well-goknown/vendor/github.com/decred/dcrd/dcrec/secp256k1/v4/loadprecomputed.go

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// Copyright 2015 The btcsuite developers
// Copyright (c) 2015-2022 The Decred developers
// Use of this source code is governed by an ISC
// license that can be found in the LICENSE file.
package secp256k1
import (
"compress/zlib"
"encoding/base64"
"io"
"strings"
"sync"
)
//go:generate go run genprecomps.go
// bytePointTable describes a table used to house pre-computed values for
// accelerating scalar base multiplication.
type bytePointTable [32][256]JacobianPoint
// compressedBytePointsFn is set to a real function by the code generation to
// return the compressed pre-computed values for accelerating scalar base
// multiplication.
var compressedBytePointsFn func() string
// s256BytePoints houses pre-computed values used to accelerate scalar base
// multiplication such that they are only loaded on first use.
var s256BytePoints = func() func() *bytePointTable {
// mustLoadBytePoints decompresses and deserializes the pre-computed byte
// points used to accelerate scalar base multiplication for the secp256k1
// curve.
//
// This approach is used since it allows the compile to use significantly
// less ram and be performed much faster than it is with hard-coding the
// final in-memory data structure. At the same time, it is quite fast to
// generate the in-memory data structure on first use with this approach
// versus computing the table.
//
// It will panic on any errors because the data is hard coded and thus any
// errors means something is wrong in the source code.
var data *bytePointTable
mustLoadBytePoints := func() {
// There will be no byte points to load when generating them.
if compressedBytePointsFn == nil {
return
}
bp := compressedBytePointsFn()
// Decompress the pre-computed table used to accelerate scalar base
// multiplication.
decoder := base64.NewDecoder(base64.StdEncoding, strings.NewReader(bp))
r, err := zlib.NewReader(decoder)
if err != nil {
panic(err)
}
serialized, err := io.ReadAll(r)
if err != nil {
panic(err)
}
// Deserialize the precomputed byte points and set the memory table to
// them.
offset := 0
var bytePoints bytePointTable
for byteNum := 0; byteNum < len(bytePoints); byteNum++ {
// All points in this window.
for i := 0; i < len(bytePoints[byteNum]); i++ {
p := &bytePoints[byteNum][i]
p.X.SetByteSlice(serialized[offset:])
offset += 32
p.Y.SetByteSlice(serialized[offset:])
offset += 32
p.Z.SetInt(1)
}
}
data = &bytePoints
}
// Return a closure that initializes the data on first access. This is done
// because the table takes a non-trivial amount of memory and initializing
// it unconditionally would cause anything that imports the package, either
// directly, or indirectly via transitive deps, to use that memory even if
// the caller never accesses any parts of the package that actually needs
// access to it.
var loadBytePointsOnce sync.Once
return func() *bytePointTable {
loadBytePointsOnce.Do(mustLoadBytePoints)
return data
}
}()