Add slow demurrage calc

python/erc20_demurrage_token/demurrage.py

@@ -43,6 +43,15 @@ class DemurrageCalculator:
 
         return adjusted_amount
 
+    
+    def amount_since_slow(self, amount, timestamp):
+        delta = datetime.datetime.utcnow() - datetime.datetime.fromtimestamp(timestamp)
+        remainder_minutes = math.floor(delta.total_seconds() / 60)
+        adjusted_amount = amount * ((1 - self.r_min) ** remainder_minutes)
+        logg.debug('adjusted for {} minutes {} -> {} delta {}'.format(remainder_minutes, amount, adjusted_amount, amount - adjusted_amount))
+
+        return adjusted_amount
+
 
     @staticmethod
     def from_contract(rpc, chain_spec, contract_address, sender_address=ZERO_ADDRESS):

python/tests/test_demurrage_ext.py

@@ -16,13 +16,19 @@ from tests.base import TestDemurrage
 class TestEmulate(TestDemurrage):
 
     def test_amount_since(self):
-
         d = datetime.datetime.utcnow() - datetime.timedelta(seconds=29, hours=5, minutes=3, days=4)
         c = DemurrageCalculator(0.00000050105908373373)
         a = c.amount_since(100, d.timestamp())
         self.assert_within_lower(a, 99.69667, 0.1)
 
 
+    def test_amount_since_slow(self):
+        d = datetime.datetime.utcnow() - datetime.timedelta(seconds=29, hours=5, minutes=3, days=4)
+        c = DemurrageCalculator(0.00000050105908373373)
+        a = c.amount_since_slow(100, d.timestamp())
+        self.assert_within_lower(a, 99.69667, 0.1)
+
+
     def test_from_contract(self):
         nonce_oracle = RPCNonceOracle(self.accounts[0], self.rpc)
         c = DemurrageToken(self.chain_spec, signer=self.signer, nonce_oracle=nonce_oracle)