Add assertions to get_target

lib/blockchain.py

@@ -50,7 +50,11 @@ class Blockchain(util.PrintError):
                 self.print_error("prev hash mismatch: %s vs %s"
                                  % (prev_hash, header.get('prev_block_hash')))
                 return False
-            bits, target = self.get_target(height/2016, chain)
+            try:
+                bits, target = self.get_target(height/2016, chain)
+            except AssertionError as e:
+                self.print_error("target calculation error: %s" % (str(e)))
+                return False
             if bits != header.get('bits'):
                 self.print_error("bits mismatch: %s vs %s"
                                  % (bits, header.get('bits')))
@@ -190,36 +194,33 @@ class Blockchain(util.PrintError):
                 if h.get('block_height') == index*2016-1:
                     last = h
 
+        # bits to target
+        bits = last.get('bits')
+        bitsN = (bits >> 24) & 0xff
+        assert bitsN >= 0x03 and bitsN <= 0x1d, "First part of bits should be in [0x03, 0x1d]"
+        bitsBase = bits & 0xffffff
+        assert bitsN >= 0x8000 and bitsN <= 0x7fffff, "Second part of bits should be in [0x8000, 0x7fffff]"
+        target = bitsBase << (8*(bitsN-3))
+
+        # new target
         nActualTimespan = last.get('timestamp') - first.get('timestamp')
         nTargetTimespan = 14*24*60*60
         nActualTimespan = max(nActualTimespan, nTargetTimespan/4)
         nActualTimespan = min(nActualTimespan, nTargetTimespan*4)
+        new_target = min(max_target, (target * nActualTimespan)/nTargetTimespan)
 
-        bits = last.get('bits')
-        # convert to bignum
-        MM = 256*256*256
-        a = bits%MM
-        if a < 0x8000:
-            a *= 256
-        target = (a) * pow(2, 8 * (bits/MM - 3))
-
-        # new target
-        new_target = min( max_target, (target * nActualTimespan)/nTargetTimespan )
-
-        # convert it to bits
-        c = ("%064X"%new_target)[2:]
-        i = 31
-        while c[0:2]=="00":
+        # convert new target to bits
+        c = ("%064x" % new_target)[2:]
+        while c[:2] == '00' and len(c) > 6:
             c = c[2:]
-            i -= 1
 
-        c = int('0x'+c[0:6],16)
-        if c >= 0x800000:
-            c /= 256
-            i += 1
+        bitsN, bitsBase = len(c)/2, int('0x'+c[:6], 16)
+        if bitsBase >= 0x800000:
+            bitsN += 1
+            bitsBase >>= 8
 
-        new_bits = c + MM * i
-        return new_bits, new_target
+        new_bits = bitsN << 24 | bitsBase
+        return new_bits, bitsBase << (8*(bitsN-3))
 
     def connect_header(self, chain, header):
         '''Builds a header chain until it connects.  Returns True if it has