function HistoryWidget(div, tabHeader) {
    this._div = div;
    this._tabHeader = tabHeader;
    this.update();
}

HistoryWidget.prototype.resize = function() {
    var body = this._div.find(".history-panel-body");
    var controls = this._div.find(".history-panel-controls");
    var nowDiv = this._div.find(".history-now");
    
    var bodyPaddings = body.outerHeight(true) - body.height();
    body.height((this._div.height() - controls.outerHeight(true) - bodyPaddings) + "px");
    body[0].scrollTop = 
        nowDiv[0].offsetTop + 
        nowDiv[0].offsetHeight - 
        body[0].offsetHeight;
};

HistoryWidget.prototype.update = function(onDone) {
    var self = this;
    Ajax.chainGetJSON(
        "/command/core/get-history?" + $.param({ project: theProject.id }), null,
        function(data) {
            self._data = data;
            self._render();
            
            if (onDone) {
                onDone();
            }
        }
    );
};

HistoryWidget.prototype._render = function() {
    var self = this;
    
    this._tabHeader.html('Undo / Redo <span class="count">' + this._data.past.length + '</span>');
    
    this._div
        .empty()
        .unbind()
        .html(
            '<div class="history-panel-controls" bind="controlsDiv">' +
                '<a class="button" href="javascript:{}" bind="extractLink">extract...</a> ' +
                '<a class="button" href="javascript:{}" bind="applyLink">apply...</a>' +
            '</div>' +
            '<div class="history-panel-help" bind="helpDiv">' +
                '<h1>Infinite undo history</h1>' +
                '<p>Don\'t worry about making mistakes. Every change you make will be shown here, and you can undo your changes anytime.</p>' +
                '<p><a href="http://code.google.com/p/google-refine/wiki/GettingStarted?tm=6" target="_blank"><b>Learn more &raquo;</b></a></p>' +
            '</div>' +
            '<div class="history-panel-body" bind="bodyDiv">' +
                '<div class="history-past" bind="pastDiv"><div class="history-highlight" bind="pastHighlightDiv"></div></div>' +
                '<div class="history-now" bind="nowDiv">done upto here</div>' +
                '<div class="history-future" bind="futureDiv"><div class="history-highlight" bind="futureHighlightDiv"></div></div>' +
            '</div>'
        );
    
    var elmts = DOM.bind(this._div);
    
    var renderEntry = function(container, index, entry, lastDoneID, past) {
        var a = $('<a href="javascript:{}"></a>').appendTo(container);
        a.addClass("history-entry")
            .text(entry.description)
            .attr("title", past ? "Undo to here" : "Redo to here")
            .click(function(evt) {
                return self._onClickHistoryEntry(evt, entry, lastDoneID);
            })
            .mouseover(function() {
                if (past) {
                    elmts.pastHighlightDiv.show().height(elmts.pastDiv.height() - this.offsetTop);
                } else {
                    elmts.futureHighlightDiv.show().height(this.offsetTop + this.offsetHeight);
                }
            })
            .mouseout(function() {
                if (past) {
                    elmts.pastHighlightDiv.hide();
                } else {
                    elmts.futureHighlightDiv.hide();
                }
            });
        
        $('<span>')
            .addClass("history-entry-index")
            .text((index + 1) + ".")
            .prependTo(a);
        
        return a;
    };
    
    if (this._data.past.length > 0 || this._data.future.length > 0) {
        if (!this._data.past.length) {
            elmts.pastDiv.html('<div class="history-panel-message">No change to undo</div>');
        } else {
            for (var i = 0; i < this._data.past.length; i++) {
                var entry = this._data.past[i];
                renderEntry(elmts.pastDiv, i, entry, i === 0 ? 0 : this._data.past[i - 1].id, true);
            }
        }
    
        if (!this._data.future.length) {
            elmts.futureDiv.html('<div class="history-panel-message">No change to redo</div>');
        } else {
            for (var i = 0; i < this._data.future.length; i++) {
                var entry = this._data.future[i];
                renderEntry(elmts.futureDiv, this._data.past.length + i, entry, entry.id, false);
            }
        }
        
        elmts.helpDiv.hide();
    } else {
        elmts.bodyDiv.hide();
    }
    
    elmts.extractLink.click(function() { self._extractOperations(); });
    elmts.applyLink.click(function() { self._showApplyOperationsDialog(); });
    
    this.resize();
};

HistoryWidget.prototype._onClickHistoryEntry = function(evt, entry, lastDoneID) {
    var self = this;
    
    Refine.postCoreProcess(
        "undo-redo",
        { lastDoneID: lastDoneID },
        null,
        { everythingChanged: true }
    );
};

HistoryWidget.prototype._extractOperations = function() {
    var self = this;
    $.getJSON(
        "/command/core/get-operations?" + $.param({ project: theProject.id }), 
        null,
        function(data) {
            if ("entries" in data) {
                self._showExtractOperationsDialog(data);
            }
        },
        "jsonp"
    );
};

HistoryWidget.prototype._showExtractOperationsDialog = function(json) {
    var self = this;
    var frame = $(DOM.loadHTML("core", "scripts/widgets/history-extract-dialog.html"));
    var elmts = DOM.bind(frame);
    
    var entryTable = elmts.entryTable[0];
    var createEntry = function(entry) {
        var tr = entryTable.insertRow(entryTable.rows.length);
        var td0 = tr.insertCell(0);
        var td1 = tr.insertCell(1);
        td0.width = "1%";
        
        if ("operation" in entry) {
            entry.selected = true;
            
            $('<input type="checkbox" checked="true" />').appendTo(td0).click(function() {
                entry.selected = !entry.selected;
                updateJson();
            });
            
            $('<span>').text(entry.operation.description).appendTo(td1);
        } else {
            $('<span>').text(entry.description).css("color", "#888").appendTo(td1);
        }
    };
    for (var i = 0; i < json.entries.length; i++) {
        createEntry(json.entries[i]);
    }
    
    var updateJson = function() {
        var a = [];
        for (var i = 0; i < json.entries.length; i++) {
            var entry = json.entries[i];
            if ("operation" in entry && entry.selected) {
                a.push(entry.operation);
            }
        }
        elmts.textarea.text(JSON.stringify(a, null, 2));
    };
    updateJson();
    
    elmts.closeButton.click(function() { DialogSystem.dismissUntil(level - 1); });
    elmts.selectAllButton.click(function() {
        for (var i = 0; i < json.entries.length; i++) {
            json.entries[i].selected = true;
        }
        
        frame.find('input[type="checkbox"]').attr("checked", "true");
        updateJson();
    });
    elmts.unselectAllButton.click(function() {
        for (var i = 0; i < json.entries.length; i++) {
            json.entries[i].selected = false;
        }
        
        frame.find('input[type="checkbox"]').removeAttr("checked");
        updateJson();
    });
    
    var level = DialogSystem.showDialog(frame);
    
    elmts.textarea[0].select();
};

HistoryWidget.prototype._showApplyOperationsDialog = function() {
    var self = this;
    var frame = DialogSystem.createDialog();
    frame.width("800px");
    
    var header = $('<div></div>').addClass("dialog-header").text("Apply Operations").appendTo(frame);
    var body = $('<div></div>').addClass("dialog-body").appendTo(frame);
    var footer = $('<div></div>').addClass("dialog-footer").appendTo(frame);
    
    var html = $(
        '<div class="grid-layout layout-normal layout-full"><table>' +
            '<tr><td>' +
                'Paste the JSON code encoding the operations to perform.' +
            '</td></tr>' +
            '<tr><td>' +
                '<div class="input-container"><textarea wrap="off" bind="textarea" class="history-operation-json" /></div>' +
            '</td></tr>' +
        '</table></div>'
    ).appendTo(body);
    
    var elmts = DOM.bind(html);
    
    $('<button class="button"></button>').text("Apply").click(function() {
        var json;
        
        try {
            json = JSON.parse(elmts.textarea[0].value);
        } catch (e) {
            alert("The JSON you pasted is invalid.");
            return;
        }
        
        Refine.postCoreProcess(
            "apply-operations",
            {},
            { operations: JSON.stringify(json) },
            { everythingChanged: true },
            {
                onDone: function(o) {
                    if (o.code == "pending") {
                        // Something might have already been done and so it's good to update
                        Refine.update({ everythingChanged: true });
                    }
                }
            }
        );
        
        DialogSystem.dismissUntil(level - 1);
    }).appendTo(footer);
    
    $('<button class="button"></button>').text("Cancel").click(function() {
        DialogSystem.dismissUntil(level - 1);
    }).appendTo(footer);
    
    var level = DialogSystem.showDialog(frame);
    
    textarea[0].focus();
};