all: unify big.Int zero checks, use common/math in more places (#3716)

* common/math: optimize PaddedBigBytes, use it more

name              old time/op    new time/op    delta
PaddedBigBytes-8    71.1ns ± 5%    46.1ns ± 1%  -35.15%  (p=0.000 n=20+19)

name              old alloc/op   new alloc/op   delta
PaddedBigBytes-8     48.0B ± 0%     32.0B ± 0%  -33.33%  (p=0.000 n=20+20)

* all: unify big.Int zero checks

Various checks were in use. This commit replaces them all with Int.Sign,
which is cheaper and less code.

eg templates:

    func before(x *big.Int) bool { return x.BitLen() == 0 }
    func after(x *big.Int) bool  { return x.Sign() == 0 }

    func before(x *big.Int) bool { return x.BitLen() > 0 }
    func after(x *big.Int) bool  { return x.Sign() != 0 }

    func before(x *big.Int) int { return x.Cmp(common.Big0) }
    func after(x *big.Int) int  { return x.Sign() }

* common/math, crypto/secp256k1: make ReadBits public in package math

2 files changed, 4 insertions, 4 deletions

diff --git a/contracts/chequebook/cheque.go b/contracts/chequebook/cheque.go
index 9a2a5a2a6..bae6f3393 100644
--- a/contracts/chequebook/cheque.go
+++ b/contracts/chequebook/cheque.go
@@ -247,7 +247,7 @@ func (self *Chequebook) Issue(beneficiary common.Address, amount *big.Int) (ch *
 	defer self.lock.Unlock()
 	self.lock.Lock()
 
-	if amount.Cmp(common.Big0) <= 0 {
+	if amount.Sign() <= 0 {
 		return nil, fmt.Errorf("amount must be greater than zero (%v)", amount)
 	}
 	if self.balance.Cmp(amount) < 0 {
@@ -515,7 +515,7 @@ func (self *Inbox) autoCash(cashInterval time.Duration) {
 		self.quit = nil
 	}
 	// if maxUncashed is set to 0, then autocash on receipt
-	if cashInterval == time.Duration(0) || self.maxUncashed != nil && self.maxUncashed.Cmp(common.Big0) == 0 {
+	if cashInterval == time.Duration(0) || self.maxUncashed != nil && self.maxUncashed.Sign() == 0 {
 		return
 	}
 
@@ -597,7 +597,7 @@ func (self *Cheque) Verify(signerKey *ecdsa.PublicKey, contract, beneficiary com
 	amount := new(big.Int).Set(self.Amount)
 	if sum != nil {
 		amount.Sub(amount, sum)
-		if amount.Cmp(common.Big0) <= 0 {
+		if amount.Sign() <= 0 {
 			return nil, fmt.Errorf("incorrect amount: %v <= 0", amount)
 		}
 	}
diff --git a/contracts/chequebook/cheque_test.go b/contracts/chequebook/cheque_test.go
index 85d923109..4029fd549 100644
--- a/contracts/chequebook/cheque_test.go
+++ b/contracts/chequebook/cheque_test.go
@@ -88,7 +88,7 @@ func TestIssueAndReceive(t *testing.T) {
 		t.Fatalf("expected no error, got %v", err)
 	}
 
-	if chbook.Balance().Cmp(common.Big0) != 0 {
+	if chbook.Balance().Sign() != 0 {
 		t.Errorf("expected: %v, got %v", "0", chbook.Balance())
 	}