Simple Rules example

This commit enforces two rules
- Size of memo (exactly 8 characters long)
- Memo has to contain all digits.

README.md

@@ -1,2 +1,42 @@
-# incoming-transfer-rules
-On EOS, incoming transfers can have rules applied to them. Exchanges and accounts can enforce their memo fields to stop transfers that don't enter the correct details on memo for example.
+# Transfer filter contract
+EOS has a method to check incoming transfers and This contract makes token transfers to it fail, if memo in those transfers does not
+satisfy certain conditions. Currently set conditions are:
+* Memo has to be exactly 8 symbols long;
+* Contain only digits;
+
+You add different asserts in ontransfer action to modify these conditions.
+
+Only transfers wher contract is receiver are currently filtered.
+
+### Build
+```commandline
+eosiocpp -o transferfilter.wast transferfilter.cpp
+```
+
+### Deploy
+```commandline
+cleos set code <your-account-name> transferfilter.wast
+```
+
+### Test
+#### Withouth memo
+```commandline
+$ cleos transfer eosio <your-account-name> "10.0000 SYS"
+Error 3050003: eosio_assert_message assertion failure
+```
+
+#### With non digit
+```commandline
+$ cleos transfer eosio <your-account-name> "10.0000 SYS" "1234567a"
+Error 3050003: eosio_assert_message assertion failure
+```
+
+#### Valid
+```commandline
+$ cleos transfer eosio <your-account-name> "10.0000 SYS" "00345670"
+executed transaction: 747ceb78a0359ffd426df96d3ce4c844bfe7402e1634638a14ebc2d91c676a91  136 bytes  607 us
+#   eosio.token <= eosio.token::transfer        {"from":"eosio","to":"<your-account-name>","quantity":"10.0000 SYS","memo":"00345670"}
+#         eosio <= eosio.token::transfer        {"from":"eosio","to":"<your-account-name>","quantity":"10.0000 SYS","memo":"00345670"}
+#      <your-account-name> <= eosio.token::transfer        {"from":"eosio","to":"<your-account-name>","quantity":"10.0000 SYS","memo":"00345670"}
+warning: transaction executed locally, but may not be confirmed by the network yet    ]
+```

transferfilter.cpp

@@ -0,0 +1,41 @@
+#include <eosiolib/eosio.hpp>
+#include <eosiolib/asset.hpp>
+#include <cctype>
+
+constexpr account_name token_contract = N(eosio.token);
+
+using namespace eosio;
+
+class transfer_filter : public contract {
+public:
+   transfer_filter(account_name self) : contract(self) {}
+
+   void ontransfer(const account_name from, const account_name to, const asset& quantity,
+                   const std::string& memo) {
+
+      if( to == _self ) {
+         const char* error_str = "Memo has to be exactly 8 symbols long and contain only digits";
+         //Rule 1: Memo has to be 8 characters long
+         eosio_assert(memo.length() == 8, error_str);
+         //Rule 2: Memo has to be all digits
+         eosio_assert(std::all_of(memo.begin(), memo.end(), ::isdigit), error_str);
+      }
+   }
+};
+
+extern "C" {
+void apply(uint64_t receiver, uint64_t code, uint64_t action) {
+
+   auto self = receiver;
+   if( action == N(onerror) ) {
+      /* onerror is only valid if it is for the "eosio" code account and authorized by "eosio"'s "active permission */ \
+      eosio_assert(code == N(eosio),
+                   "onerror action's are only valid from the \"eosio\" system account"); \
+   }
+
+   if( code == token_contract && action == N(transfer) ) {
+      transfer_filter contract(self);
+      eosio::execute_action(&contract, &transfer_filter::ontransfer);
+   }
+}
+}