4 files changed, 58 insertions, 11 deletions

diff --git a/fs/move.ts b/fs/move.ts
index eb6352bd5..007e4520b 100644
--- a/fs/move.ts
+++ b/fs/move.ts
@@ -1,20 +1,12 @@
 // Copyright 2018-2019 the Deno authors. All rights reserved. MIT license.
 import * as path from "./path/mod.ts";
 import { exists, existsSync } from "./exists.ts";
+import { isSubdir } from "./utils.ts";
 
 interface MoveOptions {
   overwrite?: boolean;
 }
 
-function isSrcSubdir(src: string, dest: string): boolean {
-  const srcArray = src.split(path.sep);
-  const destArray = dest.split(path.sep);
-
-  return srcArray.reduce((acc, current, i) => {
-    return acc && destArray[i] === current;
-  }, true);
-}
-
 /** Moves a file or directory */
 export async function move(
   src: string,
@@ -26,7 +18,7 @@ export async function move(
 
   const srcStat = await Deno.stat(src);
 
-  if (srcStat.isDirectory() && isSrcSubdir(src, dest)) {
+  if (srcStat.isDirectory() && isSubdir(src, dest)) {
     throw new Error(
       `Cannot move '${src}' to a subdirectory of itself, '${dest}'.`
     );
@@ -56,7 +48,7 @@ export function moveSync(
 
   const srcStat = Deno.statSync(src);
 
-  if (srcStat.isDirectory() && isSrcSubdir(src, dest)) {
+  if (srcStat.isDirectory() && isSubdir(src, dest)) {
     throw new Error(
       `Cannot move '${src}' to a subdirectory of itself, '${dest}'.`
     );
diff --git a/fs/utils.ts b/fs/utils.ts
new file mode 100644
index 000000000..dd7ee5fa4
--- /dev/null
+++ b/fs/utils.ts
@@ -0,0 +1,23 @@
+import * as path from "./path/mod.ts";
+
+/**
+ * Test whether or not `dest` is a sub-directory of `src`
+ * @param src src file path
+ * @param dest dest file path
+ * @param sep path separator
+ */
+export function isSubdir(
+  src: string,
+  dest: string,
+  sep: string = path.sep
+): boolean {
+  if (src === dest) {
+    return false;
+  }
+  const srcArray = src.split(sep);
+  const destArray = dest.split(sep);
+
+  return srcArray.reduce((acc, current, i) => {
+    return acc && destArray[i] === current;
+  }, true);
+}
diff --git a/fs/utils_test.ts b/fs/utils_test.ts
new file mode 100644
index 000000000..9d6959de5
--- /dev/null
+++ b/fs/utils_test.ts
@@ -0,0 +1,31 @@
+// Copyright the Browserify authors. MIT License.
+
+import { test } from "../testing/mod.ts";
+import { assertEquals } from "../testing/asserts.ts";
+import { isSubdir } from "./utils.ts";
+import * as path from "./path/mod.ts";
+
+test(function _isSubdir() {
+  const pairs = [
+    ["", "", false, path.posix.sep],
+    ["/first/second", "/first", false, path.posix.sep],
+    ["/first", "/first", false, path.posix.sep],
+    ["/first", "/first/second", true, path.posix.sep],
+    ["first", "first/second", true, path.posix.sep],
+    ["../first", "../first/second", true, path.posix.sep],
+    ["c:\\first", "c:\\first", false, path.win32.sep],
+    ["c:\\first", "c:\\first\\second", true, path.win32.sep]
+  ];
+
+  pairs.forEach(function(p) {
+    const src = p[0] as string;
+    const dest = p[1] as string;
+    const expected = p[2] as boolean;
+    const sep = p[3] as string;
+    assertEquals(
+      isSubdir(src, dest, sep),
+      expected,
+      `'${src}' should ${expected ? "" : "not"} be parent dir of '${dest}'`
+    );
+  });
+});
diff --git a/test.ts b/test.ts
index ee449a971..0cf95fde0 100755
--- a/test.ts
+++ b/test.ts
@@ -20,6 +20,7 @@ import "./fs/ensure_file_test.ts";
 import "./fs/move_test.ts";
 import "./fs/read_json_test.ts";
 import "./fs/write_json_test.ts";
+import "./fs/utils_test.ts";
 import "./io/test.ts";
 import "./http/server_test.ts";
 import "./http/file_server_test.ts";