Merge pull request #18 from jvdp1/fix_2

replace function power of 2

Author: jvdp1

src/modhash.fypp

@@ -3,7 +3,7 @@ module modhash
  use iso_fortran_env, only: int32, real32, real64
  implicit none
  private
- public :: hash, roundinguppower2
+ public :: hash, next_power_of_2
 
  integer(int32), parameter, public :: seed_hash = 305419896_int32
 
@@ -146,13 +146,30 @@ contains
   end if
  end function
 
- elemental pure function roundinguppower2(x) result(next)
-  integer(kind=int32), intent(in)::x
-  integer(kind=int32)::next
+ elemental pure function next_power_of_2(x) result(next)
+  integer(kind=int32), intent(in) :: x
+  integer(kind=int32) :: next
 
-  real(real64), parameter :: log2 = log(2._real64)
+  integer(int32) :: bits
 
-  next = 2_int32 ** ceiling( log(real(x, real64)) / log2, kind=int32 )
+  ! If x is 0 or 1, the next power of 2 is 1
+  if (x <= 1_int32) then
+   next = 1_int32
+   return
+  end if
+
+  ! Check if x is already a power of 2
+  ! A power of 2 has only one bit set, so iand(x, x-1) == 0
+  if (iand(x, x - 1_int32) == 0_int32) then
+   next = x
+   return
+  end if
+
+  ! The position of the highest set bit determines the power
+  bits = bit_size(x) - leadz(x)
+
+  ! Shift 1 into the position above the highest set bit
+  next = shiftl(1_int32, bits)
 
  end function
 

src/modtable.fypp

@@ -1,7 +1,7 @@
 #:include "common.fypp"
 module modtable
  use iso_fortran_env, only: int32, real32
- use modhash, only: roundinguppower2, seed_hash, hash
+ use modhash, only: next_power_of_2, seed_hash, hash
  implicit none
  private
  #:for t,k,l,m in TYPES
@@ -117,8 +117,8 @@ contains
    integer(kind=int32), intent(in), optional :: nel
    type(table_${k}$_t) :: this
 
-   this%nel = roundinguppower2(default_nel)
-   if (present(nel)) this%nel = roundinguppower2(nel)
+   this%nel = next_power_of_2(default_nel)
+   if (present(nel)) this%nel = next_power_of_2(nel)
 
    this%filled = 0
 
@@ -366,7 +366,7 @@ contains
    if (nfilled == 0_int32) then
     this%nel = default_nel
    else
-    this%nel = roundinguppower2(int(1.3*nfilled, kind=int32))
+    this%nel = next_power_of_2(int(1.3*nfilled, kind=int32))
    end if
    this%filled = 0
 
@@ -399,8 +399,8 @@ contains
    integer(kind=int32), intent(in), optional :: nel
    type(table_arr${k}$_t) :: this
 
-   this%nel = roundinguppower2(default_nel)
-   if (present(nel)) this%nel = roundinguppower2(nel)
+   this%nel = next_power_of_2(default_nel)
+   if (present(nel)) this%nel = next_power_of_2(nel)
 
    this%filled = 0
 
@@ -633,7 +633,7 @@ contains
    if (nfilled == 0_int32) then
     this%nel = 0_int32
    else
-    this%nel = roundinguppower2(int(1.3*nfilled, kind=int32))
+    this%nel = next_power_of_2(int(1.3*nfilled, kind=int32))
    end if
    this%filled = 0