Line data    Source code

       1             : !--------------------------------------------------------------------------------------------------!
       2             : !   CP2K: A general program to perform molecular dynamics simulations                              !
       3             : !   Copyright 2000-2024 CP2K developers group <https://cp2k.org>                                   !
       4             : !                                                                                                  !
       5             : !   SPDX-License-Identifier: GPL-2.0-or-later                                                      !
       6             : !--------------------------------------------------------------------------------------------------!
       7             : 
       8             : ! **************************************************************************************************
       9             : !> \brief  Functions which are common to different hash tables
      10             : !>         Derived from qs_fb_hash_table_types and qs_fb_hash_table_types (Mark Tucker, Jun 2016)
      11             : ! **************************************************************************************************
      12             : MODULE qs_hash_table_functions
      13             : 
      14             : #include "./base/base_uses.f90"
      15             :    IMPLICIT NONE
      16             : 
      17             :    PRIVATE
      18             : 
      19             : ! public methods
      20             :    PUBLIC :: hash_table_matching_prime
      21             : 
      22             :    CHARACTER(len=*), PARAMETER, PRIVATE :: moduleN = 'qs_hash_table_functions'
      23             : 
      24             : CONTAINS
      25             : 
      26             : ! **************************************************************************************************
      27             : !> \brief Find a prime number equal or larger than ii
      28             : !> \param ii   : input integer
      29             : !> \return : the prime number
      30             : ! **************************************************************************************************
      31       23031 :    PURE FUNCTION hash_table_matching_prime(ii) RESULT(res)
      32             :       INTEGER, INTENT(IN)                                :: ii
      33             :       INTEGER                                            :: res
      34             : 
      35             :       ! even numbers are not prime, so no point testing them, so increment by 2 each time starting
      36             :       ! from an odd number greater or equal to ii (as noted in \brief)
      37       23031 :       res = ii + 1 - MOD(ii, 2)
      38             : 
      39       34614 :       DO WHILE (.NOT. is_positive_prime(res))
      40       11583 :          res = res + 2
      41             :       END DO
      42       23031 :    END FUNCTION hash_table_matching_prime
      43             : 
      44             : ! **************************************************************************************************
      45             : !> \brief Check if a number is a positive prime
      46             : !> \param num  : number to check
      47             : !> \return : returns TRUE if num is a positive prime, FALSE otherwise
      48             : !> \author Lianheng Tong (LT) lianheng.tong@kcl.ac.uk
      49             : ! **************************************************************************************************
      50       34614 :    PURE FUNCTION is_positive_prime(num) RESULT(res)
      51             :       INTEGER, INTENT(IN)                                :: num
      52             :       LOGICAL                                            :: res
      53             : 
      54             :       INTEGER                                            :: ii
      55             : 
      56       34614 :       IF (num .LE. 3) THEN
      57             :          res = .FALSE.
      58             :          RETURN
      59             :       END IF
      60       30017 :       IF (MOD(num, 2) == 0 .OR. MOD(num, 3) == 0) THEN
      61             :          res = .FALSE.
      62             :          RETURN
      63             :       END IF
      64             : 
      65             :       ! all primes > 3 are of the form 6*kk +/- 1, kk=1,2,3...
      66             :       ! (although not all 6*kk +/- 1 is a prime);
      67             :       ! and we only have to check factors less than and equal to SQRT(num)
      68             :       ii = 5
      69       25307 :       DO WHILE (ii*ii .LE. num)
      70        2276 :          IF (MOD(num, ii) == 0 .OR. MOD(num, ii + 2) == 0) THEN
      71             :             res = .FALSE.
      72             :             RETURN
      73             :          END IF
      74        1512 :          ii = ii + 6
      75             :       END DO
      76             :       res = .TRUE.
      77             :    END FUNCTION is_positive_prime
      78             : 
      79             : END MODULE qs_hash_table_functions
      80             :