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-rw-r--r--lib/string_helpers.c63
1 files changed, 43 insertions, 20 deletions
diff --git a/lib/string_helpers.c b/lib/string_helpers.c
index 5939f63..5c88204 100644
--- a/lib/string_helpers.c
+++ b/lib/string_helpers.c
@@ -43,50 +43,73 @@ void string_get_size(u64 size, u64 blk_size, const enum string_size_units units,
[STRING_UNITS_10] = 1000,
[STRING_UNITS_2] = 1024,
};
- int i, j;
- u32 remainder = 0, sf_cap, exp;
+ static const unsigned int rounding[] = { 500, 50, 5 };
+ int i = 0, j;
+ u32 remainder = 0, sf_cap;
char tmp[8];
const char *unit;
tmp[0] = '\0';
- i = 0;
- if (!size)
+
+ if (blk_size == 0)
+ size = 0;
+ if (size == 0)
goto out;
- while (blk_size >= divisor[units]) {
- remainder = do_div(blk_size, divisor[units]);
+ /* This is Napier's algorithm. Reduce the original block size to
+ *
+ * coefficient * divisor[units]^i
+ *
+ * we do the reduction so both coefficients are just under 32 bits so
+ * that multiplying them together won't overflow 64 bits and we keep
+ * as much precision as possible in the numbers.
+ *
+ * Note: it's safe to throw away the remainders here because all the
+ * precision is in the coefficients.
+ */
+ while (blk_size >> 32) {
+ do_div(blk_size, divisor[units]);
i++;
}
- exp = divisor[units] / (u32)blk_size;
- /*
- * size must be strictly greater than exp here to ensure that remainder
- * is greater than divisor[units] coming out of the if below.
- */
- if (size > exp) {
- remainder = do_div(size, divisor[units]);
- remainder *= blk_size;
+ while (size >> 32) {
+ do_div(size, divisor[units]);
i++;
- } else {
- remainder *= size;
}
+ /* now perform the actual multiplication keeping i as the sum of the
+ * two logarithms */
size *= blk_size;
- size += remainder / divisor[units];
- remainder %= divisor[units];
+ /* and logarithmically reduce it until it's just under the divisor */
while (size >= divisor[units]) {
remainder = do_div(size, divisor[units]);
i++;
}
+ /* work out in j how many digits of precision we need from the
+ * remainder */
sf_cap = size;
for (j = 0; sf_cap*10 < 1000; j++)
sf_cap *= 10;
- if (j) {
+ if (units == STRING_UNITS_2) {
+ /* express the remainder as a decimal. It's currently the
+ * numerator of a fraction whose denominator is
+ * divisor[units], which is 1 << 10 for STRING_UNITS_2 */
remainder *= 1000;
- remainder /= divisor[units];
+ remainder >>= 10;
+ }
+
+ /* add a 5 to the digit below what will be printed to ensure
+ * an arithmetical round up and carry it through to size */
+ remainder += rounding[j];
+ if (remainder >= 1000) {
+ remainder -= 1000;
+ size += 1;
+ }
+
+ if (j) {
snprintf(tmp, sizeof(tmp), ".%03u", remainder);
tmp[j+1] = '\0';
}
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