[ADT] Add 'consume_front' and 'consume_back' methods to StringRef which are very handy when parsing text.

[chandlerc]

Summary

They are essentially a combination of startswith and a self-modifying
drop_front, or endswith and drop_back respectively.

[george.burgess.iv]

LGTM with a few nits.

include/llvm/ADT/StringRef.h

452 ↗

(On Diff #65218)

Do we want LLVM_ATTRIBUTE_UNUSED_RESULT on these, as well?

unittests/ADT/StringRefTest.cpp

337 ↗

(On Diff #65218)

Please add EXPECT_TRUE(Str.consume_front(""));

371 ↗

(On Diff #65218)

Please add EXPECT_TRUE(Str.consume_back(""));

[chandlerc]

Thanks Pete & George, submitted with your fixes

include/llvm/ADT/StringRef.h

452 ↗

(On Diff #65218)

I suspect I'll want to just use them for optional consuming some times... But I think your suggestion is right. IF the pattern becomes anonying to make explicit, can always drop it.

llvm/trunk/include/llvm/ADT/StringRef.h

public:
     /// dropped.
     LLVM_ATTRIBUTE_ALWAYS_INLINE
     LLVM_ATTRIBUTE_UNUSED_RESULT
     StringRef drop_back(size_t N = 1) const {
       assert(size() >= N && "Dropping more elements than exist");
       return substr(0, size()-N);
     }
 
+    /// Returns true if this StringRef has the given prefix and removes that
+    /// prefix.
+    LLVM_ATTRIBUTE_ALWAYS_INLINE
+    LLVM_ATTRIBUTE_UNUSED_RESULT
+    bool consume_front(StringRef Prefix) {
+      if (!startswith(Prefix))
+        return false;
+
+      *this = drop_front(Prefix.size());
+      return true;
+    }
+
+    /// Returns true if this StringRef has the given suffix and removes that
+    /// suffix.
+    LLVM_ATTRIBUTE_ALWAYS_INLINE
+    LLVM_ATTRIBUTE_UNUSED_RESULT
+    bool consume_back(StringRef Suffix) {
+      if (!endswith(Suffix))
+        return false;
+
+      *this = drop_back(Suffix.size());
+      return true;
+    }
+
     /// Return a reference to the substring from [Start, End).
     ///
     /// \param Start The index of the starting character in the substring; if
     /// the index is npos or greater than the length of the string then the
     /// empty substring will be returned.
     ///
     /// \param End The index following the last character to include in the
     /// substring. If this is npos or exceeds the number of characters

llvm/trunk/unittests/ADT/StringRefTest.cpp

TEST(StringRefTest, StartsWithLower) {
   EXPECT_TRUE(Str.startswith_lower(""));
   EXPECT_TRUE(Str.startswith_lower("he"));
   EXPECT_TRUE(Str.startswith_lower("hell"));
   EXPECT_TRUE(Str.startswith_lower("HELlo"));
   EXPECT_FALSE(Str.startswith_lower("helloworld"));
   EXPECT_FALSE(Str.startswith_lower("hi"));
 }
 
+TEST(StringRefTest, ConsumeFront) {
+  StringRef Str("hello");
+  EXPECT_TRUE(Str.consume_front(""));
+  EXPECT_EQ("hello", Str);
+  EXPECT_TRUE(Str.consume_front("he"));
+  EXPECT_EQ("llo", Str);
+  EXPECT_FALSE(Str.consume_front("lloworld"));
+  EXPECT_EQ("llo", Str);
+  EXPECT_FALSE(Str.consume_front("lol"));
+  EXPECT_EQ("llo", Str);
+  EXPECT_TRUE(Str.consume_front("llo"));
+  EXPECT_EQ("", Str);
+  EXPECT_FALSE(Str.consume_front("o"));
+  EXPECT_TRUE(Str.consume_front(""));
+}
+
 TEST(StringRefTest, EndsWith) {
   StringRef Str("hello");
   EXPECT_TRUE(Str.endswith(""));
   EXPECT_TRUE(Str.endswith("lo"));
   EXPECT_FALSE(Str.endswith("helloworld"));
   EXPECT_FALSE(Str.endswith("worldhello"));
   EXPECT_FALSE(Str.endswith("so"));
 }
 
 TEST(StringRefTest, EndsWithLower) {
   StringRef Str("heLLo");
   EXPECT_TRUE(Str.endswith_lower(""));
   EXPECT_TRUE(Str.endswith_lower("lo"));
   EXPECT_TRUE(Str.endswith_lower("LO"));
   EXPECT_TRUE(Str.endswith_lower("ELlo"));
   EXPECT_FALSE(Str.endswith_lower("helloworld"));
   EXPECT_FALSE(Str.endswith_lower("hi"));
 }
 
+TEST(StringRefTest, ConsumeBack) {
+  StringRef Str("hello");
+  EXPECT_TRUE(Str.consume_back(""));
+  EXPECT_EQ("hello", Str);
+  EXPECT_TRUE(Str.consume_back("lo"));
+  EXPECT_EQ("hel", Str);
+  EXPECT_FALSE(Str.consume_back("helhel"));
+  EXPECT_EQ("hel", Str);
+  EXPECT_FALSE(Str.consume_back("hle"));
+  EXPECT_EQ("hel", Str);
+  EXPECT_TRUE(Str.consume_back("hel"));
+  EXPECT_EQ("", Str);
+  EXPECT_FALSE(Str.consume_back("h"));
+  EXPECT_TRUE(Str.consume_back(""));
+}
+
 TEST(StringRefTest, Find) {
   StringRef Str("hello");
   EXPECT_EQ(2U, Str.find('l'));
   EXPECT_EQ(StringRef::npos, Str.find('z'));
   EXPECT_EQ(StringRef::npos, Str.find("helloworld"));
   EXPECT_EQ(0U, Str.find("hello"));
   EXPECT_EQ(1U, Str.find("ello"));
   EXPECT_EQ(StringRef::npos, Str.find("zz"));