// Matches the scheme of a URL, eg "http://"
const schemeRegex = /^[\w+.-]+:\/\//;

/**
 * Matches the parts of a URL:
 * 1. Scheme, including ":", guaranteed.
 * 2. User/password, including "@", optional.
 * 3. Host, guaranteed.
 * 4. Port, including ":", optional.
 * 5. Path, including "/", optional.
 */
const urlRegex = /^([\w+.-]+:)\/\/([^@/#?]*@)?([^:/#?]*)(:\d+)?(\/[^#?]*)?/;

/**
 * File URLs are weird. They dont' need the regular `//` in the scheme, they may or may not start
 * with a leading `/`, they can have a domain (but only if they don't start with a Windows drive).
 *
 * 1. Host, optional.
 * 2. Path, which may inclue "/", guaranteed.
 */
const fileRegex = /^file:(?:\/\/((?![a-z]:)[^/]*)?)?(\/?.*)/i;

type Url = {
  scheme: string;
  user: string;
  host: string;
  port: string;
  path: string;
  relativePath: boolean;
};

function isAbsoluteUrl(input: string): boolean {
  return schemeRegex.test(input);
}

function isSchemeRelativeUrl(input: string): boolean {
  return input.startsWith('//');
}

function isAbsolutePath(input: string): boolean {
  return input.startsWith('/');
}

function isFileUrl(input: string): boolean {
  return input.startsWith('file:');
}

function parseAbsoluteUrl(input: string): Url {
  const match = urlRegex.exec(input)!;
  return makeUrl(match[1], match[2] || '', match[3], match[4] || '', match[5] || '/');
}

function parseFileUrl(input: string): Url {
  const match = fileRegex.exec(input)!;
  const path = match[2];
  return makeUrl('file:', '', match[1] || '', '', isAbsolutePath(path) ? path : '/' + path);
}

function makeUrl(scheme: string, user: string, host: string, port: string, path: string): Url {
  return {
    scheme,
    user,
    host,
    port,
    path,
    relativePath: false,
  };
}

function parseUrl(input: string): Url {
  if (isSchemeRelativeUrl(input)) {
    const url = parseAbsoluteUrl('http:' + input);
    url.scheme = '';
    return url;
  }

  if (isAbsolutePath(input)) {
    const url = parseAbsoluteUrl('http://foo.com' + input);
    url.scheme = '';
    url.host = '';
    return url;
  }

  if (isFileUrl(input)) return parseFileUrl(input);

  if (isAbsoluteUrl(input)) return parseAbsoluteUrl(input);

  const url = parseAbsoluteUrl('http://foo.com/' + input);
  url.scheme = '';
  url.host = '';
  url.relativePath = true;
  return url;
}

function stripPathFilename(path: string): string {
  // If a path ends with a parent directory "..", then it's a relative path with excess parent
  // paths. It's not a file, so we can't strip it.
  if (path.endsWith('/..')) return path;
  const index = path.lastIndexOf('/');
  return path.slice(0, index + 1);
}

function mergePaths(url: Url, base: Url) {
  // If we're not a relative path, then we're an absolute path, and it doesn't matter what base is.
  if (!url.relativePath) return;

  normalizePath(base);

  // If the path is just a "/", then it was an empty path to begin with (remember, we're a relative
  // path).
  if (url.path === '/') {
    url.path = base.path;
  } else {
    // Resolution happens relative to the base path's directory, not the file.
    url.path = stripPathFilename(base.path) + url.path;
  }

  // If the base path is absolute, then our path is now absolute too.
  url.relativePath = base.relativePath;
}

/**
 * The path can have empty directories "//", unneeded parents "foo/..", or current directory
 * "foo/.". We need to normalize to a standard representation.
 */
function normalizePath(url: Url) {
  const { relativePath } = url;
  const pieces = url.path.split('/');

  // We need to preserve the first piece always, so that we output a leading slash. The item at
  // pieces[0] is an empty string.
  let pointer = 1;

  // Positive is the number of real directories we've output, used for popping a parent directory.
  // Eg, "foo/bar/.." will have a positive 2, and we can decrement to be left with just "foo".
  let positive = 0;

  // We need to keep a trailing slash if we encounter an empty directory (eg, splitting "foo/" will
  // generate `["foo", ""]` pieces). And, if we pop a parent directory. But once we encounter a
  // real directory, we won't need to append, unless the other conditions happen again.
  let addTrailingSlash = false;

  for (let i = 1; i < pieces.length; i++) {
    const piece = pieces[i];

    // An empty directory, could be a trailing slash, or just a double "//" in the path.
    if (!piece) {
      addTrailingSlash = true;
      continue;
    }

    // If we encounter a real directory, then we don't need to append anymore.
    addTrailingSlash = false;

    // A current directory, which we can always drop.
    if (piece === '.') continue;

    // A parent directory, we need to see if there are any real directories we can pop. Else, we
    // have an excess of parents, and we'll need to keep the "..".
    if (piece === '..') {
      if (positive) {
        addTrailingSlash = true;
        positive--;
        pointer--;
      } else if (relativePath) {
        // If we're in a relativePath, then we need to keep the excess parents. Else, in an absolute
        // URL, protocol relative URL, or an absolute path, we don't need to keep excess.
        pieces[pointer++] = piece;
      }
      continue;
    }

    // We've encountered a real directory. Move it to the next insertion pointer, which accounts for
    // any popped or dropped directories.
    pieces[pointer++] = piece;
    positive++;
  }

  let path = '';
  for (let i = 1; i < pointer; i++) {
    path += '/' + pieces[i];
  }
  if (!path || (addTrailingSlash && !path.endsWith('/..'))) {
    path += '/';
  }
  url.path = path;
}

/**
 * Attempts to resolve `input` URL/path relative to `base`.
 */
export default function resolve(input: string, base: string | undefined): string {
  if (!input && !base) return '';

  const url = parseUrl(input);

  // If we have a base, and the input isn't already an absolute URL, then we need to merge.
  if (base && !url.scheme) {
    const baseUrl = parseUrl(base);
    url.scheme = baseUrl.scheme;
    // If there's no host, then we were just a path.
    if (!url.host) {
      // The host, user, and port are joined, you can't copy one without the others.
      url.user = baseUrl.user;
      url.host = baseUrl.host;
      url.port = baseUrl.port;
    }
    mergePaths(url, baseUrl);
  }

  normalizePath(url);

  // If the input (and base, if there was one) are both relative, then we need to output a relative.
  if (url.relativePath) {
    // The first char is always a "/".
    const path = url.path.slice(1);
    if (!path) return '.';

    // If base started with a leading ".", or there is no base and input started with a ".", then we
    // need to ensure that the relative path starts with a ".". We don't know if relative starts
    // with a "..", though, so check before prepending.
    const keepRelative = (base || input).startsWith('.');
    return !keepRelative || path.startsWith('.') ? path : './' + path;
  }
  // If there's no host (and no scheme/user/port), then we need to output an absolute path.
  if (!url.scheme && !url.host) return url.path;
  // We're outputting either an absolute URL, or a protocol relative one.
  return `${url.scheme}//${url.user}${url.host}${url.port}${url.path}`;
}