aece2c5f0
Denys Vlasenko
2021-06-12
Subject
bc: tweak overflow detection in a^b
Binary
No binary (build failed)
Architecture & Compiler Flags
—
Build Log
(log yok)
Commit Diff GitHub →
miscutils/bc.c+8 -5
| @@ -5,7 +5,7 @@ | |
| * Original code copyright (c) 2018 Gavin D. Howard and contributors. | |
| */ | |
| //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_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) | |
| 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) { | |
| bc_num_one(c); | |
| 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, | |
| //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: | |
| - | // if (new_scale >= pow) is false, we had overflow, correct |
| - | // "new_scale" value is larger than ULONG_MAX, thus larger |
| - | // than any possible current value of "scale", |
| + | // if a_rdx != 0 and new_scale < pow, we had overflow, |
| + | // correct "new_scale" value is larger than ULONG_MAX, |
| + | // thus larger than any possible current value of "scale", |
| // thus "scale = new_scale" should not be done: | |
| - | if (new_scale >= pow) |
| + | if (a_rdx == 0 || new_scale >= pow) |
| if (new_scale < scale) | |
| scale = new_scale; | |
| } | |