36fbba3e22
This updates the gcs module dependencies, the copyright year in the files modified since the previous release, and serves as a base for gcs/v3.0.0. The updated direct dependencies in this commit are as follows: - github.com/decred/dcrd/blockchain/stake/v4@v4.0.0 The full list of updated direct dependencies since the previous gcs/v2.1.0 release are as follows: - github.com/dchest/siphash@v1.2.2 - github.com/decred/dcrd/blockchain/stake/v4@v4.0.0 - github.com/decred/dcrd/chaincfg/chainhash@v1.0.3 - github.com/decred/dcrd/txscript/v4@v4.0.0 - github.com/decred/dcrd/wire@v1.5.0 Finally, all modules in the repository that depend on the module are tidied to ensure they are updated to use the latest versions hoisted forward as a result.
41 lines
1.5 KiB
Go
41 lines
1.5 KiB
Go
// Copyright (c) 2019-2021 The Decred developers
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// Use of this source code is governed by an ISC
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// license that can be found in the LICENSE file.
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//go:build go1.12
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// +build go1.12
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package gcs
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import (
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"math/bits"
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)
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// fastReduce calculates a mapping that is more or less equivalent to x mod N.
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// However, instead of using a mod operation that can lead to slowness on many
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// processors when not using a power of two due to unnecessary division, this
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// uses a "multiply-and-shift" trick that eliminates all divisions as described
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// in a blog post by Daniel Lemire, located at the following site at the time
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// of this writing:
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// https://lemire.me/blog/2016/06/27/a-fast-alternative-to-the-modulo-reduction/
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//
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// Since that link might disappear, the general idea is to multiply by N and
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// shift right by log2(N). Since N is a 64-bit integer in this case, it
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// becomes:
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//
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// (x * N) / 2^64 == (x * N) >> 64
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//
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// This is a fair map since it maps integers in the range [0,2^64) to multiples
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// of N in [0, N*2^64) and then divides by 2^64 to map all multiples of N in
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// [0,2^64) to 0, all multiples of N in [2^64, 2*2^64) to 1, etc. This results
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// in either ceil(2^64/N) or floor(2^64/N) multiples of N.
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func fastReduce(x, N uint64) uint64 {
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// This uses math/bits to perform the 128-bit multiplication as the compiler
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// will replace it with the relevant intrinsic on most architectures.
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//
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// The high 64 bits in a 128-bit product is the same as shifting the entire
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// product right by 64 bits.
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hi, _ := bits.Mul64(x, N)
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return hi
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}
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