7u83/stc_rpncalc
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subtraction

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This commit is contained in:
Jeff Wang 2019-03-26 03:35:49 -04:00
parent ad9a9f55ad
commit c6ac8f5c5c
2 changed files with 46 additions and 8 deletions
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45
src/decn/decn.c
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@ -380,18 +380,14 @@ int8_t compare_decn(const dec80* a, const dec80* b){ //a<b: -1, a==b: 0, a>b: 1
return is_neg * compare_magn(a, b);
}
static void sub_mag(dec80* acc, const dec80* x){
}
//WARNING: for add_decn() function only
//rescales acc up to exponent (increase exponent of acc in abs value, while doing shifts)
//WARNING: for add_decn() and sub_mag() functions only
//rescales acc up to exponent (increase exponent of acc, while shifting right)
//the actual value of acc->exponent remains unchanged
//both acc, and the number from which exponent was taking must be stripped of leading 0s first
static void _incr_exp(dec80* acc, int16_t exponent){
int16_t curr_exp = get_exponent(acc);
uint8_t is_neg = (acc->exponent < 0 ? 1 : 0);
#ifdef DEBUG_ADD
uint8_t is_neg = (acc->exponent < 0 ? 1 : 0);
printf(" (is_neg,curr_exp,exponent)=(%d,%d,%d)\n",
is_neg, curr_exp, exponent);
#endif
@ -405,6 +401,39 @@ static void _incr_exp(dec80* acc, int16_t exponent){
//curr_exp does NOT get written back to acc->exponent
}
//for internal use only,
//acc must be larger than x in absolute value
//subtract by equal addition algorithm
static void sub_mag(dec80* acc, const dec80* x){
int8_t i;
uint8_t carry = 0;
dec80 tmp;
copy_decn(&tmp, x);
//normalize
remove_leading_zeros(acc);
remove_leading_zeros(&tmp);
_incr_exp(&tmp, get_exponent(acc));
#ifdef DEBUG_ADD
extern char buf[DECN_BUF_SIZE];
dec80_to_str(buf, &tmp);
printf("incr_exp tmp: %s\n", buf);
#endif
//do subtraction
for (i = DEC80_NUM_LSU - 1; i >=0; i--){
uint8_t digit100;
if (acc->lsu[i] > (tmp.lsu[i] + carry)){
digit100 = acc->lsu[i] - (tmp.lsu[i] + carry);
carry = 0;
} else {
digit100 = acc->lsu[i] + 100 - (tmp.lsu[i] + carry);
carry = 1;
}
assert(digit100 < 100);
acc->lsu[i] = digit100;
}
assert(carry == 0); //shouldn't be carry out if |acc| > |x|
}
void add_decn(dec80* acc, const dec80* x){
dec80 tmp;
int8_t rel;
@ -424,7 +453,6 @@ void add_decn(dec80* acc, const dec80* x){
rel = compare_magn(acc, x);
if (rel == 1){
sub_mag(acc, x);
negate_decn(acc);
return;
} else if (rel == -1){
copy_decn(&tmp, x);
@ -557,6 +585,7 @@ void dec80_to_str(char* buf, const dec80* x){
buf[i] = (x->lsu[digit100] % 10) + '0';
i++;
} else { //1 digit
exponent--;
buf[i] = x->lsu[digit100] + '0';
i++;
buf[i] = '.';

9
src/decn/decn_test.c
View File

@ -35,6 +35,15 @@ int main(void){
dec80_to_str(buf, &acc);
printf("b - a: %s\n", buf);
printf("acc: %s\n", buf);
negate_decn(&b);
dec80_to_str(buf, &b);
printf("-b: %s\n", buf);
add_decn(&acc, &b);
dec80_to_str(buf, &acc);
printf("acc - b: %s\n", buf);
return 0;
}