script: Remove magic numbers

This adds two new constants, MAX_OPS_PER_SCRIPT and
MAX_PUBKEYS_PER_MULTISIG.

src/script/interpreter.cpp

@@ -273,7 +273,7 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                 return set_error(serror, SCRIPT_ERR_PUSH_SIZE);
 
             // Note how OP_RESERVED does not count towards the opcode limit.
-            if (opcode > OP_16 && ++nOpCount > 201)
+            if (opcode > OP_16 && ++nOpCount > MAX_OPS_PER_SCRIPT)
                 return set_error(serror, SCRIPT_ERR_OP_COUNT);
 
             if (opcode == OP_CAT ||
@@ -869,10 +869,10 @@ bool EvalScript(vector<vector<unsigned char> >& stack, const CScript& script, un
                         return set_error(serror, SCRIPT_ERR_INVALID_STACK_OPERATION);
 
                     int nKeysCount = CScriptNum(stacktop(-i), fRequireMinimal).getint();
-                    if (nKeysCount < 0 || nKeysCount > 20)
+                    if (nKeysCount < 0 || nKeysCount > MAX_PUBKEYS_PER_MULTISIG)
                         return set_error(serror, SCRIPT_ERR_PUBKEY_COUNT);
                     nOpCount += nKeysCount;
-                    if (nOpCount > 201)
+                    if (nOpCount > MAX_OPS_PER_SCRIPT)
                         return set_error(serror, SCRIPT_ERR_OP_COUNT);
                     int ikey = ++i;
                     i += nKeysCount;

src/script/script.cpp

@@ -170,7 +170,7 @@ unsigned int CScript::GetSigOpCount(bool fAccurate) const
             if (fAccurate && lastOpcode >= OP_1 && lastOpcode <= OP_16)
                 n += DecodeOP_N(lastOpcode);
             else
-                n += 20;
+                n += MAX_PUBKEYS_PER_MULTISIG;
         }
         lastOpcode = opcode;
     }

src/script/script.h

@@ -17,7 +17,14 @@
 #include <string>
 #include <vector>
 
-static const unsigned int MAX_SCRIPT_ELEMENT_SIZE = 520; // bytes
+// Maximum number of bytes pushable to the stack
+static const unsigned int MAX_SCRIPT_ELEMENT_SIZE = 520;
+
+// Maximum number of non-push operations per script
+static const int MAX_OPS_PER_SCRIPT = 201;
+
+// Maximum number of public keys per multisig
+static const int MAX_PUBKEYS_PER_MULTISIG = 20;
 
 // Threshold for nLockTime: below this value it is interpreted as block number,
 // otherwise as UNIX timestamp.