in src/detail/encode_integer.cpp [124:152]
void encode_positive_integer_64(encode_context &context, uint64_t value) {
#define ENCODE(bits, digits) do { \
using type = uint_fast ## bits ## _t; \
return positive<type, digits>::encode(context, type(value)); \
} while (false)
#define CUTOFF(bits, digits, cutoff) if (value < cutoff ## ULL) ENCODE(bits, digits);
CUTOFF(32, 1, 10);
CUTOFF(32, 2, 100);
CUTOFF(32, 3, 1000);
CUTOFF(32, 4, 10000);
CUTOFF(32, 5, 100000);
CUTOFF(32, 6, 1000000);
CUTOFF(32, 7, 10000000);
CUTOFF(32, 8, 100000000);
CUTOFF(32, 9, 1000000000);
CUTOFF(64, 10, 10000000000);
CUTOFF(64, 11, 100000000000);
CUTOFF(64, 12, 1000000000000);
CUTOFF(64, 13, 10000000000000);
CUTOFF(64, 14, 100000000000000);
CUTOFF(64, 15, 1000000000000000);
CUTOFF(64, 16, 10000000000000000);
CUTOFF(64, 17, 100000000000000000);
CUTOFF(64, 18, 1000000000000000000);
CUTOFF(64, 19, 10000000000000000000);
ENCODE(64, 20);
#undef CUTOFF
#undef ENCODE
}