add a new API seek for the Cursor class in the DataExtractor.cpp

[DiggerLin]

Summary

add a new API seek to set the Cursor to a new position.

[jhenderson]

llvm/include/llvm/Support/DataExtractor.h

73

Perhaps add another sentence saying "This does not impact the error state."

llvm/unittests/Support/DataExtractorTest.cpp

181

I think you can simplify this testing: Cursor doesn't depend on DataExtractor, so you can just instantiate a Cursor and then call seek on it a couple of times to show what happens, like the following:

Cursor C(5);
C.seek(3);
EXPECT_EQ(3u, C.tell());
C.seek(8);
EXPECT_EQ(8u, C.tell());

This is different to tell, where reading data impacts the tell location.

[DiggerLin]

llvm/include/llvm/Support/DataExtractor.h

73

thanks

llvm/unittests/Support/DataExtractorTest.cpp

181

thanks

[DiggerLin]

address comments

[jhenderson]

LGTM.

[MaskRay]

Looks great!

llvm/include/llvm/Support/DataExtractor.h

public:
     /// case of errors, this does not clear the error flag -- one must call
     /// takeError() instead.
     explicit operator bool() { return !Err; }
 
     /// Return the current position of this Cursor. In the error state this is
     /// the position of the Cursor before the first error was encountered.
     uint64_t tell() const { return Offset; }
 
+    /// Set the cursor to the new offset. This does not impact the error state.
+    void seek(uint64_t NewOffSet) { Offset = NewOffSet; }
+
     /// Return error contained inside this Cursor, if any. Clears the internal
     /// Cursor state.
     Error takeError() { return std::move(Err); }
   };
 
   /// Construct with a buffer that is owned by the caller.
   ///
   /// This constructor allows us to use data that is owned by the

llvm/unittests/Support/DataExtractorTest.cpp

TEST(DataExtractorTest, Cursor_tell) {
 
   // And neither do any subsequent operations.
   EXPECT_EQ(0, DE.getU8(C));
   EXPECT_EQ(1u, C.tell());
 
   consumeError(C.takeError());
 }
 
+TEST(DataExtractorTest, Cursor_seek) {
+  DataExtractor::Cursor C(5);
+
+  C.seek(3);
+  EXPECT_EQ(3u, C.tell());
+
+  C.seek(8);
+  EXPECT_EQ(8u, C.tell());
+
+  EXPECT_THAT_ERROR(C.takeError(), Succeeded());
+}
+
 TEST(DataExtractorTest, Cursor_takeError) {
   DataExtractor DE(StringRef("AB"), false, 8);
   DataExtractor::Cursor C(0);
   // Initially, the cursor is in the "success" state.
   EXPECT_THAT_ERROR(C.takeError(), Succeeded());
 
   // It remains "success" after a successful read.
   EXPECT_EQ('A', DE.getU8(C));