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// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.// This module is browser compatible.
import { CHAR_COLON, CHAR_DOT } from "../_common/constants.ts";import { assertPath } from "../_common/assert_path.ts";import { isPathSeparator, isWindowsDeviceRoot } from "./_util.ts";
/** * Return the extension of the `path` with leading period. * @param path with extension * @returns extension (ex. for `file.ts` returns `.ts`) */export function extname(path: string): string {  assertPath(path);
  let start = 0;  let startDot = -1;  let startPart = 0;  let end = -1;  let matchedSlash = true;  // Track the state of characters (if any) we see before our first dot and  // after any path separator we find  let preDotState = 0;
  // Check for a drive letter prefix so as not to mistake the following  // path separator as an extra separator at the end of the path that can be  // disregarded
  if (    path.length >= 2 &&    path.charCodeAt(1) === CHAR_COLON &&    isWindowsDeviceRoot(path.charCodeAt(0))  ) {    start = startPart = 2;  }
  for (let i = path.length - 1; i >= start; --i) {    const code = path.charCodeAt(i);    if (isPathSeparator(code)) {      // If we reached a path separator that was not part of a set of path      // separators at the end of the string, stop now      if (!matchedSlash) {        startPart = i + 1;        break;      }      continue;    }    if (end === -1) {      // We saw the first non-path separator, mark this as the end of our      // extension      matchedSlash = false;      end = i + 1;    }    if (code === CHAR_DOT) {      // If this is our first dot, mark it as the start of our extension      if (startDot === -1) startDot = i;      else if (preDotState !== 1) preDotState = 1;    } else if (startDot !== -1) {      // We saw a non-dot and non-path separator before our dot, so we should      // have a good chance at having a non-empty extension      preDotState = -1;    }  }
  if (    startDot === -1 ||    end === -1 ||    // We saw a non-dot character immediately before the dot    preDotState === 0 ||    // The (right-most) trimmed path component is exactly '..'    (preDotState === 1 && startDot === end - 1 && startDot === startPart + 1)  ) {    return "";  }  return path.slice(startDot, end);}