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extra: avoid allocation in MultiScalarMult with two inputs #55

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Merged
FiloSottile merged 1 commit into from
Feb 17, 2026
Merged

extra: avoid allocation in MultiScalarMult with two inputs #55

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7 changes: 5 additions & 2 deletions extra.go
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Original file line number Diff line number Diff line change
Expand Up @@ -9,6 +9,7 @@ package edwards25519

import (
"errors"
"slices"

"filippo.io/edwards25519/field"
)
Expand Down Expand Up @@ -252,12 +253,14 @@ func (v *Point) MultiScalarMult(scalars []*Scalar, points []*Point) *Point {
// between each point in the multiscalar equation.

// Build lookup tables for each point
tables := make([]projLookupTable, len(points))
tables := make([]projLookupTable, 0, 2) // avoid allocation for small sizes
tables = slices.Grow(tables, len(points))[:len(points)]
for i := range tables {
tables[i].FromP3(points[i])
}
// Compute signed radix-16 digits for each scalar
digits := make([][64]int8, len(scalars))
digits := make([][64]int8, 0, 2) // avoid allocation for small sizes
digits = slices.Grow(digits, len(scalars))[:len(scalars)]
for i := range digits {
digits[i] = scalars[i].signedRadix16()
}
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9 changes: 9 additions & 0 deletions extra_test.go
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Expand Up @@ -169,6 +169,15 @@ func TestVarTimeMultiScalarMultMatchesBaseMult(t *testing.T) {
}
}

func TestMultiScalarMult2NoAllocs(t *testing.T) {
p := NewIdentityPoint()
if allocs := testing.AllocsPerRun(100, func() {
p.MultiScalarMult([]*Scalar{dalekScalar, dalekScalar}, []*Point{B, B})
}); allocs != 0 {
t.Errorf("MultiScalarMult allocated %v times, expected 0", allocs)
}
}

func TestScalarMultSlowMatchesMult(t *testing.T) {
scalarMultSlowMatchesMult := func(x, y Scalar) bool {
p := NewGeneratorPoint()
Expand Down