Add ParseRange function to parse args of the form int/[int,int]

Author: sipa

src/rpc/util.cpp

@@ -523,3 +523,17 @@ std::string RPCArg::ToString(const bool oneline) const
     }
     assert(false);
 }
+
+std::pair<int64_t, int64_t> ParseRange(const UniValue& value)
+{
+    if (value.isNum()) {
+        return {0, value.get_int64()};
+    }
+    if (value.isArray() && value.size() == 2 && value[0].isNum() && value[1].isNum()) {
+        int64_t low = value[0].get_int64();
+        int64_t high = value[1].get_int64();
+        if (low > high) throw JSONRPCError(RPC_INVALID_PARAMETER, "Range specified as [begin,end] must not have begin after end");
+        return {low, high};
+    }
+    throw JSONRPCError(RPC_INVALID_PARAMETER, "Range must be specified as end or as [begin,end]");
+}

src/rpc/util.h

@@ -38,6 +38,9 @@ unsigned int ParseConfirmTarget(const UniValue& value);
 RPCErrorCode RPCErrorFromTransactionError(TransactionError terr);
 UniValue JSONRPCTransactionError(TransactionError terr, const std::string& err_string = "");
 
+//! Parse a JSON range specified as int64, or [int64, int64]
+std::pair<int64_t, int64_t> ParseRange(const UniValue& value);
+
 struct RPCArg {
     enum class Type {
         OBJ,