Add function to determine the parent directory of a file

Author: phst

elisp/private/tools/system.cc

@@ -128,6 +128,25 @@ absl::StatusOr<FileName> FileName::FromString(const NativeStringView string) {
   return FileName(std::move(name));
 }
 
+absl::StatusOr<FileName> FileName::Parent() const {
+  NativeStringView string = string_;
+  while (string.back() == kSeparator) string.remove_suffix(1);
+  NativeStringView::size_type i = string.rfind(kSeparator);
+  if (i == string.npos || (kWindows && i == 0)) {
+    return absl::InvalidArgumentError(
+        absl::StrFormat("File %v has no parent", *this));
+  }
+  const NativeStringView element = string.substr(i + 1);
+  if (element == RULES_ELISP_NATIVE_LITERAL(".") ||
+      element == RULES_ELISP_NATIVE_LITERAL("..")) {
+    return absl::InvalidArgumentError(absl::StrFormat(
+        "Removing trailing component %s would be ambiguous", element));
+  }
+  // Root directories need to end in a separator character.
+  if (i == (kWindows ? 2 : 0)) i++;
+  return FileName::FromString(string.substr(0, i));
+}
+
 absl::StatusOr<FileName> FileName::Child(const FileName& child) const {
   const NativeString& string = child.string_;
   if (string == RULES_ELISP_NATIVE_LITERAL(".") ||

elisp/private/tools/system.h

@@ -84,6 +84,7 @@ class FileName final {
     return string_.data();
   }
 
+  absl::StatusOr<FileName> Parent() const;
   absl::StatusOr<FileName> Child(const FileName& child) const;
   absl::StatusOr<FileName> Child(NativeStringView child) const;
   absl::StatusOr<FileName> Join(const FileName& descendant) const;

tests/tools/system_test.cc

@@ -150,6 +150,67 @@ TEST(FileNameTest, IsFormattable) {
   EXPECT_EQ(absl::StrFormat("foo %v baz", bar), "foo bar äα𝐴🐈' baz");
 }
 
+TEST(FileNameTest, ParentRejectsRoot) {
+  EXPECT_THAT(FileName::FromString(kWindows ? RULES_ELISP_NATIVE_LITERAL("C:\\")
+                                            : RULES_ELISP_NATIVE_LITERAL("/"))
+                  .value()
+                  .Parent(),
+              StatusIs(absl::StatusCode::kInvalidArgument));
+}
+
+TEST(FileNameTest, ParentRejectsDot) {
+  EXPECT_THAT(FileName::FromString(RULES_ELISP_NATIVE_LITERAL("foo/."))
+                  .value()
+                  .Parent(),
+              StatusIs(absl::StatusCode::kInvalidArgument));
+}
+
+TEST(FileNameTest, ParentRejectsDotDot) {
+  EXPECT_THAT(FileName::FromString(RULES_ELISP_NATIVE_LITERAL("foo/.."))
+                  .value()
+                  .Parent(),
+              StatusIs(absl::StatusCode::kInvalidArgument));
+}
+
+TEST(FileNameTest, ParentWorksForRelativeName) {
+  EXPECT_THAT(
+      FileName::FromString(RULES_ELISP_NATIVE_LITERAL("foo/bar"))
+          .value()
+          .Parent(),
+      IsOkAndHolds(
+          FileName::FromString(RULES_ELISP_NATIVE_LITERAL("foo")).value()));
+}
+
+TEST(FileNameTest, ParentWorksForAbsoluteName) {
+  EXPECT_THAT(
+      FileName::FromString(kWindows ? RULES_ELISP_NATIVE_LITERAL("C:\\Foo")
+                                    : RULES_ELISP_NATIVE_LITERAL("/foo"))
+          .value()
+          .Parent(),
+      IsOkAndHolds(FileName::FromString(kWindows
+                                            ? RULES_ELISP_NATIVE_LITERAL("C:\\")
+                                            : RULES_ELISP_NATIVE_LITERAL("/"))
+                       .value()));
+  EXPECT_THAT(
+      FileName::FromString(kWindows ? RULES_ELISP_NATIVE_LITERAL("C:\\Foo\\Bar")
+                                    : RULES_ELISP_NATIVE_LITERAL("/foo/bar"))
+          .value()
+          .Parent(),
+      IsOkAndHolds(
+          FileName::FromString(kWindows ? RULES_ELISP_NATIVE_LITERAL("C:\\Foo")
+                                        : RULES_ELISP_NATIVE_LITERAL("/foo"))
+              .value()));
+}
+
+TEST(FileNameTest, ParentIgnoresTrailingSlashes) {
+  EXPECT_THAT(
+      FileName::FromString(RULES_ELISP_NATIVE_LITERAL("foo/bar//"))
+          .value()
+          .Parent(),
+      IsOkAndHolds(
+          FileName::FromString(RULES_ELISP_NATIVE_LITERAL("foo")).value()));
+}
+
 TEST(FileNameTest, ChildRejectsDescendant) {
   const FileName parent =
       FileName::FromString(RULES_ELISP_NATIVE_LITERAL("foo")).value();