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// Copyright 2015 The Rust Project Developers. See the COPYRIGHT // file at the top-level directory of this distribution and at // http://rust-lang.org/COPYRIGHT. // // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your // option. This file may not be copied, modified, or distributed // except according to those terms. //! The exponent estimator. /// Finds `k_0` such that `10^(k_0-1) < mant * 2^exp <= 10^(k_0+1)`. /// /// This is used to approximate `k = ceil(log_10 (mant * 2^exp))`; /// the true `k` is either `k_0` or `k_0+1`. #[doc(hidden)] pub fn estimate_scaling_factor(mant: u64, exp: i16) -> i16 { // 2^(nbits-1) < mant <= 2^nbits if mant > 0 let nbits = 64 - (mant - 1).leading_zeros() as i64; // 1292913986 = floor(2^32 * log_10 2) // therefore this always underestimates (or is exact), but not much. (((nbits + exp as i64) * 1292913986) >> 32) as i16 }