operators.f90

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!> @file
!! @defgroup group_operators Operators
!! Module implementing a full C-preprocessor-style constant expression evaluator using a top-down recursive descent parser.
!! The module provides the ability to evaluate integer constant expressions of the kind used in
!! classical preprocessor. 
!!
!! This includes support for:
!! - All C-style arithmetic, bitwise, logical, relational, and conditional operators
!! - Operator precedence and associativity
!! - Macro identifier substitution and the special `defined(identifier)` operator
!! - Integer literals in decimal, octal (`0...`), hexadecimal (`0x...`), and binary (`0b...`) bases
!! - Parenthesized sub-expressions and proper handling of unary operators
!!
!! The implementation consists of two major phases:
!!
!! 1. Tokenization
!!    The input string is scanned and converted into a sequence of @link fpx_token::token token @endlink objects.
!!    The tokenizer recognizes multi-character operators ('&&', '||', '==', '!=', '<=', '>=', '<<', '>>', '**'),
!!    the `defined` operator (with or without parentheses), numbers in all supported bases,
!!    identifiers, and parentheses. Whitespace is ignored except as a token separator.
!!
!! 2. Parsing and evaluation via top-down recursive descent
!!    A classic predictive (LL(1)) recursive descent parser is used, where each non-terminal
!!    in the grammar is implemented as a separate parsing function with the exact precedence level.
!!    The grammar is directly derived from the C standard operator precedence table:
!!
!!    parse_expression        ? parse_conditional
!!    parse_conditional       ? parse_or (parse_or '?' parse_expression ':' parse_conditional)
!!    parse_or                ? parse_and ( '||' parse_and )*
!!    parse_and               ? parse_bitwise_or ( '&&' parse_bitwise_or )*
!!    parse_bitwise_or        ? parse_bitwise_xor ( '|' parse_bitwise_xor )*
!!    parse_bitwise_xor       ? parse_bitwise_and ( '^' parse_bitwise_and )*
!!    parse_bitwise_and       ? parse_equality ( '&' parse_equality )*
!!    parse_equality          ? parse_relational ( ('==' | '!=') parse_relational )*
!!    parse_relational        ? parse_shifting ( ('<' | '>' | '<=' | '>=') parse_shifting )*
!!    parse_shifting          ? parse_additive ( ('<<' | '>>') parse_additive )*
!!    parse_additive          ? parse_multiplicative ( ('+' | '-') parse_multiplicative )*
!!    parse_multiplicative    ? parse_power ( ('*' | '/' | '%') parse_power )*
!!    parse_power             ? parse_unary ( '**' parse_unary )*          (right-associative)
!!    parse_unary             ? ('!' | '-' | '+' | '~') parse_unary
!!                             | parse_atom
!!    parse_atom              ? number
!!                             | identifier                              (macro expansion)
!!                             | 'defined' ( identifier ) | 'defined' identifier
!!                             | '(' parse_expression ')'
!!
!!    Each parsing function consumes tokens from the global position `pos` and returns the
!!    integer value of the sub-expression it recognizes. Because the grammar is factored by
!!    precedence, left-associativity is achieved naturally via left-recursive loops,
!!    while right-associativity for the power operator (`**`) is handled by calling
!!    `parse_unary` on the right-hand side first.
!!
!!    Macro expansion occurs lazily inside `parse_atom` when an identifier token is encountered:
!!    - If the identifier is defined, its replacement text is recursively evaluated.
!!    - The special `defined` operator yields 1 or 0 depending on whether the identifier exists.
!!
!!    The parser is fully re-entrant and has no global state.
!!
!!    Public interface:
!!    - @link fpx_operators::evaluate_expression evaluate_expression @endlink: high-level function that tokenizes and evaluates in
!! one
!! call
!!    - @link fpx_operators::parse_expression parse_expression @endlink: low-level entry point for already-tokenized input
!!
!!    This design guarantees correct operator precedence without the need for an explicit
!!    abstract syntax tree or stack-based shunting-yard algorithm, while remaining easy to
!!    read, maintain, and extend.
module fpx_operators
    use fpx_global
    use fpx_string
    use fpx_constants
    use fpx_macro
    use fpx_logging
    use fpx_token
    use fpx_context
 
    implicit none; private
 
    public :: evaluate_expression
 
    interface evaluate_expression
        module procedure :: evaluate_expression_default
        module procedure :: evaluate_expression_with_context
    end interface
 
contains
 
    !> Evaluates a preprocessor-style expression with macro substitution.
    !! Tokenizes the input expression, expands macros where appropriate,
    !! parses it according to operator precedence, and computes the integer result.
    !! Returns .true. if evaluation succeeded and the result is non-zero.
    !!
    !! @param[in] expr   Expression string to evaluate
    !! @param[in] macros Array of defined macros for substitution and `defined()` checks
    !! @param[out] val   (optional) integer result of the evaluation
    !! @return .true. if the expression evaluated successfully to non-zero, .false. otherwise
    !!
    !! @b Remarks
    !! @ingroup group_operators
    logical function evaluate_expression_default(expr, macros, val) result(res)
        character(*), intent(in)        :: expr
        type(macro), intent(in)         :: macros(:)
        integer, intent(out), optional  :: val
        !private
        type(context) :: ctx
 
        ctx = context(expr, 1, '')
        res = evaluate_expression(expr, macros, ctx, val)
    end function
 
    !> Evaluates a preprocessor-style expression with macro substitution.
    !! Tokenizes the input expression, expands macros where appropriate,
    !! parses it according to operator precedence, and computes the integer result.
    !! Returns .true. if evaluation succeeded and the result is non-zero.
    !!
    !! @param[in] expr   Expression string to evaluate
    !! @param[in] macros Array of defined macros for substitution and `defined()` checks
    !! @param[in] ctx Context
    !! @param[out] val   (optional) integer result of the evaluation
    !! @return .true. if the expression evaluated successfully to non-zero, .false. otherwise
    !!
    !! @b Remarks
    !! @ingroup group_operators
    logical function evaluate_expression_with_context(expr, macros, ctx, val) result(res)
        character(*), intent(in)        :: expr
        type(macro), intent(in)         :: macros(:)
        type(context), intent(in)       :: ctx
        integer, intent(out), optional  :: val
        !private
        type(token), allocatable :: tokens(:)
        integer :: ntokens, pos, result
 
        call tokenize(expr, tokens, ntokens)
        if (ntokens == 0) then
            call printf(render(diagnostic_report(level_error, &
                    message='Tokenization failed', &
                    label=label_type('No tokens found', 1, len_trim(expr)), &
                    source=trim(ctx%path)), &
                    expr, ctx%line))
            res = .false.
            return
        end if
 
        pos = 1
        result = parse_expression(expr, tokens, ntokens, pos, macros, ctx)
        if (pos <= ntokens) then
            call printf(render(diagnostic_report(level_error, &
                    message='Tokenization failed', &
                    label=label_type('Extra tokens found', tokens(pos)%start, len_trim(tokens(pos)%value)), &
                    source=trim(ctx%path)), &
                    expr, ctx%line))
            res = .false.
            return
        end if
        res = (result /= 0)
        if (present(val)) val = result
    end function
 
    !> Parses a sequence of tokens starting at position `pos` as a full expression.
    !! Entry point for the recursive descent parser. Delegates to parse_or().
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_expression(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
 
        val = parse_conditional(expr, tokens, ntokens, pos, macros, ctx)
    end function
 
    !> Parses conditional expressions (?:). Right-associative.
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_conditional(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: condition, true_val, false_val
 
        ! First parse condition at higher precedence
        condition = parse_or(expr, tokens, ntokens, pos, macros, ctx)
 
        ! Check for '?'
        if (pos <= ntokens .and. tokens(pos)%value == '?') then
            pos = pos + 1
            ! Parse true expression (full expression allowed)
            true_val = parse_expression(expr, tokens, ntokens, pos, macros, ctx)
 
            ! Expect ':'
            if (pos > ntokens .or. tokens(pos)%value /= ':') then
                call printf(render(diagnostic_report(level_error, &
                        message='Synthax error', &
                        label=label_type('Expected ":" in conditional expression', 1, len(expr)), &
                        source=trim(ctx%path)), &
                        expr, ctx%line))
                val = 0
                return
            end if
 
            pos = pos + 1
 
            ! Parse false expression (right-associative)
            false_val = parse_conditional(expr, tokens, ntokens, pos, macros, ctx)
 
            ! Evaluate condition
            val = merge(true_val, false_val, condition /= 0)
        else
            val = condition
        end if
 
    end function
 
    !> Parses logical OR expressions (`||`).
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_or(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: left
 
        left = parse_and(expr, tokens, ntokens, pos, macros, ctx)
        do while (pos <= ntokens .and. tokens(pos)%value == '||')
            pos = pos + 1
            val = merge(1, 0, left /= 0 .or. parse_and(expr, tokens, ntokens, pos, macros, ctx) /= 0)
            left = val
        end do
        val = left
    end function
 
    !> Parses logical AND expressions (`&&`).
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_and(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: left
 
        left = parse_bitwise_or(expr, tokens, ntokens, pos, macros, ctx)
        do while (pos <= ntokens .and. tokens(pos)%value == '&&')
            pos = pos + 1
            val = merge(1, 0, left /= 0 .and. parse_bitwise_or(expr, tokens, ntokens, pos, macros, ctx) /= 0)
            left = val
        end do
        val = left
    end function
 
    !> Parses bitwise OR expressions (`|`).
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_bitwise_or(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: left
 
        left = parse_bitwise_xor(expr, tokens, ntokens, pos, macros, ctx)
        do while (pos <= ntokens .and. tokens(pos)%value == '|')
            pos = pos + 1
            val = parse_bitwise_xor(expr, tokens, ntokens, pos, macros, ctx)
            left = ior(left, val)
        end do
        val = left
    end function
 
    !> Parses bitwise XOR expressions (`^`).
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_bitwise_xor(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: left
 
        left = parse_bitwise_and(expr, tokens, ntokens, pos, macros, ctx)
        do while (pos <= ntokens .and. tokens(pos)%value == '^')
            pos = pos + 1
            val = parse_bitwise_and(expr, tokens, ntokens, pos, macros, ctx)
            left = ieor(left, val)
        end do
        val = left
    end function
 
    !> Parses bitwise AND expressions (`&`).
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_bitwise_and(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: left
 
        left = parse_equality(expr, tokens, ntokens, pos, macros, ctx)
        do while (pos <= ntokens .and. tokens(pos)%value == '&')
            pos = pos + 1
            val = parse_equality(expr, tokens, ntokens, pos, macros, ctx)
            left = iand(left, val)
        end do
        val = left
    end function
 
    !> Parses equality/inequality expressions (`==`, `!=`).
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_equality(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: left, right
 
        left = parse_relational(expr, tokens, ntokens, pos, macros, ctx)
        do while (pos <= ntokens .and. (tokens(pos)%value == '==' .or. tokens(pos)%value == '!='))
            if (tokens(pos)%value == '==') then
                pos = pos + 1
                right = parse_relational(expr, tokens, ntokens, pos, macros, ctx)
                val = merge(1, 0, left == right)
            else
                pos = pos + 1
                right = parse_relational(expr, tokens, ntokens, pos, macros, ctx)
                val = merge(1, 0, left /= right)
            end if
            left = val
        end do
        val = left
    end function
 
    !> Parses relational expressions (`<`, `>`, `<=`, `>=`).
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_relational(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: left, right
 
        left = parse_shifting(expr, tokens, ntokens, pos, macros, ctx)
        do while (pos <= ntokens .and. (tokens(pos)%value == '<' .or. tokens(pos)%value == '>' .or. &
                tokens(pos)%value == '<=' .or. tokens(pos)%value == '>='))
            if (tokens(pos)%value == '<') then
                pos = pos + 1
                right = parse_shifting(expr, tokens, ntokens, pos, macros, ctx)
                val = merge(1, 0, left < right)
            else if (tokens(pos)%value == '>') then
                pos = pos + 1
                right = parse_shifting(expr, tokens, ntokens, pos, macros, ctx)
                val = merge(1, 0, left > right)
            else if (tokens(pos)%value == '<=') then
                pos = pos + 1
                right = parse_shifting(expr, tokens, ntokens, pos, macros, ctx)
                val = merge(1, 0, left <= right)
            else
                pos = pos + 1
                right = parse_shifting(expr, tokens, ntokens, pos, macros, ctx)
                val = merge(1, 0, left >= right)
            end if
            left = val
        end do
        val = left
    end function
 
    !> Parses shift expressions (`<<`, `>>`).
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_shifting(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: left, right
 
        left = parse_additive(expr, tokens, ntokens, pos, macros, ctx)
        do while (pos <= ntokens .and. (tokens(pos)%value == '<<' .or. tokens(pos)%value == '>>'))
            if (tokens(pos)%value == '<<') then
                pos = pos + 1
                right = parse_additive(expr, tokens, ntokens, pos, macros, ctx)
                val = lshift(left, right)
            else
                pos = pos + 1
                right = parse_additive(expr, tokens, ntokens, pos, macros, ctx)
                val = rshift(left, right)
            end if
            left = val
        end do
        val = left
    end function
 
    !> Parses additive expressions (`+`, `-`).
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_additive(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: left, right
 
        left = parse_multiplicative(expr, tokens, ntokens, pos, macros, ctx)
        do while (pos <= ntokens .and. (tokens(pos)%value == '+' .or. tokens(pos)%value == '-'))
            if (tokens(pos)%value == '+') then
                pos = pos + 1
                right = parse_multiplicative(expr, tokens, ntokens, pos, macros, ctx)
                val = left + right
            else
                pos = pos + 1
                right = parse_multiplicative(expr, tokens, ntokens, pos, macros, ctx)
                val = left - right
            end if
            left = val
        end do
        val = left
    end function
 
    !> Parses multiplicative expressions (`*`, `/`, `%`).
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx    Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_multiplicative(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: left, right
 
        left = parse_power(expr, tokens, ntokens, pos, macros, ctx)
        do while (pos <= ntokens .and. (tokens(pos)%value == '*' .or. tokens(pos)%value == '/' .or. tokens(pos)%value == '%'))
            if (tokens(pos)%value == '*') then
                pos = pos + 1
                right = parse_power(expr, tokens, ntokens, pos, macros, ctx)
                val = left * right
            else if (tokens(pos)%value == '/') then
                pos = pos + 1
                right = parse_power(expr, tokens, ntokens, pos, macros, ctx)
                val = left / right
            else
                pos = pos + 1
                right = parse_power(expr, tokens, ntokens, pos, macros, ctx)
                val = modulo(left, right)
            end if
            left = val
        end do
        val = left
    end function
 
    !> Parses exponentiation (`**`). Right-associative.
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx    Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_power(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: left, right
 
        left = parse_unary(expr, tokens, ntokens, pos, macros, ctx)
        do while (pos <= ntokens .and. (tokens(pos)%value == '**'))
            pos = pos + 1
            right = parse_unary(expr, tokens, ntokens, pos, macros, ctx)
            val = left**right
            left = val
        end do
        val = left
    end function
 
    !> Parses unary operators (`!`, `-`, `+`, `~`).
    !! @param[in] expr Expression to be processed
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx    Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_unary(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
 
        if (pos <= ntokens .and. tokens(pos)%value == '!') then
            pos = pos + 1
            val = merge(0, 1, parse_unary(expr, tokens, ntokens, pos, macros, ctx) /= 0)
        else if (pos <= ntokens .and. tokens(pos)%value == '-') then
            pos = pos + 1
            val = -parse_unary(expr, tokens, ntokens, pos, macros, ctx)
        else if (pos <= ntokens .and. tokens(pos)%value == '+') then
            pos = pos + 1
            val = parse_unary(expr, tokens, ntokens, pos, macros, ctx)
        else if (pos <= ntokens .and. tokens(pos)%value == '~') then
            pos = pos + 1
            val = not(parse_unary(expr, tokens, ntokens, pos, macros, ctx))
        else
            val = parse_atom(expr, tokens, ntokens, pos, macros, ctx)
        end if
    end function
 
    !> Parses primary expressions: numbers, identifiers, `defined(...)`, parentheses.
    !! @param[in] expr      Input expression
    !! @param[in] tokens    Array of tokens to parse
    !! @param[in] ntokens   Number of valid tokens in the array
    !! @param[inout] pos    Current parsing position (updated as tokens are consumed)
    !! @param[in] macros    Defined macros for expansion and `defined()` checks
    !! @param[in] ctx    Context
    !! @return Integer value of the parsed expression
    !!
    !! @b Remarks
    !! @ingroup group_operators
    recursive integer function parse_atom(expr, tokens, ntokens, pos, macros, ctx) result(val)
        character(*), intent(in)    :: expr
        type(token), intent(in)     :: tokens(:)
        integer, intent(in)         :: ntokens
        integer, intent(inout)      :: pos
        type(macro), intent(in)     :: macros(:)
        type(context), intent(in)   :: ctx
        !private
        integer :: i
        character(:), allocatable :: expanded
        logical :: stitch
 
        if (pos > ntokens) then
            call printf(render(diagnostic_report(level_error, &
                    message='Synthax error', &
                    label=label_type('Unexpected end of expression', pos, 1), &
                    source=trim(ctx%path)), &
                    expr, ctx%line))
            val = 0
            return
        end if
 
        if (tokens(pos)%type == 0) then
            val = strtol(tokens(pos)%value)
            pos = pos + 1
        else if (tokens(pos)%type == 2) then
            if (is_defined(tokens(pos)%value, macros)) then
                expanded = expand_macros(tokens(pos)%value, macros, stitch, global%implicit_continuation, ctx)
                if (.not. evaluate_expression(expanded, macros, ctx, val)) val = 0
            else
                val = 0
            end if
            pos = pos + 1
        else if (tokens(pos)%value == '(') then
            pos = pos + 1
            val = parse_expression(expr, tokens, ntokens, pos, macros, ctx)
            if (pos > ntokens .or. tokens(pos)%value /= ')') then
                call printf(render(diagnostic_report(level_error, &
                        message='Synthax error', &
                        label=label_type('Missing closing parenthesis in expression', len(expr), 1), &
                        source=trim(ctx%path)), &
                        expr, ctx%line))
                val = 0
            else
                pos = pos + 1
            end if
        else if (tokens(pos)%type == 4) then
            expanded = trim(tokens(pos)%value)
            val = merge(1, 0, is_defined(expanded, macros))
            pos = pos + 1
        else
            call printf(render(diagnostic_report(level_error, &
                    message='Invalid expression', &
                    label=label_type('Unknown token', 1, len_trim(tokens(pos)%value)), &
                    source=trim(ctx%path)), &
                    expr, ctx%line))
            val = 0
            pos = pos + 1
        end if
    end function
end module