Link Header Parser For RFC 8288 Relations

export interface ParsedLink {
  url: string;
  rel: string[];
  params: Record<string, string>;
}

function skipWhitespace(header: string, index: number): number {
  let cursor = index;
  while (cursor < header.length && /\s/.test(header[cursor])) cursor += 1;
  return cursor;
}

function parseParameters(
  header: string,
  start: number,
): { params: Record<string, string>; next: number } {
  const params: Record<string, string> = {};
  let index = start;
  while (index < header.length && header[index] !== ",") {
    index = skipWhitespace(header, index);
    if (index >= header.length || header[index] === ",") break;
    if (header[index] !== ";") throw new SyntaxError("expected ';' at index " + index);
    index = skipWhitespace(header, index + 1);

    const keyStart = index;
    while (index < header.length && /[A-Za-z0-9!#$%&'*+\-.^_\x60|~]/.test(header[index])) {
      index += 1;
    }
    if (index === keyStart) throw new SyntaxError("missing link parameter name");
    const key = header.slice(keyStart, index).toLowerCase();
    index = skipWhitespace(header, index);
    if (header[index] !== "=") throw new SyntaxError("missing '=' for " + key);
    index = skipWhitespace(header, index + 1);

    let value = "";
    if (header[index] === '"') {
      index += 1;
      let closed = false;
      while (index < header.length) {
        const character = header[index];
        if (character === "\\") {
          index += 1;
          if (index >= header.length) throw new SyntaxError("unterminated escape");
          value += header[index];
          index += 1;
        } else if (character === '"') {
          index += 1;
          closed = true;
          break;
        } else {
          value += character;
          index += 1;
        }
      }
      if (!closed) throw new SyntaxError("unterminated quoted parameter");
    } else {
      const valueStart = index;
      while (index < header.length && header[index] !== ";" && header[index] !== ",") {
        index += 1;
      }
      value = header.slice(valueStart, index).trim();
      if (value === "") throw new SyntaxError("empty link parameter value");
    }
    params[key] = value;
  }
  return { params, next: index };
}

export function parseLinkHeader(header: string, baseUrl?: string): ParsedLink[] {
  if (typeof header !== "string") throw new TypeError("header must be a string");
  const links: ParsedLink[] = [];
  let index = 0;
  while (index < header.length) {
    index = skipWhitespace(header, index);
    while (header[index] === ",") index = skipWhitespace(header, index + 1);
    if (index >= header.length) break;
    if (header[index] !== "<") throw new SyntaxError("expected '<' at index " + index);
    const end = header.indexOf(">", index + 1);
    if (end === -1) throw new SyntaxError("unterminated link target");
    const target = header.slice(index + 1, end).trim();
    if (target === "") throw new SyntaxError("empty link target");
    index = end + 1;

    const { params, next } = parseParameters(header, index);
    index = next;
    if (header[index] === ",") index += 1;

    let url = target;
    if (baseUrl !== undefined) {
      try {
        url = new URL(target, baseUrl).toString();
      } catch {
        throw new SyntaxError("invalid link URL: " + target);
      }
    }
    links.push({
      url,
      rel: (params.rel ?? "").split(/\s+/).filter((entry) => entry.length > 0),
      params,
    });
  }
  return links;
}

export function findLink(links: readonly ParsedLink[], rel: string): string | undefined {
  return links.find((link) => link.rel.includes(rel))?.url;
}