Line data    Source code

       1              : !--------------------------------------------------------------------------------------------------!
       2              : !   CP2K: A general program to perform molecular dynamics simulations                              !
       3              : !   Copyright 2000-2025 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        26647 :    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        26647 :       res = ii + 1 - MOD(ii, 2)
      38              : 
      39        40583 :       DO WHILE (.NOT. is_positive_prime(res))
      40        13936 :          res = res + 2
      41              :       END DO
      42        26647 :    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        40583 :    PURE FUNCTION is_positive_prime(num) RESULT(res)
      51              :       INTEGER, INTENT(IN)                                :: num
      52              :       LOGICAL                                            :: res
      53              : 
      54              :       INTEGER                                            :: ii
      55              : 
      56        40583 :       IF (num <= 3) THEN
      57              :          res = .FALSE.
      58              :          RETURN
      59              :       END IF
      60        34228 :       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        29027 :       DO WHILE (ii*ii <= num)
      70         2380 :          IF (MOD(num, ii) == 0 .OR. MOD(num, ii + 2) == 0) THEN
      71              :             res = .FALSE.
      72              :             RETURN
      73              :          END IF
      74         1588 :          ii = ii + 6
      75              :       END DO
      76              :       res = .TRUE.
      77              :    END FUNCTION is_positive_prime
      78              : 
      79              : END MODULE qs_hash_table_functions
      80              :