// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
// This module is browser compatible.
import { CHAR_DOT } from "../_common/constants.ts";
import { assertPath } from "../_common/assert_path.ts";
import { isPosixPathSeparator } from "./_util.ts";
/**
 * Return the extension of the `path` with leading period.
 *
 * @example
 * ```ts
 * import { extname } from "@std/path/extname";
 *
 * console.log(extname("/home/user/Documents/")); // ""
 * console.log(extname("/home/user/Documents/image.png")); // ".png"
 * ```
 *
 * @param path with extension
 * @returns extension (ex. for `file.ts` returns `.ts`)
 */ export function extname(path) {
  assertPath(path);
  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;
  for(let i = path.length - 1; i >= 0; --i){
    const code = path.charCodeAt(i);
    if (isPosixPathSeparator(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);
}
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