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

1 files changed, 2 insertions, 3 deletions

diff --git a/swarm/services/swap/swap/swap.go b/swarm/services/swap/swap/swap.go
index d04194960..a78f1f0e2 100644
--- a/swarm/services/swap/swap/swap.go
+++ b/swarm/services/swap/swap/swap.go
@@ -22,7 +22,6 @@ import (
 	"sync"
 	"time"
 
-	"github.com/ethereum/go-ethereum/common"
 	"github.com/ethereum/go-ethereum/log"
 )
 
@@ -120,11 +119,11 @@ func (self *Swap) SetRemote(remote *Profile) {
 	self.lock.Lock()
 
 	self.remote = remote
-	if self.Sells && (remote.BuyAt.Cmp(common.Big0) <= 0 || self.local.SellAt.Cmp(common.Big0) <= 0 || remote.BuyAt.Cmp(self.local.SellAt) < 0) {
+	if self.Sells && (remote.BuyAt.Sign() <= 0 || self.local.SellAt.Sign() <= 0 || remote.BuyAt.Cmp(self.local.SellAt) < 0) {
 		self.Out.Stop()
 		self.Sells = false
 	}
-	if self.Buys && (remote.SellAt.Cmp(common.Big0) <= 0 || self.local.BuyAt.Cmp(common.Big0) <= 0 || self.local.BuyAt.Cmp(self.remote.SellAt) < 0) {
+	if self.Buys && (remote.SellAt.Sign() <= 0 || self.local.BuyAt.Sign() <= 0 || self.local.BuyAt.Cmp(self.remote.SellAt) < 0) {
 		self.In.Stop()
 		self.Buys = false
 	}