detectDateFormat function exported ✓ 100.0%

export function detectDateFormat(field: {
  inputType?: string;
  placeholder?: string;
  pattern?: string;
}): DateFormat {
  // Native <input type="date"> always requires YYYY-MM-DD
  if (field.inputType === "date") return "iso";

  const ph = (field.placeholder ?? "").toUpperCase();
  if (ph.length > 0) {
    const ddPos = ph.indexOf("DD");
    const mmPos = ph.indexOf("MM");
    const yyyyPos = Math.max(ph.indexOf("YYYY"), ph.indexOf("AAAA"));

    // YYYY/AAAA appears first → ISO (e.g. "YYYY-MM-DD")
    if (yyyyPos >= 0 && (mmPos < 0 || yyyyPos < mmPos)) return "iso";

    // DD appears before MM → BR (e.g. "DD/MM/YYYY" or "dd/mm/aaaa")
    if (ddPos >= 0 && mmPos >= 0 && ddPos < mmPos) return "br";

    // MM appears before DD → US (e.g. "MM/DD/YYYY")
    if (ddPos >= 0 && mmPos >= 0 && mmPos < ddPos) return "us";
  }

  // Inspect HTML `pattern` attribute for structural clues
  const pat = field.pattern ?? "";
  if (pat) {
    // Starts with 4-digit year: \d{4} or [0-9]{4} → ISO
    if (/^(\\d\{4\}|\[0-9\]\{4\})/.test(pat)) return "iso";
    // Year at end: \d{4}$ → likely BR or US, default to BR (Brazilian context)
    if (/(\\d\{4\}|\[0-9\]\{4\})\s*$/.test(pat)) return "br";
  }

  return "iso";
}