113 lines
2.4 KiB
C
113 lines
2.4 KiB
C
#include "os.h"
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#include <mp.h>
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#include "dat.h"
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// division ala knuth, seminumerical algorithms, pp 237-238
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// the numbers are stored backwards to what knuth expects so j
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// counts down rather than up.
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void
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mpdiv(mpint *dividend, mpint *divisor, mpint *quotient, mpint *remainder)
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{
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int j, s, vn, sign;
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mpdigit qd, *up, *vp, *qp;
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mpint *u, *v, *t;
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// divide bv zero
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if(divisor->top == 0)
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abort();
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// quick check
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if(mpmagcmp(dividend, divisor) < 0){
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if(remainder != nil)
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mpassign(dividend, remainder);
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if(quotient != nil)
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mpassign(mpzero, quotient);
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return;
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}
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// D1: shift until divisor, v, has hi bit set (needed to make trial
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// divisor accurate)
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qd = divisor->p[divisor->top-1];
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for(s = 0; (qd & mpdighi) == 0; s++)
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qd <<= 1;
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u = mpnew((dividend->top+2)*Dbits + s);
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if(s == 0 && divisor != quotient && divisor != remainder) {
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mpassign(dividend, u);
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v = divisor;
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} else {
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mpleft(dividend, s, u);
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v = mpnew(divisor->top*Dbits);
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mpleft(divisor, s, v);
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}
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up = u->p+u->top-1;
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vp = v->p+v->top-1;
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vn = v->top;
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// D1a: make sure high digit of dividend is less than high digit of divisor
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if(*up >= *vp){
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*++up = 0;
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u->top++;
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}
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// storage for multiplies
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t = mpnew(4*Dbits);
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qp = nil;
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if(quotient != nil){
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mpbits(quotient, (u->top - v->top)*Dbits);
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quotient->top = u->top - v->top;
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qp = quotient->p+quotient->top-1;
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}
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// D2, D7: loop on length of dividend
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for(j = u->top; j > vn; j--){
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// D3: calculate trial divisor
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mpdigdiv(up-1, *vp, &qd);
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// D3a: rule out trial divisors 2 greater than real divisor
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if(vn > 1) for(;;){
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memset(t->p, 0, 3*Dbytes); // mpvecdigmuladd adds to what's there
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mpvecdigmuladd(vp-1, 2, qd, t->p);
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if(mpveccmp(t->p, 3, up-2, 3) > 0)
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qd--;
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else
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break;
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}
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// D4: u -= v*qd << j*Dbits
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sign = mpvecdigmulsub(v->p, vn, qd, up-vn);
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if(sign < 0){
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// D6: trial divisor was too high, add back borrowed
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// value and decrease divisor
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mpvecadd(up-vn, vn+1, v->p, vn, up-vn);
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qd--;
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}
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// D5: save quotient digit
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if(qp != nil)
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*qp-- = qd;
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// push top of u down one
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u->top--;
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*up-- = 0;
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}
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if(qp != nil){
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mpnorm(quotient);
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if(dividend->sign != divisor->sign)
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quotient->sign = -1;
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}
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if(remainder != nil){
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mpright(u, s, remainder); // u is the remainder shifted
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remainder->sign = dividend->sign;
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}
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mpfree(t);
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mpfree(u);
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if(v != divisor)
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mpfree(v);
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}
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