in src/detail/encode_integer.cpp [55:73]
void encode_negative_integer_32(encode_context &context, int32_t value) {
#define ENCODE(bits, digits) do { \
using type = int_fast ## bits ## _t; \
return negative<type, digits>::encode(context, type(value)); \
} while (false)
#define CUTOFF(bits, digits, cutoff) if (value > cutoff ## LL) 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);
ENCODE(32, 10);
#undef CUTOFF
#undef ENCODE
}