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