diff --git a/src/line_modification.cc b/src/line_modification.cc
new file mode 100644
index 00000000..5da287fc
--- /dev/null
+++ b/src/line_modification.cc
@@ -0,0 +1,105 @@
+#include "line_modification.hh"
+
+#include "buffer.hh"
+
+namespace Kakoune
+{
+
+namespace
+{
+
+struct LineChange
+{
+    LineChange(const Buffer::Change& change)
+    {
+        ByteCoord begin = change.begin;
+        ByteCoord end = change.end;
+        if (change.type == Buffer::Change::Insert)
+        {
+            if (change.at_end and begin != ByteCoord{0,0})
+            {
+                kak_assert(begin.column == 0);
+                --begin.line;
+            }
+            pos = begin.line;
+            num = end.line - begin.line;
+        }
+        else
+        {
+            if (change.at_end and begin != ByteCoord{0,0})
+            {
+                kak_assert(begin.column == 0);
+                --begin.line;
+            }
+            pos = begin.line;
+            num = begin.line - end.line;
+        }
+    }
+
+    LineCount pos;
+    LineCount num;
+};
+
+}
+
+std::vector<LineModification> compute_line_modifications(const Buffer& buffer, size_t timestamp)
+{
+    std::vector<LineModification> res;
+    for (auto& buf_change : buffer.changes_since(timestamp))
+    {
+        const LineChange change(buf_change);
+
+        auto pos = std::upper_bound(res.begin(), res.end(), change.pos,
+                                    [](const LineCount& l, const LineModification& c)
+                                    { return l < c.new_line; });
+
+        if (pos != res.begin())
+        {
+            auto& prev = *(pos-1);
+            if (change.pos <= prev.new_line + prev.num_added)
+                --pos;
+            else
+                pos = res.insert(pos, {change.pos - prev.diff(), change.pos, 0, 0});
+        }
+        else
+            pos = res.insert(pos, {change.pos, change.pos, 0, 0});
+
+        auto& modif = *pos;
+        auto next = pos + 1;
+        if (change.num > 0)
+        {
+            modif.num_added += change.num;
+            for (auto it = next; it != res.end(); ++it)
+                it->new_line += change.num;
+        }
+        if (change.num < 0)
+        {
+            const LineCount num_removed = -change.num;
+
+            auto delend = std::upper_bound(next, res.end(), change.pos + num_removed,
+                                           [](const LineCount& l, const LineModification& c)
+                                           { return l < c.new_line; });
+
+            for (auto it = next; it != delend; ++it)
+            {
+                LineCount removed_from_it = (change.pos + num_removed - it->new_line);
+                modif.num_removed += it->num_removed - std::min(removed_from_it, it->num_added);
+                modif.num_added += std::max(0_line, it->num_added - removed_from_it);
+            }
+            next = res.erase(next, delend);
+
+            const LineCount num_added_after_pos =
+                modif.new_line + modif.num_added - change.pos;
+            const LineCount num_removed_from_added =
+                std::min(num_removed, num_added_after_pos);
+            modif.num_added -= num_removed_from_added;
+            modif.num_removed += num_removed - num_removed_from_added;
+
+            for (auto it = next; it != res.end(); ++it)
+                it->new_line -= num_removed;
+        }
+    }
+    return res;
+}
+
+}
diff --git a/src/line_modification.hh b/src/line_modification.hh
new file mode 100644
index 00000000..3cd2d669
--- /dev/null
+++ b/src/line_modification.hh
@@ -0,0 +1,26 @@
+#ifndef line_change_watcher_hh_INCLUDED
+#define line_change_watcher_hh_INCLUDED
+
+#include "units.hh"
+#include "utils.hh"
+
+namespace Kakoune
+{
+
+class Buffer;
+
+struct LineModification
+{
+    LineCount old_line; // line position in the old buffer
+    LineCount new_line; // new line position
+    LineCount num_removed; // number of lines removed after this one
+    LineCount num_added; // number of lines added after this one
+
+    LineCount diff() const { return new_line - old_line + num_added - num_removed; }
+};
+
+std::vector<LineModification> compute_line_modifications(const Buffer& buffer, size_t timestamp);
+
+}
+
+#endif // line_change_watcher_hh_INCLUDED