Snake stabilization

// See result at http://gfycat.com/ValuableIdioticAfricanpiedkingfisher
// Settings
var cellWidth = 15;
var cellHeight = 15;
var cellOffsetX = -1;
var cellOffsetY = -1;
var rows = 20;
var cols = 20;
var startX = 7;
var startY = 0;
var startDir = 2;
var rotateDivide = 5;

// Return 0 for empty, 1 for snake, 2 for apple
function getCellContent(frame, cellX, cellY) {
    if (cellX < 0 || cellX >= cols ||
        cellY < 0 || cellY >= rows) {
        return 0;
    }
    var pixelX = cellX * cellWidth + cellOffsetX;
    var pixelY = cellY * cellHeight + cellOffsetY;
    var snakeX = pixelX + 7;
    var snakeY = pixelY + 7;
    var appleX = pixelX + 11;
    var appleY = pixelY + 11;
    var snakeTestPixel = frame.getPixel(snakeX, snakeY);
    var appleTestPixel = frame.getPixel(appleX, appleY);
    if (appleTestPixel.red == 255) {
        return 2;
    } else if (snakeTestPixel.red == 255) {
        return 1;
    } else {
        return 0;
    }
}

function getDirOffsetX(dir) {
    if (dir == 2) {
        return -1;
    } else if (dir == 3) {
        return 1;
    } else {
        return 0;
    }
}

function getDirOffsetY(dir) {
    if (dir == 0) {
        return -1;
    } else if (dir == 1) {
        return 1;
    } else {
        return 0;
    }
}

// Return the next position as a triplet
// 0 = up, 1 = down, 2 = left, 3 = right, -1 = no head movement
// [x, y, dir] (coordinates in cells unit)
function findNextMove(previousFrame, frame, lastX, lastY) {
    for (var dir = 0; dir <= 3; ++dir) {
        var checkX = lastX + getDirOffsetX(dir);
        var checkY = lastY + getDirOffsetY(dir);
        var oldContent = getCellContent(previousFrame, checkX, checkY);
        if (oldContent == 1) {
            continue;
        }
        var newContent = getCellContent(frame, checkX, checkY);
        if (newContent == 1) {
            return [checkX, checkY, dir];
        }
    }
    return [lastX, lastY, -1];
}

function fillMoves(source) {
    var frameCount = source.frameCount;
    var previousFrame = source.getFrame(0);
    var lastX = startX;
    var lastY = startY;
    var moves = [[startX, startY, startDir]];
    for (var currentFrame = 1; currentFrame < frameCount; ++currentFrame) {
        var frame = source.getFrame(currentFrame);
        var move = findNextMove(previousFrame, frame, lastX, lastY);
        moves[moves.length] = move;
        lastX = move[0];
        lastY = move[1];
        previousFrame = frame;
    }
    return moves;
}

function dirToAngle(dir) {
    switch (dir) {
        case 0: return 0;
        case 1: return Math.PI;
        case 2: return Math.PI / 2;
        case 3: return -Math.PI / 2;
    }
}

function transformCenter(canvas, cellX, cellY, angle) {
    canvas.identity();
    canvas.translate(canvas.width / 2 -cellX * cellWidth - cellOffsetX - cellWidth / 2, canvas.height / 2 -cellY * cellHeight - cellOffsetY - cellHeight / 2);
    if (angle != 0) {
        canvas.rotatecenter(angle, cellX * cellWidth + cellOffsetX + cellWidth / 2, cellY * cellHeight + cellOffsetY + cellHeight / 2);
    }
}

function blankCanvas(canvas, color) {
    canvas.identity();
    canvas.scale(canvas.width, canvas.height);
    canvas.fillRect(color);
}

function computeAngleOffset(oldDir, newDir) {
    // TODO wake up brain to write this correctly
    var dir;
    switch (oldDir) {
        case 0:
            switch(newDir) {
                case 2: dir = 1; break;
                case 3: dir = -1; break;
            }
            break;
        case 1:
            switch(newDir) {
                case 2: dir = -1; break;
                case 3: dir = 1; break; // OK
            }
            break;
        case 2:
            switch(newDir) {
                case 0: dir = -1; break;
                case 1: dir = 1; break; // OK
            }
            break;
        case 3:
            switch(newDir) {
                case 0: dir = 1; break; // OK
                case 1: dir = -1; break;
            }
            break;
    }
    return dir * Math.PI / 2;
}

function render(source, dest, moves, doRotate) {
    var black = new Color(0, 0, 0, 255);
    var frame = source.getFrame(0);
    var w = frame.width * 2;
    var h = frame.height * 2;
    var canvas = new Canvas(w, h);
    var frameCount = source.frameCount;
    var previousDir = moves[0][2];
    if (moves[0][2] == -1) {
        // Failsafe
        moves[0][2] = 0;
    }
    if (!doRotate) {
        previousDir = 0;
    }
    blankCanvas(canvas, black);
    transformCenter(canvas, moves[0][0], moves[0][1], dirToAngle(moves[0][2]));
    canvas.drawImage(frame);
    dest.addCanvas(canvas, source.getFrameDelay(0));
    for (var currentFrame = 1; currentFrame < frameCount; ++currentFrame) {
        print("Rendering frame " + (currentFrame + 1) + " out of " + frameCount);
        if (doRotate) {
            var newDir = moves[currentFrame][2];
            if (newDir != previousDir && newDir != -1) {
                // Do a rotation sequence
                var oldAngle = dirToAngle(previousDir);
                var offset = computeAngleOffset(previousDir, newDir) / rotateDivide;
                for (var angleIter = 1; angleIter < (rotateDivide + 1); ++angleIter) {
                    blankCanvas(canvas, black);
                    transformCenter(canvas, moves[currentFrame - 1][0], moves[currentFrame - 1][1], oldAngle + offset * angleIter);
                    canvas.drawImage(frame);
                    dest.addCanvas(canvas, source.getFrameDelay(currentFrame - 1));
                }
                previousDir = newDir;
            }
        }
        blankCanvas(canvas, black);
        frame = source.getFrame(currentFrame);
        transformCenter(canvas, moves[currentFrame][0], moves[currentFrame][1], dirToAngle(previousDir));
        canvas.drawImage(frame);
        dest.addCanvas(canvas, source.getFrameDelay(currentFrame));
    }
}

var source = new FrameSource("e:/src.gif");
var moves = fillMoves(source);
//var dest = new FrameSink("e:/dst1.gif");
var dest2 = new FrameSink("e:/dst2.gif");
//var dest2 = new FrameSink("e:/t2");
//render(source, dest, moves, false);
render(source, dest2, moves, true);
//dest.close();
dest2.close();