var __PDF_DOC,
            __CURRENT_PAGE,
            __TOTAL_PAGES,
            __PAGE_RENDERING_IN_PROGRESS = 0,
            __CANVAS = $('#pdf-canvas').get(0),
            __CANVAS_CTX = __CANVAS.getContext('2d');

        function showPDF(pdf_url) {
            $("#pdf-loader").show();

            PDFJS.getDocument({ url: pdf_url }).then(function(pdf_doc) {
                __PDF_DOC = pdf_doc;
                __TOTAL_PAGES = __PDF_DOC.numPages;

                // Hide the pdf loader and show pdf container in HTML
                $("#pdf-loader").hide();
                $("#pdf-contents").show();
                $("#pdf-total-pages").text(__TOTAL_PAGES);

                // Show the first page
                showPage(1);
            }).catch(function(error) {
                // If error re-show the upload button
                $("#pdf-loader").hide();
                $("#upload-button").show();

                alert(error.message);
            });;
        }

        function showPage(page_no) {
            __PAGE_RENDERING_IN_PROGRESS = 1;
            __CURRENT_PAGE = page_no;

            // Disable Prev & Next buttons while page is being loaded
            $("#pdf-next, #pdf-prev").attr('disabled', 'disabled');

            // While page is being rendered hide the canvas and show a loading message
            $("#pdf-canvas").hide();
            $("#page-loader").show();

            // Update current page in HTML
            $("#pdf-current-page").text(page_no);

            // Fetch the page
            __PDF_DOC.getPage(page_no).then(function(page) {
                // As the canvas is of a fixed width we need to set the scale of the viewport accordingly
                var scale_required = __CANVAS.width / page.getViewport(0.5).width;


                // Get viewport of the page at required scale
                var viewport = page.getViewport(scale_required);

                // Set canvas height
                __CANVAS.height = viewport.height;
                __CANVAS.width = viewport.width;

                var renderContext = {
                    canvasContext: __CANVAS_CTX,
                    viewport: viewport
                };

                // Render the page contents in the canvas
                page.render(renderContext).then(function() {
                    __PAGE_RENDERING_IN_PROGRESS = 0;

                    // Re-enable Prev & Next buttons
                    $("#pdf-next, #pdf-prev").removeAttr('disabled');

                    // Show the canvas and hide the page loader
                    $("#pdf-canvas").show();
                    $("#page-loader").hide();
                });
            });
        }

        // Upon click this should should trigger click on the #file-to-upload file input element
        // This is better than showing the not-good-looking file input element
        $("#upload-button").on('click', function() {
            $("#file-to-upload").trigger('click');
        });

        // When user chooses a PDF file
        $("#file-to-upload").on('change', function() {
            // Validate whether PDF
            if(['application/pdf'].indexOf($("#file-to-upload").get(0).files[0].type) === -1) {
                alert('Error : Not a PDF');
                return;
            }

            $("#upload-button").hide();

            // Send the object url of the pdf
            showPDF(URL.createObjectURL($("#file-to-upload").get(0).files[0]));
        });


        // Previous page of the PDF
        $("#pdf-prev").on('click', function() {
            if(__CURRENT_PAGE !== 1)
                showPage(--__CURRENT_PAGE);
        });

        // Next page of the PDF
        $("#pdf-next").on('click', function() {
            if(__CURRENT_PAGE !== __TOTAL_PAGES)
                showPage(++__CURRENT_PAGE);
        });