flat assembler
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vid 20 Nov 2007, 18:19
hi, i wrote an article how to do basic operations with bignums (big numbers, more than 32 / 64 bits):
http://www.x86asm.net/articles/workingwithbignumbersusingx86instructions/ 

20 Nov 2007, 18:19 

asmfan 20 Nov 2007, 18:30
Quote:
Mult via addition and div via subtraction) 

20 Nov 2007, 18:30 

realcr 20 Nov 2007, 20:26
Hey vid.
I really like reading your pieces of code , so first great thanks for sharing it. Asm people like speed , so you probably want to use an efficient algorithm for multiplication , like fast fourier transform for the discrete case. realcr. 

20 Nov 2007, 20:26 

vid 20 Nov 2007, 20:36
Unfortunately i don't know FFT yet. I thought FFT is used for exponentiation, not multiplication.
My purpose in this article was to demonstrate real basics. Like how to work with 64bit numbers on 32bit processor, etc. If you are really after speed, then use some readymade library, which has been developed for several years. It will be likely faster than what you would write. 

20 Nov 2007, 20:36 

Garthower 21 Nov 2007, 18:35
Hi vid!
Some time back I too was engaged in a similar problem. I shall not write the theory, you and have well painted it. Here my code for various operations, can, it will be useful to someone. Add numbers Code: ; input: ESI=big a ; EDI=big b ; ECX=length in DWORDs ; output:CF ; action:b = b + a bn_add: clc ; CF=0 __cycle: lodsd adc eax, [edi] stosd loop __cycle retn [/b]Sub numbers:[/b] Code: ; input: ESI=big a ; EDI=big b ; ECX=length in DWORDs ; output:CF ; action:b = b  a bn_sub: clc ; CF=0 __cycle: lodsd sbb [edi], eax lea edi, [edi+4] loop __cycle retn Compare numbers: Code: ; input: ESI=big a ; EDI=big b ; ECX=length in DWORDs ; output:CF==1 (jb)  a < b ; ZF==1 (jz)  a = b ; CF==0 (ja)  a > b bn_cmp: __cycle: mov eax, [esi+ecx*44] cmp eax, [edi+ecx*44] loopz __cycle __exit: retn Mul numbers: Code: MulNumbers: ;Num1,Num2,Res,Size pushad mov edi,[esp+2Ch] xor eax,eax mov ecx,LEN_DWORD*2 rep stosd mov ebx,[esp+24h] mov ecx,[esp+28h] xor esi,esi align 4 @@First_Cicle: xor edi,edi align 4 @@Second_Cicle: mov eax,[ebx+edi] mul 4 ptr [ecx+esi] lea ebp,[esi+edi] add ebp,[esp+2Ch] add [ebp],eax adc [ebp+4],edx lea ebp,[ebp+8] jnc @@No_Add @@Add_Again: adc 4 ptr [ebp],0 lea ebp,[ebp+4] jc @@Add_Again @@No_Add: add edi,4 cmp edi,[esp+30h] jb @@Second_Cicle add esi,4 cmp esi,[esp+30h] jb @@First_Cicle popad ret 10h DIV numbers: Code: DivNumbers: ;Num1:dword,Num2:dword,Res:dword,Rem:dword pushad mov ecx,LEN_DWORD mov edi,[esp+2Ch] xor eax,eax push ecx rep stosd pop ecx mov edi,[esp+30h] rep stosd mov ebx,LEN_BITS1 align 4 @@Next_Bit: mov esi,[esp+2Ch] clc mov ecx,LEN_DWORD align 4 @@Next_Shl: rcl 4 ptr [esi],1 lea esi,[esi+4] loop @@Next_Shl mov eax,[esp+24h] bt [eax],ebx mov esi,[esp+30h] mov ecx,LEN_DWORD push esi align 4 @@Next_Rcl: rcl 4 ptr [esi],1 lea esi,[esi+4] loop @@Next_Rcl pop edi mov esi,[esp+28h] mov ecx,LEN_DWORD align 4 @@Next_Cmp: mov eax,[edi+ecx*44] cmp eax,[esi+ecx*44] jb @@No_Sub ja @@Sub loop @@Next_Cmp @@Sub: mov ecx,LEN_DWORD clc align 4 @@Next_Sub: lodsd sbb [edi],eax lea edi,[edi+4] loop @@Next_Sub mov eax,[esp+2Ch] or 1 ptr [eax],1 @@No_Sub: dec ebx jns @@Next_Bit popad ret 10h 

21 Nov 2007, 18:35 

hckr83 21 Nov 2007, 23:51
hey, Vid, is English your first language? You talk understandably, but some stuff is kinda awkward to read...like
Quote: To invert all bits, we have not instruction would be at least To invert all bits, we have a not instruction...and possibly phrased a bit differently... 

21 Nov 2007, 23:51 

vid 21 Nov 2007, 23:55
nope, i just learnt english by using. I always omit "a" and "the" , for some reason.
and i would use "the" in that case 

21 Nov 2007, 23:55 

MazeGen 22 Nov 2007, 08:55
hckr83 wrote:
I'm the editor of x86asm.net. Note that "not" is set up using Curier  that should make the difference. Vid is used to use quotes to make mnemonics different, but I don't like this way. And yes, there should be additional a... 

22 Nov 2007, 08:55 

rugxulo 05 Dec 2007, 14:10
Code: ;  AMD Optimizations (c) 2001  2006 Advanced Micro Devices, Inc. All rights reserved. http://developer.amd.com /htmlhelp/optimization/wwhelp/wwhimpl/java/html/wwhelp.htm (below taken from "Integer Optimizations" section): ;  Efficient 64Bit Integer Arithmetic in 32Bit Mode Optimization The following section contains a collection of code snippets and subroutines showing the efficient implementation of 64bit arithmetic in 32bit mode. Note that these are 32bit recommendations, in 64bit mode it is important to use 64bit integer instructions for best performance. Addition, subtraction, negation, and shifting are best handled by inline code. Multiplication, division, and the computation of remainders are less common operations and are usually implemented as subroutines. If these subroutines are used often, the programmer should consider inlining them. Except for division and remainder calculations, the following code works for both signed and unsigned integers. The division and remainder code shown works for unsigned integers, but can easily be extended to handle signed integers. 64Bit Addition ; Add ECX:EBX to EDX:EAX, and place sum in EDX:EAX. add eax, ebx adc edx, ecx 64Bit Subtraction ; Subtract ECX:EBX from EDX:EAX and place difference in EDX:EAX. sub eax, ebx sbb edx, ecx 64Bit Negation ; Negate EDX:EAX. not edx neg eax sbb edx, 1 ; Fix: Increment high word if low word was 0. 64Bit Left Shift ; Shift EDX:EAX left, ??shift count in ECX (count ; applied modulo 64). shld edx, eax, cl ; First apply shift count. shl eax, cl ; ??mod 32 to EDX:EAX test ecx, 32 ; Need to shift by another 32? jz lshift_done ; No, done. mov edx, eax ; Left shift EDX:EAX xor eax, eax ; by 32 bits lshift_done: 64Bit Right Shift shrd eax, edx, cl ; First apply shift count. shr edx, cl ; ??mod 32 to EDX:EAX test ecx, 32 ; Need to shift by another 32? jz rshift_done ; No, done. mov eax, edx ; Left shift EDX:EAX xor edx, edx ; by 32 bits. rshift_done: 64Bit Multiplication ; _llmul computes the loworder half of the product of its ; arguments, two 64bit integers. ; ; In: [ESP+8]:[ESP+4] = multiplicand ; [ESP+16]:[ESP+12] = multiplier ; Out: EDX:EAX = (multiplicand * multiplier) % 2^64 ; Destroys: EAX, ECX, EDX, EFlags _llmul PROC mov edx, [esp+8] ; multiplicand_hi mov ecx, [esp+16] ; multiplier_hi or edx, ecx ; One operand >= 2^32? mov edx, [esp+12] ; multiplier_lo mov eax, [esp+4] ; multiplicand_lo jnz twomul ; Yes, need two multiplies. mul edx ; multiplicand_lo * multiplier_lo ret ; Done, return to caller. twomul: imul edx, [esp+8] ; p3_lo = multiplicand_hi * multiplier_lo imul ecx, eax ; p2_lo = multiplier_hi * multiplicand_lo add ecx, edx ; p2_lo + p3_lo mul dword ptr [esp+12] ; p1 = multiplicand_lo * multiplier_lo add edx, ecx ; p1 + p2_lo + p3_lo = result in EDX:EAX ret ; Done, return to caller. _llmul ENDP 64Bit Unsigned Division ; _ulldiv divides two unsigned 64bit integers and returns the quotient. ; ; In: [ESP+8]:[ESP+4] = dividend ; [ESP+16]:[ESP+12] = divisor ; Out: EDX:EAX = quotient of division ; Destroys: EAX, ECX, EDX, EFlags _ulldiv PROC push ebx ; Save EBX as per calling convention. mov ecx, [esp+20] ; divisor_hi mov ebx, [esp+16] ; divisor_lo mov edx, [esp+12] ; dividend_hi mov eax, [esp+8] ; dividend_lo test ecx, ecx ; divisor > (2^32  1)? jnz big_divisor ; Yes, divisor > 2^32  1. cmp edx, ebx ; Only one division needed (ECX = 0)? jae two_divs ; Need two divisions. div ebx ; EAX = quotient_lo mov edx, ecx ; EDX = quotient_hi = 0 (quotient in EDX:EAX) pop ebx ; Restore EBX as per calling convention. ret ; Done, return to caller. two_divs: mov ecx, eax ; Save dividend_lo in ECX. mov eax, edx ; Get dividend_hi. xor edx, edx ; Zeroextend it into EDX:EAX. div ebx ; quotient_hi in EAX xchg eax, ecx ; ECX = quotient_hi, EAX = dividend_lo div ebx ; EAX = quotient_lo mov edx, ecx ; EDX = quotient_hi (quotient in EDX:EAX) pop ebx ; Restore EBX as per calling convention. ret ; Done, return to caller. big_divisor: push edi ; Save EDI as per calling convention. mov edi, ecx ; Save divisor_hi. shr edx, 1 ; Shift both divisor and dividend right rcr eax, 1 ; by 1 bit. ror edi, 1 rcr ebx, 1 bsr ecx, ecx ; ECX = number of remaining shifts shrd ebx, edi, cl ; Scale down divisor and dividend shrd eax, edx, cl ; such that divisor is less than shr edx, cl ; 2^32 (that is, it fits in EBX). rol edi, 1 ; Restore original divisor_hi. div ebx ; Compute quotient. mov ebx, [esp+12] ; dividend_lo mov ecx, eax ; Save quotient. imul edi, eax ; quotient * divisor high word (??low only) mul dword ptr [esp+20] ; quotient * divisor low word add edx, edi ; EDX:EAX = quotient * divisor sub ebx, eax ; dividend_lo  (quot.*divisor)_lo mov eax, ecx ; Get quotient. mov ecx, [esp+16] ; dividend_hi sbb ecx, edx ; Subtract (divisor * quot.) from dividend. sbb eax, 0 ; Adjust quotient if remainder negative. xor edx, edx ; Clear high word of quot. (EAX<=FFFFFFFFh). pop edi ; Restore EDI as per calling convention. pop ebx ; Restore EBX as per calling convention. ret ; Done, return to caller. _ulldiv ENDP 64Bit Signed Division ; _lldiv divides two signed 64bit numbers and delivers the quotient ; ; In: [ESP+8]:[ESP+4] = dividend ; [ESP+16]:[ESP+12] = divisor ; Out: EDX:EAX = quotient of division ; Destroys: EAX, ECX,E DX, EFlags _lldiv PROC push ebx ; Save EBX as per calling convention. push esi ; Save ESI as per calling convention. push edi ; Save EDI as per calling convention. mov ecx, [esp+28] ; divisor_hi mov ebx, [esp+24] ; divisor_lo mov edx, [esp+20] ; dividend_hi mov eax, [esp+16] ; dividend_lo mov esi, ecx ; divisor_hi xor esi, edx ; divisor_hi ^ dividend_hi sar esi, 31 ; (quotient < 0) ? 1 : 0 mov edi, edx ; dividend_hi sar edi, 31 ; (dividend < 0) ? 1 : 0 xor eax, edi ; If (dividend < 0), xor edx, edi ; compute 1's complement of dividend. sub eax, edi ; If (dividend < 0), sbb edx, edi ; compute 2's complement of dividend. mov edi, ecx ; divisor_hi sar edi, 31 ; (divisor < 0) ? 1 : 0 xor ebx, edi ; If (divisor < 0), xor ecx, edi ; compute 1's complement of divisor. sub ebx, edi ; If (divisor < 0), sbb ecx, edi ; compute 2's complement of divisor. jnz big_divisor ; divisor > 2^32  1 cmp edx, ebx ; Only one division needed (ECX = 0)? jae two_divs ; Need two divisions. div ebx ; EAX = quotient_lo mov edx, ecx ; EDX = quotient_hi = 0 (quotient in EDX:EAX) xor eax, esi ; If (quotient < 0), xor edx, esi ; compute 1's complement of result. sub eax, esi ; If (quotient < 0), sbb edx, esi ; compute 2's complement of result. pop edi ; Restore EDI as per calling convention. pop esi ; Restore ESI as per calling convention. pop ebx ; Restore EBX as per calling convention. ret ; Done, return to caller. two_divs: mov ecx, eax ; Save dividend_lo in ECX. mov eax, edx ; Get dividend_hi. xor edx, edx ; Zeroextend it into EDX:EAX. div ebx ; quotient_hi in EAX xchg eax, ecx ; ECX = quotient_hi, EAX = dividend_lo div ebx ; EAX = quotient_lo mov edx, ecx ; EDX = quotient_hi (quotient in EDX:EAX) jmp make_sign ; Make quotient signed. big_divisor: sub esp, 12 ; Create three local variables. mov [esp], eax ; dividend_lo mov [esp+4], ebx ; divisor_lo mov [esp+8], edx ; dividend_hi mov edi, ecx ; Save divisor_hi. shr edx, 1 ; Shift both rcr eax, 1 ; divisor and ror edi, 1 ; and dividend rcr ebx, 1 ; right by 1 bit. bsr ecx, ecx ; ECX = number of remaining shifts shrd ebx, edi, cl ; Scale down divisor and shrd eax, edx, cl ; dividend such that divisor is shr edx, cl ; less than 2^32 (that is, fits in EBX). rol edi, 1 ; Restore original divisor_hi. div ebx ; Compute quotient. mov ebx, [esp] ; dividend_lo mov ecx, eax ; Save quotient. imul edi, eax ; quotient * divisor high word (??low only) mul DWORD PTR [esp+4] ; quotient * divisor low word add edx, edi ; EDX:EAX = quotient * divisor sub ebx, eax ; dividend_lo  (quot.*divisor)_lo mov eax, ecx ; Get quotient. mov ecx, [esp+8] ; dividend_hi sbb ecx, edx ; Subtract (divisor * quot.) from dividend sbb eax, 0 ; Adjust quotient if remainder is negative. xor edx, edx ; Clear high word of quotient. add esp, 12 ; Remove local variables. make_sign: xor eax, esi ; If (quotient < 0), xor edx, esi ; compute 1's complement of result. sub eax, esi ; If (quotient < 0), sbb edx, esi ; compute 2's complement of result. pop edi ; Restore EDI as per calling convention. pop esi ; Restore ESI as per calling convention. pop ebx ; Restore EBX as per calling convention. ret ; Done, return to caller. _lldiv ENDP 64Bit Unsigned Remainder Computation ; _ullrem divides two unsigned 64bit integers and returns the remainder. ; ; In: [ESP+8]:[ESP+4] = dividend ; [ESP+16]:[ESP+12] = divisor ; ; Out: EDX:EAX = remainder of division ; ; Destroys: EAX, ECX, EDX, EFlags _ullrem PROC push ebx ; Save EBX as per calling convention. mov ecx, [esp+20] ; divisor_hi mov ebx, [esp+16] ; divisor_lo mov edx, [esp+12] ; dividend_hi mov eax, [esp+8] ; dividend_lo test ecx, ecx ; divisor > 2^32  1? jnz r_big_divisor ; Yes, divisor > 32^32  1. cmp edx, ebx ; Only one division needed (ECX = 0)? jae r_two_divs ; Need two divisions. div ebx ; EAX = quotient_lo mov eax, edx ; EAX = remainder_lo mov edx, ecx ; EDX = remainder_hi = 0 pop ebx ; Restore EBX per calling convention. ret ; Done, return to caller. r_two_divs: mov ecx, eax ; Save dividend_lo in ECX. mov eax, edx ; Get dividend_hi. xor edx, edx ; Zeroextend it into EDX:EAX. div ebx ; EAX = quotient_hi, EDX = intermediate remainder mov eax, ecx ; EAX = dividend_lo div ebx ; EAX = quotient_lo mov eax, edx ; EAX = remainder_lo xor edx, edx ; EDX = remainder_hi = 0 pop ebx ; Restore EBX as per calling convention. ret ; Done, return to caller. r_big_divisor: push edi ; Save EDI as per calling convention. mov edi, ecx ; Save divisor_hi. shr edx, 1 ; Shift both divisor and dividend right rcr eax, 1 ; by 1 bit. ror edi, 1 rcr ebx, 1 bsr ecx, ecx ; ECX = number of remaining shifts shrd ebx, edi, cl ; Scale down divisor and dividend such shrd eax, edx, cl ; that divisor is less than 2^32 shr edx, cl ; (that is, it fits in EBX). rol edi, 1 ; Restore original divisor (EDI:ESI). div ebx ; Compute quotient. mov ebx, [esp+12] ; dividend low word mov ecx, eax ; Save quotient. imul edi, eax ; quotient * divisor high word (??low only) mul DWORD PTR [esp+20] ; quotient * divisor low word add edx, edi ; EDX:EAX = quotient * divisor sub ebx, eax ; dividend_lo  (quot.*divisor)_lo mov ecx, [esp+16] ; dividend_hi mov eax, [esp+20] ; divisor_lo sbb ecx, edx ; Subtract divisor * quot. from dividend. sbb edx, edx ; (remainder < 0) ? 0xFFFFFFFF : 0 and eax, edx ; (remainder < 0) ? divisor_lo : 0 and edx, [esp+24] ; (remainder < 0) ? divisor_hi : 0 add eax, ebx ; remainder += (remainder < 0) ? divisor : 0 pop edi ; Restore EDI as per calling convention. pop ebx ; Restore EBX as per calling convention. ret ; Done, return to caller. _ullrem ENDP 64Bit Signed Remainder Computation ; _llrem divides two signed 64bit numbers and returns the remainder. ; ; In: [ESP+8]:[ESP+4] = dividend ; [ESP+16]:[ESP+12] = divisor ; ; Out: EDX:EAX = remainder of division ; ; Destroys: EAX, ECX, EDX, EFlags push ebx ; Save EBX as per calling convention. push esi ; Save ESI as per calling convention. push edi ; Save EDI as per calling convention. mov ecx, [esp+28] ; divisorhi mov ebx, [esp+24] ; divisorlo mov edx, [esp+20] ; dividendhi mov eax, [esp+16] ; dividendlo mov esi, edx ; sign(remainder) == sign(dividend) sar esi, 31 ; (remainder < 0) ? 1 : 0 mov edi, edx ; dividendhi sar edi, 31 ; (dividend < 0) ? 1 : 0 xor eax, edi ; If (dividend < 0), xor edx, edi ; compute 1's complement of dividend. sub eax, edi ; If (dividend < 0), sbb edx, edi ; compute 2's complement of dividend. mov edi, ecx ; divisorhi sar edi, 31 ; (divisor < 0) ? 1 : 0 xor ebx, edi ; If (divisor < 0), xor ecx, edi ; compute 1's complement of divisor. sub ebx, edi ; If (divisor < 0), sbb ecx, edi ; compute 2's complement of divisor. jnz sr_big_divisor ; divisor > 2^32  1 cmp edx, ebx ; Only one division needed (ECX = 0)? jae sr_two_divs ; No, need two divisions. div ebx ; EAX = quotient_lo mov eax, edx ; EAX = remainder_lo mov edx, ecx ; EDX = remainder_lo = 0 xor eax, esi ; If (remainder < 0), xor edx, esi ; compute 1's complement of result. sub eax, esi ; If (remainder < 0), sbb edx, esi ; compute 2's complement of result. pop edi ; Restore EDI as per calling convention. pop esi ; Restore ESI as per calling convention. pop ebx ; Restore EBX as per calling convention. ret ; Done, return to caller. sr_two_divs: mov ecx, eax ; Save dividend_lo in ECX. mov eax, edx ; Get dividend_hi. xor edx, edx ; Zeroextend it into EDX:EAX. div ebx ; EAX = quotient_hi, EDX = intermediate remainder mov eax, ecx ; EAX = dividend_lo div ebx ; EAX = quotient_lo mov eax, edx ; remainder_lo xor edx, edx ; remainder_hi = 0 jmp sr_makesign ; Make remainder signed. sr_big_divisor: sub esp, 16 ; Create three local variables. mov [esp], eax ; dividend_lo mov [esp+4], ebx ; divisor_lo mov [esp+8], edx ; dividend_hi mov [esp+12], ecx ; divisor_hi mov edi, ecx ; Save divisor_hi. shr edx, 1 ; Shift both rcr eax, 1 ; divisor and ror edi, 1 ; and dividend rcr ebx, 1 ; right by 1 bit. bsr ecx, ecx ; ECX = number of remaining shifts shrd ebx, edi, cl ; Scale down divisor and shrd eax, edx, cl ; dividend such that divisor is shr edx, cl ; less than 2^32 (that is, fits in EBX). rol edi, 1 ; Restore original divisor_hi. div ebx ; Compute quotient. mov ebx, [esp] ; dividend_lo mov ecx, eax ; Save quotient. imul edi, eax ; quotient * divisor high word (??low only) mul DWORD PTR [esp+4] ; quotient * divisor low word add edx, edi ; EDX:EAX = quotient * divisor sub ebx, eax ; dividend_lo  (quot.*divisor)_lo mov ecx, [esp+8] ; dividend_hi sbb ecx, edx ; Subtract divisor * quot. from dividend. sbb eax, eax ; remainder < 0 ? 0xffffffff : 0 mov edx, [esp+12] ; divisor_hi and edx, eax ; remainder < 0 ? divisor_hi : 0 and eax, [esp+4] ; remainder < 0 ? divisor_lo : 0 add eax, ebx ; remainder_lo add edx, ecx ; remainder_hi add esp, 16 ; Remove local variables. sr_makesign: xor eax, esi ; If (remainder < 0), xor edx, esi ; compute 1's complement of result. sub eax, esi ; If (remainder < 0), sbb edx, esi ; compute 2's complement of result. pop edi ; Restore EDI as per calling convention. pop esi ; Restore ESI as per calling convention. pop ebx ; Restore EBX as per calling convention. ret ; Done, return to caller. 

05 Dec 2007, 14:10 

vid 05 Dec 2007, 14:30
Thanks. Don't you happen to have direct link to this manual, and name of chapter/section where this is? I'd add it as reference to article.
Things in article are basically done same way, except for NEG and that was already pointed out by bitrake on x86asm board. Those bignum divisions are really interesting, I'd have to study them someday. 

05 Dec 2007, 14:30 

rugxulo 05 Dec 2007, 14:45
Sorry, I posted as close to a direct reference (URL) as I could.
Software Optimization Guide for AMD64 Processors  Integer Optimizations  Efficient 64Bit Integer Arithmatic in 32Bit Mode 

05 Dec 2007, 14:45 

vid 05 Dec 2007, 14:50
Thanks a lot.
I plan to add bignum multiplication, and with your help even division someday to article. 

05 Dec 2007, 14:50 

MCD 07 Jan 2008, 05:41


07 Jan 2008, 05:41 

vid 07 Jan 2008, 11:25
MCD:
not sure about math stuff, but my O(n*log(n)) is number of MUL instructions used, probably not what math people intended. My idea is something like this: c = a*b (all number consist of 4 dwords, c0 = first dword of destination, c1 = second, etc... edx is upper 32bits of previous multiplication) Code: c0 = a0 * b0 c1 = a1 * b0 + edx c2 = a2 * b0 + edx c3 = a3 * b0 + edx c1 += a0 * b1 c2 += a1 * b1 + edx c3 += a2 * b1 + edx c2 += a0 * b2 c3 += a1 * b2 + edx c3 += a0 * b3 Isn't this O(n*log(n)) number of MUL instructions? 

07 Jan 2008, 11:25 

Madis731 07 Jan 2008, 13:19
But if you want this code to scale without loops, you need to scale your code that does the operation.
If you add 128bit numbers with 32bit registers you need a loop ECX=4 If you mul 128bit numbers with 64bit registers you can do this manually but MUL128 with R32 is tricky. You still need an algorithm, which means shifting is needed on subcalculations. 

07 Jan 2008, 13:19 

vid 07 Jan 2008, 14:13
of course that it would be done with loops.
but number of MULs is N + (N1) + (N2) + (N3) + ... + 1 isn't this O(n*log(n))? I never really knew bigO notation 

07 Jan 2008, 14:13 

LocoDelAssembly 07 Jan 2008, 14:38
The "N + (N1) + (N2) + (N3) + ... + 1" is sometimes refered as the sum of the first N natural numbers (though, you wrote it in reverse order ).
Doing some maths: Code: 1 + 2 + ... + N1 + N = ((1 + 2 + 3 + ... + N2 + N1 + N) + (N + N1 + N2 + ... + 3 + 2 + 1))/2 = (1+N + 2+N1 + 3+N2 + ... + N2+3 + N1+2 + N+1)/2 = (N+1 + N+1 + N+1 + ... + N+1 + N+1 + N+1)/2 = N(N+1)/2 However, while N(N+1)/2 for N=100 is 5050, log(100) is 2 and log2(100) is 6,64.. Not sure if the notation refers to log2 or log10 but non of those matches your calculation of the number of MULs 

07 Jan 2008, 14:38 

vid 07 Jan 2008, 14:54
okay, so it is O(N^2)... my bad
I was just quessing anyway... 

07 Jan 2008, 14:54 

LocoDelAssembly 07 Jan 2008, 14:57
Doesn't exists O(N(N+1)/2)? N^2 is ultra big on big Ns, much much bigger than N(N+1)/2


07 Jan 2008, 14:57 

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