fix(path): fix common() results that depend on the order and/or numbe…

…r of input paths (#4414)

path/_common/common.ts

@@ -3,24 +3,24 @@
 
 export function _common(paths: string[], sep: string): 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;
+  let append = "";
   for (const path of remaining) {
     const compare = path.split(sep);
+    if (compare.length <= endOfPrefix) {
+      endOfPrefix = compare.length;
+      append = "";
+    }
+
     for (let i = 0; i < endOfPrefix; i++) {
       if (compare[i] !== parts[i]) {
         endOfPrefix = i;
+        append = i === 0 ? "" : sep;
+        break;
       }
     }
-
-    if (endOfPrefix === 0) {
-      return "";
-    }
   }
-  const prefix = parts.slice(0, endOfPrefix).join(sep);
-  return prefix.endsWith(sep) ? prefix : `${prefix}${sep}`;
+  return parts.slice(0, endOfPrefix).join(sep) + append;
 }

path/common_test.ts

@@ -42,3 +42,45 @@ Deno.test({
     assertEquals(actual, "c:\\deno\\");
   },
 });
+
+Deno.test({
+  name: "common(['', '/'], '/') returns ''",
+  fn() {
+    const actual = common(["", "/"], "/");
+    assertEquals(actual, "");
+  },
+});
+
+Deno.test({
+  name: "common(['/', ''], '/') returns ''",
+  fn() {
+    const actual = common([
+      "/",
+      "",
+    ], "/");
+    assertEquals(actual, "");
+  },
+});
+
+Deno.test({
+  name: "common() returns the first path unmodified when it's the only path",
+  fn() {
+    const actual = common(["./deno/std/path/mod.ts"], "/");
+    assertEquals(actual, "./deno/std/path/mod.ts");
+  },
+});
+
+Deno.test({
+  name: "common() returns the first path unmodified if all paths are equal",
+  fn() {
+    const actual = common(
+      [
+        "./deno/std/path/mod.ts",
+        "./deno/std/path/mod.ts",
+        "./deno/std/path/mod.ts",
+      ],
+      "/",
+    );
+    assertEquals(actual, "./deno/std/path/mod.ts");
+  },
+});