qt: Network-specific example address

Generate an (invalid) example address for in the bitcoin address
widgets, based on the network prefix, instead of hardcoding a mainnet
address.

- `1NS17iag9jJgTHD1VXjvLCEnZuQ3rJDE9L` for mainnet (same as now)
- `n2wxQmfexkjwEPgdD6iJA7T7RtzkmHxhFc` for testnet

src/qt/guiutil.cpp

@@ -107,6 +107,23 @@ QFont fixedPitchFont()
 #endif
 }
 
+// Just some dummy data to generate an convincing random-looking (but consistent) address
+static const uint8_t dummydata[] = {0xeb,0x15,0x23,0x1d,0xfc,0xeb,0x60,0x92,0x58,0x86,0xb6,0x7d,0x06,0x52,0x99,0x92,0x59,0x15,0xae,0xb1,0x72,0xc0,0x66,0x47};
+
+// Generate a dummy address with invalid CRC, starting with the network prefix.
+static std::string DummyAddress(const CChainParams &params)
+{
+    std::vector<unsigned char> sourcedata = params.Base58Prefix(CChainParams::PUBKEY_ADDRESS);
+    sourcedata.insert(sourcedata.end(), dummydata, dummydata + sizeof(dummydata));
+    for(int i=0; i<256; ++i) { // Try every trailing byte
+        std::string s = EncodeBase58(begin_ptr(sourcedata), end_ptr(sourcedata));
+        if (!CBitcoinAddress(s).IsValid())
+            return s;
+        sourcedata[sourcedata.size()-1] += 1;
+    }
+    return "";
+}
+
 void setupAddressWidget(QValidatedLineEdit *widget, QWidget *parent)
 {
     parent->setFocusProxy(widget);
@@ -115,7 +132,8 @@ void setupAddressWidget(QValidatedLineEdit *widget, QWidget *parent)
 #if QT_VERSION >= 0x040700
     // We don't want translators to use own addresses in translations
     // and this is the only place, where this address is supplied.
-    widget->setPlaceholderText(QObject::tr("Enter a Bitcoin address (e.g. %1)").arg("1NS17iag9jJgTHD1VXjvLCEnZuQ3rJDE9L"));
+    widget->setPlaceholderText(QObject::tr("Enter a Bitcoin address (e.g. %1)").arg(
+        QString::fromStdString(DummyAddress(Params()))));
 #endif
     widget->setValidator(new BitcoinAddressEntryValidator(parent));
     widget->setCheckValidator(new BitcoinAddressCheckValidator(parent));