/*  sifissub.cpp  by K.Tsuru  */
// function ID = 423    BRADIX
/************************************************
SInteger class
It provides a subtraction "SInteger-unsigned short".
It is used in operator--();
result = m - s  ( s < BRADIX)
*************************************************/
#ifndef SN_H
#include "sn.h"
#endif

void IsSub(const SInteger& m, fType s, SInteger& result){
    if(s >= BRADIX) m.SetError(m.OUT_OF_RANGE, "IsSub", 423);
    result = m;     // copy
    if(!s) return;
// result == m
    if(m.Sign(423)<0){  // m < 0
    // m - s = -(|m|+s)
        result.SetSign(1);  IsAdd(result, s, result);   result.SetSign(-1);
        return;
    }
// ------ In the case of m > 0 ------
    fType u = result.figure(0) - s; //u>BRADIX for result.figure(0)<s

    if( (m.aHead == 0) && (u == 0) ){   // m == s
        result.SetZero();   return;
    }

    register uint i = 0;
    if(u < BRADIX){
        result.figure[0] = u;
        while( !result.figure(i) ) i++;
        result.aTail = i;
        return;
    }
// u >= BRADIX ( m(0) < s )
    if(!m.aHead){   // m has one figure and m < s.
        result.figure[0] = s - m[0];
        result.SetSign(-1);
        return;
    }
// Normalize u >= BRADIX and m.aHead >= 1 
    fType* rv = result.figure.Elements();
    rv[0] = u & BRADIX1;
    u = u >> BRADIX_BITS;       //borrow u =1

    i = 0;
    while(u){
        i++;
        u = rv[i] - u;
        rv[i] = u & BRADIX1;
        u = u >> BRADIX_BITS;
    }
    i = m.aHead;
#ifndef NDEBUG
result.figure(i);
#endif
    while(!rv[i]) i--;
    result.aHead = i;
    i = 0;
    while(!rv[i]) i++;
    result.aTail = i;
    if(2u*(result.aHead+1) <= result.figure.size()) result.DoCutDown();
}