From 63c3ab6b736535cb9b7f633cd6c5a09db52fd9f0 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?R=C3=A9my=20Oudompheng?= <remyoudompheng@gmail.com>
Date: Sun, 19 Mar 2023 10:09:22 +0100
Subject: [PATCH] Lighter construction of finite field elements from lists

When doing intensive polynomial arithmetic with the NTL implementation
the constructor with lists is called a large number of times
and may spend a lot of time constructing the vector_space and
FreeModuleElement objects.

The very common call to vector_space(map=False) is optimized to be as cheap as
possible using the already cached object.

The common case of lists of length 0 and 1 is replaced by cheaper
shortcuts.
---
 src/sage/rings/finite_rings/element_pari_ffelt.pyx | 14 +++++++++++++-
 src/sage/rings/finite_rings/finite_field_base.pyx  |  9 +++++++--
 2 files changed, 20 insertions(+), 3 deletions(-)

diff --git a/src/sage/rings/finite_rings/element_pari_ffelt.pyx b/src/sage/rings/finite_rings/element_pari_ffelt.pyx
index be70aa56d39..4c0b02011a8 100644
--- a/src/sage/rings/finite_rings/element_pari_ffelt.pyx
+++ b/src/sage/rings/finite_rings/element_pari_ffelt.pyx
@@ -270,13 +270,19 @@ cdef class FiniteFieldElement_pari_ffelt(FinitePolyExtElement):
         sage: k = FiniteField(3^11, 't', impl='pari_ffelt')
         sage: k([ 0, 1/2 ])
         2*t
+        sage: k([ 0, 1/2, 0, 0, 0, 0, 0, 0, 0, -1, 0 ])
+        2*t^9 + 2*t
         sage: k([ k(0), k(1) ])
         t
         sage: k([ GF(3)(2), GF(3^5,'u')(1) ])
         t + 2
         sage: R.<x> = PolynomialRing(k)
+        sage: k([ x/x ])
+        1
         sage: k([ R(-1), x/x ])
         t + 2
+        sage: k([ R(-1), R(0), 0 ])
+        2
 
     Check that zeros are created correctly (:trac:`11685`)::
 
@@ -496,7 +502,13 @@ cdef class FiniteFieldElement_pari_ffelt(FinitePolyExtElement):
             self.construct_from(x.constant_coefficient())
 
         elif isinstance(x, list):
-            if len(x) == self._parent.degree():
+            n = len(x)
+            if n == 0:
+                self.construct_from(None)
+            elif n == 1:
+                Fp = self._parent.base_ring()
+                self.construct_from(Fp(x[0]))
+            elif n == self._parent.degree():
                 self.construct_from(self._parent.vector_space(map=False)(x))
             else:
                 Fp = self._parent.base_ring()
diff --git a/src/sage/rings/finite_rings/finite_field_base.pyx b/src/sage/rings/finite_rings/finite_field_base.pyx
index 06e57a04952..fa9c8862ddb 100644
--- a/src/sage/rings/finite_rings/finite_field_base.pyx
+++ b/src/sage/rings/finite_rings/finite_field_base.pyx
@@ -1230,8 +1230,6 @@ cdef class FiniteField(Field):
             sage: all(to_V(h(c) * e) == c * to_V(e) for e in E for c in F)
             True
         """
-        from sage.modules.all import VectorSpace
-        from sage.categories.morphism import is_Morphism
         if subfield is not None:
             if base is not None:
                 raise ValueError
@@ -1241,6 +1239,13 @@ cdef class FiniteField(Field):
             deprecation(28481, "The default value for map will be changing to True.  To keep the current behavior, explicitly pass map=False.")
             map = False
 
+        if base is None and self.__vector_space is not None and not map:
+            # A very common case: return as early as possible.
+            return self.__vector_space
+
+        from sage.modules.all import VectorSpace
+        from sage.categories.morphism import is_Morphism
+
         if base is None:
             base = self.prime_subfield()
             s = self.degree()