT_CUSTOM_SYSTEM_PROMPT) + SYSTEM_SUFFIX },
      { role: 'user', content: `**Email Contents**\n\`\`\`\n${text}\n\`\`\`\nClassification Criterion: ${criterion}` }
    ], ...config.aiParams
  };
  if (config.model) payload.model = config.model;
  return payload;
}
export function parseResponse(result) {
  const content = result?.choices?.[0]?.message?.content;
  if (typeof content !== 'string') throw new Error('Chat response missing text content.');
  const candidate = extractLastJsonObject(content);
  if (!candidate) throw new Error('No JSON object found in AI response.');
  const obj = JSON.parse(candidate);
  const matched = Object.hasOwn(obj, 'match') ? obj.match : obj.matched;
  if (typeof matched !== 'boolean') throw new Error('AI response missing valid match boolean.');
  const reason = obj.reason ?? obj.reasoning ?? obj.explaination;
  return { matched, reason: typeof reason === 'string' ? reason : null, probability: null };
}