string: add character_position function

`character_position` computes the line and column numbers for an
iterator inside a view.

string.hpp

@@ -3,16 +3,14 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/.
  *
- * Copyright 2011-2016 Danny Robson <danny@nerdcruft.net>
+ * Copyright 2011-2018 Danny Robson <danny@nerdcruft.net>
  */
 
-#ifndef CRUFT_UTIL_STRING_HPP
-#define CRUFT_UTIL_STRING_HPP
+#pragma once
 
 #include "debug.hpp"
 #include "view.hpp"
 
-
 namespace cruft {
     std::string to_utf8 (const wchar_t*);
     std::string to_utf8 (const std::wstring&);
@@ -147,7 +145,37 @@ namespace cruft {
     {
         return tokeniser { std::begin (data), std::end (data), separator };
     }
+
+
+    ///////////////////////////////////////////////////////////////////////////
+    /// Calculate the line and column of an iterator within a view.
+    template <typename IteratorT>
+    auto
+    character_position (
+        cruft::view<IteratorT> data,
+        IteratorT const cursor
+    ) {
+        using difference_type = typename std::iterator_traits<IteratorT>::difference_type;
+        difference_type line_count = 0;
+        auto current_line = data.begin ();
+
+        do {
+            auto next_line = std::find (current_line, data.end (), '\n');
+            if (next_line == data.end () || next_line > cursor)
+                break;
+
+            ++line_count;
+            current_line = next_line + 1;
+        } while (1);
+
+        struct {
+            difference_type line;
+            difference_type column;
+        } val = {
+            .line = line_count,
+            .column = cursor - current_line,
+        };
+
+        return val;
+    }
 }
-
-#endif // __UTIL_STRING_HPP
-