Fix the cdiv() and cmod() definitions in Shadow.py (GH-3429)

Closes #2643.

Cython/Shadow.py

@@ -146,14 +146,16 @@ def compile(f):
 # Special functions
 
 def cdiv(a, b):
-    q = a / b
-    if q < 0:
-        q += 1
-    return q
+    if a < 0:
+        a = -a
+        b = -b
+    if b < 0:
+        return (a + b + 1) // b
+    return a // b
 
 def cmod(a, b):
     r = a % b
-    if (a*b) < 0:
+    if (a * b) < 0 and r:
         r -= b
     return r
 

tests/run/cdivision_CEP_516.pyx

@@ -27,6 +27,9 @@ True
 >>> [test_cdiv_cmod(a, b) for a, b in v]
 [(1, 7), (-1, -7), (1, -7), (-1, 7)]
 
+>>> [test_cdiv_cmod(a, b) for a, b in [(4, -4), (4, -2), (4, -1)]]
+[(-1, 0), (-2, 0), (-4, 0)]
+
 >>> all([mod_int_py(a,b) == a % b for a in range(-10, 10) for b in range(-10, 10) if b != 0])
 True
 >>> all([div_int_py(a,b) == a // b for a in range(-10, 10) for b in range(-10, 10) if b != 0])