How to resolve the algorithm Compiler/AST interpreter step by step in the Fortran programming language
Published on 12 May 2024 09:40 PM
How to resolve the algorithm Compiler/AST interpreter step by step in the Fortran programming language
Table of Contents
Problem Statement
An AST interpreter interprets an Abstract Syntax Tree (AST) produced by a Syntax Analyzer. Take the AST output from the Syntax analyzer task, and interpret it as appropriate. Refer to the Syntax analyzer task for details of the AST. Notes: Because of the simple nature of our tiny language, Semantic analysis is not needed. Your interpreter should use C like division semantics, for both division and modulus. For division of positive operands, only the non-fractional portion of the result should be returned. In other words, the result should be truncated towards 0. This means, for instance, that 3 / 2 should result in 1. For division when one of the operands is negative, the result should be truncated towards 0. This means, for instance, that 3 / -2 should result in -1. Your solution should pass all the test cases above and the additional tests found Here. The C and Python versions can be considered reference implementations.
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Step by Step solution about How to resolve the algorithm Compiler/AST interpreter step by step in the Fortran programming language
Source code in the fortran programming language
!!!
!!! An implementation of the Rosetta Code interpreter task:
!!! https://rosettacode.org/wiki/Compiler/AST_interpreter
!!!
!!! The implementation is based on the published pseudocode.
!!!
module compiler_type_kinds
use, intrinsic :: iso_fortran_env, only: int32
use, intrinsic :: iso_fortran_env, only: int64
implicit none
private
! Synonyms.
integer, parameter, public :: size_kind = int64
integer, parameter, public :: length_kind = size_kind
integer, parameter, public :: nk = size_kind
! Synonyms for character capable of storing a Unicode code point.
integer, parameter, public :: unicode_char_kind = selected_char_kind ('ISO_10646')
integer, parameter, public :: ck = unicode_char_kind
! Synonyms for integers capable of storing a Unicode code point.
integer, parameter, public :: unicode_ichar_kind = int32
integer, parameter, public :: ick = unicode_ichar_kind
! Synonyms for integers in the runtime code.
integer, parameter, public :: runtime_int_kind = int64
integer, parameter, public :: rik = runtime_int_kind
end module compiler_type_kinds
module helper_procedures
use, non_intrinsic :: compiler_type_kinds, only: nk, ck
implicit none
private
public :: new_storage_size
public :: next_power_of_two
public :: isspace
character(1, kind = ck), parameter :: horizontal_tab_char = char (9, kind = ck)
character(1, kind = ck), parameter :: linefeed_char = char (10, kind = ck)
character(1, kind = ck), parameter :: vertical_tab_char = char (11, kind = ck)
character(1, kind = ck), parameter :: formfeed_char = char (12, kind = ck)
character(1, kind = ck), parameter :: carriage_return_char = char (13, kind = ck)
character(1, kind = ck), parameter :: space_char = ck_' '
contains
elemental function new_storage_size (length_needed) result (size)
integer(kind = nk), intent(in) :: length_needed
integer(kind = nk) :: size
! Increase storage by orders of magnitude.
if (2_nk**32 < length_needed) then
size = huge (1_nk)
else
size = next_power_of_two (length_needed)
end if
end function new_storage_size
elemental function next_power_of_two (x) result (y)
integer(kind = nk), intent(in) :: x
integer(kind = nk) :: y
!
! It is assumed that no more than 64 bits are used.
!
! The branch-free algorithm is that of
! https://archive.is/nKxAc#RoundUpPowerOf2
!
! Fill in bits until one less than the desired power of two is
! reached, and then add one.
!
y = x - 1
y = ior (y, ishft (y, -1))
y = ior (y, ishft (y, -2))
y = ior (y, ishft (y, -4))
y = ior (y, ishft (y, -8))
y = ior (y, ishft (y, -16))
y = ior (y, ishft (y, -32))
y = y + 1
end function next_power_of_two
elemental function isspace (ch) result (bool)
character(1, kind = ck), intent(in) :: ch
logical :: bool
bool = (ch == horizontal_tab_char) .or. &
& (ch == linefeed_char) .or. &
& (ch == vertical_tab_char) .or. &
& (ch == formfeed_char) .or. &
& (ch == carriage_return_char) .or. &
& (ch == space_char)
end function isspace
end module helper_procedures
module string_buffers
use, intrinsic :: iso_fortran_env, only: error_unit
use, intrinsic :: iso_fortran_env, only: int64
use, non_intrinsic :: compiler_type_kinds, only: nk, ck, ick
use, non_intrinsic :: helper_procedures
implicit none
private
public :: strbuf_t
public :: skip_whitespace
public :: skip_non_whitespace
public :: skip_whitespace_backwards
public :: at_end_of_line
type :: strbuf_t
integer(kind = nk), private :: len = 0
!
! ‘chars’ is made public for efficient access to the individual
! characters.
!
character(1, kind = ck), allocatable, public :: chars(:)
contains
procedure, pass, private :: ensure_storage => strbuf_t_ensure_storage
procedure, pass :: to_unicode_full_string => strbuf_t_to_unicode_full_string
procedure, pass :: to_unicode_substring => strbuf_t_to_unicode_substring
procedure, pass :: length => strbuf_t_length
procedure, pass :: set => strbuf_t_set
procedure, pass :: append => strbuf_t_append
generic :: to_unicode => to_unicode_full_string
generic :: to_unicode => to_unicode_substring
generic :: assignment(=) => set
end type strbuf_t
contains
function strbuf_t_to_unicode_full_string (strbuf) result (s)
class(strbuf_t), intent(in) :: strbuf
character(:, kind = ck), allocatable :: s
!
! This does not actually ensure that the string is valid Unicode;
! any 31-bit ‘character’ is supported.
!
integer(kind = nk) :: i
allocate (character(len = strbuf%len, kind = ck) :: s)
do i = 1, strbuf%len
s(i:i) = strbuf%chars(i)
end do
end function strbuf_t_to_unicode_full_string
function strbuf_t_to_unicode_substring (strbuf, i, j) result (s)
!
! ‘Extreme’ values of i and j are allowed, as shortcuts for ‘from
! the beginning’, ‘up to the end’, or ‘empty substring’.
!
class(strbuf_t), intent(in) :: strbuf
integer(kind = nk), intent(in) :: i, j
character(:, kind = ck), allocatable :: s
!
! This does not actually ensure that the string is valid Unicode;
! any 31-bit ‘character’ is supported.
!
integer(kind = nk) :: i1, j1
integer(kind = nk) :: n
integer(kind = nk) :: k
i1 = max (1_nk, i)
j1 = min (strbuf%len, j)
n = max (0_nk, (j1 - i1) + 1_nk)
allocate (character(n, kind = ck) :: s)
do k = 1, n
s(k:k) = strbuf%chars(i1 + (k - 1_nk))
end do
end function strbuf_t_to_unicode_substring
elemental function strbuf_t_length (strbuf) result (n)
class(strbuf_t), intent(in) :: strbuf
integer(kind = nk) :: n
n = strbuf%len
end function strbuf_t_length
subroutine strbuf_t_ensure_storage (strbuf, length_needed)
class(strbuf_t), intent(inout) :: strbuf
integer(kind = nk), intent(in) :: length_needed
integer(kind = nk) :: len_needed
integer(kind = nk) :: new_size
type(strbuf_t) :: new_strbuf
len_needed = max (length_needed, 1_nk)
if (.not. allocated (strbuf%chars)) then
! Initialize a new strbuf%chars array.
new_size = new_storage_size (len_needed)
allocate (strbuf%chars(1:new_size))
else if (ubound (strbuf%chars, 1) < len_needed) then
! Allocate a new strbuf%chars array, larger than the current
! one, but containing the same characters.
new_size = new_storage_size (len_needed)
allocate (new_strbuf%chars(1:new_size))
new_strbuf%chars(1:strbuf%len) = strbuf%chars(1:strbuf%len)
call move_alloc (new_strbuf%chars, strbuf%chars)
end if
end subroutine strbuf_t_ensure_storage
subroutine strbuf_t_set (dst, src)
class(strbuf_t), intent(inout) :: dst
class(*), intent(in) :: src
integer(kind = nk) :: n
integer(kind = nk) :: i
select type (src)
type is (character(*, kind = ck))
n = len (src, kind = nk)
call dst%ensure_storage(n)
do i = 1, n
dst%chars(i) = src(i:i)
end do
dst%len = n
type is (character(*))
n = len (src, kind = nk)
call dst%ensure_storage(n)
do i = 1, n
dst%chars(i) = src(i:i)
end do
dst%len = n
class is (strbuf_t)
n = src%len
call dst%ensure_storage(n)
dst%chars(1:n) = src%chars(1:n)
dst%len = n
class default
error stop
end select
end subroutine strbuf_t_set
subroutine strbuf_t_append (dst, src)
class(strbuf_t), intent(inout) :: dst
class(*), intent(in) :: src
integer(kind = nk) :: n_dst, n_src, n
integer(kind = nk) :: i
select type (src)
type is (character(*, kind = ck))
n_dst = dst%len
n_src = len (src, kind = nk)
n = n_dst + n_src
call dst%ensure_storage(n)
do i = 1, n_src
dst%chars(n_dst + i) = src(i:i)
end do
dst%len = n
type is (character(*))
n_dst = dst%len
n_src = len (src, kind = nk)
n = n_dst + n_src
call dst%ensure_storage(n)
do i = 1, n_src
dst%chars(n_dst + i) = src(i:i)
end do
dst%len = n
class is (strbuf_t)
n_dst = dst%len
n_src = src%len
n = n_dst + n_src
call dst%ensure_storage(n)
dst%chars((n_dst + 1):n) = src%chars(1:n_src)
dst%len = n
class default
error stop
end select
end subroutine strbuf_t_append
function skip_whitespace (strbuf, i) result (j)
class(strbuf_t), intent(in) :: strbuf
integer(kind = nk), intent(in) :: i
integer(kind = nk) :: j
logical :: done
j = i
done = .false.
do while (.not. done)
if (at_end_of_line (strbuf, j)) then
done = .true.
else if (.not. isspace (strbuf%chars(j))) then
done = .true.
else
j = j + 1
end if
end do
end function skip_whitespace
function skip_non_whitespace (strbuf, i) result (j)
class(strbuf_t), intent(in) :: strbuf
integer(kind = nk), intent(in) :: i
integer(kind = nk) :: j
logical :: done
j = i
done = .false.
do while (.not. done)
if (at_end_of_line (strbuf, j)) then
done = .true.
else if (isspace (strbuf%chars(j))) then
done = .true.
else
j = j + 1
end if
end do
end function skip_non_whitespace
function skip_whitespace_backwards (strbuf, i) result (j)
class(strbuf_t), intent(in) :: strbuf
integer(kind = nk), intent(in) :: i
integer(kind = nk) :: j
logical :: done
j = i
done = .false.
do while (.not. done)
if (j == -1) then
done = .true.
else if (.not. isspace (strbuf%chars(j))) then
done = .true.
else
j = j - 1
end if
end do
end function skip_whitespace_backwards
function at_end_of_line (strbuf, i) result (bool)
class(strbuf_t), intent(in) :: strbuf
integer(kind = nk), intent(in) :: i
logical :: bool
bool = (strbuf%length() < i)
end function at_end_of_line
end module string_buffers
module reading_one_line_from_a_stream
use, intrinsic :: iso_fortran_env, only: input_unit
use, intrinsic :: iso_fortran_env, only: error_unit
use, non_intrinsic :: compiler_type_kinds, only: nk, ck, ick
use, non_intrinsic :: string_buffers
implicit none
private
! get_line_from_stream: read an entire input line from a stream into
! a strbuf_t.
public :: get_line_from_stream
character(1, kind = ck), parameter :: linefeed_char = char (10, kind = ck)
! The following is correct for Unix and its relatives.
character(1, kind = ck), parameter :: newline_char = linefeed_char
contains
subroutine get_line_from_stream (unit_no, eof, no_newline, strbuf)
integer, intent(in) :: unit_no
logical, intent(out) :: eof ! End of file?
logical, intent(out) :: no_newline ! There is a line but it has no
! newline? (Thus eof also must
! be .true.)
class(strbuf_t), intent(inout) :: strbuf
character(1, kind = ck) :: ch
strbuf = ''
call get_ch (unit_no, eof, ch)
do while (.not. eof .and. ch /= newline_char)
call strbuf%append (ch)
call get_ch (unit_no, eof, ch)
end do
no_newline = eof .and. (strbuf%length() /= 0)
end subroutine get_line_from_stream
subroutine get_ch (unit_no, eof, ch)
!
! Read a single code point from the stream.
!
! Currently this procedure simply inputs ‘ASCII’ bytes rather than
! Unicode code points.
!
integer, intent(in) :: unit_no
logical, intent(out) :: eof
character(1, kind = ck), intent(out) :: ch
integer :: stat
character(1) :: c = '*'
eof = .false.
if (unit_no == input_unit) then
call get_input_unit_char (c, stat)
else
read (unit = unit_no, iostat = stat) c
end if
if (stat < 0) then
ch = ck_'*'
eof = .true.
else if (0 < stat) then
write (error_unit, '("Input error with status code ", I0)') stat
stop 1
else
ch = char (ichar (c, kind = ick), kind = ck)
end if
end subroutine get_ch
!!!
!!! If you tell gfortran you want -std=f2008 or -std=f2018, you likely
!!! will need to add also -fall-intrinsics or -U__GFORTRAN__
!!!
!!! The first way, you get the FGETC intrinsic. The latter way, you
!!! get the C interface code that uses getchar(3).
!!!
#ifdef __GFORTRAN__
subroutine get_input_unit_char (c, stat)
!
! The following works if you are using gfortran.
!
! (FGETC is considered a feature for backwards compatibility with
! g77. However, I know of no way to reconfigure input_unit as a
! Fortran 2003 stream, for use with ordinary ‘read’.)
!
character, intent(inout) :: c
integer, intent(out) :: stat
call fgetc (input_unit, c, stat)
end subroutine get_input_unit_char
#else
subroutine get_input_unit_char (c, stat)
!
! An alternative implementation of get_input_unit_char. This
! actually reads input from the C standard input, which might not
! be the same as input_unit.
!
use, intrinsic :: iso_c_binding, only: c_int
character, intent(inout) :: c
integer, intent(out) :: stat
interface
!
! Use getchar(3) to read characters from standard input. This
! assumes there is actually such a function available, and that
! getchar(3) does not exist solely as a macro. (One could write
! one’s own getchar() if necessary, of course.)
!
function getchar () result (c) bind (c, name = 'getchar')
use, intrinsic :: iso_c_binding, only: c_int
integer(kind = c_int) :: c
end function getchar
end interface
integer(kind = c_int) :: i_char
i_char = getchar ()
!
! The C standard requires that EOF have a negative value. If the
! value returned by getchar(3) is not EOF, then it will be
! representable as an unsigned char. Therefore, to check for end
! of file, one need only test whether i_char is negative.
!
if (i_char < 0) then
stat = -1
else
stat = 0
c = char (i_char)
end if
end subroutine get_input_unit_char
#endif
end module reading_one_line_from_a_stream
module ast_reader
!
! The AST will be read into an array. Perhaps that will improve
! locality, compared to storing the AST as many linked heap nodes.
!
! In any case, implementing the AST this way is an interesting
! problem.
!
use, intrinsic :: iso_fortran_env, only: input_unit
use, intrinsic :: iso_fortran_env, only: output_unit
use, intrinsic :: iso_fortran_env, only: error_unit
use, non_intrinsic :: compiler_type_kinds, only: nk, ck, ick, rik
use, non_intrinsic :: helper_procedures, only: next_power_of_two
use, non_intrinsic :: helper_procedures, only: new_storage_size
use, non_intrinsic :: string_buffers
use, non_intrinsic :: reading_one_line_from_a_stream
implicit none
private
public :: symbol_table_t
public :: interpreter_ast_node_t
public :: interpreter_ast_t
public :: read_ast
integer, parameter, public :: node_Nil = 0
integer, parameter, public :: node_Identifier = 1
integer, parameter, public :: node_String = 2
integer, parameter, public :: node_Integer = 3
integer, parameter, public :: node_Sequence = 4
integer, parameter, public :: node_If = 5
integer, parameter, public :: node_Prtc = 6
integer, parameter, public :: node_Prts = 7
integer, parameter, public :: node_Prti = 8
integer, parameter, public :: node_While = 9
integer, parameter, public :: node_Assign = 10
integer, parameter, public :: node_Negate = 11
integer, parameter, public :: node_Not = 12
integer, parameter, public :: node_Multiply = 13
integer, parameter, public :: node_Divide = 14
integer, parameter, public :: node_Mod = 15
integer, parameter, public :: node_Add = 16
integer, parameter, public :: node_Subtract = 17
integer, parameter, public :: node_Less = 18
integer, parameter, public :: node_LessEqual = 19
integer, parameter, public :: node_Greater = 20
integer, parameter, public :: node_GreaterEqual = 21
integer, parameter, public :: node_Equal = 22
integer, parameter, public :: node_NotEqual = 23
integer, parameter, public :: node_And = 24
integer, parameter, public :: node_Or = 25
type :: symbol_table_element_t
character(:, kind = ck), allocatable :: str
end type symbol_table_element_t
type :: symbol_table_t
integer(kind = nk), private :: len = 0_nk
type(symbol_table_element_t), allocatable, private :: symbols(:)
contains
procedure, pass, private :: ensure_storage => symbol_table_t_ensure_storage
procedure, pass :: look_up_index => symbol_table_t_look_up_index
procedure, pass :: look_up_name => symbol_table_t_look_up_name
procedure, pass :: length => symbol_table_t_length
generic :: look_up => look_up_index
generic :: look_up => look_up_name
end type symbol_table_t
type :: interpreter_ast_node_t
integer :: node_variety
integer(kind = rik) :: int ! Runtime integer or symbol index.
character(:, kind = ck), allocatable :: str ! String value.
! The left branch begins at the next node. The right branch
! begins at the address of the left branch, plus the following.
integer(kind = nk) :: right_branch_offset
end type interpreter_ast_node_t
type :: interpreter_ast_t
integer(kind = nk), private :: len = 0_nk
type(interpreter_ast_node_t), allocatable, public :: nodes(:)
contains
procedure, pass, private :: ensure_storage => interpreter_ast_t_ensure_storage
end type interpreter_ast_t
contains
subroutine symbol_table_t_ensure_storage (symtab, length_needed)
class(symbol_table_t), intent(inout) :: symtab
integer(kind = nk), intent(in) :: length_needed
integer(kind = nk) :: len_needed
integer(kind = nk) :: new_size
type(symbol_table_t) :: new_symtab
len_needed = max (length_needed, 1_nk)
if (.not. allocated (symtab%symbols)) then
! Initialize a new symtab%symbols array.
new_size = new_storage_size (len_needed)
allocate (symtab%symbols(1:new_size))
else if (ubound (symtab%symbols, 1) < len_needed) then
! Allocate a new symtab%symbols array, larger than the current
! one, but containing the same symbols.
new_size = new_storage_size (len_needed)
allocate (new_symtab%symbols(1:new_size))
new_symtab%symbols(1:symtab%len) = symtab%symbols(1:symtab%len)
call move_alloc (new_symtab%symbols, symtab%symbols)
end if
end subroutine symbol_table_t_ensure_storage
elemental function symbol_table_t_length (symtab) result (len)
class(symbol_table_t), intent(in) :: symtab
integer(kind = nk) :: len
len = symtab%len
end function symbol_table_t_length
function symbol_table_t_look_up_index (symtab, symbol_name) result (index)
class(symbol_table_t), intent(inout) :: symtab
character(*, kind = ck), intent(in) :: symbol_name
integer(kind = rik) :: index
!
! This implementation simply stores the symbols sequentially into
! an array. Obviously, for large numbers of symbols, one might
! wish to do something more complex.
!
! Standard Fortran does not come, out of the box, with a massive
! runtime library for doing such things. They are, however, no
! longer nearly as challenging to implement in Fortran as they
! used to be.
!
integer(kind = nk) :: i
i = 1
index = 0
do while (index == 0)
if (i == symtab%len + 1) then
! The symbol is new and must be added to the table.
i = symtab%len + 1
if (huge (1_rik) < i) then
! Symbol indices are assumed to be storable as runtime
! integers.
write (error_unit, '("There are more symbols than can be handled.")')
stop 1
end if
call symtab%ensure_storage(i)
symtab%len = i
allocate (symtab%symbols(i)%str, source = symbol_name)
index = int (i, kind = rik)
else if (symtab%symbols(i)%str == symbol_name) then
index = int (i, kind = rik)
else
i = i + 1
end if
end do
end function symbol_table_t_look_up_index
function symbol_table_t_look_up_name (symtab, index) result (symbol_name)
class(symbol_table_t), intent(inout) :: symtab
integer(kind = rik), intent(in) :: index
character(:, kind = ck), allocatable :: symbol_name
!
! This is the reverse of symbol_table_t_look_up_index: given an
! index, it finds the symbol’s name.
!
if (index < 1 .or. symtab%len < index) then
! In correct code, this branch should never be reached.
error stop
else
allocate (symbol_name, source = symtab%symbols(index)%str)
end if
end function symbol_table_t_look_up_name
subroutine interpreter_ast_t_ensure_storage (ast, length_needed)
class(interpreter_ast_t), intent(inout) :: ast
integer(kind = nk), intent(in) :: length_needed
integer(kind = nk) :: len_needed
integer(kind = nk) :: new_size
type(interpreter_ast_t) :: new_ast
len_needed = max (length_needed, 1_nk)
if (.not. allocated (ast%nodes)) then
! Initialize a new ast%nodes array.
new_size = new_storage_size (len_needed)
allocate (ast%nodes(1:new_size))
else if (ubound (ast%nodes, 1) < len_needed) then
! Allocate a new ast%nodes array, larger than the current one,
! but containing the same nodes.
new_size = new_storage_size (len_needed)
allocate (new_ast%nodes(1:new_size))
new_ast%nodes(1:ast%len) = ast%nodes(1:ast%len)
call move_alloc (new_ast%nodes, ast%nodes)
end if
end subroutine interpreter_ast_t_ensure_storage
subroutine read_ast (unit_no, strbuf, ast, symtab)
integer, intent(in) :: unit_no
type(strbuf_t), intent(inout) :: strbuf
type(interpreter_ast_t), intent(inout) :: ast
type(symbol_table_t), intent(inout) :: symtab
logical :: eof
logical :: no_newline
integer(kind = nk) :: after_ast_address
symtab%len = 0
ast%len = 0
call build_subtree (1_nk, after_ast_address)
contains
recursive subroutine build_subtree (here_address, after_subtree_address)
integer(kind = nk), value :: here_address
integer(kind = nk), intent(out) :: after_subtree_address
integer :: node_variety
integer(kind = nk) :: i, j
integer(kind = nk) :: left_branch_address
integer(kind = nk) :: right_branch_address
! Get a line from the parser output.
call get_line_from_stream (unit_no, eof, no_newline, strbuf)
if (eof) then
call ast_error
else
! Prepare to store a new node.
call ast%ensure_storage(here_address)
ast%len = here_address
! What sort of node is it?
i = skip_whitespace (strbuf, 1_nk)
j = skip_non_whitespace (strbuf, i)
node_variety = strbuf_to_node_variety (strbuf, i, j - 1)
ast%nodes(here_address)%node_variety = node_variety
select case (node_variety)
case (node_Nil)
after_subtree_address = here_address + 1
case (node_Identifier)
i = skip_whitespace (strbuf, j)
j = skip_non_whitespace (strbuf, i)
ast%nodes(here_address)%int = &
& strbuf_to_symbol_index (strbuf, i, j - 1, symtab)
after_subtree_address = here_address + 1
case (node_String)
i = skip_whitespace (strbuf, j)
j = skip_whitespace_backwards (strbuf, strbuf%length())
ast%nodes(here_address)%str = strbuf_to_string (strbuf, i, j)
after_subtree_address = here_address + 1
case (node_Integer)
i = skip_whitespace (strbuf, j)
j = skip_non_whitespace (strbuf, i)
ast%nodes(here_address)%int = strbuf_to_int (strbuf, i, j - 1)
after_subtree_address = here_address + 1
case default
! The node is internal, and has left and right branches.
! The left branch will start at left_branch_address; the
! right branch will start at left_branch_address +
! right_side_offset.
left_branch_address = here_address + 1
! Build the left branch.
call build_subtree (left_branch_address, right_branch_address)
! Build the right_branch.
call build_subtree (right_branch_address, after_subtree_address)
ast%nodes(here_address)%right_branch_offset = &
& right_branch_address - left_branch_address
end select
end if
end subroutine build_subtree
end subroutine read_ast
function strbuf_to_node_variety (strbuf, i, j) result (node_variety)
class(strbuf_t), intent(in) :: strbuf
integer(kind = nk), intent(in) :: i, j
integer :: node_variety
!
! This function has not been optimized in any way, unless the
! Fortran compiler can optimize it.
!
! Something like a ‘radix tree search’ could be done on the
! characters of the strbuf. Or a perfect hash function. Or a
! binary search. Etc.
!
if (j == i - 1) then
call ast_error
else
select case (strbuf%to_unicode(i, j))
case (ck_";")
node_variety = node_Nil
case (ck_"Identifier")
node_variety = node_Identifier
case (ck_"String")
node_variety = node_String
case (ck_"Integer")
node_variety = node_Integer
case (ck_"Sequence")
node_variety = node_Sequence
case (ck_"If")
node_variety = node_If
case (ck_"Prtc")
node_variety = node_Prtc
case (ck_"Prts")
node_variety = node_Prts
case (ck_"Prti")
node_variety = node_Prti
case (ck_"While")
node_variety = node_While
case (ck_"Assign")
node_variety = node_Assign
case (ck_"Negate")
node_variety = node_Negate
case (ck_"Not")
node_variety = node_Not
case (ck_"Multiply")
node_variety = node_Multiply
case (ck_"Divide")
node_variety = node_Divide
case (ck_"Mod")
node_variety = node_Mod
case (ck_"Add")
node_variety = node_Add
case (ck_"Subtract")
node_variety = node_Subtract
case (ck_"Less")
node_variety = node_Less
case (ck_"LessEqual")
node_variety = node_LessEqual
case (ck_"Greater")
node_variety = node_Greater
case (ck_"GreaterEqual")
node_variety = node_GreaterEqual
case (ck_"Equal")
node_variety = node_Equal
case (ck_"NotEqual")
node_variety = node_NotEqual
case (ck_"And")
node_variety = node_And
case (ck_"Or")
node_variety = node_Or
case default
call ast_error
end select
end if
end function strbuf_to_node_variety
function strbuf_to_symbol_index (strbuf, i, j, symtab) result (int)
class(strbuf_t), intent(in) :: strbuf
integer(kind = nk), intent(in) :: i, j
type(symbol_table_t), intent(inout) :: symtab
integer(kind = rik) :: int
if (j == i - 1) then
call ast_error
else
int = symtab%look_up(strbuf%to_unicode (i, j))
end if
end function strbuf_to_symbol_index
function strbuf_to_int (strbuf, i, j) result (int)
class(strbuf_t), intent(in) :: strbuf
integer(kind = nk), intent(in) :: i, j
integer(kind = rik) :: int
integer :: stat
character(:, kind = ck), allocatable :: str
if (j < i) then
call ast_error
else
allocate (character(len = (j - i) + 1_nk, kind = ck) :: str)
str = strbuf%to_unicode (i, j)
read (str, *, iostat = stat) int
if (stat /= 0) then
call ast_error
end if
end if
end function strbuf_to_int
function strbuf_to_string (strbuf, i, j) result (str)
class(strbuf_t), intent(in) :: strbuf
integer(kind = nk), intent(in) :: i, j
character(:, kind = ck), allocatable :: str
character(1, kind = ck), parameter :: linefeed_char = char (10, kind = ck)
character(1, kind = ck), parameter :: backslash_char = char (92, kind = ck)
! The following is correct for Unix and its relatives.
character(1, kind = ck), parameter :: newline_char = linefeed_char
integer(kind = nk) :: k
integer(kind = nk) :: count
if (strbuf%chars(i) /= ck_'"' .or. strbuf%chars(j) /= ck_'"') then
call ast_error
else
! Count how many characters are needed.
count = 0
k = i + 1
do while (k < j)
count = count + 1
if (strbuf%chars(k) == backslash_char) then
k = k + 2
else
k = k + 1
end if
end do
allocate (character(len = count, kind = ck) :: str)
count = 0
k = i + 1
do while (k < j)
if (strbuf%chars(k) == backslash_char) then
if (k == j - 1) then
call ast_error
else
select case (strbuf%chars(k + 1))
case (ck_'n')
count = count + 1
str(count:count) = newline_char
case (backslash_char)
count = count + 1
str(count:count) = backslash_char
case default
call ast_error
end select
k = k + 2
end if
else
count = count + 1
str(count:count) = strbuf%chars(k)
k = k + 1
end if
end do
end if
end function strbuf_to_string
subroutine ast_error
!
! It might be desirable to give more detail.
!
write (error_unit, '("The AST input seems corrupted.")')
stop 1
end subroutine ast_error
end module ast_reader
module ast_interpreter
use, intrinsic :: iso_fortran_env, only: input_unit
use, intrinsic :: iso_fortran_env, only: output_unit
use, intrinsic :: iso_fortran_env, only: error_unit
use, non_intrinsic :: compiler_type_kinds
use, non_intrinsic :: ast_reader
implicit none
private
public :: value_t
public :: variable_table_t
public :: nil_value
public :: interpret_ast_node
integer, parameter, public :: v_Nil = 0
integer, parameter, public :: v_Integer = 1
integer, parameter, public :: v_String = 2
type :: value_t
integer :: tag = v_Nil
integer(kind = rik) :: int_val = -(huge (1_rik))
character(:, kind = ck), allocatable :: str_val
end type value_t
type :: variable_table_t
type(value_t), allocatable :: vals(:)
contains
procedure, pass :: initialize => variable_table_t_initialize
end type variable_table_t
! The canonical nil value.
type(value_t), parameter :: nil_value = value_t ()
contains
elemental function int_value (int_val) result (val)
integer(kind = rik), intent(in) :: int_val
type(value_t) :: val
val%tag = v_Integer
val%int_val = int_val
end function int_value
elemental function str_value (str_val) result (val)
character(*, kind = ck), intent(in) :: str_val
type(value_t) :: val
val%tag = v_String
allocate (val%str_val, source = str_val)
end function str_value
subroutine variable_table_t_initialize (vartab, symtab)
class(variable_table_t), intent(inout) :: vartab
type(symbol_table_t), intent(in) :: symtab
allocate (vartab%vals(1:symtab%length()), source = nil_value)
end subroutine variable_table_t_initialize
recursive subroutine interpret_ast_node (outp, ast, symtab, vartab, address, retval)
integer, intent(in) :: outp
type(interpreter_ast_t), intent(in) :: ast
type(symbol_table_t), intent(in) :: symtab
type(variable_table_t), intent(inout) :: vartab
integer(kind = nk) :: address
type(value_t), intent(inout) :: retval
integer(kind = rik) :: variable_index
type(value_t) :: val1, val2, val3
select case (ast%nodes(address)%node_variety)
case (node_Nil)
retval = nil_value
case (node_Integer)
retval = int_value (ast%nodes(address)%int)
case (node_Identifier)
variable_index = ast%nodes(address)%int
retval = vartab%vals(variable_index)
case (node_String)
retval = str_value (ast%nodes(address)%str)
case (node_Assign)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val1)
variable_index = ast%nodes(left_branch (address))%int
vartab%vals(variable_index) = val1
retval = nil_value
case (node_Multiply)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
call multiply (val1, val2, val3)
retval = val3
case (node_Divide)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
call divide (val1, val2, val3)
retval = val3
case (node_Mod)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
call pseudo_remainder (val1, val2, val3)
retval = val3
case (node_Add)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
call add (val1, val2, val3)
retval = val3
case (node_Subtract)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
call subtract (val1, val2, val3)
retval = val3
case (node_Less)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
call less_than (val1, val2, val3)
retval = val3
case (node_LessEqual)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
call less_than_or_equal_to (val1, val2, val3)
retval = val3
case (node_Greater)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
call greater_than (val1, val2, val3)
retval = val3
case (node_GreaterEqual)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
call greater_than_or_equal_to (val1, val2, val3)
retval = val3
case (node_Equal)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
call equal_to (val1, val2, val3)
retval = val3
case (node_NotEqual)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
call not_equal_to (val1, val2, val3)
retval = val3
case (node_Negate)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
retval = int_value (-(rik_cast (val1, ck_'unary ''-''')))
case (node_Not)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
retval = int_value (bool2int (rik_cast (val1, ck_'unary ''!''') == 0_rik))
case (node_And)
! For similarity to C, we make this a ‘short-circuiting AND’,
! which is really a branching construct rather than a binary
! operation.
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
if (rik_cast (val1, ck_'''&&''') == 0_rik) then
retval = int_value (0_rik)
else
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
retval = int_value (bool2int (rik_cast (val2, ck_'''&&''') /= 0_rik))
end if
case (node_Or)
! For similarity to C, we make this a ‘short-circuiting OR’,
! which is really a branching construct rather than a binary
! operation.
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
if (rik_cast (val1, ck_'''||''') /= 0_rik) then
retval = int_value (1_rik)
else
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
retval = int_value (bool2int (rik_cast (val2, ck_'''||''') /= 0_rik))
end if
case (node_If)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
if (rik_cast (val1, ck_'''if-else'' construct') /= 0_rik) then
call interpret_ast_node (outp, ast, symtab, vartab, &
& left_branch (right_branch (address)), &
& val2)
else
call interpret_ast_node (outp, ast, symtab, vartab, &
& right_branch (right_branch (address)), &
& val2)
end if
retval = nil_value
case (node_While)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
do while (rik_cast (val1, ck_'''while'' construct') /= 0_rik)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
end do
retval = nil_value
case (node_Prtc)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
write (outp, '(A1)', advance = 'no') &
& char (rik_cast (val1, ck_'''putc'''), kind = ck)
retval = nil_value
case (node_Prti, node_Prts)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
select case (val1%tag)
case (v_Integer)
write (outp, '(I0)', advance = 'no') val1%int_val
case (v_String)
write (outp, '(A)', advance = 'no') val1%str_val
case (v_Nil)
write (outp, '("(no value)")', advance = 'no')
case default
error stop
end select
retval = nil_value
case (node_Sequence)
call interpret_ast_node (outp, ast, symtab, vartab, left_branch (address), val1)
call interpret_ast_node (outp, ast, symtab, vartab, right_branch (address), val2)
retval = nil_value
case default
write (error_unit, '("unknown node type")')
stop 1
end select
contains
elemental function left_branch (here_addr) result (left_addr)
integer(kind = nk), intent(in) :: here_addr
integer(kind = nk) :: left_addr
left_addr = here_addr + 1
end function left_branch
elemental function right_branch (here_addr) result (right_addr)
integer(kind = nk), intent(in) :: here_addr
integer(kind = nk) :: right_addr
right_addr = here_addr + 1 + ast%nodes(here_addr)%right_branch_offset
end function right_branch
end subroutine interpret_ast_node
subroutine multiply (x, y, z)
type(value_t), intent(in) :: x, y
type(value_t), intent(out) :: z
character(*, kind = ck), parameter :: op = ck_'*'
z = int_value (rik_cast (x, op) * rik_cast (y, op))
end subroutine multiply
subroutine divide (x, y, z)
type(value_t), intent(in) :: x, y
type(value_t), intent(out) :: z
character(*, kind = ck), parameter :: op = ck_'/'
! Fortran integer division truncates towards zero, as C’s does.
z = int_value (rik_cast (x, op) / rik_cast (y, op))
end subroutine divide
subroutine pseudo_remainder (x, y, z)
type(value_t), intent(in) :: x, y
type(value_t), intent(out) :: z
!
! I call this ‘pseudo-remainder’ because I consider ‘remainder’ to
! mean the *non-negative* remainder in A = (B * Quotient) +
! Remainder. See https://doi.org/10.1145%2F128861.128862
!
! The pseudo-remainder gives the actual remainder, if both
! operands are positive.
!
character(*, kind = ck), parameter :: op = ck_'binary ''%'''
! Fortran’s MOD intrinsic, when given integer arguments, works
! like C ‘%’.
z = int_value (mod (rik_cast (x, op), rik_cast (y, op)))
end subroutine pseudo_remainder
subroutine add (x, y, z)
type(value_t), intent(in) :: x, y
type(value_t), intent(out) :: z
character(*, kind = ck), parameter :: op = ck_'binary ''+'''
z = int_value (rik_cast (x, op) + rik_cast (y, op))
end subroutine add
subroutine subtract (x, y, z)
type(value_t), intent(in) :: x, y
type(value_t), intent(out) :: z
character(*, kind = ck), parameter :: op = ck_'binary ''-'''
z = int_value (rik_cast (x, op) - rik_cast (y, op))
end subroutine subtract
subroutine less_than (x, y, z)
type(value_t), intent(in) :: x, y
type(value_t), intent(out) :: z
character(*, kind = ck), parameter :: op = ck_'binary ''<'''
z = int_value (bool2int (rik_cast (x, op) < rik_cast (y, op)))
end subroutine less_than
subroutine less_than_or_equal_to (x, y, z)
type(value_t), intent(in) :: x, y
type(value_t), intent(out) :: z
character(*, kind = ck), parameter :: op = ck_'binary ''<='''
z = int_value (bool2int (rik_cast (x, op) <= rik_cast (y, op)))
end subroutine less_than_or_equal_to
subroutine greater_than (x, y, z)
type(value_t), intent(in) :: x, y
type(value_t), intent(out) :: z
character(*, kind = ck), parameter :: op = ck_'binary ''>'''
z = int_value (bool2int (rik_cast (x, op) > rik_cast (y, op)))
end subroutine greater_than
subroutine greater_than_or_equal_to (x, y, z)
type(value_t), intent(in) :: x, y
type(value_t), intent(out) :: z
character(*, kind = ck), parameter :: op = ck_'binary ''>='''
z = int_value (bool2int (rik_cast (x, op) >= rik_cast (y, op)))
end subroutine greater_than_or_equal_to
subroutine equal_to (x, y, z)
type(value_t), intent(in) :: x, y
type(value_t), intent(out) :: z
character(*, kind = ck), parameter :: op = ck_'binary ''=='''
z = int_value (bool2int (rik_cast (x, op) == rik_cast (y, op)))
end subroutine equal_to
subroutine not_equal_to (x, y, z)
type(value_t), intent(in) :: x, y
type(value_t), intent(out) :: z
character(*, kind = ck), parameter :: op = ck_'binary ''!='''
z = int_value (bool2int (rik_cast (x, op) /= rik_cast (y, op)))
end subroutine not_equal_to
function rik_cast (val, operation_name) result (i_val)
class(*), intent(in) :: val
character(*, kind = ck), intent(in) :: operation_name
integer(kind = rik) :: i_val
select type (val)
class is (value_t)
if (val%tag == v_Integer) then
i_val = val%int_val
else
call type_error (operation_name)
end if
type is (integer(kind = rik))
i_val = val
class default
call type_error (operation_name)
end select
end function rik_cast
elemental function bool2int (bool) result (int)
logical, intent(in) :: bool
integer(kind = rik) :: int
if (bool) then
int = 1_rik
else
int = 0_rik
end if
end function bool2int
subroutine type_error (operation_name)
character(*, kind = ck), intent(in) :: operation_name
write (error_unit, '("type error in ", A)') operation_name
stop 1
end subroutine type_error
end module ast_interpreter
program Interp
use, intrinsic :: iso_fortran_env, only: input_unit
use, intrinsic :: iso_fortran_env, only: output_unit
use, intrinsic :: iso_fortran_env, only: error_unit
use, non_intrinsic :: compiler_type_kinds
use, non_intrinsic :: string_buffers
use, non_intrinsic :: ast_reader
use, non_intrinsic :: ast_interpreter
implicit none
integer, parameter :: inp_unit_no = 100
integer, parameter :: outp_unit_no = 101
integer :: arg_count
character(200) :: arg
integer :: inp
integer :: outp
type(strbuf_t) :: strbuf
type(interpreter_ast_t) :: ast
type(symbol_table_t) :: symtab
type(variable_table_t) :: vartab
type(value_t) :: retval
arg_count = command_argument_count ()
if (3 <= arg_count) then
call print_usage
else
if (arg_count == 0) then
inp = input_unit
outp = output_unit
else if (arg_count == 1) then
call get_command_argument (1, arg)
inp = open_for_input (trim (arg))
outp = output_unit
else if (arg_count == 2) then
call get_command_argument (1, arg)
inp = open_for_input (trim (arg))
call get_command_argument (2, arg)
outp = open_for_output (trim (arg))
end if
call read_ast (inp, strbuf, ast, symtab)
if (1 <= ubound (ast%nodes, 1)) then
call vartab%initialize(symtab)
call interpret_ast_node (outp, ast, symtab, vartab, 1_nk, retval)
end if
end if
contains
function open_for_input (filename) result (unit_no)
character(*), intent(in) :: filename
integer :: unit_no
integer :: stat
open (unit = inp_unit_no, file = filename, status = 'old', &
& action = 'read', access = 'stream', form = 'unformatted', &
& iostat = stat)
if (stat /= 0) then
write (error_unit, '("Error: failed to open ", 1A, " for input")') filename
stop 1
end if
unit_no = inp_unit_no
end function open_for_input
function open_for_output (filename) result (unit_no)
character(*), intent(in) :: filename
integer :: unit_no
integer :: stat
open (unit = outp_unit_no, file = filename, action = 'write', iostat = stat)
if (stat /= 0) then
write (error_unit, '("Error: failed to open ", 1A, " for output")') filename
stop 1
end if
unit_no = outp_unit_no
end function open_for_output
subroutine print_usage
character(200) :: progname
call get_command_argument (0, progname)
write (output_unit, '("Usage: ", 1A, " [INPUT_FILE [OUTPUT_FILE]]")') &
& trim (progname)
end subroutine print_usage
end program Interp
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