deno/std/path/common.ts

// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
/** This module is browser compatible. */

import { SEP } from "./separator.ts";

/** Determines the common path from a set of paths, using an optional separator,
 * which defaults to the OS default separator.
 *
 *       import { common } from "https://deno.land/std/path/mod.ts";
 *       const p = common([
 *         "./deno/std/path/mod.ts",
 *         "./deno/std/fs/mod.ts",
 *       ]);
 *       console.log(p); // "./deno/std/"
 *
 */
export function common(paths: string[], sep = SEP): string {
  const [first = "", ...remaining] = paths;
  if (first === "" || remaining.length === 0) {
    return first.substring(0, first.lastIndexOf(sep) + 1);
  }
  const parts = first.split(sep);

  let endOfPrefix = parts.length;
  for (const path of remaining) {
    const compare = path.split(sep);
    for (let i = 0; i < endOfPrefix; i++) {
      if (compare[i] !== parts[i]) {
        endOfPrefix = i;
      }
    }

    if (endOfPrefix === 0) {
      return "";
    }
  }
  const prefix = parts.slice(0, endOfPrefix).join(sep);
  return prefix.endsWith(sep) ? prefix : `${prefix}${sep}`;
}