Landing http://codereview.chromium.org/3387008 for sadrul:

third_party/undoview/undo_manager.c

Index: third_party/undoview/undo_manager.c
diff --git a/third_party/undoview/undo_manager.c b/third_party/undoview/undo_manager.c
new file mode 100644
index 0000000000000000000000000000000000000000..9136b37bc263f9aa9f1adb97390d0689ed8ae7fe
--- /dev/null
+++ b/third_party/undoview/undo_manager.c
@@ -0,0 +1,1084 @@
+/*
+ * Copyright (C) 1998, 1999 Alex Roberts, Evan Lawrence
+ * Copyright (C) 2000, 2001 Chema Celorio, Paolo Maggi
+ * Copyright (C) 2002-2005 Paolo Maggi
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ * Lesser General Public License for more details.
+
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#ifdef HAVE_CONFIG_H
+#include <config.h>
+#endif
+
+#include <glib.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "undo_manager.h"
+
+#define DEFAULT_MAX_UNDO_LEVELS 25
+
+typedef struct _GtkSourceUndoAction GtkSourceUndoAction;
+typedef struct _GtkSourceUndoInsertAction GtkSourceUndoInsertAction;
+typedef struct _GtkSourceUndoDeleteAction GtkSourceUndoDeleteAction;
+
+typedef enum {
+  GTK_SOURCE_UNDO_ACTION_INSERT,
+  GTK_SOURCE_UNDO_ACTION_DELETE,
+} GtkSourceUndoActionType;
+
+/*
+ * We use offsets instead of GtkTextIters because the last ones
+ * require to much memory in this context without giving us any advantage.
+ */
+
+struct _GtkSourceUndoInsertAction {
+  gint pos;
+  gchar *text;
+  gint length;
+  gint chars;
+};
+
+struct _GtkSourceUndoDeleteAction {
+  gint start;
+  gint end;
+  gchar *text;
+  gboolean forward;
+};
+
+struct _GtkSourceUndoAction {
+  GtkSourceUndoActionType action_type;
+
+  union {
+    GtkSourceUndoInsertAction insert;
+    GtkSourceUndoDeleteAction delete;
+  } action;
+
+  gint order_in_group;
+
+  /* It is TRUE whether the action can be merged with the following action. */
+  guint mergeable : 1;
+
+  /* It is TRUE whether the action is marked as "modified".
+   * An action is marked as "modified" if it changed the
+   * state of the buffer from "not modified" to "modified". Only the first
+   * action of a group can be marked as modified.
+   * There can be a single action marked as "modified" in the actions list.
+   */
+  guint modified : 1;
+};
+
+/* INVALID is a pointer to an invalid action */
+#define INVALID ((void *) "IA")
+
+struct _GtkSourceUndoManagerPrivate {
+  GtkTextBuffer *document;
+
+  GList* actions;
+  gint next_redo;
+
+  gint actions_in_current_group;
+
+  gint running_not_undoable_actions;
+
+  gint num_of_groups;
+
+  gint max_undo_levels;
+
+  guint can_undo : 1;
+  guint can_redo : 1;
+
+  /* It is TRUE whether, while undoing an action of the current group (with order_in_group > 1),
+   * the state of the buffer changed from "not modified" to "modified".
+   */
+  guint modified_undoing_group : 1;
+
+  /* Pointer to the action (in the action list) marked as "modified".
+   * It is NULL when no action is marked as "modified".
+   * It is INVALID when the action marked as "modified" has been removed
+   * from the action list (freeing the list or resizing it) */
+  GtkSourceUndoAction *modified_action;
+};
+
+enum {
+  CAN_UNDO,
+  CAN_REDO,
+  LAST_SIGNAL
+};
+
+static void gtk_source_undo_manager_class_init(GtkSourceUndoManagerClass *klass);
+static void gtk_source_undo_manager_init(GtkSourceUndoManager *um);
+static void gtk_source_undo_manager_finalize(GObject *object);
+
+static void gtk_source_undo_manager_insert_text_handler(GtkTextBuffer *buffer,
+                  GtkTextIter *pos,
+                                         const gchar *text,
+                  gint length,
+                  GtkSourceUndoManager *um);
+static void gtk_source_undo_manager_delete_range_handler(GtkTextBuffer *buffer,
+                  GtkTextIter *start,
+                                          GtkTextIter *end,
+                  GtkSourceUndoManager *um);
+static void gtk_source_undo_manager_begin_user_action_handler(GtkTextBuffer *buffer,
+                 GtkSourceUndoManager *um);
+static void gtk_source_undo_manager_modified_changed_handler(GtkTextBuffer *buffer,
+                 GtkSourceUndoManager *um);
+
+static void gtk_source_undo_manager_free_action_list(GtkSourceUndoManager *um);
+
+static void gtk_source_undo_manager_add_action(GtkSourceUndoManager *um,
+                                                const GtkSourceUndoAction *undo_action);
+static void gtk_source_undo_manager_free_first_n_actions(GtkSourceUndoManager *um,
+                 gint n);
+static void gtk_source_undo_manager_check_list_size(GtkSourceUndoManager *um);
+
+static gboolean gtk_source_undo_manager_merge_action(GtkSourceUndoManager *um,
+                                               const GtkSourceUndoAction *undo_action);
+
+static GObjectClass *parent_class = NULL;
+static guint undo_manager_signals [LAST_SIGNAL] = { 0 };
+
+GType
+gtk_source_undo_manager_get_type(void) {
+  static GType undo_manager_type = 0;
+
+    if(undo_manager_type == 0)
+      {
+          static const GTypeInfo our_info =
+          {
+            sizeof(GtkSourceUndoManagerClass),
+            NULL, /* base_init */
+            NULL, /* base_finalize */
+            (GClassInitFunc) gtk_source_undo_manager_class_init,
+            NULL, /* class_finalize */
+            NULL, /* class_data */
+            sizeof(GtkSourceUndoManager),
+            0, /* n_preallocs */
+            (GInstanceInitFunc) gtk_source_undo_manager_init,
+            NULL /* value_table */
+          };
+
+          undo_manager_type = g_type_register_static(G_TYPE_OBJECT,
+              "GtkSourceUndoManager",
+              &our_info,
+              0);
+      }
+
+  return undo_manager_type;
+}
+
+static void
+gtk_source_undo_manager_class_init(GtkSourceUndoManagerClass *klass) {
+  GObjectClass *object_class = G_OBJECT_CLASS(klass);
+
+  parent_class = g_type_class_peek_parent(klass);
+
+  object_class->finalize = gtk_source_undo_manager_finalize;
+
+  klass->can_undo = NULL;
+  klass->can_redo = NULL;
+
+  undo_manager_signals[CAN_UNDO] =
+       g_signal_new("can_undo",
+            G_OBJECT_CLASS_TYPE(object_class),
+            G_SIGNAL_RUN_LAST,
+            G_STRUCT_OFFSET(GtkSourceUndoManagerClass, can_undo),
+            NULL, NULL,
+            g_cclosure_marshal_VOID__BOOLEAN,
+            G_TYPE_NONE,
+            1,
+            G_TYPE_BOOLEAN);
+
+  undo_manager_signals[CAN_REDO] =
+       g_signal_new("can_redo",
+            G_OBJECT_CLASS_TYPE(object_class),
+            G_SIGNAL_RUN_LAST,
+            G_STRUCT_OFFSET(GtkSourceUndoManagerClass, can_redo),
+            NULL, NULL,
+            g_cclosure_marshal_VOID__BOOLEAN,
+            G_TYPE_NONE,
+            1,
+            G_TYPE_BOOLEAN);
+}
+
+static void
+gtk_source_undo_manager_init(GtkSourceUndoManager *um) {
+  um->priv = g_new0(GtkSourceUndoManagerPrivate, 1);
+
+  um->priv->actions = NULL;
+  um->priv->next_redo = 0;
+
+  um->priv->can_undo = FALSE;
+  um->priv->can_redo = FALSE;
+
+  um->priv->running_not_undoable_actions = 0;
+
+  um->priv->num_of_groups = 0;
+
+  um->priv->max_undo_levels = DEFAULT_MAX_UNDO_LEVELS;
+
+  um->priv->modified_action = NULL;
+
+  um->priv->modified_undoing_group = FALSE;
+}
+
+static void
+gtk_source_undo_manager_finalize(GObject *object) {
+  GtkSourceUndoManager *um;
+
+  g_return_if_fail(object != NULL);
+  g_return_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(object));
+
+  um = GTK_SOURCE_UNDO_MANAGER(object);
+
+  g_return_if_fail(um->priv != NULL);
+
+  if(um->priv->actions != NULL)
+    gtk_source_undo_manager_free_action_list(um);
+
+  g_signal_handlers_disconnect_by_func(G_OBJECT(um->priv->document),
+        G_CALLBACK(gtk_source_undo_manager_delete_range_handler),
+        um);
+
+  g_signal_handlers_disconnect_by_func(G_OBJECT(um->priv->document),
+        G_CALLBACK(gtk_source_undo_manager_insert_text_handler),
+        um);
+
+  g_signal_handlers_disconnect_by_func(G_OBJECT(um->priv->document),
+        G_CALLBACK(gtk_source_undo_manager_begin_user_action_handler),
+        um);
+
+  g_signal_handlers_disconnect_by_func(G_OBJECT(um->priv->document),
+        G_CALLBACK(gtk_source_undo_manager_modified_changed_handler),
+        um);
+
+  g_free(um->priv);
+
+  G_OBJECT_CLASS(parent_class)->finalize(object);
+}
+
+GtkSourceUndoManager*
+gtk_source_undo_manager_new(GtkTextBuffer* buffer) {
+  GtkSourceUndoManager *um;
+
+  um = GTK_SOURCE_UNDO_MANAGER(g_object_new(GTK_SOURCE_TYPE_UNDO_MANAGER, NULL));
+
+  g_return_val_if_fail(um->priv != NULL, NULL);
+  um->priv->document = buffer;
+
+  g_signal_connect(G_OBJECT(buffer), "insert_text",
+        G_CALLBACK(gtk_source_undo_manager_insert_text_handler),
+        um);
+
+  g_signal_connect(G_OBJECT(buffer), "delete_range",
+        G_CALLBACK(gtk_source_undo_manager_delete_range_handler),
+        um);
+
+  g_signal_connect(G_OBJECT(buffer), "begin_user_action",
+        G_CALLBACK(gtk_source_undo_manager_begin_user_action_handler),
+        um);
+
+  g_signal_connect(G_OBJECT(buffer), "modified_changed",
+        G_CALLBACK(gtk_source_undo_manager_modified_changed_handler),
+        um);
+  return um;
+}
+
+void
+gtk_source_undo_manager_begin_not_undoable_action(GtkSourceUndoManager *um) {
+  g_return_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um));
+  g_return_if_fail(um->priv != NULL);
+
+  ++um->priv->running_not_undoable_actions;
+}
+
+static void
+gtk_source_undo_manager_end_not_undoable_action_internal(GtkSourceUndoManager *um) {
+  g_return_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um));
+  g_return_if_fail(um->priv != NULL);
+
+  g_return_if_fail(um->priv->running_not_undoable_actions > 0);
+
+  --um->priv->running_not_undoable_actions;
+}
+
+void
+gtk_source_undo_manager_end_not_undoable_action(GtkSourceUndoManager *um) {
+  g_return_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um));
+  g_return_if_fail(um->priv != NULL);
+
+  gtk_source_undo_manager_end_not_undoable_action_internal(um);
+
+  if(um->priv->running_not_undoable_actions == 0)
+  {
+    gtk_source_undo_manager_free_action_list(um);
+
+    um->priv->next_redo = -1;
+
+    if(um->priv->can_undo)
+    {
+      um->priv->can_undo = FALSE;
+      g_signal_emit(G_OBJECT(um),
+               undo_manager_signals [CAN_UNDO],
+               0,
+               FALSE);
+    }
+
+    if(um->priv->can_redo)
+    {
+      um->priv->can_redo = FALSE;
+      g_signal_emit(G_OBJECT(um),
+               undo_manager_signals [CAN_REDO],
+               0,
+               FALSE);
+    }
+  }
+}
+
+gboolean
+gtk_source_undo_manager_can_undo(const GtkSourceUndoManager *um) {
+  g_return_val_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um), FALSE);
+  g_return_val_if_fail(um->priv != NULL, FALSE);
+
+  return um->priv->can_undo;
+}
+
+gboolean
+gtk_source_undo_manager_can_redo(const GtkSourceUndoManager *um) {
+  g_return_val_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um), FALSE);
+  g_return_val_if_fail(um->priv != NULL, FALSE);
+
+  return um->priv->can_redo;
+}
+
+static void
+set_cursor(GtkTextBuffer *buffer, gint cursor) {
+  GtkTextIter iter;
+
+  /* Place the cursor at the requested position */
+  gtk_text_buffer_get_iter_at_offset(buffer, &iter, cursor);
+  gtk_text_buffer_place_cursor(buffer, &iter);
+}
+
+static void
+insert_text(GtkTextBuffer *buffer, gint pos, const gchar *text, gint len) {
+  GtkTextIter iter;
+
+  gtk_text_buffer_get_iter_at_offset(buffer, &iter, pos);
+  gtk_text_buffer_insert(buffer, &iter, text, len);
+}
+
+static void
+delete_text(GtkTextBuffer *buffer, gint start, gint end) {
+  GtkTextIter start_iter;
+  GtkTextIter end_iter;
+
+  gtk_text_buffer_get_iter_at_offset(buffer, &start_iter, start);
+
+  if(end < 0)
+    gtk_text_buffer_get_end_iter(buffer, &end_iter);
+  else
+    gtk_text_buffer_get_iter_at_offset(buffer, &end_iter, end);
+
+  gtk_text_buffer_delete(buffer, &start_iter, &end_iter);
+}
+
+static gchar*
+get_chars(GtkTextBuffer *buffer, gint start, gint end) {
+  GtkTextIter start_iter;
+  GtkTextIter end_iter;
+
+  gtk_text_buffer_get_iter_at_offset(buffer, &start_iter, start);
+
+  if(end < 0)
+    gtk_text_buffer_get_end_iter(buffer, &end_iter);
+  else
+    gtk_text_buffer_get_iter_at_offset(buffer, &end_iter, end);
+
+  return gtk_text_buffer_get_slice(buffer, &start_iter, &end_iter, TRUE);
+}
+
+void
+gtk_source_undo_manager_undo(GtkSourceUndoManager *um) {
+  GtkSourceUndoAction *undo_action;
+  gboolean modified = FALSE;
+
+  g_return_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um));
+  g_return_if_fail(um->priv != NULL);
+  g_return_if_fail(um->priv->can_undo);
+
+  um->priv->modified_undoing_group = FALSE;
+
+  gtk_source_undo_manager_begin_not_undoable_action(um);
+
+  do
+  {
+    undo_action = g_list_nth_data(um->priv->actions, um->priv->next_redo + 1);
+    g_return_if_fail(undo_action != NULL);
+
+    /* undo_action->modified can be TRUE only if undo_action->order_in_group <= 1 */
+    g_return_if_fail((undo_action->order_in_group <= 1) ||
+        ((undo_action->order_in_group > 1) && !undo_action->modified));
+
+    if(undo_action->order_in_group <= 1)
+    {
+      /* Set modified to TRUE only if the buffer did not change its state from
+       * "not modified" to "modified" undoing an action(with order_in_group > 1)
+       * in current group. */
+      modified =(undo_action->modified && !um->priv->modified_undoing_group);
+    }
+
+    switch(undo_action->action_type)
+    {
+      case GTK_SOURCE_UNDO_ACTION_DELETE:
+        insert_text(
+          um->priv->document,
+          undo_action->action.delete.start,
+          undo_action->action.delete.text,
+          strlen(undo_action->action.delete.text));
+
+        if(undo_action->action.delete.forward)
+          set_cursor(
+            um->priv->document,
+            undo_action->action.delete.start);
+        else
+          set_cursor(
+            um->priv->document,
+            undo_action->action.delete.end);
+
+        break;
+
+      case GTK_SOURCE_UNDO_ACTION_INSERT:
+        delete_text(
+          um->priv->document,
+          undo_action->action.insert.pos,
+          undo_action->action.insert.pos +
+            undo_action->action.insert.chars);
+
+        set_cursor(
+          um->priv->document,
+          undo_action->action.insert.pos);
+        break;
+
+      default:
+        /* Unknown action type. */
+        g_return_if_reached();
+    }
+
+    ++um->priv->next_redo;
+
+  } while(undo_action->order_in_group > 1);
+
+  if(modified)
+  {
+    --um->priv->next_redo;
+    gtk_text_buffer_set_modified(um->priv->document, FALSE);
+    ++um->priv->next_redo;
+  }
+
+  gtk_source_undo_manager_end_not_undoable_action_internal(um);
+
+  um->priv->modified_undoing_group = FALSE;
+
+  if(!um->priv->can_redo)
+  {
+    um->priv->can_redo = TRUE;
+    g_signal_emit(G_OBJECT(um),
+             undo_manager_signals [CAN_REDO],
+             0,
+             TRUE);
+  }
+
+  if(um->priv->next_redo >=(gint)(g_list_length(um->priv->actions) - 1))
+  {
+    um->priv->can_undo = FALSE;
+    g_signal_emit(G_OBJECT(um),
+             undo_manager_signals [CAN_UNDO],
+             0,
+             FALSE);
+  }
+}
+
+void
+gtk_source_undo_manager_redo(GtkSourceUndoManager *um) {
+  GtkSourceUndoAction *undo_action;
+  gboolean modified = FALSE;
+
+  g_return_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um));
+  g_return_if_fail(um->priv != NULL);
+  g_return_if_fail(um->priv->can_redo);
+
+  undo_action = g_list_nth_data(um->priv->actions, um->priv->next_redo);
+  g_return_if_fail(undo_action != NULL);
+
+  gtk_source_undo_manager_begin_not_undoable_action(um);
+
+  do
+  {
+    if(undo_action->modified)
+    {
+      g_return_if_fail(undo_action->order_in_group <= 1);
+      modified = TRUE;
+    }
+
+    --um->priv->next_redo;
+
+    switch(undo_action->action_type)
+    {
+      case GTK_SOURCE_UNDO_ACTION_DELETE:
+        delete_text(
+          um->priv->document,
+          undo_action->action.delete.start,
+          undo_action->action.delete.end);
+
+        set_cursor(
+          um->priv->document,
+          undo_action->action.delete.start);
+
+        break;
+
+      case GTK_SOURCE_UNDO_ACTION_INSERT:
+        set_cursor(
+          um->priv->document,
+          undo_action->action.insert.pos);
+
+        insert_text(
+          um->priv->document,
+          undo_action->action.insert.pos,
+          undo_action->action.insert.text,
+          undo_action->action.insert.length);
+
+        break;
+
+      default:
+        /* Unknown action type */
+        ++um->priv->next_redo;
+        g_return_if_reached();
+    }
+
+    if(um->priv->next_redo < 0)
+      undo_action = NULL;
+    else
+      undo_action = g_list_nth_data(um->priv->actions, um->priv->next_redo);
+
+  } while((undo_action != NULL) &&(undo_action->order_in_group > 1));
+
+  if(modified)
+  {
+    ++um->priv->next_redo;
+    gtk_text_buffer_set_modified(um->priv->document, FALSE);
+    --um->priv->next_redo;
+  }
+
+  gtk_source_undo_manager_end_not_undoable_action_internal(um);
+
+  if(um->priv->next_redo < 0)
+  {
+    um->priv->can_redo = FALSE;
+    g_signal_emit(G_OBJECT(um), undo_manager_signals [CAN_REDO], 0, FALSE);
+  }
+
+  if(!um->priv->can_undo)
+  {
+    um->priv->can_undo = TRUE;
+    g_signal_emit(G_OBJECT(um), undo_manager_signals [CAN_UNDO], 0, TRUE);
+  }
+}
+
+static void
+gtk_source_undo_action_free(GtkSourceUndoAction *action) {
+  if(action == NULL)
+    return;
+
+  if(action->action_type == GTK_SOURCE_UNDO_ACTION_INSERT)
+    g_free(action->action.insert.text);
+  else if(action->action_type == GTK_SOURCE_UNDO_ACTION_DELETE)
+    g_free(action->action.delete.text);
+  else
+    g_return_if_reached();
+
+  g_free(action);
+}
+
+static void
+gtk_source_undo_manager_free_action_list(GtkSourceUndoManager *um) {
+  GList *l;
+
+  l = um->priv->actions;
+
+  while(l != NULL)
+  {
+    GtkSourceUndoAction *action = l->data;
+
+    if(action->order_in_group == 1)
+      --um->priv->num_of_groups;
+
+    if(action->modified)
+      um->priv->modified_action = INVALID;
+
+    gtk_source_undo_action_free(action);
+
+    l = g_list_next(l);
+  }
+
+  g_list_free(um->priv->actions);
+  um->priv->actions = NULL;
+}
+
+static void
+gtk_source_undo_manager_insert_text_handler(GtkTextBuffer *buffer,
+    GtkTextIter *pos,
+    const gchar *text,
+    gint length,
+    GtkSourceUndoManager *um) {
+  GtkSourceUndoAction undo_action;
+
+  if(um->priv->running_not_undoable_actions > 0)
+    return;
+
+  undo_action.action_type = GTK_SOURCE_UNDO_ACTION_INSERT;
+
+  undo_action.action.insert.pos = gtk_text_iter_get_offset(pos);
+  undo_action.action.insert.text =(gchar*) text;
+  undo_action.action.insert.length = length;
+  undo_action.action.insert.chars = g_utf8_strlen(text, length);
+
+  if((undo_action.action.insert.chars > 1) ||(g_utf8_get_char(text) == '\n'))
+
+           undo_action.mergeable = FALSE;
+  else
+    undo_action.mergeable = TRUE;
+
+  undo_action.modified = FALSE;
+
+  gtk_source_undo_manager_add_action(um, &undo_action);
+}
+
+static void
+gtk_source_undo_manager_delete_range_handler(GtkTextBuffer *buffer,
+    GtkTextIter *start,
+    GtkTextIter *end,
+    GtkSourceUndoManager *um) {
+  GtkSourceUndoAction undo_action;
+  GtkTextIter insert_iter;
+
+  if(um->priv->running_not_undoable_actions > 0)
+    return;
+
+  undo_action.action_type = GTK_SOURCE_UNDO_ACTION_DELETE;
+
+  gtk_text_iter_order(start, end);
+
+  undo_action.action.delete.start = gtk_text_iter_get_offset(start);
+  undo_action.action.delete.end = gtk_text_iter_get_offset(end);
+
+  undo_action.action.delete.text = get_chars(
+            buffer,
+            undo_action.action.delete.start,
+            undo_action.action.delete.end);
+
+  /* figure out if the user used the Delete or the Backspace key */
+  gtk_text_buffer_get_iter_at_mark(buffer, &insert_iter,
+            gtk_text_buffer_get_insert(buffer));
+  if(gtk_text_iter_get_offset(&insert_iter) <= undo_action.action.delete.start)
+    undo_action.action.delete.forward = TRUE;
+  else
+    undo_action.action.delete.forward = FALSE;
+
+  if(((undo_action.action.delete.end - undo_action.action.delete.start) > 1) ||
+      (g_utf8_get_char(undo_action.action.delete.text ) == '\n'))
+           undo_action.mergeable = FALSE;
+  else
+    undo_action.mergeable = TRUE;
+
+  undo_action.modified = FALSE;
+
+  gtk_source_undo_manager_add_action(um, &undo_action);
+
+  g_free(undo_action.action.delete.text);
+
+}
+
+static void
+gtk_source_undo_manager_begin_user_action_handler(GtkTextBuffer *buffer, GtkSourceUndoManager *um) {
+  g_return_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um));
+  g_return_if_fail(um->priv != NULL);
+
+  if(um->priv->running_not_undoable_actions > 0)
+    return;
+
+  um->priv->actions_in_current_group = 0;
+}
+
+static void
+gtk_source_undo_manager_add_action(GtkSourceUndoManager *um,
+            const GtkSourceUndoAction *undo_action) {
+  GtkSourceUndoAction* action;
+
+  if(um->priv->next_redo >= 0)
+  {
+    gtk_source_undo_manager_free_first_n_actions(um, um->priv->next_redo + 1);
+  }
+
+  um->priv->next_redo = -1;
+
+  if(!gtk_source_undo_manager_merge_action(um, undo_action))
+  {
+    action = g_new(GtkSourceUndoAction, 1);
+    *action = *undo_action;
+
+    if(action->action_type == GTK_SOURCE_UNDO_ACTION_INSERT)
+      action->action.insert.text = g_strndup(undo_action->action.insert.text, undo_action->action.insert.length);
+    else if(action->action_type == GTK_SOURCE_UNDO_ACTION_DELETE)
+      action->action.delete.text = g_strdup(undo_action->action.delete.text);
+    else
+    {
+      g_free(action);
+      g_return_if_reached();
+    }
+
+    ++um->priv->actions_in_current_group;
+    action->order_in_group = um->priv->actions_in_current_group;
+
+    if(action->order_in_group == 1)
+      ++um->priv->num_of_groups;
+
+    um->priv->actions = g_list_prepend(um->priv->actions, action);
+  }
+
+  gtk_source_undo_manager_check_list_size(um);
+
+  if(!um->priv->can_undo)
+  {
+    um->priv->can_undo = TRUE;
+    g_signal_emit(G_OBJECT(um), undo_manager_signals [CAN_UNDO], 0, TRUE);
+  }
+
+  if(um->priv->can_redo)
+  {
+    um->priv->can_redo = FALSE;
+    g_signal_emit(G_OBJECT(um), undo_manager_signals [CAN_REDO], 0, FALSE);
+  }
+}
+
+static void
+gtk_source_undo_manager_free_first_n_actions(GtkSourceUndoManager *um,
+                gint n) {
+  gint i;
+
+  if(um->priv->actions == NULL)
+    return;
+
+  for(i = 0; i < n; i++)
+  {
+    GtkSourceUndoAction *action = g_list_first(um->priv->actions)->data;
+
+    if(action->order_in_group == 1)
+      --um->priv->num_of_groups;
+
+    if(action->modified)
+      um->priv->modified_action = INVALID;
+
+    gtk_source_undo_action_free(action);
+
+    um->priv->actions = g_list_delete_link(um->priv->actions,
+              um->priv->actions);
+
+    if(um->priv->actions == NULL)
+      return;
+  }
+}
+
+static void
+gtk_source_undo_manager_check_list_size(GtkSourceUndoManager *um) {
+  gint undo_levels;
+
+  g_return_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um));
+  g_return_if_fail(um->priv != NULL);
+
+  undo_levels = gtk_source_undo_manager_get_max_undo_levels(um);
+
+  if(undo_levels < 1)
+    return;
+
+  if(um->priv->num_of_groups > undo_levels)
+  {
+    GtkSourceUndoAction *undo_action;
+    GList *last;
+
+    last = g_list_last(um->priv->actions);
+    undo_action =(GtkSourceUndoAction*) last->data;
+
+    do
+    {
+      GList *tmp;
+
+      if(undo_action->order_in_group == 1)
+        --um->priv->num_of_groups;
+
+      if(undo_action->modified)
+        um->priv->modified_action = INVALID;
+
+      gtk_source_undo_action_free(undo_action);
+
+      tmp = g_list_previous(last);
+      um->priv->actions = g_list_delete_link(um->priv->actions, last);
+      last = tmp;
+      g_return_if_fail(last != NULL);
+
+      undo_action =(GtkSourceUndoAction*) last->data;
+
+    } while((undo_action->order_in_group > 1) ||
+      (um->priv->num_of_groups > undo_levels));
+  }
+}
+
+/**
+ * gtk_source_undo_manager_merge_action:
+ * @um: a #GtkSourceUndoManager.
+ * @undo_action: a #GtkSourceUndoAction.
+ *
+ * This function tries to merge the undo action at the top of
+ * the stack with a new undo action. So when we undo for example
+ * typing, we can undo the whole word and not each letter by itself.
+ *
+ * Return Value: %TRUE is merge was successful, %FALSE otherwise.
+ **/
+static gboolean
+gtk_source_undo_manager_merge_action(GtkSourceUndoManager *um,
+              const GtkSourceUndoAction *undo_action) {
+  GtkSourceUndoAction *last_action;
+
+  g_return_val_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um), FALSE);
+  g_return_val_if_fail(um->priv != NULL, FALSE);
+
+  if(um->priv->actions == NULL)
+    return FALSE;
+
+  last_action =(GtkSourceUndoAction*) g_list_nth_data(um->priv->actions, 0);
+
+  if(!last_action->mergeable)
+    return FALSE;
+
+  if((!undo_action->mergeable) ||
+     (undo_action->action_type != last_action->action_type))
+  {
+    last_action->mergeable = FALSE;
+    return FALSE;
+  }
+
+  if(undo_action->action_type == GTK_SOURCE_UNDO_ACTION_DELETE)
+  {
+    if((last_action->action.delete.forward != undo_action->action.delete.forward) ||
+       ((last_action->action.delete.start != undo_action->action.delete.start) &&
+        (last_action->action.delete.start != undo_action->action.delete.end)))
+    {
+      last_action->mergeable = FALSE;
+      return FALSE;
+    }
+
+    if(last_action->action.delete.start == undo_action->action.delete.start)
+    {
+      gchar *str;
+
+#define L (last_action->action.delete.end - last_action->action.delete.start - 1)
+#define g_utf8_get_char_at(p,i) g_utf8_get_char(g_utf8_offset_to_pointer((p),(i)))
+
+      /* Deleted with the delete key */
+      if((g_utf8_get_char(undo_action->action.delete.text) != ' ') &&
+         (g_utf8_get_char(undo_action->action.delete.text) != '\t') &&
+                           ((g_utf8_get_char_at(last_action->action.delete.text, L) == ' ') ||
+          (g_utf8_get_char_at(last_action->action.delete.text, L) == '\t')))
+      {
+        last_action->mergeable = FALSE;
+        return FALSE;
+      }
+
+      str = g_strdup_printf("%s%s", last_action->action.delete.text,
+        undo_action->action.delete.text);
+
+      g_free(last_action->action.delete.text);
+      last_action->action.delete.end +=(undo_action->action.delete.end -
+                 undo_action->action.delete.start);
+      last_action->action.delete.text = str;
+    }
+    else
+    {
+      gchar *str;
+
+      /* Deleted with the backspace key */
+      if((g_utf8_get_char(undo_action->action.delete.text) != ' ') &&
+         (g_utf8_get_char(undo_action->action.delete.text) != '\t') &&
+                           ((g_utf8_get_char(last_action->action.delete.text) == ' ') ||
+          (g_utf8_get_char(last_action->action.delete.text) == '\t')))
+      {
+        last_action->mergeable = FALSE;
+        return FALSE;
+      }
+
+      str = g_strdup_printf("%s%s", undo_action->action.delete.text,
+        last_action->action.delete.text);
+
+      g_free(last_action->action.delete.text);
+      last_action->action.delete.start = undo_action->action.delete.start;
+      last_action->action.delete.text = str;
+    }
+  }
+  else if(undo_action->action_type == GTK_SOURCE_UNDO_ACTION_INSERT)
+  {
+    gchar* str;
+
+#define I (last_action->action.insert.chars - 1)
+
+    if((undo_action->action.insert.pos !=
+          (last_action->action.insert.pos + last_action->action.insert.chars)) ||
+       ((g_utf8_get_char(undo_action->action.insert.text) != ' ') &&
+         (g_utf8_get_char(undo_action->action.insert.text) != '\t') &&
+        ((g_utf8_get_char_at(last_action->action.insert.text, I) == ' ') ||
+         (g_utf8_get_char_at(last_action->action.insert.text, I) == '\t')))
+       )
+    {
+      last_action->mergeable = FALSE;
+      return FALSE;
+    }
+
+    str = g_strdup_printf("%s%s", last_action->action.insert.text,
+        undo_action->action.insert.text);
+
+    g_free(last_action->action.insert.text);
+    last_action->action.insert.length += undo_action->action.insert.length;
+    last_action->action.insert.text = str;
+    last_action->action.insert.chars += undo_action->action.insert.chars;
+
+  }
+  else
+    /* Unknown action inside undo merge encountered */
+    g_return_val_if_reached(TRUE);
+
+  return TRUE;
+}
+
+gint
+gtk_source_undo_manager_get_max_undo_levels(GtkSourceUndoManager *um) {
+  g_return_val_if_fail(um != NULL, 0);
+  g_return_val_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um), 0);
+
+  return um->priv->max_undo_levels;
+}
+
+void
+gtk_source_undo_manager_set_max_undo_levels(GtkSourceUndoManager *um,
+                 gint max_undo_levels) {
+  gint old_levels;
+
+  g_return_if_fail(um != NULL);
+  g_return_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um));
+
+  old_levels = um->priv->max_undo_levels;
+  um->priv->max_undo_levels = max_undo_levels;
+
+  if(max_undo_levels < 1)
+    return;
+
+  if(old_levels > max_undo_levels)
+  {
+    /* strip redo actions first */
+    while(um->priv->next_redo >= 0 &&(um->priv->num_of_groups > max_undo_levels))
+    {
+      gtk_source_undo_manager_free_first_n_actions(um, 1);
+      um->priv->next_redo--;
+    }
+
+    /* now remove undo actions if necessary */
+    gtk_source_undo_manager_check_list_size(um);
+
+    /* emit "can_undo" and/or "can_redo" if appropiate */
+    if(um->priv->next_redo < 0 && um->priv->can_redo)
+    {
+      um->priv->can_redo = FALSE;
+      g_signal_emit(G_OBJECT(um), undo_manager_signals [CAN_REDO], 0, FALSE);
+    }
+
+    if(um->priv->can_undo &&
+        um->priv->next_redo >=(gint)(g_list_length(um->priv->actions) - 1))
+    {
+      um->priv->can_undo = FALSE;
+      g_signal_emit(G_OBJECT(um), undo_manager_signals [CAN_UNDO], 0, FALSE);
+    }
+  }
+}
+
+static void
+gtk_source_undo_manager_modified_changed_handler(GtkTextBuffer *buffer,
+                                                GtkSourceUndoManager *um) {
+  GtkSourceUndoAction *action;
+  GList *list;
+
+  g_return_if_fail(GTK_SOURCE_IS_UNDO_MANAGER(um));
+  g_return_if_fail(um->priv != NULL);
+
+  if(um->priv->actions == NULL)
+    return;
+
+  list = g_list_nth(um->priv->actions, um->priv->next_redo + 1);
+
+  if(list != NULL)
+    action =(GtkSourceUndoAction*) list->data;
+  else
+    action = NULL;
+
+  if(gtk_text_buffer_get_modified(buffer) == FALSE)
+  {
+    if(action != NULL)
+      action->mergeable = FALSE;
+
+    if(um->priv->modified_action != NULL)
+    {
+      if(um->priv->modified_action != INVALID)
+        um->priv->modified_action->modified = FALSE;
+
+      um->priv->modified_action = NULL;
+    }
+
+    return;
+  }
+
+  if(action == NULL)
+  {
+    g_return_if_fail(um->priv->running_not_undoable_actions > 0);
+
+    return;
+  }
+
+  /* gtk_text_buffer_get_modified(buffer) == TRUE */
+
+  g_return_if_fail(um->priv->modified_action == NULL);
+
+  if(action->order_in_group > 1)
+    um->priv->modified_undoing_group = TRUE;
+
+  while(action->order_in_group > 1)
+  {
+    list = g_list_next(list);
+    g_return_if_fail(list != NULL);
+
+    action =(GtkSourceUndoAction*) list->data;
+    g_return_if_fail(action != NULL);
+  }
+
+  action->modified = TRUE;
+  um->priv->modified_action = action;
+}
+