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bc: tweak overflow detection in a^b

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function                                             old     new   delta
zbc_num_p                                            506     510      +4

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
This commit is contained in:
Denys Vlasenko 2021-06-12 14:03:24 +02:00
parent e36c39a46d
commit aece2c5f0f
1 changed files with 8 additions and 5 deletions
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13
miscutils/bc.c
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@ -5,7 +5,7 @@
* Original code copyright (c) 2018 Gavin D. Howard and contributors. * Original code copyright (c) 2018 Gavin D. Howard and contributors.
*/ */
//TODO: //TODO:
// maybe implement a^b for non-integer b? // maybe implement a^b for non-integer b? (see zbc_num_p())
#define DEBUG_LEXER 0 #define DEBUG_LEXER 0
#define DEBUG_COMPILE 0 #define DEBUG_COMPILE 0
@ -2163,6 +2163,9 @@ static FAST_FUNC BC_STATUS zbc_num_p(BcNum *a, BcNum *b, BcNum *restrict c, size
if (b->num[i] != 0) if (b->num[i] != 0)
RETURN_STATUS(bc_error("not an integer")); RETURN_STATUS(bc_error("not an integer"));
// a^b for non-integer b (for a>0) can be implemented as exp(ln(a)*b).
// Possibly better precision would be given by a^int(b) * exp(ln(a)*frac(b)).
if (b->len == 0) { if (b->len == 0) {
bc_num_one(c); bc_num_one(c);
RETURN_STATUS(BC_STATUS_SUCCESS); RETURN_STATUS(BC_STATUS_SUCCESS);
@ -2199,11 +2202,11 @@ static FAST_FUNC BC_STATUS zbc_num_p(BcNum *a, BcNum *b, BcNum *restrict c, size
//but it can be improved to detect zero results etc. Example: with scale=0, //but it can be improved to detect zero results etc. Example: with scale=0,
//result of 0.01^N for any N>1 is 0: 0.01^2 = 0.0001 ~= 0.00 (trunc to scale) //result of 0.01^N for any N>1 is 0: 0.01^2 = 0.0001 ~= 0.00 (trunc to scale)
//then this would matter: //then this would matter:
// if (new_scale >= pow) is false, we had overflow, correct // if a_rdx != 0 and new_scale < pow, we had overflow,
// "new_scale" value is larger than ULONG_MAX, thus larger // correct "new_scale" value is larger than ULONG_MAX,
// than any possible current value of "scale", // thus larger than any possible current value of "scale",
// thus "scale = new_scale" should not be done: // thus "scale = new_scale" should not be done:
if (new_scale >= pow) if (a_rdx == 0 || new_scale >= pow)
if (new_scale < scale) if (new_scale < scale)
scale = new_scale; scale = new_scale;
} }