series: support Piecewise expressions in mrv()

Closes sympy/sympy#18492

diofant/series/gruntz.py

@@ -134,6 +134,16 @@ def mrv(e, x):
         return mrv(e.base, x)
     elif isinstance(e, log):
         return mrv(e.args[0], x)
+    elif e.is_Piecewise:
+        for a, c in e.args:
+            if not c.is_Atom:
+                c = c.func(c.lhs - c.rhs)
+                c = c.func(limitinf(c.lhs, x))
+                if not c.is_Atom:
+                    raise NotImplementedError("Parametric limits aren't supported yet.")
+                if c:
+                    break
+        return mrv(a, x)
     elif e.is_Function and not isinstance(e.func, UndefinedFunction):
         return functools.reduce(lambda a, b: mrv_max(a, b, x),
                                 [mrv(a, x) for a in e.args])

diofant/tests/series/test_limits.py

@@ -99,6 +99,19 @@ def test_basic4():
     e = ((n**(n + 1) + (n + 1)**n)/n**(n + 1))**n
     assert limit(e, n, oo) == E**E
 
+    # issue sympy/sympy#18492
+    e1 = 2*sqrt(x)*Piecewise(((4*x - 2)/abs(sqrt(4 - 4*(2*x - 1)**2)),
+                              4*x - 2 >= 0),
+                             ((2 - 4*x)/abs(sqrt(4 - 4*(2*x - 1)**2)), True))
+    e2 = Piecewise((x**2/2, x <= Rational(1, 2)), (x/2 - Rational(1, 8), True))
+    e3 = Piecewise(((x - 9)/5, x < -1), ((x - 9)/5, x > 4),
+                   (sqrt(abs(x - 3)), True))
+    assert limit(e1, x, 0) == 1
+    assert limit(e2, x, 0) == 0
+    assert limit(e2, x, oo) == oo
+    assert limit(e3, x, -1) == 2
+    assert limit(e3, x, oo) == oo
+
 
 def test_basic5():
     class MyFunction(Function):

docs/release/notes-0.14.rst

@@ -7,6 +7,8 @@ Not Released Yet
 New features
 ============
 
+* Support calculating limits with :class:`~diofant.functions.elementary.piecewise.Piecewise` functions, see :pull:`1214`.
+
 Major changes
 =============
 
@@ -67,3 +69,4 @@ These Sympy issues also were addressed:
 * :sympyissue:`14387`: Tutorial on limits creates impression that they are two-sided by default
 * :sympyissue:`8166`: Limit assumes function is continuous?
 * :sympyissue:`14502`: Problem with limit including factorial.
+* :sympyissue:`18492`: Limit of Piecewise function - NotImplementedError: Don't know how to calculate the mrv