BeginPackage["Undo`"]
SetUpUndo::usage="";

Begin["`Private`"]

getCellID[]:="CellID"/.Developer`CellInformation[EvaluationNotebook[]]

ClearAll[appendEvents];
appendEvents[evList_,(Rule|RuleDelayed)[lhs_,rhs_]]/;MemberQ[evList, (Rule|RuleDelayed)[lhs,_]]:=evList/.(Rule|RuleDelayed)[lhs,sth_]:>(lhs:>(sth;rhs));
appendEvents[evList_, ev_]:=Append[evList, ev]

checkCreateUndoNotebook[]:=Refresh[
With[{nb:=CurrentValue[EvaluationNotebook[], {TaggingRules,"undoNotebook"}]},
If[(* if the undo notebook is not created or open *)!TrueQ@NotebookTools`NotebookOpenQ@nb,
nb=CreateDocument[{}, CellGrouping->Manual,Visible->False,ShowSelection->False,
WindowTitle->FileBaseName[WindowTitle/.AbsoluteOptions[EvaluationNotebook[], WindowTitle]]<>"_undoData"];
]], None]

nbdeRules={Dynamic[rhs_]:>Dynamic[rhs; Needs["Undo`"];Undo`Private`checkCreateUndoNotebook[]], Null:>Dynamic[Needs["Undo`"];Undo`Private`checkCreateUndoNotebook[]],rhs_:>Dynamic[rhs;Needs["Undo`"];Undo`Private`checkCreateUndoNotebook[]]};

parseNBEOptions[nb_]:=ReleaseHold@Replace[Hold[NotebookDynamicExpression]/.Options[nb, NotebookDynamicExpression], nbdeRules, {1}]

appendNDE[nb_NotebookObject]:=SetOptions[nb, parseNBEOptions[nb]/.newNDE_:>NotebookDynamicExpression:>newNDE]

appendNEA[nb_NotebookObject]:=(* appends the notebook event action WindowClose *)
appendEvents[Replace[NotebookEventActions/.Options[nb, NotebookEventActions], None:>{}], "WindowClose":>NotebookClose@CurrentValue[EvaluationNotebook[], {TaggingRules,"undoNotebook"}]]/.newOpts_:>SetOptions[nb, NotebookEventActions:>newOpts]

SetUpUndo[nb_NotebookObject,styles_List]:=SetOptions[initializeNotebook[nb];nb, StyleDefinitions->Fold[
#1/.{s_String:>Notebook[{Cell[StyleData[StyleDefinitions->s]],Cell[StyleData[#2],cellEventActions
]}],
Notebook[{bef___,Cell[StyleData[#2],cellRest___],aft___}, rest___]:>Notebook[{bef,Cell[StyleData[#2], cellRest, cellEventActions],aft}, rest],
Notebook[{all___}, rest___]:>Notebook[{all, Cell[StyleData[#2], cellEventActions]},rest]
}&,
FullOptions[nb, StyleDefinitions],styles]];

SetUpUndo[nb_,styles_]:=SetUpUndo[nb, {styles}];

$defaultFrequency=15;

initializeNotebook[nb_NotebookObject]:=Through@{appendNDE, appendNEA,
SetOptions[#, CreateCellID->True]&,
(CurrentValue[#, {TaggingRules, "undoSettings", "frequency"}]=$defaultFrequency)&}[
nb];

cellEventActions=CellEventActions:>{PassEventsDown -> True, "KeyDown" :> If[CurrentValue["EventKey"]=!=None,undoSave[CurrentValue[CellID]]],"MouseClicked":>If[CurrentValue["AltKey"], undoLoad@CurrentValue@CellID]};

ClearAll[undoSave];
undoSave[id_Integer]:=
If[Mod[ ++ idCounter[id],
        CurrentValue[EvaluationNotebook[], {TaggingRules, "undoSettings", "frequency"}]] === 0,
      undoSave[ToString@id, CurrentValue[EvaluationNotebook[], {TaggingRules,"undoNotebook"}]]];

undoSave[id_, undoNb_]:=(If[
NotebookFind[undoNb, ToString@id<>"End", All, CellTags]===$Failed,
createCellIDZone[id][undoNb],
SelectionMove[undoNb, Before, Cell]];
copyCell[EvaluationNotebook[], undoNb]);

createCellIDZone[id_][nb_NotebookObject]:=(ToString@id/.idStr_:>{idStr<>"Start", idStr<>"End"}/.{stag_, etag_}:>FrontEndExecute[{
FrontEnd`SelectionMove[nb,After,CellGroup],
FrontEnd`NotebookWrite[nb,Cell@CellGroupData[{Cell["Start", CellTags->stag],Cell["End", CellTags->etag]}]],
FrontEnd`SelectionMove[undoNb, Previous, Cell],
FrontEnd`SelectionMove[undoNb, Before, Cell]}];
nb)

undoNb:=CurrentValue[EvaluationNotebook[], {TaggingRules, "undoNotebook"}];

idCounter[_]=0;

ClearAll[doSthWithWholeCellSelected];
doSthWithWholeCellSelected[sth_,what_:Cell][nb_]:=With[{val=CurrentValue[nb, ShowSelection],
cursor=("CursorPosition"/.Developer`CellInformation[nb])[[1,1]]},
FrontEndExecute[{
FrontEnd`SetOptions[nb, ShowSelection->False],
FrontEnd`SelectionMove[nb, All, what,AutoScroll->False]}];
With[{cell=sth[nb]},
{FrontEnd`SelectionMove[nb, Before, CellContents, AutoScroll->False],
FrontEnd`SelectionMove[nb, Next, Character, cursor, AutoScroll->False],
FrontEnd`SetOptions[nb, ShowSelection->val]}/.l:{a_, b_, c_}:>If[cursor>0,FrontEndExecute@l,FrontEndExecute[{a,c}]];
cell
]
]

getCurrentCell[nb_NotebookObject]:=doSthWithWholeCellSelected[NotebookRead][nb];

copyCell[nbFrom_, nbTo_]:=NotebookWrite[nbTo, getCurrentCell[nbFrom]]

replaceCurrentCellContents[nb_NotebookObject, newCell_]:=doSthWithWholeCellSelected[NotebookWrite[#, newCell]&, CellContents][nb];

undoLoad[id_Integer]:=undoLoad[ToString@id,CurrentValue[EvaluationNotebook[], {TaggingRules, "undoNotebook"}]];

undoLoad[id_, nb_]:=If[(* if it is found *)
NotebookFind[undoNb, ToString@id<>"End", All, CellTags]=!=$Failed,
SelectionMove[undoNb, Previous, Cell];
replaceCurrentCellContents[EvaluationNotebook[],If[CurrentValue[NotebookSelection@undoNb, CellTags]=!=ToString@id<>"Start",(NotebookDelete[undoNb];#)&@NotebookRead[undoNb],SelectionMove[undoNb, After, Cell];""]]]

End[];
EndPackage[];