perf(polint): add uniform-grid fast path for barycentric n=6

AMSS_NCKU_source/fmisc.f90

@@ -1189,8 +1189,10 @@ end subroutine d2dump
   real*8, intent(out) :: y, dy
 
   integer :: i, j
+  logical :: is_uniform
   real*8, dimension(6) :: lambda
-  real*8 :: dx, den_i, term, num, den
+  real*8 :: dx, den_i, term, num, den, step, tol
+  real*8, parameter :: c_uniform(6) = (/ -1.d0, 5.d0, -10.d0, 10.d0, -5.d0, 1.d0 /)
 
   do i = 1, 6
     if (x == xa(i)) then
@@ -1200,6 +1202,31 @@ end subroutine d2dump
     end if
   end do
 
+  step = xa(2) - xa(1)
+  is_uniform = (step /= 0.d0)
+  if (is_uniform) then
+    tol = 64.d0 * epsilon(1.d0) * max(1.d0, abs(step))
+    do i = 3, 6
+      if (abs((xa(i) - xa(i-1)) - step) > tol) then
+        is_uniform = .false.
+        exit
+      end if
+    end do
+  end if
+
+  if (is_uniform) then
+    num = 0.d0
+    den = 0.d0
+    do i = 1, 6
+      term = c_uniform(i) / (x - xa(i))
+      num = num + term * ya(i)
+      den = den + term
+    end do
+    y = num / den
+    dy = 0.d0
+    return
+  end if
+
   do i = 1, 6
     den_i = 1.d0
     do j = 1, 6