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cleaner log calc algorithm

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fireice-uk 2016-12-22 20:29:41 +00:00
parent dd580d7bc7
commit 474c249c90
1 changed files with 18 additions and 20 deletions
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38
src/crypto/tree-hash.c
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@ -40,27 +40,28 @@
#include <stdlib.h>
#endif
/// Quick check if this is power of two (use on unsigned types; in this case for size_t only)
bool ispowerof2_size_t(size_t x) {
return x && !(x & (x - 1));
}
/***
* Round to power of two, for count>=3 and for count being not too large (as reasonable for tree hash calculations)
*/
size_t tree_hash_cnt(size_t count) {
assert( count >= 3); // cases for 0,1,2 are handled elsewhere
// Round down the count size: fun(2**n)= 2**(n-1) to round down to power of two
size_t tmp = count - 1;
size_t jj = 1;
for (jj=1 ; tmp != 0 ; ++jj) {
tmp /= 2; // dividing by 2 until to get how many powers of 2 fits size_to tmp
}
size_t cnt = 1 << (jj-2); // cnt is the count, but rounded down to power of two
// printf("count=%zu cnt=%zu jj=%zu tmp=%zu \n" , count,cnt,jj,tmp);
assert( cnt > 0 ); assert( cnt >= count/2 ); assert( cnt <= count );
assert( ispowerof2_size_t( cnt ));
return cnt;
// This algo has some bad history but all we are doing is 1 << floor(log2(count))
// There are _many_ ways to do log2, for some reason the one selected was the most obscure one,
// and fixing it made it even more obscure.
//
// Iterative method implemented below aims for clarity over speed, if performance is needed
// then my advice is to use the BSR instruction on x86
//
// All the paranoid asserts have been removed since it is trivial to mathematically prove that
// the return will always be a power of 2.
// Problem space has been defined as 3 <= count <= 2^28. Of course quarter of a billion transactions
// is not a sane upper limit for a block, so there will be tighter limits in other parts of the code
assert( count >= 3 ); // cases for 0,1,2 are handled elsewhere
assert( count <= 0x10000000 ); // sanity limit to 2^28, MSB=1 will cause an inf loop
size_t pow = 2;
while(pow < count) pow <<= 1;
return pow >> 1;
}
void tree_hash(const char (*hashes)[HASH_SIZE], size_t count, char *root_hash) {
@ -86,9 +87,6 @@ void tree_hash(const char (*hashes)[HASH_SIZE], size_t count, char *root_hash) {
size_t i, j;
size_t cnt = tree_hash_cnt( count );
size_t max_size_t = (size_t) -1; // max allowed value of size_t
assert( cnt < max_size_t/2 ); // reasonable size to avoid any overflows. /2 is extra; Anyway should be limited much stronger by logical code
// as we have sane limits on transactions counts in blockchain rules
char (*ints)[HASH_SIZE];
size_t ints_size = cnt * HASH_SIZE;