compose.go

package evmprecompiles

import (
	"fmt"
	"math/big"
)

func recompose(inputs []*big.Int, nbBits uint) *big.Int {
	res := new(big.Int)
	if len(inputs) == 0 {
		return res
	}
	for i := range inputs {
		res.Lsh(res, nbBits)
		res.Add(res, inputs[len(inputs)-i-1])
	}
	return res
}

func decompose(input *big.Int, nbBits uint, res []*big.Int) error {
	// limb modulus
	if input.BitLen() > len(res)*int(nbBits) {
		return fmt.Errorf("decomposed integer does not fit into res")
	}
	for _, r := range res {
		if r == nil {
			return fmt.Errorf("result slice element uninitalized")
		}
	}
	base := new(big.Int).Lsh(big.NewInt(1), nbBits)
	tmp := new(big.Int).Set(input)
	for i := 0; i < len(res); i++ {
		res[i].Mod(tmp, base)
		tmp.Rsh(tmp, nbBits)
	}
	return nil
}