add Ufunc::diffcentred - #516

Changes

@@ -8,6 +8,7 @@
 - now an error to upd_data if datasv set to different size from nbytes
 - add PDL::readonly method (#516) - thanks @wlmb for suggestion
 - rename Ufunc::diffover to Ufunc::numdiff (#516)
+- add Ufunc::diffcentred (#516)
 
 2.098 2025-01-03
 - fix Windows build problems

lib/PDL/Ufunc.pd

@@ -226,6 +226,38 @@ Compare to L</cumusumover>, which acts as the converse of this.
 EOF
 );
 
+pp_def('diffcentred',
+  HandleBad => 1,
+  Pars => 'a(n); [o]o(nminus2=CALC($SIZE(n) - 2))',
+  GenericTypes => [ppdefs_all],
+  Code => <<'EOF',
+loop(nminus2) %{
+  PDL_IF_BAD(if ($ISBAD($a(n=>nminus2)) || $ISBAD($a(n=>nminus2+2))) {
+    $SETBAD(o()); continue;
+  },)
+  $o() = ($a(n=>nminus2+2) - $a(n=>nminus2))/2;
+%}
+EOF
+  Doc => <<'EOF',
+=for ref
+
+Calculates centred differences along a vector's 0th dimension.
+Currently takes no account of boundary values, so produces 2 fewer
+values than on the input.
+
+By using L<PDL::Slices/xchg> etc. it is possible to use I<any> dimension.
+
+=for usage
+
+  print pdl(q[3 4 2 3 2 3 1])->diff2;
+  # [1 -2 1 -1 1 -2]
+EOF
+  BadDoc => <<'EOF',
+A bad value at C<n> means the affected output values at C<n-2>,C<n>
+(if in boounds) are set bad.
+EOF
+);
+
 pp_def('diff2',
   HandleBad => 1,
   Pars => 'a(n); [o]o(nminus1=CALC($SIZE(n) - 1))',

t/ufunc.t

@@ -246,6 +246,14 @@ subtest numdiff => sub {
   is_pdl $a, pdl(2, 1, 1, 1, 1), "numdiff inplace";
 };
 
+subtest diffcentred => sub {
+  my $a = sequence(6) + 2;
+  is_pdl $a->diffcentred, pdl(1, 1, 1, 1), "diffcentred";
+  is_pdl +($a**2)->diffcentred, pdl('6 8 10 12'), "diffcentred of x^2";
+  $a->setbadat(2);
+  is_pdl +($a**2)->diffcentred, pdl('BAD 8 BAD 12'), "diffcentred of x^2 with bad";
+};
+
 subtest diff2 => sub {
   my $got = pdl('[BAD 2 3 4]')->diff2;
   is "$got", "[2 1 1]", 'first bad';