1 files changed, 38 insertions, 0 deletions
diff --git a/std/path/common.ts b/std/path/common.ts
new file mode 100644
index 000000000..0a4de3f0c
--- /dev/null
+++ b/std/path/common.ts
@@ -0,0 +1,38 @@
+// Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
+
+import { SEP } from "./constants.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}`;
+}