package zkgm

import (
	"errors"
	"math"

	u256 "gno.land/p/onbloc/math/uint256"
)

// ErrAmountOverflow is returned when a (downscaled) amount does not fit int64,
// the width of the grc20/banker ledger.
var ErrAmountOverflow = errors.New("zkgm: amount overflows int64")

// ErrAmountNotDivisible is returned when a source-precision amount carries precision
// below the ledger's canonical scale,
// so dividing by the token's scale factor leaves a remainder.
// It is rejected so cross-chain value is conserved exactly.
var ErrAmountNotDivisible = errors.New("zkgm: amount not divisible by token scale factor")

// Pow10 returns 10^exp as a uint256.
// exp is small (a token's decimal excess over the canonical precision),
// so float pow is exact and sufficient.
func Pow10(exp int) *u256.Uint {
	result := math.Pow10(exp)

	return u256.NewUint(uint64(result))
}

// ScaleDownToInt64 narrows an origin-precision amount to an int64 ledger amount
// by dividing by 10^scaleExp.
// scaleExp <= 0 is the identity narrow, used by native and low-decimal tokens.
func ScaleDownToInt64(amount *u256.Uint, scaleExp int) (int64, error) {
	value := amount

	if scaleExp > 0 {
		quotient, remainder := u256.Zero().DivMod(amount, Pow10(scaleExp), u256.Zero())
		if !remainder.IsZero() {
			return 0, ErrAmountNotDivisible
		}

		value = quotient
	}

	valueUint64, overflow := value.Uint64WithOverflow()
	if overflow || valueUint64 > uint64(math.MaxInt64) {
		return 0, ErrAmountOverflow
	}

	return int64(valueUint64), nil
}