(function() {

// Dimensions of the whole book
var BOOK_WIDTH = 1391;
var BOOK_HEIGHT = 950;

// Dimensions of one page in the book
var PAGE_WIDTH = 651;
var PAGE_HEIGHT = 890;

// Vertical spacing between the top edge of the book and the papers
var PAGE_Y = ( BOOK_HEIGHT - PAGE_HEIGHT ) / 2;

// The canvas size equals to the book dimensions + this padding
var CANVAS_PADDING = 20;

var page = 0;

var canvas = document.getElementById( "pageflip-canvas" );
var context = canvas.getContext( "2d" );

var mouse = { x: 0, y: 0 };

var flips = [];

var book = document.getElementById( "book" );

// List of all the page elements in the DOM
var pages = book.getElementsByTagName( "section" );

// Organize the depth of our pages and create the flip definitions
for( var i = 0, len = pages.length; i < len; i++ ) {
    pages[i].style.zIndex = len - i;

    flips.push( {
        // Current progress of the flip (left -1 to right +1)
        progress: 1,
        // The target value towards which progress is always moving
        target: 1,
        // The page DOM element related to this flip
        page: pages[i],
        // True while the page is being dragged
        dragging: false
    } );
}

// Resize the canvas to match the book size
canvas.width = BOOK_WIDTH + ( CANVAS_PADDING * 2 );
canvas.height = BOOK_HEIGHT + ( CANVAS_PADDING * 2 );

// Offset the canvas so that it's padding is evenly spread around the book
canvas.style.top = -CANVAS_PADDING + "px";
canvas.style.left = -CANVAS_PADDING + "px";

// Render the page flip 60 times a second
setInterval( render, 1000 / 60 );

document.addEventListener( "mousemove", mouseMoveHandler, false );
document.addEventListener( "mousedown", mouseDownHandler, false );
document.addEventListener( "mouseup", mouseUpHandler, false );

function mouseMoveHandler( event ) {
    // Offset mouse position so that the top of the book spine is 0,0
    mouse.x = event.clientX - book.offsetLeft - ( BOOK_WIDTH / 2 );
    mouse.y = event.clientY - book.offsetTop;
}

function mouseDownHandler( event ) {
    // Make sure the mouse pointer is inside of the book
    if (Math.abs(mouse.x) < PAGE_WIDTH) {
        if (mouse.x < 0 && page - 1 >= 0) {
            // We are on the left side, drag the previous page
            flips[page - 1].dragging = true;
        }
        else if (mouse.x > 0 && page + 1 < flips.length) {
            // We are on the right side, drag the current page
            flips[page].dragging = true;
        }
    }

    // Prevents the text selection
    event.preventDefault();
}

function mouseUpHandler( event ) {
    for( var i = 0; i < flips.length; i++ ) {
        // If this flip was being dragged, animate to its destination
        if( flips[i].dragging ) {
            // Figure out which page we should navigate to
            if( mouse.x < 0 ) {
                flips[i].target = -1;
                page = Math.min( page + 1, flips.length );
            }
            else {
                flips[i].target = 1;
                page = Math.max( page - 1, 0 );
            }
        }

        flips[i].dragging = false;
    }
}

function render() {

    // Reset all pixels in the canvas
    context.clearRect( 0, 0, canvas.width, canvas.height );

    for( var i = 0, len = flips.length; i < len; i++ ) {
        var flip = flips[i];

        if( flip.dragging ) {
            flip.target = Math.max( Math.min( mouse.x / PAGE_WIDTH, 1 ), -1 );
        }

        // Ease progress towards the target value
        flip.progress += ( flip.target - flip.progress ) * 0.2;

        // If the flip is being dragged or is somewhere in the middle of the book, render it
        if( flip.dragging || Math.abs( flip.progress ) < 0.997 ) {
            drawFlip( flip );
        }

    }

}

function drawFlip( flip ) {
    // Strength of the fold is strongest in the middle of the book
    var strength = 1 - Math.abs( flip.progress );

    // Width of the folded paper
    var foldWidth = ( PAGE_WIDTH * 0.5 ) * ( 1 - flip.progress );

    // X position of the folded paper
    var foldX = PAGE_WIDTH * flip.progress + foldWidth;

    // How far the page should outdent vertically due to perspective
    var verticalOutdent = 20 * strength;

    // The maximum width of the left and right side shadows
    var paperShadowWidth = ( PAGE_WIDTH * 0.5 ) * Math.max( Math.min( 1 - flip.progress, 0.5 ), 0 );
    var rightShadowWidth = ( PAGE_WIDTH * 0.5 ) * Math.max( Math.min( strength, 0.5 ), 0 );
    var leftShadowWidth = ( PAGE_WIDTH * 0.5 ) * Math.max( Math.min( strength, 0.5 ), 0 );


    // Change page element width to match the x position of the fold
    flip.page.style.width = Math.max(foldX, 0) + "px";

    context.save();
    context.translate( CANVAS_PADDING + ( BOOK_WIDTH / 2 ), PAGE_Y + CANVAS_PADDING );


    // Draw a sharp shadow on the left side of the page
    context.strokeStyle = 'rgba(0,0,0,'+(0.05 * strength)+')';
    context.lineWidth = 30 * strength;
    context.beginPath();
    context.moveTo(foldX - foldWidth, -verticalOutdent * 0.5);
    context.lineTo(foldX - foldWidth, PAGE_HEIGHT + (verticalOutdent * 0.5));
    context.stroke();


    // Right side drop shadow
    var rightShadowGradient = context.createLinearGradient(foldX, 0, foldX + rightShadowWidth, 0);
    rightShadowGradient.addColorStop(0, 'rgba(0,0,0,'+(strength*0.2)+')');
    rightShadowGradient.addColorStop(0.8, 'rgba(0,0,0,0.0)');

    context.fillStyle = rightShadowGradient;
    context.beginPath();
    context.moveTo(foldX, 0);
    context.lineTo(foldX + rightShadowWidth, 0);
    context.lineTo(foldX + rightShadowWidth, PAGE_HEIGHT);
    context.lineTo(foldX, PAGE_HEIGHT);
    context.fill();


    // Left side drop shadow
    var leftShadowGradient = context.createLinearGradient(foldX - foldWidth - leftShadowWidth, 0, foldX - foldWidth, 0);
    leftShadowGradient.addColorStop(0, 'rgba(0,0,0,0.0)');
    leftShadowGradient.addColorStop(1, 'rgba(0,0,0,'+(strength*0.15)+')');

    context.fillStyle = leftShadowGradient;
    context.beginPath();
    context.moveTo(foldX - foldWidth - leftShadowWidth, 0);
    context.lineTo(foldX - foldWidth, 0);
    context.lineTo(foldX - foldWidth, PAGE_HEIGHT);
    context.lineTo(foldX - foldWidth - leftShadowWidth, PAGE_HEIGHT);
    context.fill();


    // Gradient applied to the folded paper (highlights & shadows)
    var foldGradient = context.createLinearGradient(foldX - paperShadowWidth, 0, foldX, 0);
    foldGradient.addColorStop(0.35, '#fafafa');
    foldGradient.addColorStop(0.73, '#eeeeee');
    foldGradient.addColorStop(0.9, '#fafafa');
    foldGradient.addColorStop(1.0, '#e2e2e2');

    context.fillStyle = foldGradient;
    context.strokeStyle = 'rgba(0,0,0,0.06)';
    context.lineWidth = 0.5;

    // Draw the folded piece of paper
    context.beginPath();
    context.moveTo(foldX, 0);
    context.lineTo(foldX, PAGE_HEIGHT);
    context.quadraticCurveTo(foldX, PAGE_HEIGHT + (verticalOutdent * 2), foldX - foldWidth, PAGE_HEIGHT + verticalOutdent);
    context.lineTo(foldX - foldWidth, -verticalOutdent);
    context.quadraticCurveTo(foldX, -verticalOutdent * 2, foldX, 0);

    context.fill();
    context.stroke();


    context.restore();
}

})();