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shell/math: avoid $((3**999999999999999999)) to take years

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function                                             old     new   delta
arith_apply                                          991    1030     +39

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2023-07-02 19:32:12 +02:00
parent 045924ed7d
commit cc9543fed1
1 changed files with 14 additions and 3 deletions
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17
shell/math.c
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@ -155,7 +155,7 @@ typedef unsigned char operator;
#define PREC_ASSIGN2 3
#define TOK_MUL_ASSIGN tok_decl(3,0)
/* "/" and "/=" ops have the same id bits */
#define DIV_ID1 1
#define DIV_ID1 1
#define TOK_DIV_ASSIGN tok_decl(3,DIV_ID1)
#define TOK_REM_ASSIGN tok_decl(3,2)
@ -422,8 +422,19 @@ arith_apply(arith_state_t *math_state, operator op, var_or_num_t *numstack, var_
if (right_side_val < 0)
return "exponent less than 0";
c = 1;
while (--right_side_val >= 0)
while (right_side_val != 0) {
if ((right_side_val & 1) == 0) {
/* this if() block is not necessary for correctness,
* but otherwise echo $((3**999999999999999999))
* takes a VERY LONG time
* (and it's not interruptible by ^C)
*/
rez *= rez;
right_side_val >>= 1;
}
c *= rez;
right_side_val--;
}
rez = c;
}
else /*if (op == TOK_DIV || op == TOK_DIV_ASSIGN
@ -550,7 +561,7 @@ static arith_t parse_with_base(const char *nptr, char **endptr, unsigned base)
/* current char is one of :;<=>?@A..Z[\]^_`a..z */
/* in bases up to 36, case does not matter for a-z,
* map A..Z and a..z to 10..35: */
* map @A..Z and `a..z to 9..35: */
digit = (unsigned)(*nptr | 0x20) - ('a' - 10);
if (base > 36 && *nptr <= '_') {
/* base > 36: A-Z,@,_ are 36-61,62,63 */