blockchain+chaincfg: disable retargeting for regtest

This commit emulates the behavior of Bitcoin Core introduced in
bitcoin/bitcoin#6853 that disables retargeting
of the required proof of work for regtest.

blockchain/difficulty.go

@@ -42,18 +42,21 @@ func HashToBig(hash *chainhash.Hash) *big.Int {
 // Like IEEE754 floating point, there are three basic components: the sign,
 // the exponent, and the mantissa.  They are broken out as follows:
 //
-//	* the most significant 8 bits represent the unsigned base 256 exponent
-// 	* bit 23 (the 24th bit) represents the sign bit
-//	* the least significant 23 bits represent the mantissa
+//   - the most significant 8 bits represent the unsigned base 256 exponent
 //
-//	-------------------------------------------------
-//	|   Exponent     |    Sign    |    Mantissa     |
-//	-------------------------------------------------
-//	| 8 bits [31-24] | 1 bit [23] | 23 bits [22-00] |
-//	-------------------------------------------------
+//   - bit 23 (the 24th bit) represents the sign bit
+//
+//   - the least significant 23 bits represent the mantissa
+//
+//     -------------------------------------------------
+//     |   Exponent     |    Sign    |    Mantissa     |
+//     -------------------------------------------------
+//     | 8 bits [31-24] | 1 bit [23] | 23 bits [22-00] |
+//     -------------------------------------------------
 //
 // The formula to calculate N is:
-// 	N = (-1^sign) * mantissa * 256^(exponent-3)
+//
+//	N = (-1^sign) * mantissa * 256^(exponent-3)
 //
 // This compact form is only used in bitcoin to encode unsigned 256-bit numbers
 // which represent difficulty targets, thus there really is not a need for a
@@ -218,7 +221,15 @@ func (b *BlockChain) findPrevTestNetDifficulty(startNode *blockNode) uint32 {
 // This function differs from the exported CalcNextRequiredDifficulty in that
 // the exported version uses the current best chain as the previous block node
 // while this function accepts any block node.
-func (b *BlockChain) calcNextRequiredDifficulty(lastNode *blockNode, newBlockTime time.Time) (uint32, error) {
+func (b *BlockChain) calcNextRequiredDifficulty(lastNode *blockNode,
+	newBlockTime time.Time) (uint32, error) {
+
+	// Emulate the same behavior as Bitcoin Core that for regtest there is
+	// no difficulty retargeting.
+	if b.chainParams.PoWNoRetargeting {
+		return b.chainParams.PowLimitBits, nil
+	}
+
 	// Genesis block.
 	if lastNode == nil {
 		return b.chainParams.PowLimitBits, nil

chaincfg/params.go

@@ -183,6 +183,11 @@ type Params struct {
 	// block in compact form.
 	PowLimitBits uint32
 
+	// PoWNoRetargeting defines whether the network has difficulty
+	// retargeting enabled or not. This should only be set to true for
+	// regtest like networks.
+	PoWNoRetargeting bool
+
 	// These fields define the block heights at which the specified softfork
 	// BIP became active.
 	BIP0034Height int32
@@ -432,6 +437,7 @@ var RegressionNetParams = Params{
 	GenesisHash:              &regTestGenesisHash,
 	PowLimit:                 regressionPowLimit,
 	PowLimitBits:             0x207fffff,
+	PoWNoRetargeting:         true,
 	CoinbaseMaturity:         100,
 	BIP0034Height:            100000000, // Not active - Permit ver 1 blocks
 	BIP0065Height:            1351,      // Used by regression tests
@@ -1007,8 +1013,9 @@ func IsBech32SegwitPrefix(prefix string) bool {
 // ErrInvalidHDKeyID error will be returned.
 //
 // Reference:
-//   SLIP-0132 : Registered HD version bytes for BIP-0032
-//   https://github.com/satoshilabs/slips/blob/master/slip-0132.md
+//
+//	SLIP-0132 : Registered HD version bytes for BIP-0032
+//	https://github.com/satoshilabs/slips/blob/master/slip-0132.md
 func RegisterHDKeyID(hdPublicKeyID []byte, hdPrivateKeyID []byte) error {
 	if len(hdPublicKeyID) != 4 || len(hdPrivateKeyID) != 4 {
 		return ErrInvalidHDKeyID