kernel: add LookupPrimePower helper

src/finfield.c

@@ -42,29 +42,20 @@ static Obj TYPE_FFE0;
 // used for successor bags
 static Obj TYPE_KERNEL_OBJECT;
 
+
 /****************************************************************************
 **
-*F  FiniteField(<p>,<d>)  . . . make the small finite field with <q> elements
+*F  LookupPrimePower(<q>) . . . . . . . .  search for a prime power in tables
 **
-**  'FiniteField' returns the small finite field with <p>^<d> elements.
+**  Searches the tables of prime powers from ffdata.c for q, returns the
+**  index of q in SizeFF if it is present and 0 if not (the 0 position of
+**  SizeFF is unused).
 */
-FF              FiniteField (
-    UInt                p,
-    UInt                d )
+static FF LookupPrimePower(UInt q)
 {
-    FF    ff;            /* finite field, result            */
-    Obj   tmp;           /* temporary bag                   */
-    Obj   succBag;       /* successor table bag             */
-    FFV * succ;          /* successor table                 */
-    FFV * indx;          /* index table                     */
-    UInt  q;             /* size of finite field            */
-    UInt  poly;          /* Conway polynomial of extension  */
-    UInt  i, l, f, n, e; /* loop variables                  */
-
-    /* calculate size of field */
-    q = 1;
-    for (i = 1; i <= d; i++)
-        q *= p;
+    UInt l, n;
+    FF   ff;
+    UInt e;
 
     /* search through the finite field table                               */
     l = 1;
@@ -85,10 +76,39 @@ FF              FiniteField (
     }
     if (ff < 1 || ff > NUM_SHORT_FINITE_FIELDS)
         return 0;
-    if (CharFF[ff] != p)
-        return 0;
     if (SizeFF[ff] != q)
         return 0;
+    return ff;
+}
+
+
+/****************************************************************************
+**
+*F  FiniteField(<p>,<d>)  . . . make the small finite field with <q> elements
+**
+**  'FiniteField' returns the small finite field with <p>^<d> elements.
+*/
+FF              FiniteField (
+    UInt                p,
+    UInt                d )
+{
+    FF    ff;            /* finite field, result            */
+    Obj   tmp;           /* temporary bag                   */
+    Obj   succBag;       /* successor table bag             */
+    FFV * succ;          /* successor table                 */
+    FFV * indx;          /* index table                     */
+    UInt  q;             /* size of finite field            */
+    UInt  poly;          /* Conway polynomial of extension  */
+    UInt  i, l, f, n, e; /* loop variables                  */
+
+    /* calculate size of field */
+    q = 1;
+    for (i = 1; i <= d; i++)
+        q *= p;
+
+    ff = LookupPrimePower(q);
+    if (CharFF[ff] != p)
+        return 0;
 #ifdef HPCGAP
     /* Important correctness concern here:
      *