Function core::num::flt2dec::estimate_max_buf_len

fn estimate_max_buf_len(exp: i16) -> usize

🔬This is a nightly-only experimental API. (flt2dec)

Returns a rather crude approximation (upper bound) for the maximum buffer size calculated from the given decoded exponent.

The exact limit is:

• when exp < 0, the maximum length is ceil(log_10 (5^-exp * (2^64 - 1))).
• when exp >= 0, the maximum length is ceil(log_10 (2^exp * (2^64 - 1))).

ceil(log_10 (x^exp * (2^64 - 1))) is less than ceil(log_10 (2^64 - 1)) + ceil(exp * log_10 x), which is in turn less than 20 + (1 + exp * log_10 x). We use the facts that log_10 2 < 5/16 and log_10 5 < 12/16, which is enough for our purposes.

Why do we need this? format_exact functions will fill the entire buffer unless limited by the last digit restriction, but it is possible that the number of digits requested is ridiculously large (say, 30,000 digits). The vast majority of buffer will be filled with zeroes, so we don’t want to allocate all the buffer beforehand. Consequently, for any given arguments, 826 bytes of buffer should be sufficient for f64. Compare this with the actual number for the worst case: 770 bytes (when exp = -1074).