Add get_y similar to stableswap

curvesim/pool/cryptoswap/pool.py

@@ -500,6 +500,41 @@ def get_dy(self, i: int, j: int, dx: int) -> int:
 
         return dy
 
+    def get_y(self, i, j, x, xp):
+        r"""
+        Calculate x[j] if one makes x[i] = x.
+
+        Parameters
+        ----------
+        i: int
+            index of coin; usually the "in"-token
+        j: int
+            index of coin; usually the "out"-token
+        x: int
+            balance of i-th coin in units of D
+        xp: list of int
+            coin balances in units of D
+
+        Returns
+        -------
+        int
+            The balance of the j-th coin, in units of D, for the other
+            coin balances given.
+
+        Note
+        ----
+        This is a "view" function; it doesn't change the state of the pool.
+        """
+        A: int = self.A
+        gamma: int = self.gamma
+        D: int = _newton_D(A, gamma, xp)
+
+        xp = xp.copy()
+        xp[i] = x
+
+        y, _ = _get_y(A, gamma, xp, D, j)
+        return y
+
     def _fee(self, xp: List[int]) -> int:
         """
         f = fee_gamma / (fee_gamma + (1 - K))

test/unit/test_tricrypto.py

@@ -337,6 +337,36 @@ def test__get_y(vyper_tricrypto, A, gamma, x0, x1, x2, pair, dx_perc):
     assert y_out[1] == expected_y_out[1]
 
 
+def test_get_y(vyper_tricrypto):
+    """
+    Test `get_y`.
+
+    Note `_get_y`, which is the pure version, is already tested
+    thoroughly in its own test against the vyper.
+
+    This test is a sanity check to make sure we pass values in correctly
+    to the underlying `_get_y` implementation.
+    """
+    pool = initialize_pool(vyper_tricrypto)
+
+    xp = pool._xp()
+    A = pool.A
+    gamma = pool.gamma
+    D = _newton_D(A, gamma, xp)
+
+    i = 0
+    j = 1
+
+    # `get_y` will set i-th balance to `x`
+    x = xp[i] * 102 // 100
+    y = pool.get_y(i, j, x, xp)
+
+    xp[i] = x
+    expected_y, _ = _get_y(A, gamma, xp, D, j)
+
+    assert y == expected_y
+
+
 @given(st.integers(min_value=-42139678854452767551, max_value=135305999368893231589))
 @settings(
     suppress_health_check=[HealthCheck.function_scoped_fixture],