modify difficultyRatio impl to mirror dcrd's

internal/rpc/jsonrpc/methods.go

@@ -1558,10 +1558,21 @@ func createVoutList(mtx *wire.MsgTx, chainParams *chaincfg.Params, filterAddrMap
 // difficultyRatio returns the proof-of-work difficulty as a multiple of the
 // minimum difficulty using the passed bits field from the header of a block.
 func difficultyRatio(bits uint32, params *chaincfg.Params) float64 {
+	// The minimum difficulty is the max possible proof-of-work limit bits
+	// converted back to a number.  Note this is not the same as the proof
+	// of work limit directly because the block difficulty is encoded in a
+	// block with the compact form which loses precision.
 	max := blockchain.CompactToBig(params.PowLimitBits)
 	target := blockchain.CompactToBig(bits)
-	ratio, _ := new(big.Rat).SetFrac(max, target).Float64()
-	return ratio
+
+	difficulty := new(big.Rat).SetFrac(max, target)
+	outString := difficulty.FloatString(8)
+	diff, err := strconv.ParseFloat(outString, 64)
+	if err != nil {
+		log.Errorf("Cannot get difficulty: %v", err)
+		return 0
+	}
+	return diff
 }
 
 // getInfo handles a getinfo request by returning a structure containing