diff --git a/gdb/kakoune.py b/gdb/kakoune.py
index f9ef11f3..fb284864 100644
--- a/gdb/kakoune.py
+++ b/gdb/kakoune.py
@@ -51,9 +51,9 @@ class BufferIterator:
 
     def to_string(self):
         if self.val['m_buffer'] != 0:
-            return "buffer<%s>@%d" % (self.val['m_buffer'].dereference()['m_name'], self.val['m_position'])
+            return "buffer<%s>@(%d, %d)" % (self.val['m_buffer'].dereference()['m_name'], self.val['m_coord']['line'], self.val['m_coord']['column'])
         else:
-            return "buffer<none>@%s" % (self.val['m_position'])
+            return "buffer<none>@(%d, %d)" % (self.val['m_coord']['line'], self.val['m_coord']['column'])
 
 def build_pretty_printer():
     pp = gdb.printing.RegexpCollectionPrettyPrinter("kakoune")
diff --git a/src/buffer.cc b/src/buffer.cc
index 824006df..75416712 100644
--- a/src/buffer.cc
+++ b/src/buffer.cc
@@ -47,37 +47,25 @@ Buffer::~Buffer()
 
 BufferIterator Buffer::iterator_at(const BufferCoord& line_and_column) const
 {
-    if (m_lines.empty())
-        return begin();
-
-    BufferCoord clamped = clamp(line_and_column);
-    return BufferIterator(*this, m_lines[clamped.line] + clamped.column);
+    return BufferIterator(*this, clamp(line_and_column));
 }
 
 BufferCoord Buffer::line_and_column_at(const BufferIterator& iterator) const
 {
-    BufferCoord result;
-    if (not m_lines.empty())
-    {
-        result.line = line_at(iterator);
-        result.column = iterator.m_position - m_lines[result.line];
-    }
-    return result;
+    return iterator.m_coord;
 }
 
 BufferPos Buffer::line_at(const BufferIterator& iterator) const
 {
-    auto it = std::upper_bound(m_lines.begin(), m_lines.end(),
-                               iterator.m_position);
-    return it - m_lines.begin() - 1;
+    return iterator.line();
 }
 
 BufferSize Buffer::line_length(BufferPos line) const
 {
-    assert(not m_lines.empty());
+    assert(line < line_count());
     BufferPos end = (line < m_lines.size() - 1) ?
-                    m_lines[line + 1] : length();
-    return end - m_lines[line];
+                    m_lines[line + 1].start : length();
+    return end - m_lines[line].start;
 }
 
 BufferCoord Buffer::clamp(const BufferCoord& line_and_column) const
@@ -87,36 +75,39 @@ BufferCoord Buffer::clamp(const BufferCoord& line_and_column) const
 
     BufferCoord result(line_and_column.line, line_and_column.column);
     result.line = Kakoune::clamp<int>(0, m_lines.size() - 1, result.line);
-    int max_col = std::max(0, line_length(result.line)-2);
+    int max_col = std::max(0, line_length(result.line) - 2);
     result.column = Kakoune::clamp<int>(0, max_col, result.column);
     return result;
 }
 
 BufferIterator Buffer::iterator_at_line_begin(const BufferIterator& iterator) const
 {
-    return BufferIterator(*this, m_lines[line_at(iterator)]);
+    return BufferIterator(*this, { iterator.line(), 0 });
 }
 
 BufferIterator Buffer::iterator_at_line_end(const BufferIterator& iterator) const
 {
-    BufferPos line = line_at(iterator) + 1;
-    return line < m_lines.size() ? BufferIterator(*this, m_lines[line]) : end();
+    BufferPos line = iterator.line();
+    return ++BufferIterator(*this, { line, std::max(line_length(line) - 1, 0) });
 }
 
-
 BufferIterator Buffer::begin() const
 {
-    return BufferIterator(*this, 0);
+    return BufferIterator(*this, { 0, 0 });
 }
 
 BufferIterator Buffer::end() const
 {
-    return BufferIterator(*this, length());
+    if (m_lines.empty())
+        return BufferIterator(*this, { 0, 0 });
+    return BufferIterator(*this, { (int)line_count()-1, (int)m_lines.back().content.length() });
 }
 
 BufferSize Buffer::length() const
 {
-    return m_content.size();
+    if (m_lines.empty())
+        return 0;
+    return m_lines.back().start + m_lines.back().content.length();
 }
 
 BufferSize Buffer::line_count() const
@@ -126,7 +117,18 @@ BufferSize Buffer::line_count() const
 
 String Buffer::string(const BufferIterator& begin, const BufferIterator& end) const
 {
-    return m_content.substr(begin.m_position, end - begin);
+    String res;
+    for (BufferPos line = begin.line(); line <= end.line(); ++line)
+    {
+       size_t start = 0;
+       if (line == begin.line())
+           start = begin.column();
+       size_t count = -1;
+       if (line == end.line())
+           count = end.column() - start;
+       res += m_lines[line].content.substr(start, count);
+    }
+    return res;
 }
 
 void Buffer::begin_undo_group()
@@ -182,79 +184,115 @@ bool Buffer::redo()
     ++m_history_cursor;
 }
 
-void Buffer::update_lines(const Modification& modification)
+void Buffer::check_invariant() const
 {
-    size_t length = modification.content.length();
-    const BufferPos pos = modification.position.m_position;
-    const BufferPos endpos = pos + length;
-
-    // find the first line beginning after modification position
-    auto line_it = std::upper_bound(m_lines.begin(), m_lines.end(), pos);
-
-    if (modification.type == Modification::Insert)
+    BufferSize start = 0;
+    for (auto& line : m_lines)
     {
-        // all following lines advanced by length
-        for (auto it = line_it; it != m_lines.end(); ++it)
-            *it += length;
-
-        std::vector<BufferPos> new_lines;
-        // if we inserted at the end of the buffer, we may have created a new
-        // line without inserting a '\n'
-        if (endpos == m_content.size() and
-            (pos == 0 or m_content[pos-1] == '\n'))
-            new_lines.push_back(pos);
-
-        // every \n inserted that was not the last buffer character created a
-        // new line
-        for (BufferPos i = pos; i < endpos and i + 1 < m_content.size(); ++i)
-        {
-            if (m_content[i] == '\n')
-                new_lines.push_back(i + 1);
-        }
-        m_lines.insert(line_it, new_lines.begin(), new_lines.end());
+        assert(line.start == start);
+        start += line.content.length();
     }
-    else if (modification.type == Modification::Erase)
+}
+
+void Buffer::insert(const BufferIterator& pos, const String& content)
+{
+    BufferSize offset = pos.offset();
+
+    // all following lines advanced by length
+    for (size_t i = pos.line()+1; i < line_count(); ++i)
+        m_lines[i].start += content.length();
+
+    // if we inserted at the end of the buffer, we may have created a new
+    // line without inserting a '\n'
+    if (pos == end() and (pos == begin() or *(pos-1) == '\n'))
     {
-        // find the first line beginning after endpos
-        auto end = std::upper_bound(line_it, m_lines.end(), endpos);
-
-        // all the lines until the end moved back by length
-        for (auto it = end; it != m_lines.end(); ++it)
+        int start = 0;
+        for (int i = 0; i < content.length(); ++i)
         {
-            *it -= length;
-            assert(m_content[(*it)-1] == '\n');
+            if (content[i] == '\n')
+            {
+                m_lines.push_back({ offset + start, content.substr(start, i + 1 - start) });
+                start = i + 1;
+            }
         }
-
-        // if we erased from the beginning of a line until the end of
-        // the buffer, that line also needs to be erased
-        if (pos == m_content.size() and *(line_it-1) == pos)
-            --line_it;
-        m_lines.erase(line_it, end);
+        if (start != content.length())
+            m_lines.push_back({ offset + start, content.substr(start) });
     }
     else
-        assert(false);
+    {
+        String prefix = m_lines[pos.line()].content.substr(0, pos.column());
+        String suffix = m_lines[pos.line()].content.substr(pos.column());
+
+        auto line_it = m_lines.begin() + pos.line();
+        line_it = m_lines.erase(line_it);
+
+        int start = 0;
+        for (int i = 0; i < content.length(); ++i)
+        {
+            if (content[i] == '\n')
+            {
+                String line_content = content.substr(start, i + 1 - start);
+                if (start == 0)
+                {
+                    line_content = prefix + line_content;
+                    line_it = m_lines.insert(line_it, { offset + start - (int)prefix.length(),
+                                                        std::move(line_content) });
+                }
+                else
+                    line_it = m_lines.insert(line_it, { offset + start,
+                                                        std::move(line_content) });
+
+                ++line_it;
+                start = i + 1;
+            }
+        }
+        if (start == 0)
+            m_lines.insert(line_it, { offset + start - (int)prefix.length(), prefix + content + suffix });
+        else
+            m_lines.insert(line_it, { offset + start, content.substr(start) + suffix });
+    }
+
+    check_invariant();
+}
+
+void Buffer::erase(const BufferIterator& pos, BufferSize length)
+{
+    BufferIterator end = pos + length;
+    String prefix = m_lines[pos.line()].content.substr(0, pos.column());
+    String suffix = m_lines[end.line()].content.substr(end.column());
+    Line new_line = { m_lines[pos.line()].start, prefix + suffix };
+
+    m_lines.erase(m_lines.begin() + pos.line(), m_lines.begin() + end.line() + 1);
+    m_lines.insert(m_lines.begin() + pos.line(), new_line);
+
+    for (size_t i = pos.line()+1; i < line_count(); ++i)
+        m_lines[i].start -= length;
+
+    check_invariant();
 }
 
 void Buffer::apply_modification(const Modification& modification)
 {
+    const String& content = modification.content;
+    const BufferIterator& pos = modification.position;
+
     switch (modification.type)
     {
     case Modification::Insert:
-        m_content.insert(modification.position.m_position,
-                         modification.content);
+        insert(modification.position, modification.content);
         break;
     case Modification::Erase:
     {
         size_t size = modification.content.size();
         assert(string(modification.position, modification.position + size)
                == modification.content);
-        m_content.erase(modification.position.m_position, size);
+        erase(modification.position, size);
         break;
     }
     default:
         assert(false);
     }
-    update_lines(modification);
+
     for (auto listener : m_modification_listeners)
         listener->on_modification(modification);
 }
diff --git a/src/buffer.hh b/src/buffer.hh
index 60e6f7e2..526110fd 100644
--- a/src/buffer.hh
+++ b/src/buffer.hh
@@ -12,6 +12,7 @@ namespace Kakoune
 {
 
 class Buffer;
+class Modification;
 class Window;
 
 typedef int      BufferPos;
@@ -39,8 +40,8 @@ public:
     typedef const value_type& reference;
     typedef std::bidirectional_iterator_tag iterator_category;
 
-    BufferIterator() : m_buffer(NULL), m_position(0) {}
-    BufferIterator(const Buffer& buffer, BufferPos position);
+    BufferIterator() : m_buffer(NULL) {}
+    BufferIterator(const Buffer& buffer, BufferCoord coord);
     BufferIterator& operator=(const BufferIterator& iterator);
 
     bool operator== (const BufferIterator& iterator) const;
@@ -66,11 +67,17 @@ public:
     bool is_end() const;
     bool is_valid() const;
 
+    void update(const Modification& modification);
+
     const Buffer& buffer() const;
 
 private:
+    BufferSize line() const { return m_coord.line; }
+    BufferSize column() const { return m_coord.column; }
+    BufferSize offset() const;
+
     const Buffer* m_buffer;
-    BufferPos     m_position;
+    BufferCoord   m_coord;
     friend class Buffer;
 };
 
@@ -145,8 +152,6 @@ public:
 
     const std::string& name() const { return m_name; }
 
-    const String& content() const { return m_content; }
-
     Window* get_or_create_window();
     void delete_window(Window* window);
 
@@ -166,17 +171,26 @@ public:
     // not on the last one)
     BufferIterator iterator_at_line_end(const BufferIterator& iterator) const;
 
+    const String& line_content(size_t l) const { return m_lines[l].content; }
+
 private:
     friend class BufferIterator;
 
-    std::vector<BufferPos> m_lines;
-    void update_lines(const Modification& modification);
+    void check_invariant() const;
+
+    struct Line
+    {
+        BufferPos start;
+        String    content;
+    };
+    std::vector<Line> m_lines;
+
+    void insert(const BufferIterator& pos, const String& content);
+    void erase(const BufferIterator& pos, BufferSize length);
 
     BufferPos line_at(const BufferIterator& iterator) const;
     BufferSize line_length(BufferPos line) const;
 
-    String m_content;
-
     std::string  m_name;
     const Type   m_type;
 
diff --git a/src/buffer_iterator.inl.hh b/src/buffer_iterator.inl.hh
index d7def812..fa9e8f67 100644
--- a/src/buffer_iterator.inl.hh
+++ b/src/buffer_iterator.inl.hh
@@ -6,10 +6,10 @@
 namespace Kakoune
 {
 
-inline BufferIterator::BufferIterator(const Buffer& buffer, BufferPos position)
-    : m_buffer(&buffer),
-      m_position(std::max(0, std::min(position, (BufferPos)buffer.length())))
+inline BufferIterator::BufferIterator(const Buffer& buffer, BufferCoord coord)
+    : m_buffer(&buffer), m_coord(coord)
 {
+    assert(is_valid());
 }
 
 inline const Buffer& BufferIterator::buffer() const
@@ -20,90 +20,175 @@ inline const Buffer& BufferIterator::buffer() const
 
 inline bool BufferIterator::is_valid() const
 {
-    return m_buffer;
+    return m_buffer and
+           ((line() < m_buffer->line_count() and
+             column() < m_buffer->m_lines[line()].content.length()) or
+            ((line() == m_buffer->line_count() and column() == 0)) or
+             (line() == m_buffer->line_count() - 1 and
+              column() == m_buffer->m_lines.back().content.length()));
 }
 
 inline BufferIterator& BufferIterator::operator=(const BufferIterator& iterator)
 {
     m_buffer = iterator.m_buffer;
-    m_position = iterator.m_position;
+    m_coord = iterator.m_coord;
     return *this;
 }
 
 inline bool BufferIterator::operator==(const BufferIterator& iterator) const
 {
     assert(m_buffer == iterator.m_buffer);
-    return (m_position == iterator.m_position);
+    return (m_coord == iterator.m_coord);
 }
 
 inline bool BufferIterator::operator!=(const BufferIterator& iterator) const
 {
     assert(m_buffer == iterator.m_buffer);
-    return (m_position != iterator.m_position);
+    return (m_coord != iterator.m_coord);
 }
 
 inline bool BufferIterator::operator<(const BufferIterator& iterator) const
 {
     assert(m_buffer == iterator.m_buffer);
-    return (m_position < iterator.m_position);
+    return (m_coord < iterator.m_coord);
 }
 
 inline bool BufferIterator::operator<=(const BufferIterator& iterator) const
 {
     assert(m_buffer == iterator.m_buffer);
-    return (m_position <= iterator.m_position);
+    return (m_coord <= iterator.m_coord);
 }
 
 inline bool BufferIterator::operator>(const BufferIterator& iterator) const
 {
     assert(m_buffer == iterator.m_buffer);
-    return (m_position > iterator.m_position);
+    return (m_coord > iterator.m_coord);
 }
 
 inline bool BufferIterator::operator>=(const BufferIterator& iterator) const
 {
     assert(m_buffer == iterator.m_buffer);
-    return (m_position >= iterator.m_position);
+    return (m_coord >= iterator.m_coord);
+}
+
+inline BufferCoord measure_string(const String& string)
+{
+    BufferCoord res;
+    for (auto c : string)
+    {
+        if (c == '\n')
+        {
+            ++res.line;
+            res.column = 0;
+        }
+        else
+           ++res.column;
+    }
+    return res;
+}
+
+inline BufferCoord advance(const BufferCoord& base, const BufferCoord& offset)
+{
+    if (offset.line == 0)
+        return BufferCoord{base.line, base.column + offset.column};
+    else
+        return BufferCoord{base.line + offset.line, offset.column};
+}
+
+inline void BufferIterator::update(const Modification& modification)
+{
+    const BufferIterator& pos = modification.position;
+    if (*this < pos)
+        return;
+
+    BufferCoord measure = measure_string(modification.content);
+    if (modification.type == Modification::Erase)
+    {
+        BufferCoord end = advance(pos.m_coord, measure);
+        if (m_coord <= end)
+            m_coord = pos.m_coord;
+        else
+        {
+            m_coord.line -= measure.line;
+            if (end.line == m_coord.line)
+                m_coord.column -= measure.column;
+        }
+
+        if (is_end())
+            operator--();
+    }
+    else
+    {
+        assert(modification.type == Modification::Insert);
+        if (pos.line() == line())
+        {
+            BufferCoord end = advance(pos.m_coord, measure);
+            m_coord.column = end.column + column() - pos.column();
+        }
+        m_coord.line += measure.line;
+    }
+    assert(is_valid());
 }
 
 inline BufferChar BufferIterator::operator*() const
 {
     assert(m_buffer);
-    return m_buffer->m_content[m_position];
+    return m_buffer->m_lines[line()].content[column()];
+}
+
+inline BufferSize BufferIterator::offset() const
+{
+    assert(m_buffer);
+    return line() == 0 ? column() : m_buffer->m_lines[line()].start + column();
 }
 
 inline BufferSize BufferIterator::operator-(const BufferIterator& iterator) const
 {
     assert(m_buffer == iterator.m_buffer);
-    return m_position - iterator.m_position;
+    return offset() - iterator.offset();
 }
 
 inline BufferIterator BufferIterator::operator+(BufferSize size) const
 {
     assert(m_buffer);
-    return BufferIterator(*m_buffer, m_position + size);
+    if (size >= 0)
+    {
+        BufferSize o = std::min(m_buffer->length(), offset() + size);
+        for (int i = line() + 1; i < m_buffer->line_count(); ++i)
+        {
+            if (m_buffer->m_lines[i].start > o)
+                return BufferIterator(*m_buffer, { i-1, o - m_buffer->m_lines[i-1].start });
+        }
+        int last_line = m_buffer->line_count() - 1;
+        return BufferIterator(*m_buffer, { last_line, o - m_buffer->m_lines[last_line].start });
+    }
+    return operator-(-size);
 }
 
 inline BufferIterator BufferIterator::operator-(BufferSize size) const
 {
     assert(m_buffer);
-    return BufferIterator(*m_buffer, m_position - size);
+    if (size >= 0)
+    {
+        BufferSize o = std::max(0, offset() - size);
+        for (int i = line(); i >= 0; --i)
+        {
+            if (m_buffer->m_lines[i].start <= o)
+                return BufferIterator(*m_buffer, { i, o - m_buffer->m_lines[i].start });
+        }
+        assert(false);
+    }
+    return operator+(-size);
 }
 
 inline BufferIterator& BufferIterator::operator+=(BufferSize size)
 {
-    assert(m_buffer);
-    m_position = std::max(0, std::min((BufferSize)m_position + size,
-                                      m_buffer->length()));
-    return *this;
+    return *this = (*this + size);
 }
 
 inline BufferIterator& BufferIterator::operator-=(BufferSize size)
 {
-    assert(m_buffer);
-    m_position = std::max(0, std::min((BufferSize)m_position - size,
-                                      m_buffer->length()));
-    return *this;
+    return *this = (*this - size);
 }
 
 inline BufferIterator& BufferIterator::operator++()
@@ -119,13 +204,13 @@ inline BufferIterator& BufferIterator::operator--()
 inline bool BufferIterator::is_begin() const
 {
     assert(m_buffer);
-    return m_position == 0;
+    return m_coord.line == 0 and m_coord.column == 0;
 }
 
 inline bool BufferIterator::is_end() const
 {
     assert(m_buffer);
-    return m_position == m_buffer->length();
+    return offset() == m_buffer->length();
 }
 
 }
diff --git a/src/file.cc b/src/file.cc
index 08ce8ae6..fdb50927 100644
--- a/src/file.cc
+++ b/src/file.cc
@@ -88,18 +88,21 @@ void write_buffer_to_file(const Buffer& buffer, const std::string& filename)
     if (fd == -1)
         throw file_access_error(filename, strerror(errno));
 
-    const String& content = buffer.content();
-    ssize_t count = content.length() * sizeof(BufferChar);
-    const char* ptr = content.c_str();
-
-    while (count)
+    for (size_t i = 0; i < buffer.line_count(); ++i)
     {
-        ssize_t written = write(fd, ptr, count);
-        ptr += written;
-        count -= written;
+        const String& content = buffer.line_content(i);
+        ssize_t count = content.length() * sizeof(BufferChar);
+        const char* ptr = content.c_str();
 
-        if (written == -1)
-            throw file_access_error(filename, strerror(errno));
+        while (count)
+        {
+            ssize_t written = write(fd, ptr, count);
+            ptr += written;
+            count -= written;
+
+            if (written == -1)
+                throw file_access_error(filename, strerror(errno));
+        }
     }
     close(fd);
 }
diff --git a/src/selection.cc b/src/selection.cc
index 1d4e1765..c34c1362 100644
--- a/src/selection.cc
+++ b/src/selection.cc
@@ -54,36 +54,10 @@ void Selection::merge_with(const Selection& selection)
     m_last = selection.m_last;
 }
 
-static void update_iterator(const Modification& modification,
-                            BufferIterator& iterator)
-{
-    if (iterator < modification.position)
-        return;
-
-    size_t length = modification.content.length();
-    if (modification.type == Modification::Erase)
-    {
-        // do not move length on the other side of the inequality,
-        // as modification.position + length may be after buffer end
-        if (iterator - length <= modification.position)
-            iterator = modification.position;
-        else
-            iterator -= length;
-
-        if (iterator.is_end())
-            --iterator;
-    }
-    else
-    {
-        assert(modification.type == Modification::Insert);
-        iterator += length;
-    }
-}
-
 void Selection::on_modification(const Modification& modification)
 {
-    update_iterator(modification, m_first);
-    update_iterator(modification, m_last);
+    m_first.update(modification);
+    m_last.update(modification);
 }
 
 void Selection::register_with_buffer()