prime_birthday.sf

#!/usr/bin/ruby

# Daniel "Trizen" Șuteu
# Date: 11 October 2018
# https://github.com/trizen

# Find the most "prime" birthday:
#
#   Y / M / D
#
# where all the following expressions are prime numbers:
#
#   D
#   D - M
#
#   D + M
#   D + Y
#
#   Y * M - D
#   Y + M + 1
#
#   Y*D + M + 1
#   Y*D - M - 1
#
#   Y + M*D - 1
#   Y - M*D + 1
#
#   Y*M*D - 1
#   Y*M*D + 1
#
#   Y*D - 1
#   Y*M + 1
#   M*D - 1
#   M*D + 1
#
#   Y + M + D
#   Y - M - D
#   Y + M - D
#   Y - M + D

# Only one such birthday exists between the years 1 and 10000, and that is:
#   Y = 1980
#   M = 12
#   D = 19

#  Jake Gyllenhaal, famous American actor and producer, was born on this date.

for y in (1900..2018), m in (1..12), d in (1..31) {

    d.is_prime || next

    d - m -> is_prime || next
    d + m -> is_prime || next
    d + y -> is_prime || next

    y * m - d -> is_prime || next
    y + m + 1 -> is_prime || next

    y*d + m + 1 -> is_prime || next
    y*d - m - 1 -> is_prime || next

    y + m*d - 1 -> is_prime || next
    y - m*d + 1 -> is_prime || next

    y * m * d - 1 -> is_prime || next
    y * m * d + 1 -> is_prime || next

    y * d - 1 -> is_prime || next
    y * m + 1 -> is_prime || next
    m * d - 1 -> is_prime || next
    m * d + 1 -> is_prime || next

    y + m + d -> is_prime || next
    y - m - d -> is_prime || next
    y + m - d -> is_prime || next
    y - m + d -> is_prime || next

    say "#{y} #{m} #{d}"
}

__END__
1980 12 19