From: Daniel Sheppard <daniels@...>
Date: 2005-11-24T08:16:50+09:00
Subject: Re: How convert an integer to a bit array

>> bits = Array.new(8) { |i| 7[i] }.reverse!

This code will call:
Integer#[]  8 times
Array.new  once
Array.reverse!  once

> def to_ba(num, size=8)
>   (-size+1..0).inject({}) {|x,i| x << num[-i]} end

This code will call:
Integer#[]  8 times
Array.new  twice

Range.new  once
Integer#@-  once
Integer#+  once
Array#inject once
Array#<<  8 times

All those calls are slower than the single call to Array.reverse!

> Or perhaps, modifying James':
> 
> def to_ba(num, size=8)
>   Array.new(size) {|i| num[-i+size-1]}
> end

Integer#[]  8 times
Array.new  once

Integer#@- 8 times
Integer#+ 8 times
Integer#- 8 times

Once again, those extra calls take longer than the single call to
Array#reverse!.

It's an easy trap to fall into - experience in most other languages
teaches you that simple operations on primitives are much faster than
calling of to a method. The problem with porting that knowledge to ruby
is that there are not operations on primitives - everything is going
through the method dispatching. 

I believe most (all?) of the methods above are implemented in C - best
thing you can do for your code is to make it faster is to spend the
least amount of time possible in ruby-land - in 99% of situations, one
call to C code will outperform anything that takes more than 1 call to
get to C code (it's only if the algorithm in the C code is drastically
bad that you'll be able to outperform it, and I trust that ruby's code
core contains no algorithms that bad). 

24 calls through the message dispatching or 1 call through message
dispatching - it's easy to see the speed improvement.

Benchmark.bm(10) {|bm|
bm.report('james') {100000.times {Array.new(8) { |i| 7[i] }.rev
bm.report('ed1') {100000.times {(-8+1..0).inject([]) {|x,i| x <<
7[-i]}}}
bm.report('ed2') {100000.times {Array.new(8) {|i| 7[-i+8-1]}}}
}
                user     system      total        real
james       0.781000   0.000000   0.781000 (  0.781000)
ed1         2.859000   0.000000   2.859000 (  2.922000)
ed2         1.219000   0.000000   1.219000 (  1.250000)

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