export type ExportMode = "searchable" | "flattened";

export interface ExportFileNameInput {
  /** Tokens: {number}, {client}, {date}. */
  pattern?: string;
  number: string;
  client: string;
  date: string;
  mode: ExportMode;
}

const DEFAULT_PATTERN = "{number}_{client}_{date}";
const MAX_STEM = 120;
const FALLBACK = "invoice";

// Characters Windows forbids in file names, plus control characters.
// eslint-disable-next-line no-control-regex
const FORBIDDEN = /[<>:"/\\|?*\u0000-\u001f\u007f]/g;
// The device name matters even with an extension ("CON.txt" is still the console).
const RESERVED = /^(con|prn|aux|nul|com[1-9¹²³]|lpt[1-9¹²³])$/i;
const EDGE_JUNK = /^[.\s_-]+|[.\s_-]+$/g;

function collapse(value: string): string {
  return value
    .replace(FORBIDDEN, "-")
    .replace(/\s+/g, " ")
    .replace(/-{2,}/g, "-")
    .replace(/_{2,}/g, "_")
    .replace(/ ?([-_]) ?/g, "$1");
}

function cleanPart(value: string): string {
  return collapse(value.normalize("NFC")).replace(EDGE_JUNK, "");
}

/** Cut to at most `max` UTF-16 units without splitting a surrogate pair. */
function truncate(value: string, max: number): string {
  let out = "";
  for (const ch of value) {
    if (out.length + ch.length > max) break;
    out += ch;
  }
  return out;
}

function fill(pattern: string, parts: { number: string; client: string; date: string }): string {
  return pattern.replace(/\{(number|client|date)\}/g, (_, key: keyof typeof parts) => parts[key]);
}

/** A file name that is valid on Windows, macOS and Linux for an exported invoice PDF. */
export function exportFileName(input: ExportFileNameInput): string {
  const pattern = input.pattern?.trim() ? input.pattern : DEFAULT_PATTERN;
  const parts = {
    number: cleanPart(input.number),
    client: cleanPart(input.client),
    date: cleanPart(input.date),
  };

  let stem = fill(pattern, parts);
  if (stem.length > MAX_STEM) {
    // The invoice number is what identifies the file, so the client name gives way first.
    const overflow = stem.length - MAX_STEM;
    const client = truncate(parts.client, Math.max(0, parts.client.length - overflow));
    stem = fill(pattern, { ...parts, client });
  }
  stem = cleanPart(truncate(cleanPart(stem), MAX_STEM));
  if (!stem) stem = FALLBACK;
  if (RESERVED.test(stem.split(".")[0])) stem = `_${stem}`;

  return `${stem}${input.mode === "flattened" ? "-flat" : ""}.pdf`;
}