undo.ts

let undoStack: ImageState[];
let undoPosition: number;

function InitUndo()
{
    // First image is created by the first CreateImage()
    undoStack = [];
    undoPosition = -1;

    let saved = localStorage.getItem("undoMaxEntries");
    settings.maxUndoEntries = saved == null ? 200 : parseInt(saved);
}

function RecordUndo()
{
    if (undoPosition + 1 < undoStack.length)
        undoStack = undoStack.slice(0, undoPosition + 1);

    undoStack.push(ImageState.Get());
    undoPosition++;

    while (undoStack.length > settings.maxUndoEntries)
    {
        undoStack.shift();
        undoPosition--;
    }

    SaveHistory();
}

function Undo()
{
    if (undoPosition <= 0) return;

    undoPosition--;
    undoStack[undoPosition].Set();
}

function Redo()
{
    if (undoPosition >= undoStack.length - 1) return;

    undoPosition++;
    undoStack[undoPosition].Set();
}