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By Root 4007 0

one. In the inefficient code, we’ve gone out of our way to use as many dots as possible. Here’s a section (don’t worry too much about what the equations mean!): var grav=(earth.physics.mass*moon.physics.mass)

/(dist*dist*gravF);

var xGrav=grav*(distX/dist);

var yGrav=grav*(distY/dist);

moon.physics.acc.x=-xGrav/(moon.physics.mass);

moon.physics.acc.y=-yGrav/(moon.physics.mass);

moon.physics.vel.x+=moon.physics.acc.x;

moon.physics.vel.y+=moon.physics.acc.y;

moon.physics.pos.x+=moon.physics.vel.x;

moon.physics.pos.y+=moon.physics.vel.y;

Licensed to jonathan zheng

298

CHAPTER 8

Performance

This is something of a caricature—we’ve deliberately used as many deep references down the object graphs as possible, making for verbose and slow code. There is certainly plenty of room for improvement! Here’s the same code from the optimized version:

var mp=moon.physics;

var mpa=mp.acc;

var mpv=mp.vel;

var mpp=mp.pos;

var mpm=mp.mass;

...

var grav=(epm*mpm)/(dist*dist*gravF);

var xGrav=grav*(distX/dist);

var yGrav=grav*(distY/dist);

mpa.x=-xGrav/(mpm);

mpa.y=-yGrav/(mpm);

mpv.x+=mpa.x;

mpv.y+=mpa.y;

mpp.x+=mpv.x;

mpp.y+=mpv.y;

We’ve simply resolved all the necessary references at the start of the calculation as local variables. This makes the code more readable and, more important, reduces the work that the interpreter needs to do. Listing 8.7 shows the code for the complete web page that allows the two algorithms to be profiled side by side. Listing 8.7 Profiling variable resolution