common: move big integer math to common/math (#3699)

* common: remove CurrencyToString

Move denomination values to params instead.

* common: delete dead code

* common: move big integer operations to common/math

This commit consolidates all big integer operations into common/math and
adds tests and documentation.

There should be no change in semantics for BigPow, BigMin, BigMax, S256,
U256, Exp and their behaviour is now locked in by tests.

The BigD, BytesToBig and Bytes2Big functions don't provide additional
value, all uses are replaced by new(big.Int).SetBytes().

BigToBytes is now called PaddedBigBytes, its minimum output size
parameter is now specified as the number of bytes instead of bits. The
single use of this function is in the EVM's MSTORE instruction.

Big and String2Big are replaced by ParseBig, which is slightly stricter.
It previously accepted leading zeros for hexadecimal inputs but treated
decimal inputs as octal if a leading zero digit was present.

ParseUint64 is used in places where String2Big was used to decode a
uint64.

The new functions MustParseBig and MustParseUint64 are now used in many
places where parsing errors were previously ignored.

* common: delete unused big integer variables

* accounts/abi: replace uses of BytesToBig with use of encoding/binary

* common: remove BytesToBig

* common: remove Bytes2Big

* common: remove BigTrue

* cmd/utils: add BigFlag and use it for error-checked integer flags

While here, remove environment variable processing for DirectoryFlag
because we don't use it.

* core: add missing error checks in genesis block parser

* common: remove String2Big

* cmd/evm: use utils.BigFlag

* common/math: check for 256 bit overflow in ParseBig

This is supposed to prevent silent overflow/truncation of values in the
genesis block JSON. Without this check, a genesis block that set a
balance larger than 256 bits would lead to weird behaviour in the VM.

* cmd/utils: fixup import

1 files changed, 7 insertions, 133 deletions

diff --git a/common/big.go b/common/big.go
index 4ce87ee0c..b552608bc 100644
--- a/common/big.go
+++ b/common/big.go
@@ -20,137 +20,11 @@ import "math/big"
 
 // Common big integers often used
 var (
-	Big1     = big.NewInt(1)
-	Big2     = big.NewInt(2)
-	Big3     = big.NewInt(3)
-	Big0     = big.NewInt(0)
-	BigTrue  = Big1
-	BigFalse = Big0
-	Big32    = big.NewInt(32)
-	Big36    = big.NewInt(36)
-	Big97    = big.NewInt(97)
-	Big98    = big.NewInt(98)
-	Big256   = big.NewInt(0xff)
-	Big257   = big.NewInt(257)
-	MaxBig   = String2Big("0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff")
+	Big1   = big.NewInt(1)
+	Big2   = big.NewInt(2)
+	Big3   = big.NewInt(3)
+	Big0   = big.NewInt(0)
+	Big32  = big.NewInt(32)
+	Big256 = big.NewInt(0xff)
+	Big257 = big.NewInt(257)
 )
-
-// Big pow
-//
-// Returns the power of two big integers
-func BigPow(a, b int) *big.Int {
-	c := new(big.Int)
-	c.Exp(big.NewInt(int64(a)), big.NewInt(int64(b)), big.NewInt(0))
-
-	return c
-}
-
-// Big
-//
-// Shortcut for new(big.Int).SetString(..., 0)
-func Big(num string) *big.Int {
-	n := new(big.Int)
-	n.SetString(num, 0)
-
-	return n
-}
-
-// Bytes2Big
-//
-func BytesToBig(data []byte) *big.Int {
-	n := new(big.Int)
-	n.SetBytes(data)
-
-	return n
-}
-func Bytes2Big(data []byte) *big.Int { return BytesToBig(data) }
-func BigD(data []byte) *big.Int      { return BytesToBig(data) }
-
-func String2Big(num string) *big.Int {
-	n := new(big.Int)
-	n.SetString(num, 0)
-	return n
-}
-
-func BitTest(num *big.Int, i int) bool {
-	return num.Bit(i) > 0
-}
-
-// To256
-//
-// "cast" the big int to a 256 big int (i.e., limit to)
-var tt256 = new(big.Int).Lsh(big.NewInt(1), 256)
-var tt256m1 = new(big.Int).Sub(new(big.Int).Lsh(big.NewInt(1), 256), big.NewInt(1))
-var tt255 = new(big.Int).Lsh(big.NewInt(1), 255)
-
-func U256(x *big.Int) *big.Int {
-	//if x.Cmp(Big0) < 0 {
-	//		return new(big.Int).Add(tt256, x)
-	//	}
-
-	x.And(x, tt256m1)
-
-	return x
-}
-
-func S256(x *big.Int) *big.Int {
-	if x.Cmp(tt255) < 0 {
-		return x
-	} else {
-		// We don't want to modify x, ever
-		return new(big.Int).Sub(x, tt256)
-	}
-}
-
-func FirstBitSet(v *big.Int) int {
-	for i := 0; i < v.BitLen(); i++ {
-		if v.Bit(i) > 0 {
-			return i
-		}
-	}
-
-	return v.BitLen()
-}
-
-// Big to bytes
-//
-// Returns the bytes of a big integer with the size specified by **base**
-// Attempts to pad the byte array with zeros.
-func BigToBytes(num *big.Int, base int) []byte {
-	ret := make([]byte, base/8)
-
-	if len(num.Bytes()) > base/8 {
-		return num.Bytes()
-	}
-
-	return append(ret[:len(ret)-len(num.Bytes())], num.Bytes()...)
-}
-
-// Big copy
-//
-// Creates a copy of the given big integer
-func BigCopy(src *big.Int) *big.Int {
-	return new(big.Int).Set(src)
-}
-
-// Big max
-//
-// Returns the maximum size big integer
-func BigMax(x, y *big.Int) *big.Int {
-	if x.Cmp(y) < 0 {
-		return y
-	}
-
-	return x
-}
-
-// Big min
-//
-// Returns the minimum size big integer
-func BigMin(x, y *big.Int) *big.Int {
-	if x.Cmp(y) > 0 {
-		return y
-	}
-
-	return x
-}