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diff --git a/effcee/cursor.h b/effcee/cursor.h
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+// Copyright 2017 The Effcee Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#ifndef EFFCEE_CURSOR_H
+#define EFFCEE_CURSOR_H
+
+#include <sstream>
+#include <string>
+
+#include "re2/stringpiece.h"
+
+namespace effcee {
+
+using StringPiece = re2::StringPiece;
+
+// Represents a position in a StringPiece, while tracking line number.
+class Cursor {
+
+ public:
+  explicit Cursor(StringPiece str)
+      : remaining_(str), line_num_(1) {}
+
+  StringPiece remaining() const { return remaining_; }
+  // Returns the current 1-based line number.
+  int line_num() const { return line_num_; }
+
+  // Returns true if the remaining text is empty.
+  bool Exhausted() const { return remaining_.empty(); }
+
+  // Returns a string piece from the current position until the end of the line
+  // or the end of input, up to and including the newline.
+  StringPiece RestOfLine() const {
+    const auto newline_pos = remaining_.find('\n');
+    return remaining_.substr(0, newline_pos + (newline_pos != StringPiece::npos));
+  }
+
+  // Advance |n| characters.  Does not adjust line count.  The next |n|
+  // characters should not contain newlines if line numbering is to remain
+  // up to date.  Returns this object.
+  Cursor& Advance(size_t n) { remaining_.remove_prefix(n); return *this; }
+
+  // Advances the cursor by a line.  If no text remains, then does nothing.
+  // Otherwise removes the first line (including newline) and increments the
+  // line count.  If there is no newline then the remaining string becomes
+  // empty.  Returns this object.
+  Cursor& AdvanceLine() {
+    if (remaining_.size()) {
+      Advance(RestOfLine().size());
+      ++line_num_;
+    }
+    return *this;
+  };
+
+ private:
+  // The remaining text, after all previous advancements.  References the
+  // original string storage.
+  StringPiece remaining_;
+  // The current 1-based line number.
+  int line_num_;
+};
+
+// Returns string containing a description of the line containing a given
+// subtext, with a message, and a caret displaying the subtext position.
+// Assumes subtext does not contain a newline.
+inline std::string LineMessage(StringPiece text, StringPiece subtext,
+                               StringPiece message) {
+  Cursor c(text);
+  StringPiece full_line = c.RestOfLine();
+  while (subtext.end() - full_line.end() > 0) {
+    c.AdvanceLine();
+    full_line = c.RestOfLine();
+  }
+  const char* full_line_newline =
+      full_line.find('\n') == StringPiece::npos ? "\n" : "";
+  const size_t column = subtext.begin() - full_line.begin();
+
+  std::ostringstream out;
+  out << ":" << c.line_num() << ":" << (1 + column) << ": " << message << "\n"
+      << full_line << full_line_newline << std::string(column, ' ') << "^\n";
+
+  return out.str();
+}
+
+}  // namespace effcee
+
+#endif