css3模仿雨滴（附源码）

/**
*/

function RainyDay(canvasid, sourceid, width, height, opacity, blur) {
this.canvasid = canvasid;
this.canvas = document.getElementById(canvasid);

this.sourceid = sourceid;
this.img = document.getElementById(sourceid);

// draw and blur the backgroiund image
this.prepareBackground(blur ? blur : 20, width, height);
this.w = this.canvas.width;
this.h = this.canvas.height;

// create the glass canvas
this.prepareGlass(opacity ? opacity : 1);

// assume default reflection mechanism
this.reflection = this.REFLECTION_MINIATURE;

// assume default trail mechanism
this.trail = this.TRAIL_DROPS;

// assume default gravity
this.gravity = this.GRAVITY_NON_LINEAR;

// drop size threshold for the gravity algorhitm
this.VARIABLE_GRAVITY_THRESHOLD = 3;

// gravity angle
this.VARIABLE_GRAVITY_ANGLE = Math.PI / 2;

// frames per second animation speed
this.VARIABLE_FPS = 25;

// context fill style when no REFLECTION_NONE is used
this.VARIABLE_FILL_STYLE = '#8ED6FF';

// collisions enabled by default
this.VARIABLE_COLLISIONS = false;

// assume default collision algorhitm
this.collision = this.COLLISION_SIMPLE;
}

/**
* Create the helper canvas for rendering raindrop reflections.
*/
RainyDay.prototype.prepareReflections = function() {
// new canvas
this.reflected = document.createElement('canvas');
this.reflected.width = this.canvas.width;
this.reflected.height = this.canvas.height;

var ctx = this.reflected.getContext('2d');

// rotate by 180 degress
ctx.translate(this.reflected.width / 2, this.reflected.height / 2);
ctx.rotate(Math.PI);

ctx.drawImage(this.img, -this.reflected.width / 2, -this.reflected.height / 2, this.reflected.width, this.reflected.height);
};

/**
* Create the glass canvas and position it directly over the main one.
* @param opacity opacity attribute value of the glass canvas
*/
RainyDay.prototype.prepareGlass = function(opacity) {
this.glass = document.createElement('canvas');
this.glass.width = this.canvas.width;
this.glass.height = this.canvas.height;
this.glass.style.position = "absolute";
this.glass.style.top = this.canvas.offsetTop;
this.glass.style.left = this.canvas.offsetLeft;
this.glass.style.zIndex = this.canvas.style.zIndex + 100;
this.canvas.parentNode.appendChild(this.glass);
this.context = this.glass.getContext('2d');
this.glass.style.opacity = opacity;
};

/**
* Creates a new preset object with given attributes.
* @param min minimum size of a drop
* @param base base value for randomizing drop size
* @param quan probability of selecting this preset (must be between 0 and 1)
* @returns present object with given attributes
*/
RainyDay.prototype.preset = function(min, base, quan) {
return {
"min": min,
"base": base,
"quan": quan
}
};

/**
* Main function for starting rain rendering.
* @param presets list of presets to be applied
* @param speed speed of the animation (if not provided or 0 static image will be generated)
*/
RainyDay.prototype.rain = function(presets, speed) {
// prepare canvas for drop reflections
if (this.reflection != this.REFLECTION_NONE) {
this.prepareReflections();
}

if (speed > 0) {
// animation
this.presets = presets;

this.PRIVATE_GRAVITY_FORCE_FACTOR_Y = (this.VARIABLE_FPS * 0.005) / 25;
this.PRIVATE_GRAVITY_FORCE_FACTOR_X = ((Math.PI / 2) - this.VARIABLE_GRAVITY_ANGLE) * (this.VARIABLE_FPS * 0.005) / 50;

// prepare gravity matrix
if (this.VARIABLE_COLLISIONS) {

// calculate max radius of a drop to establish gravity matrix resolution
var maxDropRadius = 0;
for (var i = 0; i < presets.length; i++) {
if (presets[i].base + presets[i].min > maxDropRadius) {
maxDropRadius = Math.floor(presets[i].base + presets[i].min);
}
}

if (maxDropRadius > 0) {
// initialize the gravity matrix
var mwi = Math.ceil(this.w / maxDropRadius);
var mhi = Math.ceil(this.h / maxDropRadius);
this.matrix = new CollisionMatrix(mwi, mhi, maxDropRadius);
} else {
this.VARIABLE_COLLISIONS = false;
}
}

setInterval(
(function(self) {
return function() {
var random = Math.random();
// select matching preset
var preset;
for (var i = 0; i < presets.length; i++) {
if (random < presets[i].quan) {
preset = presets[i];
break;
}
}
if (preset) {
self.putDrop(new Drop(self, Math.random() * self.w, Math.random() * self.h, preset.min, preset.base));
}
}
})(this),
speed
);
} else {
// static picture
for (var i = 0; i < presets.length; i++) {
var preset = presets[i];
for (var c = 0; c < preset.quan; ++c) {
this.putDrop(new Drop(this, Math.random() * this.w, Math.random() * this.h, preset.min, preset.base));
}
}
}
};
/**
* Adds a new raindrop to the animation.
* @param drop drop object to be added to the animation
*/
RainyDay.prototype.putDrop = function(drop) {
drop.draw();
if (this.gravity && drop.r1 > this.VARIABLE_GRAVITY_THRESHOLD) {

if (this.VARIABLE_COLLISIONS) {
// put on the gravity matrix
this.matrix.update(drop);
}

drop.animate();
}
};
/**
* Imperfectly approximates shape of a circle.
* @param iterations number of iterations applied to the size approximation algorithm
* @returns list of points approximating a circle shape
*/
RainyDay.prototype.getLinepoints = function(iterations) {
var pointList = {};
pointList.first = {
x: 0,
y: 1
};
var lastPoint = {
x: 1,
y: 1
}
var minY = 1;
var maxY = 1;
var point;
var nextPoint;
var dx, newX, newY;

pointList.first.next = lastPoint;
for (var i = 0; i < iterations; i++) {
point = pointList.first;
while (point.next != null) {
nextPoint = point.next;

dx = nextPoint.x - point.x;
newX = 0.5 * (point.x + nextPoint.x);
newY = 0.5 * (point.y + nextPoint.y);
newY += dx * (Math.random() * 2 - 1);

var newPoint = {
x: newX,
y: newY
};

//min, max
if (newY < minY) {
minY = newY;
} else if (newY > maxY) {
maxY = newY;
}

//put between points
newPoint.next = nextPoint;
point.next = newPoint;

point = nextPoint;
}
}

//normalize to values between 0 and 1
if (maxY != minY) {
var normalizeRate = 1 / (maxY - minY);
point = pointList.first;
while (point != null) {
point.y = normalizeRate * (point.y - minY);
point = point.next;
}
} else {
point = pointList.first;
while (point != null) {
point.y = 1;
point = point.next;
}
}

return pointList;
};

/**
* Defines a new raindrop object.
* @param rainyday reference to the parent object
* @param centerX x position of the center of this drop
* @param centerY y position of the center of this drop
* @param min minimum size of a drop
* @param base base value for randomizing drop size
*/

function Drop(rainyday, centerX, centerY, min, base) {
this.x = Math.floor(centerX);
this.y = Math.floor(centerY);
this.r1 = (Math.random() * base) + min;
this.rainyday = rainyday;
var iterations = 4;
this.r2 = 0.8 * this.r1;
this.linepoints = rainyday.getLinepoints(iterations);
this.context = rainyday.context;
this.reflection = rainyday.reflected;
}

/**
* Draws a raindrop on canvas at the current position.
*/
Drop.prototype.draw = function() {
var phase = 0;
var point;
var rad, theta;
var x0, y0;

this.context.save();
this.context.beginPath();
point = this.linepoints.first;
theta = phase;
rad = this.r2 + 0.5 * Math.random() * (this.r2 - this.r1);
x0 = this.x + rad * Math.cos(theta);
y0 = this.y + rad * Math.sin(theta);
this.context.lineTo(x0, y0);
while (point.next != null) {
point = point.next;
theta = (Math.PI * 2 * point.x) + phase;
rad = this.r2 + 0.5 * Math.random() * (this.r2 - this.r1);
x0 = this.x + rad * Math.cos(theta);
y0 = this.y + rad * Math.sin(theta);
this.context.lineTo(x0, y0);
}

this.context.clip();

if (this.rainyday.reflection) {
this.rainyday.reflection(this);
}

this.context.restore();
};

/**
* Clears the raindrop region.
* @param force force stop
* @returns true if the animation is stopped
*/
Drop.prototype.clear = function(force) {
this.context.clearRect(this.x - this.r1 - 1, this.y - this.r1 - 1, 2 * this.r1 + 2, 2 * this.r1 + 2);
if (force) {
// forced
clearInterval(this.intid);
return true;
}
if (this.y - this.r1 > this.rainyday.h) {
// over the bottom edge, stop the thread
clearInterval(this.intid);
return true;
}
if ((this.x - this.r1 > this.rainyday.w) || (this.x + this.r1 < 0)) {
// over the right or left edge, stop the thread
clearInterval(this.intid);
return true;
}
return false;
};

/**
* Moves the raindrop to a new position according to the gravity.
*/
Drop.prototype.animate = function() {
this.intid = setInterval(
(function(self) {
return function() {
var stopped = self.rainyday.gravity(self);
if (!stopped && self.rainyday.trail) {
self.rainyday.trail(self);
}
if (self.rainyday.VARIABLE_COLLISIONS) {
var collision = self.rainyday.matrix.update(self, stopped);
if (collision) {
self.rainyday.collision(self, collision.drop);
}
}
}
})(this),
Math.floor(1000 / this.rainyday.VARIABLE_FPS)
);
};

/**
* TRAIL function: no trail at all
* @param drop raindrop object
*/
RainyDay.prototype.TRAIL_NONE = function(drop) {
// nothing going on here
};

/**
* TRAIL function: trail of small drops (default)
* @param drop raindrop object
*/
RainyDay.prototype.TRAIL_DROPS = function(drop) {
if (!drop.trail_y || drop.y - drop.trail_y >= Math.random() * 10 * drop.r1) {
drop.trail_y = drop.y;
this.putDrop(new Drop(this, drop.x, drop.y - drop.r1 - 5, 0, Math.ceil(drop.r1 / 5)));
}
};
/**
* GRAVITY function: no gravity at all
* @param drop raindrop object
* @returns true if the animation is stopped
*/
RainyDay.prototype.GRAVITY_NONE = function(drop) {
return true;
};

/**
* GRAVITY function: linear gravity
* @param drop raindrop object
* @returns true if the animation is stopped
*/
RainyDay.prototype.GRAVITY_LINEAR = function(drop) {
if (drop.clear()) {
return true;
}

if (drop.yspeed) {
drop.yspeed += this.PRIVATE_GRAVITY_FORCE_FACTOR_Y * Math.floor(drop.r1);
drop.xspeed += this.PRIVATE_GRAVITY_FORCE_FACTOR_X * Math.floor(drop.r1);
} else {
drop.yspeed = this.PRIVATE_GRAVITY_FORCE_FACTOR_Y;
drop.xspeed = this.PRIVATE_GRAVITY_FORCE_FACTOR_X;
}

drop.y += drop.yspeed;
drop.draw();
return false;
};

/**
* GRAVITY function: non-linear gravity (default)
* @param drop raindrop object
* @returns true if the animation is stopped
*/
RainyDay.prototype.GRAVITY_NON_LINEAR = function(drop) {
if (drop.clear()) {
return true;
}

if (!drop.seed || drop.seed < 0) {
drop.seed = Math.floor(Math.random() * this.VARIABLE_FPS);
drop.skipping = drop.skipping == false ? true : false;
drop.slowing = true;
}

drop.seed--;

if (drop.yspeed) {
if (drop.slowing) {
drop.yspeed /= 1.1;
drop.xspeed /= 1.1;
if (drop.yspeed < this.PRIVATE_GRAVITY_FORCE_FACTOR_Y) {
drop.slowing = false;
}
} else if (drop.skipping) {
drop.yspeed = this.PRIVATE_GRAVITY_FORCE_FACTOR_Y;
drop.xspeed = this.PRIVATE_GRAVITY_FORCE_FACTOR_X;
} else {
drop.yspeed += 10 * this.PRIVATE_GRAVITY_FORCE_FACTOR_Y * Math.floor(drop.r1);
drop.xspeed += 10 * this.PRIVATE_GRAVITY_FORCE_FACTOR_X * Math.floor(drop.r1);
}
} else {
drop.yspeed = this.PRIVATE_GRAVITY_FORCE_FACTOR_Y;
drop.xspeed = this.PRIVATE_GRAVITY_FORCE_FACTOR_X;
}

drop.y += drop.yspeed;
drop.x += drop.xspeed;

drop.draw();
return false;
};
/**
* REFLECTION function: no reflection at all
* @param drop raindrop object
*/
RainyDay.prototype.REFLECTION_NONE = function(drop) {
this.context.fillStyle = this.VARIABLE_FILL_STYLE;
this.context.fill();
};

/**
* REFLECTION function: miniature reflection (default)
* @param drop raindrop object
*/
RainyDay.prototype.REFLECTION_MINIATURE = function(drop) {
this.context.drawImage(this.reflected, drop.x - drop.r1, drop.y - drop.r1, drop.r1 * 2, drop.r1 * 2);
};

/**
* COLLISION function: default collision implementation
* @param drop1 one of the drops colliding
* @param drop2 the other one
*/
RainyDay.prototype.COLLISION_SIMPLE = function(drop1, drop2) {
drop1.clear();
// force stopping the second drop
drop2.clear(true);

drop1.x = (drop1.x + drop2.x) / 2;
drop1.y = (drop1.y + drop2.y) / 2;
};

var mul_table = [
512, 512, 456, 512, 328, 456, 335, 512, 405, 328, 271, 456, 388, 335, 292, 512,
454, 405, 364, 328, 298, 271, 496, 456, 420, 388, 360, 335, 312, 292, 273, 512,
482, 454, 428, 405, 383, 364, 345, 328, 312, 298, 284, 271, 259, 496, 475, 456,
437, 420, 404, 388, 374, 360, 347, 335, 323, 312, 302, 292, 282, 273, 265, 512,
497, 482, 468, 454, 441, 428, 417, 405, 394, 383, 373, 364, 354, 345, 337, 328,
320, 312, 305, 298, 291, 284, 278, 271, 265, 259, 507, 496, 485, 475, 465, 456,
446, 437, 428, 420, 412, 404, 396, 388, 381, 374, 367, 360, 354, 347, 341, 335,
329, 323, 318, 312, 307, 302, 297, 292, 287, 282, 278, 273, 269, 265, 261, 512,
505, 497, 489, 482, 475, 468, 461, 454, 447, 441, 435, 428, 422, 417, 411, 405,
399, 394, 389, 383, 378, 373, 368, 364, 359, 354, 350, 345, 341, 337, 332, 328,
324, 320, 316, 312, 309, 305, 301, 298, 294, 291, 287, 284, 281, 278, 274, 271,
268, 265, 262, 259, 257, 507, 501, 496, 491, 485, 480, 475, 470, 465, 460, 456,
451, 446, 442, 437, 433, 428, 424, 420, 416, 412, 408, 404, 400, 396, 392, 388,
385, 381, 377, 374, 370, 367, 363, 360, 357, 354, 350, 347, 344, 341, 338, 335,
332, 329, 326, 323, 320, 318, 315, 312, 310, 307, 304, 302, 299, 297, 294, 292,
289, 287, 285, 282, 280, 278, 275, 273, 271, 269, 267, 265, 263, 261, 259
];

var shg_table = [
9, 11, 12, 13, 13, 14, 14, 15, 15, 15, 15, 16, 16, 16, 16, 17,
17, 17, 17, 17, 17, 17, 18, 18, 18, 18, 18, 18, 18, 18, 18, 19,
19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 19, 20, 20, 20,
20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 20, 21,
21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 21,
21, 21, 21, 21, 21, 21, 21, 21, 21, 21, 22, 22, 22, 22, 22, 22,
22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22,
22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 22, 23,
23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23,
23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23,
23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23, 23,
23, 23, 23, 23, 23, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24,
24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24, 24
];

/**
* Resizes canvas, draws original image and applies bluring algorithm.
* @param radius blur radius to be applied
* @param width width of the canvas
* @param height height of the canvas
*/
RainyDay.prototype.prepareBackground = function(radius, width, height) {
if (width && height) {
this.canvas.style.width = width + "px";
this.canvas.style.height = height + "px";
this.canvas.width = width;
this.canvas.height = height;
} else {
width = this.canvas.width;
height = this.canvas.height;
}

var context = this.canvas.getContext("2d");
context.clearRect(0, 0, width, height);
context.drawImage(this.img, 0, 0, width, height);

if (isNaN(radius) || radius < 1) return;

this.stackBlurCanvasRGB(0, 0, width, height, radius);
};

/**
* Implements the Stack Blur Algorithm (@see http://www.quasimondo.com/StackBlurForCanvas/StackBlurDemo.html).
* @param top_x x of top-left corner of the blurred rectangle
* @param top_y y of top-left corner of the blurred rectangle
* @param width width of the canvas
* @param height height of the canvas
* @param radius blur radius
*/
RainyDay.prototype.stackBlurCanvasRGB = function(top_x, top_y, width, height, radius) {
radius |= 0;

var context = this.canvas.getContext("2d");
var imageData = context.getImageData(top_x, top_y, width, height);

var pixels = imageData.data;

var x, y, i, p, yp, yi, yw, r_sum, g_sum, b_sum,
r_out_sum, g_out_sum, b_out_sum,
r_in_sum, g_in_sum, b_in_sum,
pr, pg, pb, rbs;

var div = radius + radius + 1;
var w4 = width << 2;
var widthMinus1 = width - 1;
var heightMinus1 = height - 1;
var radiusPlus1 = radius + 1;
var sumFactor = radiusPlus1 * (radiusPlus1 + 1) / 2;

var stackStart = new BlurStack();
var stack = stackStart;
for (i = 1; i < div; i++) {
stack = stack.next = new BlurStack();
if (i == radiusPlus1) var stackEnd = stack;
}
stack.next = stackStart;
var stackIn = null;
var stackOut = null;

yw = yi = 0;

var mul_sum = mul_table[radius];
var shg_sum = shg_table[radius];

for (y = 0; y < height; y++) {
r_in_sum = g_in_sum = b_in_sum = r_sum = g_sum = b_sum = 0;

r_out_sum = radiusPlus1 * (pr = pixels[yi]);
g_out_sum = radiusPlus1 * (pg = pixels[yi + 1]);
b_out_sum = radiusPlus1 * (pb = pixels[yi + 2]);

r_sum += sumFactor * pr;
g_sum += sumFactor * pg;
b_sum += sumFactor * pb;

stack = stackStart;

for (i = 0; i < radiusPlus1; i++) {
stack.r = pr;
stack.g = pg;
stack.b = pb;
stack = stack.next;
}

for (i = 1; i < radiusPlus1; i++) {
p = yi + ((widthMinus1 < i ? widthMinus1 : i) << 2);
r_sum += (stack.r = (pr = pixels[p])) * (rbs = radiusPlus1 - i);
g_sum += (stack.g = (pg = pixels[p + 1])) * rbs;
b_sum += (stack.b = (pb = pixels[p + 2])) * rbs;

r_in_sum += pr;
g_in_sum += pg;
b_in_sum += pb;

stack = stack.next;
}

stackIn = stackStart;
stackOut = stackEnd;
for (x = 0; x < width; x++) {
pixels[yi] = (r_sum * mul_sum) >> shg_sum;
pixels[yi + 1] = (g_sum * mul_sum) >> shg_sum;
pixels[yi + 2] = (b_sum * mul_sum) >> shg_sum;

r_sum -= r_out_sum;
g_sum -= g_out_sum;
b_sum -= b_out_sum;

r_out_sum -= stackIn.r;
g_out_sum -= stackIn.g;
b_out_sum -= stackIn.b;

p = (yw + ((p = x + radius + 1) < widthMinus1 ? p : widthMinus1)) << 2;

r_in_sum += (stackIn.r = pixels[p]);
g_in_sum += (stackIn.g = pixels[p + 1]);
b_in_sum += (stackIn.b = pixels[p + 2]);

r_sum += r_in_sum;
g_sum += g_in_sum;
b_sum += b_in_sum;

stackIn = stackIn.next;

r_out_sum += (pr = stackOut.r);
g_out_sum += (pg = stackOut.g);
b_out_sum += (pb = stackOut.b);

r_in_sum -= pr;
g_in_sum -= pg;
b_in_sum -= pb;

stackOut = stackOut.next;

yi += 4;
}
yw += width;
}

for (x = 0; x < width; x++) {
g_in_sum = b_in_sum = r_in_sum = g_sum = b_sum = r_sum = 0;

yi = x << 2;
r_out_sum = radiusPlus1 * (pr = pixels[yi]);
g_out_sum = radiusPlus1 * (pg = pixels[yi + 1]);
b_out_sum = radiusPlus1 * (pb = pixels[yi + 2]);

r_sum += sumFactor * pr;
g_sum += sumFactor * pg;
b_sum += sumFactor * pb;

stack = stackStart;

for (i = 0; i < radiusPlus1; i++) {
stack.r = pr;
stack.g = pg;
stack.b = pb;
stack = stack.next;
}

yp = width;

for (i = 1; i <= radius; i++) {
yi = (yp + x) << 2;

r_sum += (stack.r = (pr = pixels[yi])) * (rbs = radiusPlus1 - i);
g_sum += (stack.g = (pg = pixels[yi + 1])) * rbs;
b_sum += (stack.b = (pb = pixels[yi + 2])) * rbs;

r_in_sum += pr;
g_in_sum += pg;
b_in_sum += pb;

stack = stack.next;

if (i < heightMinus1) {
yp += width;
}
}

yi = x;
stackIn = stackStart;
stackOut = stackEnd;
for (y = 0; y < height; y++) {
p = yi << 2;
pixels[p] = (r_sum * mul_sum) >> shg_sum;
pixels[p + 1] = (g_sum * mul_sum) >> shg_sum;
pixels[p + 2] = (b_sum * mul_sum) >> shg_sum;

r_sum -= r_out_sum;
g_sum -= g_out_sum;
b_sum -= b_out_sum;

r_out_sum -= stackIn.r;
g_out_sum -= stackIn.g;
b_out_sum -= stackIn.b;

p = (x + (((p = y + radiusPlus1) < heightMinus1 ? p : heightMinus1) * width)) << 2;

r_sum += (r_in_sum += (stackIn.r = pixels[p]));
g_sum += (g_in_sum += (stackIn.g = pixels[p + 1]));
b_sum += (b_in_sum += (stackIn.b = pixels[p + 2]));

stackIn = stackIn.next;

r_out_sum += (pr = stackOut.r);
g_out_sum += (pg = stackOut.g);
b_out_sum += (pb = stackOut.b);

r_in_sum -= pr;
g_in_sum -= pg;
b_in_sum -= pb;

stackOut = stackOut.next;

yi += width;
}
}

context.putImageData(imageData, top_x, top_y);

};

/**
* Defines a new helper object for Stack Blur Algorithm.
*/

function BlurStack() {
this.r = 0;
this.g = 0;
this.b = 0;
this.a = 0;
this.next = null;
}

/**
* Defines a gravity matrix object which handles collision detection.
* @param x number of columns in the matrix
* @param y number of rows in the matrix
* @param r grid size
*/

/**
* Updates position of the given drop on the collision matrix.
* @param drop raindrop to be positioned/repositioned
* @forceDelete if true the raindrop will be removed from the matrix
* @returns collisions if any
*/
CollisionMatrix.prototype.update = function(drop, forceDelete) {
if (drop.gid) {
this.matrix[drop.gmx][drop.gmy].remove(drop);
if (forceDelete) {
return null;
}

drop.gmx = Math.floor(drop.x / this.resolution);
drop.gmy = Math.floor(drop.y / this.resolution);
this.matrix[drop.gmx][drop.gmy].add(drop);

var collisions = this.collisions(drop);
if (collisions && collisions.next != null) {
return collisions.next;
}
} else {
drop.gid = Math.random().toString(36).substr(2, 9);
drop.gmx = Math.floor(drop.x / this.resolution);
drop.gmy = Math.floor(drop.y / this.resolution);
this.matrix[drop.gmx][drop.gmy].add(drop);
}
return null;
};

/**
* Looks for collisions with the given raindrop.
* @param drop raindrop to be checked
* @returns list of drops that collide with it
*/
CollisionMatrix.prototype.collisions = function(drop) {
var item = new DropItem(null);
var first = item;

item = this.addAll(item, drop.gmx - 1, drop.gmy);
item = this.addAll(item, drop.gmx - 1, drop.gmy + 1);
item = this.addAll(item, drop.gmx, drop.gmy + 1);
item = this.addAll(item, drop.gmx + 1, drop.gmy + 1);
item = this.addAll(item, drop.gmx + 1, drop.gmy);

return first;
};

/**
* Appends all found drop at a given location to the given item.
* @param to item to which the results will be appended to
* @param x x position in the matrix
* @param y y position in the matrix
* @returns last discovered item on the list
*/
CollisionMatrix.prototype.addAll = function(to, x, y) {
if (x > 0 && y > 0 && x < this.xc && y < this.yc) {
var items = this.matrix[x][y];
while (items.next != null) {
items = items.next;
to.next = new DropItem(items.drop);
to = to.next;
}
}
return to;
};

/**
* Defines a linked list item.
*/

function DropItem(drop) {
this.drop = drop;
this.next = null;
}

/**
* Adds the raindrop to the end of the list.
* @param drop raindrop to be added
*/
DropItem.prototype.add = function(drop) {
var item = this;
while (item.next != null) {
item = item.next;
}
item.next = new DropItem(drop);
};

/**
* Removes the raindrop from the list.
* @param drop raindrop to be removed
*/
DropItem.prototype.remove = function(drop) {
var item = this;
var prevItem = null;
while (item.next != null) {
prevItem = item;
item = item.next;
if (item.drop.gid == drop.gid) {
prevItem.next = item.next;
}
}
};

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