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blob_share_commitment_rules.go
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blob_share_commitment_rules.go
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package inclusion
import (
"fmt"
"math"
"golang.org/x/exp/constraints"
)
// BlobSharesUsedNonInteractiveDefaults returns the number of shares used by a
// given set of blobs share lengths. It follows the blob share commitment rules
// and returns the total shares used and share indexes for each blob.
func BlobSharesUsedNonInteractiveDefaults(cursor, subtreeRootThreshold int, blobShareLens ...int) (sharesUsed int, indexes []uint32) {
start := cursor
indexes = make([]uint32, len(blobShareLens))
for i, blobLen := range blobShareLens {
cursor = NextShareIndex(cursor, blobLen, subtreeRootThreshold)
indexes[i] = uint32(cursor)
cursor += blobLen
}
return cursor - start, indexes
}
// NextShareIndex determines the next index in a square that can be used. It
// follows the blob share commitment rules defined in ADR-013. Assumes that all
// args are non negative, that squareSize is a power of two and that the blob can
// fit in the square. The cursor is expected to be the index after the end of
// the previous blob.
//
// See https://github.com/celestiaorg/celestia-app/blob/main/specs/src/specs/data_square_layout.md
// for more information.
func NextShareIndex(cursor, blobShareLen, subtreeRootThreshold int) int {
// Calculate the subtreewidth. This is the width of the first mountain in the
// merkle mountain range that makes up the blob share commitment (given the
// subtreeRootThreshold and the BlobMinSquareSize).
treeWidth := SubTreeWidth(blobShareLen, subtreeRootThreshold)
// Round up the cursor to the next multiple of treeWidth. For example, if
// the cursor was at 13 and the tree width is 4, return 16.
return RoundUpByMultipleOf(cursor, treeWidth)
}
// RoundUpByMultipleOf rounds cursor up to the next multiple of v. If cursor is divisible
// by v, then it returns cursor.
func RoundUpByMultipleOf(cursor, v int) int {
if cursor%v == 0 {
return cursor
}
return ((cursor / v) + 1) * v
}
// RoundUpPowerOfTwo returns the next power of two greater than or equal to input.
func RoundUpPowerOfTwo[I constraints.Integer](input I) I {
var result I = 1
for result < input {
result <<= 1
}
return result
}
// RoundDownPowerOfTwo returns the next power of two less than or equal to input.
func RoundDownPowerOfTwo[I constraints.Integer](input I) (I, error) {
if input <= 0 {
return 0, fmt.Errorf("input %v must be positive", input)
}
roundedUp := RoundUpPowerOfTwo(input)
if roundedUp == input {
return roundedUp, nil
}
return roundedUp / 2, nil
}
// BlobMinSquareSize returns the minimum square size that can contain shareCount
// number of shares.
func BlobMinSquareSize(shareCount int) int {
return RoundUpPowerOfTwo(int(math.Ceil(math.Sqrt(float64(shareCount)))))
}
// SubTreeWidth returns the maximum number of leaves per subtree in the share
// commitment over a given blob. The input should be the total number of shares
// used by that blob. See ADR-013.
func SubTreeWidth(shareCount, subtreeRootThreshold int) int {
// Per ADR-013, we use a predetermined threshold to determine width of sub
// trees used to create share commitments
s := (shareCount / subtreeRootThreshold)
// round up if the width is not an exact multiple of the threshold
if shareCount%subtreeRootThreshold != 0 {
s++
}
// use a power of two equal to or larger than the multiple of the subtree
// root threshold
s = RoundUpPowerOfTwo(s)
// use the minimum of the subtree width and the min square size, this
// gurarantees that a valid value is returned
return min(s, BlobMinSquareSize(shareCount))
}
func min[T constraints.Integer](i, j T) T {
if i < j {
return i
}
return j
}