d1mach <- function ( i )

#*****************************************************************************80
#
## D1MACH returns double precision real machine constants.
#
#  Discussion:
#
#    Assume that double precision real numbers are stored with a mantissa
#    of T digits in base B, with an exponent whose value must lie
#    between EMIN and EMAX.  Then for values of I between 1 and 5,
#    D1MACH will return the following values:
#
#      D1MACH(1) <- B^(EMIN-1), the smallest positive magnitude.
#      D1MACH(2) <- B^EMAX*(1-B^(-T)), the largest magnitude.
#      D1MACH(3) <- B^(-T), the smallest relative spacing.
#      D1MACH(4) <- B^(1-T), the largest relative spacing.
#      D1MACH(5) <- log10(B)
#
#  Licensing:
#
#    This code is distributed under the MIT license.
#
#  Modified:
#
#    05 February 2020
#
#  Author:
#
#    Original FORTRAN77 version by Phyllis Fox, Andrew Hall, Norman Schryer
#    R version by John Burkardt.
#
#  Reference:
#
#    Phyllis Fox, Andrew Hall, Norman Schryer,
#    Algorithm 528,
#    Framework for a Portable Library,
#    ACM Transactions on Mathematical Software,
#    Volume 4, Number 2, June 1978, page 176-188.
#
#  Input:
#
#    integer I, chooses the parameter to be returned.
#    1 <= I <= 5.
#
#  Output:
#
#    real VALUE, the value of the chosen parameter.
#
{
  if ( i < 1 )
  {
    cat ( "\n" )
    cat ( "D1MACH - Fatal error!\n" )
    cat ( "  The input argument I is out of bounds.\n" )
    cat ( "  Legal values satisfy 1 <= I <= 5.\n" )
    cat ( "  I <- ", i )
    stop ( "D1MACH - Fatal error!\n" )
  } else if ( i == 1 ) {
    value <- .Machine$double.xmax
  } else if ( i == 2 ) {
    value <- .Machine$double.xmin
  } else if ( i == 3 ) {
    value <- .Machine$double.eps / 2.0
  } else if ( i == 4 ) {
    value <- .Machine$double.eps
  } else if ( i == 5 ) {
    value <- log10 ( .Machine$double.base )
  } else if ( 5 < i ) {
    cat ( "\n" )
    cat ( "D1MACH - Fatal error!\n" )
    cat ( "  The input argument I is out of bounds.\n" )
    cat ( "  Legal values satisfy 1 <= I <= 5.\n" )
    cat ( "  I <- ", i )
    stop ( "D1MACH - Fatal error!" )
  }

  return ( value )
}