Simplify bitmask creation

Separate clearnet and onion tests

cerberus.go

@@ -13,8 +13,7 @@ import (
 type Challenge struct {
 	Salt  string // Challenge salt from server.
 	Diff  uint32 // Difficulty level.
-	Steps int8   // Each step consists of a Challenge and a Solution. More than 1 may be required.
-	// Stored as a signed int to get past underflow issues later on.
+	Steps uint8  // Each step consists of a Challenge and a Solution. More than 1 may be required.
 
 	host *url.URL
 }
@@ -44,7 +43,12 @@ func checkZeros(diff uint32, hash []byte) bool {
 		rem    = diff % 8         // Remainder after dividing diff (given in bits) to bytes.
 		nbytes = (diff - rem) / 8 // Amount of 0x0 bytes we can divide difficulty bits into.
 
-		mask uint8 // Mask to check remaining bits.
+		// Bitmask by setting a 1 bit (on LHS) for each remaining bit to check.
+		mask uint8 = ^((1 << (8 - rem)) - 1)
+		// Example (rem = 3): mask = ^((1 << (8 - 3)) - 1)
+		//                         = ^(00100000 - 1)
+		//                         = ^(00011111)
+		//                         = 11100000
 	)
 
 	// Check bounds for the loops found below.
@@ -63,15 +67,6 @@ func checkZeros(diff uint32, hash []byte) bool {
 		return true
 	}
 
-	// Create bitmask by setting a bit to 1 for each remaining bit to check.
-	// The mask is built from right-to-left and then shifted to the LHS of the octet.
-	for range rem {
-		mask <<= 1
-		mask += 1
-	}
-	// Shift 1s we just added to the LHS of the octet.
-	mask <<= 8 - rem
-
 	return hash[nbytes]&mask == 0x0
 }