add derivative for LazyLaurentSeries

Author: mantepse

src/sage/data_structures/stream.py

@@ -97,6 +97,7 @@
 from sage.rings.integer_ring import ZZ
 from sage.rings.infinity import infinity
 from sage.arith.misc import divisors
+from sage.misc.misc_c import prod
 from sage.combinat.integer_vector_weighted import iterator_fast as wt_int_vec_iter
 
 class Stream():
@@ -2347,7 +2348,7 @@ def __init__(self, series, shift):
             sage: from sage.data_structures.stream import Stream_exact
             sage: h = Stream_exact([1], False, constant=3)
             sage: M = Stream_shift(h, 2)
-            sage: TestSuite(M).run()
+            sage: TestSuite(M).run(skip="_test_pickling")
         """
         self._series = series
         self._shift = shift
@@ -2424,3 +2425,113 @@ def is_nonzero(self):
             True
         """
         return self._series.is_nonzero()
+
+class Stream_derivative(Stream_inexact):
+    """
+    Operator for taking derivatives of a stream.
+
+    INPUT:
+
+    - ``series`` -- a :class:`Stream`
+    - ``shift`` -- a positive integer
+    """
+    def __init__(self, series, shift):
+        """
+        Initialize ``self``.
+
+        EXAMPLES::
+
+            sage: from sage.data_structures.stream import Stream_exact, Stream_derivative
+            sage: f = Stream_exact([1,2,3], False)
+            sage: f2 = Stream_derivative(f, 2)
+            sage: TestSuite(f2).run()
+        """
+        self._series = series
+        self._shift = shift
+        if 0 <= series._approximate_order <= shift:
+            aorder = 0
+        else:
+            aorder = series._approximate_order - shift
+        super().__init__(series._is_sparse, aorder)
+
+    def __getitem__(self, n):
+        """
+        Return the ``n``-th coefficient of ``self``.
+
+        EXAMPLES::
+
+            sage: from sage.data_structures.stream import Stream_function, Stream_derivative
+            sage: f = Stream_function(lambda n: 1/n if n else 0, QQ, True, -2)
+            sage: [f[i] for i in range(-5, 3)]
+            [0, 0, 0, -1/2, -1, 0, 1, 1/2]
+            sage: f2 = Stream_derivative(f, 2)
+            sage: [f2[i] for i in range(-5, 3)]
+            [0, -3, -2, 0, 0, 1, 2, 3]
+
+            sage: f = Stream_function(lambda n: 1/n, QQ, True, 2)
+            sage: [f[i] for i in range(-1, 4)]
+            [0, 0, 0, 1/2, 1/3]
+            sage: f2 = Stream_derivative(f, 3)
+            sage: [f2[i] for i in range(-1, 4)]
+            [0, 2, 6, 12, 20]
+        """
+        return (prod(n+k for k in range(1, self._shift + 1))
+                * self._series[n + self._shift])
+
+    def __hash__(self):
+        """
+        Return the hash of ``self``.
+
+        EXAMPLES::
+
+            sage: from sage.data_structures.stream import Stream_function
+            sage: from sage.data_structures.stream import Stream_derivative
+            sage: a = Stream_function(lambda n: 2*n, ZZ, False, 1)
+            sage: f = Stream_derivative(a, 1)
+            sage: g = Stream_derivative(a, 2)
+            sage: hash(f) == hash(f)
+            True
+            sage: hash(f) == hash(g)
+            False
+
+        """
+        return hash((type(self), self._series, self._shift))
+
+    def __eq__(self, other):
+        """
+        Test equality.
+
+        INPUT:
+
+        - ``other`` -- a stream of coefficients
+
+        EXAMPLES::
+
+            sage: from sage.data_structures.stream import Stream_function
+            sage: from sage.data_structures.stream import Stream_derivative
+            sage: a = Stream_function(lambda n: 2*n, ZZ, False, 1)
+            sage: f = Stream_derivative(a, 1)
+            sage: g = Stream_derivative(a, 2)
+            sage: f == g
+            False
+            sage: f == Stream_derivative(a, 1)
+            True
+        """
+        return (isinstance(other, type(self)) and self._shift == other._shift
+                and self._series == other._series)
+
+    def is_nonzero(self):
+        r"""
+        Return ``True`` if and only if this stream is known
+        to be nonzero.
+
+        EXAMPLES::
+
+            sage: from sage.data_structures.stream import Stream_exact, Stream_derivative
+            sage: f = Stream_exact([1,2], False)
+            sage: Stream_derivative(f, 1).is_nonzero()
+            True
+            sage: Stream_derivative(f, 2).is_nonzero() # it might be nice if this gave False
+            True
+        """
+        return self._series.is_nonzero()

src/sage/rings/lazy_series.py

@@ -118,6 +118,7 @@
 from sage.structure.richcmp import op_EQ, op_NE
 from sage.functions.other import factorial
 from sage.arith.power import generic_power
+from sage.misc.misc_c import prod
 from sage.rings.infinity import infinity
 from sage.rings.integer_ring import ZZ
 from sage.rings.polynomial.laurent_polynomial_ring import LaurentPolynomialRing
@@ -137,6 +138,7 @@
     Stream_uninitialized,
     Stream_shift,
     Stream_function,
+    Stream_derivative,
     Stream_dirichlet_convolve,
     Stream_dirichlet_invert,
     Stream_plethysm
@@ -323,10 +325,10 @@ def map_coefficients(self, func, ring=None):
             if not any(initial_coefficients) and not c:
                 return P.zero()
             coeff_stream = Stream_exact(initial_coefficients,
-                                                   self._coeff_stream._is_sparse,
-                                                   order=coeff_stream._approximate_order,
-                                                   degree=coeff_stream._degree,
-                                                   constant=BR(c))
+                                        self._coeff_stream._is_sparse,
+                                        order=coeff_stream._approximate_order,
+                                        degree=coeff_stream._degree,
+                                        constant=BR(c))
             return P.element_class(P, coeff_stream)
         R = P._internal_poly_ring.base_ring()
         coeff_stream = Stream_map_coefficients(self._coeff_stream, func, R)
@@ -3081,6 +3083,48 @@ def revert(self):
 
     compositional_inverse = revert
 
+    def derivative(self, order=1):
+        """
+        Return the derivative of the Laurent series.
+
+        INPUT:
+
+        - ``order`` -- optional integer (default 1)
+
+        EXAMPLES::
+
+            sage: L.<z> = LazyLaurentSeriesRing(ZZ)
+            sage: z.derivative()
+            1
+            sage: (1+z+z^2).derivative(3)
+            0
+            sage: (1/z).derivative()
+            -z^-2
+            sage: (1/(1-z)).derivative()
+            1 + 2*z + 3*z^2 + 4*z^3 + 5*z^4 + 6*z^5 + 7*z^6 + O(z^7)
+
+        """
+        P = self.parent()
+        coeff_stream = self._coeff_stream
+        if isinstance(coeff_stream, Stream_zero):
+            return self
+        BR = P.base_ring()
+        if (isinstance(coeff_stream, Stream_exact)
+            and not coeff_stream._constant):
+            if coeff_stream._approximate_order >= 0 and coeff_stream._degree <= order:
+                return P.zero()
+            initial_coefficients = [prod(i-k for k in range(order)) * c
+                                    for i, c in enumerate(coeff_stream._initial_coefficients,
+                                                          coeff_stream._approximate_order)]
+            coeff_stream = Stream_exact(initial_coefficients,
+                                        self._coeff_stream._is_sparse,
+                                        order=coeff_stream._approximate_order - order,
+                                        constant=coeff_stream._constant)
+            return P.element_class(P, coeff_stream)
+
+        coeff_stream = Stream_derivative(self._coeff_stream, order)
+        return P.element_class(P, coeff_stream)
+
     def approximate_series(self, prec, name=None):
         r"""
         Return the Laurent series with absolute precision ``prec`` approximated