Introduce a simple line-by-line iterator type into the Support library.

This is an iterator which you can build around a MemoryBuffer. It will
iterate through the non-empty, non-comment lines of the buffer as
a forward iterator. It should be small and reasonably fast (although it
could be made much faster if anyone cares, I don't really...).

This will be used to more simply support the text-based sample
profile file format, and is largely based on the original patch by
Diego. I've re-worked the style of it and separated it from the work of
producing a MemoryBuffer from a file which both simplifies the interface
and makes it easier to test.

The style of the API follows the C++ standard naming conventions to fit
in better with iterators in general, much like the Path and FileSystem
interfaces follow standard-based naming conventions.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@198068 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/LineIterator.h

@@ -0,0 +1,73 @@
+//===- LineIterator.h - Iterator to read a text buffer's lines --*- C++ -*-===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/ADT/StringRef.h"
+#include <iterator>
+
+namespace llvm {
+
+class MemoryBuffer;
+
+/// \brief A forward iterator which reads non-blank text lines from a buffer.
+///
+/// This class provides a forward iterator interface for reading one line at
+/// a time from a buffer. When default constructed the iterator will be the
+/// "end" iterator.
+///
+/// The iterator also is aware of what line number it is currently processing
+/// and can strip comment lines given the comment-starting character.
+///
+/// Note that this iterator requires the buffer to be nul terminated.
+class line_iterator
+    : public std::iterator<std::forward_iterator_tag, StringRef, ptrdiff_t> {
+  const MemoryBuffer *Buffer;
+  char CommentMarker;
+
+  unsigned LineNumber;
+  StringRef CurrentLine;
+
+public:
+  /// \brief Default construct an "end" iterator.
+  line_iterator() : Buffer(0) {}
+
+  /// \brief Construct a new iterator around some memory buffer.
+  explicit line_iterator(const MemoryBuffer &Buffer, char CommentMarker = '\0');
+
+  /// \brief Return true if we've reached EOF or are an "end" iterator.
+  bool is_at_eof() const { return !Buffer; }
+
+  /// \brief Return true if we're an "end" iterator or have reached EOF.
+  bool is_at_end() const { return is_at_eof(); }
+
+  /// \brief Return the current line number. May return any number at EOF.
+  int64_t line_number() const { return LineNumber; }
+
+  /// \brief Advance to the next (non-empty, non-comment) line.
+  line_iterator &operator++() {
+    advance();
+    return *this;
+  }
+
+  /// \brief Get the current line as a \c StringRef.
+  StringRef operator*() const { return CurrentLine; }
+
+  friend bool operator==(const line_iterator &LHS, const line_iterator &RHS) {
+    return LHS.Buffer == RHS.Buffer &&
+           LHS.CurrentLine.begin() == RHS.CurrentLine.begin();
+  }
+
+  friend bool operator!=(const line_iterator &LHS, const line_iterator &RHS) {
+    return !(LHS == RHS);
+  }
+
+private:
+  /// \brief Advance the iterator to the next line.
+  void advance();
+};
+}