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program day_03
implicit none
logical :: find_a
integer ::io, ios, a, b, state, res
character(len=1) :: c
character(len=10) :: buffer
character(len=4) :: mul_cmd = 'mul('
res = 0
open(newunit=io, file='./day_03_input.txt', status='old', action='read', access='stream')
state = 0 ! 0 = find command, 1 = find number, 2 = calculate
find_a = .true.
buffer = ''
do
read(io, iostat=ios) c
if (ios /= 0) exit
select case(state)
case (0) ! find command
buffer = trim(buffer) // c
if(buffer == mul_cmd) then
state = 1
buffer = ''
else if(trim(buffer) /= mul_cmd(1:len(trim(buffer)))) then
state = 0
buffer = ''
end if
case (1) ! find digits
select case (iachar(c))
case (48:57) ! 0-9
buffer = trim(buffer) // c
case (44) ! ,
if (find_a .and. len(trim(buffer)) > 0) then
read(buffer, *) a
buffer = ''
find_a = .false.
else
state = 0 ! invalid
find_a = .true.
buffer = ''
end if
case (41) ! )
if(.not. find_a .and. len(trim(buffer)) > 0) then
read(buffer, *) b
state = 2
find_a = .true.
buffer = ''
else
state = 0
find_a = .true.
buffer = ''
end if
case default
state = 0
find_a = .true.
buffer = ''
end select
end select
if(state == 2) then
res = res + (a * b)
state = 0
buffer = ''
end if
end do
print*, res
end program day_03
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