tier 1

#include <eosio/eosio.hpp>

using namespace std;
using namespace eosio;

CONTRACT test : public contract {
  public:
    using contract::contract;

    ACTION hi(name from, string message);
    ACTION clear();

  private:
    TABLE messages {
      name    user;
      string  text;
      auto primary_key() const { return user.value; }
    };
    typedef multi_index<name("messages"), messages> messages_table;
};

#include <test.hpp>

ACTION test::hi(name from, string message) {
  require_auth(from);

  // Init the _message table
  messages_table _messages(get_self(), get_self().value);

  // Find the record from _messages table
  auto msg_itr = _messages.find(from.value);
  if (msg_itr == _messages.end()) {
    // Create a message record if it does not exist
    _messages.emplace(from, [&](auto& msg) {
      msg.user = from;
      msg.text = message;
    });
  } else {
    // Modify a message record if it exists
    _messages.modify(msg_itr, from, [&](auto& msg) {
      msg.text = message;
    });
  }
}

ACTION test::clear() {
  require_auth(get_self());

  messages_table _messages(get_self(), get_self().value);

  // Delete all records in _messages table
  auto msg_itr = _messages.begin();
  while (msg_itr != _messages.end()) {
    msg_itr = _messages.erase(msg_itr);
  }
}

EOSIO_DISPATCH(test, (hi)(clear))

tier 2

#include <eosio/eosio.hpp>

using namespace std;
using namespace eosio;

CONTRACT test : public contract {
  public:
    using contract::contract;

    ACTION hi(name from, name to, string message);
    ACTION clear(name from);

  private:
    TABLE messages {
      name    user_to;
      string  text;

      auto primary_key() const { return user_to.value; }
    };
    typedef multi_index<name("messages"), messages> messages_table;
};

#include <test.hpp>

ACTION test::hi(name from, name to, string message) {
  require_auth(from);

  // Init the _message table
  messages_table _messages(get_self(), from.value);

  // Find the record from _messages table
  auto msg_itr = _messages.find(to.value);
  if (msg_itr == _messages.end()) {
    // Create a message record if it does not exist
    _messages.emplace(to, [&](auto& msg) {
      msg.user_to = to;
      msg.text = message;
    });
  } else {
    // Modify a message record if it exists
    _messages.modify(msg_itr, from, [&](auto& msg) {
      msg.text = message;
    });
  }
}

ACTION test::clear(name from) {
  require_auth(from);

  messages_table _messages(get_self(), from.value);

  // Delete all records in _messages table
  auto msg_itr = _messages.begin();
  while (msg_itr != _messages.end()) {
    msg_itr = _messages.erase(msg_itr);
  }
}

EOSIO_DISPATCH(test, (hi)(clear))

#include <eosio/eosio.hpp>
#include <eosio/asset.hpp>

using namespace std;
using namespace eosio;

CONTRACT test : public contract {
  public:
    using contract::contract;

    ACTION hi(name from, name to, string message);
    ACTION clear(name from);
    ACTION clear2(name from, uint32_t limit);

    [[eosio::on_notify("eosio.token::transfer")]]
    void deposit(name from, name to, eosio::asset quantity, std::string memo);

  private:
    TABLE messages {
      name    user_to;
      string  text;

      auto primary_key() const { return user_to.value; }
    };
    typedef multi_index<name("messages"), messages> messages_table;

    TABLE payments {
      name    user;
      asset   amount;

      auto primary_key() const { return user.value; }
    };
    typedef multi_index<name("payments"), payments> payments_table;
};

tier 3

#include <test.hpp>

ACTION test::hi(name from, name to, string message) {
  require_auth(from);

  payments_table payments(get_self(), get_self().value);
  const auto &payment = payments.get(from.value);
  check(payment.amount >= asset(10, symbol("EOS", 4)), "fail"); 
  payments.modify(payment, get_self(), [&](auto& row) {
    row.amount -= asset(10, symbol("EOS", 4));
  });

  // Init the _message table
  messages_table _messages(get_self(), from.value);

  // Find the record from _messages table
  auto msg_itr = _messages.find(to.value);
  if (msg_itr == _messages.end()) {
    // Create a message record if it does not exist
    _messages.emplace(to, [&](auto& msg) {
      msg.user_to = to;
      msg.text = message;
    });
  } else {
    // Modify a message record if it exists
    _messages.modify(msg_itr, from, [&](auto& msg) {
      msg.text = message;
    });
  }
}

ACTION test::clear(name from) {
  require_auth(from);

  messages_table _messages(get_self(), from.value);

  // Delete all records in _messages table
  auto msg_itr = _messages.begin();
  while (msg_itr != _messages.end()) {
    msg_itr = _messages.erase(msg_itr);
  }
}

ACTION test::clear2(name from, uint32_t limit) {
  require_auth(from);

  messages_table _messages(get_self(), from.value);

  // Delete all records in _messages table
  auto msg_itr = _messages.begin();
  while (msg_itr != _messages.end() && limit > 0) {
    msg_itr = _messages.erase(msg_itr);
    --limit;
  }
}


void test::deposit(name from, name to, eosio::asset quantity, std::string memo) {
  if (to == get_self()) {
    return;
  }

  payments_table payment(get_self(), get_self().value);
  auto payment_itr = payment.find(from.value);

  if (payment_itr != payment.end())
    payment.modify(payment_itr, get_self(), [&](auto &row) {
      row.amount += quantity;
    });
  else
    payment.emplace(get_self(), [&](auto &row) {
      row.amount = quantity;
    });
}

EOSIO_DISPATCH(test, (hi)(clear)(clear2))