Arbitrary precision arithmetic with Ruby
https://stackoverflow.com/questions/2867288
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30-09-2019 - |
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How the heck does Ruby do this? Does Jörg or anyone else know what's happening behind the scenes?
Unfortunately I don't know C very well so bignum.c is of little help to me. I was just kind of curious it someone could explain (in plain English) the theory behind whatever miracle algorithm its using.
irb(main):001:0> 999**999
368063488259223267894700840060521865838338232037353204655959621437025609300472231530103873614505175218691345257589896391130393189447969771645832382192366076536631132001776175977932178658703660778465765811830827876982014124022948671975678131724958064427949902810498973271030787716781467419524180040734398996952930832508934116945966120176735120823151959779536852290090377452502236990839453416790640456116471139751546750048602189291028640970574762600185950226138244530187489211615864021135312077912018844630780307462205252807737757672094320692373101032517459518497524015120165166724189816766397247824175394802028228160027100623998873667435799073054618906855460488351426611310634023489044291860510352301912426608488807462312126590206830413782664554260411266378866626653755763627796569082931785645600816236891168141774993267488171702172191072731069216881668294625679492696148976999868715671440874206427212056717373099639711168901197440416590226524192782842896415414611688187391232048327738965820265934093108172054875188246591760877131657895633586576611857277011782497943522945011248430439201297015119468730712364007639373910811953430309476832453230123996750235710787086641070310288725389595138936784715274150426495416196669832679980253436807864187160054589045664027158817958549374490512399055448819148487049363674611664609890030088549591992466360050042566270348330911795487647045949301286614658650071299695652245266080672989921799342509291635330827874264789587306974472327718704306352445925996155619153783913237212716010410294999877569745287353422903443387562746452522860420416689019732913798073773281533570910205207767157128174184873357050830752777900041943256738499067821488421053870869022738698816059810579221002560882999884763252161747566893835178558961142349304466506402373556318707175710866983035313122068321102457824112014969387225476259342872866363550383840720010832906695360553556647545295849966279980830561242960013654529514995113584909050813015198928283202189194615501403435553060147713139766323195743324848047347575473228198492343231496580885057330510949058490527738662697480293583612233134502078182014347192522391449087738579081585795613547198599661273567662441490401862839817822686573112998663038868314974259766039340894024308383451039874674061160538242392803580758232755749310843694194787991556647907091849600704712003371103926967137408125713631396699343733288014254084819379380555174777020843568689927348949484201042595271932630685747613835385434424807024615161848223715989797178155169951121052285149157137697718850449708843330475301440373094611119631361702936342263219382793996895988331701890693689862459020775599439506870005130750427949747071390095256759203426671803377068109744629909769176319526837824364926844730545524646494321826241925107158040561607706364484910978348669388142016838792902926158979355432483611517588605967745393958061959024834251565197963477521095821435651996730128376734574843289089682710350244222290017891280419782767803785277960834729869249991658417000499998999
Solution
Simple: it does it the same way you do, ever since first grade. Except it doesn't compute in base 10, it computes in base 4 billion (and change).
Think about it: with our number system, we can only represent numbers from 0 to 9. So, how can we compute 6+7 without overflowing? Easy: we do actually overflow! We cannot represent the result of 6+7 as a number between 0 and 9, but we can overflow to the next place and represent it as two numbers between 0 and 9: 3×100 + 1×101. If you want to add two numbers, you add them digit-wise from the right and overflow ("carry") to the left. If you want to multiply two numbers, you have to multiply every digit of one number individually with the other number, then add up the intermediate results.
BigNum arithmetic (this is what this kind of arithmetic where the numbers are bigger than the native machine numbers is usually called) works basically the same way. Except that the base is not 10, and its not 2, either – it's the size of a native machine integer. So, on a 32 bit machine, it would be base 232 or 4 294 967 296.
Specifically, in Ruby Integer is actually an abstract class that is never instianted. Instead, it has two subclasses, Fixnum and Bignum, and numbers automagically migrate between them, depending on their size. In MRI and YARV, Fixnum can hold a 31 or 63 bit signed integer (one bit is used for tagging) depending on the native word size of the machine. In JRuby, a Fixnum can hold a full 64 bit signed integer, even on an 32 bit machine.
The simplest operation is adding two numbers. And if you look at the implementation of + or rather bigadd_core in YARV's bignum.c, it's not too bad to follow. I can't read C either, but you can cleary see how it loops over the individual digits.
OTHER TIPS
You could read the source for bignum.c...
At a very high level, without going into any implementation details, bignums are calculated "by hand" like you used to do in grade school. Now, there are certainly many optimizations that can be applied, but that's the gist of it.
I don't know of the implementation details so I'll cover how a basic Big Number implementation would work.
Basically instead of relying on CPU "integers" it will create it's own using multiple CPU integers. To store arbritrary precision, well lets say you have 2 bits. So the current integer is 11. You want to add one. In normal CPU integers, this would roll over to 00
But, for big number, instead of rolling over and keeping a "fixed" integer width, it would allocate another bit and simulate an addition so that the number becomes the correct 100.
Try looking up how binary math can be done on paper. It's very simple and is trivial to convert to an algorithm.
Beaconaut APICalc 2 just released on Jan.18, 2011, which is an arbitrary-precision integer calculator for bignum arithmetic, cryptography analysis and number theory research......
