/**
 *  payloads/stack.c
 *
 *  The execution stack.
 *
 *  (c) 2026 Simon Brooke <simon@journeyman.cc>
 *  Licensed under GPL version 2.0, or, at your option, any later version.
 */

#include "memory/node.h"
#include "memory/pointer.h"
#include "memory/pso2.h"
#include "memory/pso4.h"
#include "memory/tags.h"

#include "payloads/cons.h"
#include "payloads/stack.h"

/** 
 * @brief The maximum depth of stack before we throw an exception.
 *
 * `0` is interpeted as `unlimited`.
 */
uint32_t stack_limit = 0;

/**
 * Fetch a pointer to the value of the local variable at this index.
 *
 * TODO: I think the first argument would be better as a pso_pointer.
 */
struct pso_pointer fetch_arg( struct pso4 *frame, unsigned int index ) {
    struct pso_pointer result = nil;

    // TODO check that the frame is indeed a frame!
    if ( index < frame->payload.stack_frame.args ) {
        result = frame->payload.stack_frame.arg[index];
    } else {
        struct pso_pointer p = frame->payload.stack_frame.more;

        for ( int i = args_in_frame; i < index; i++ ) {
            p = pointer_to_object( p )->payload.cons.cdr;
        }

        result = pointer_to_object( p )->payload.cons.car;
    }

    return result;
}

/**
 * @brief Return the environment from the stack frame identified by this
 * `frame_pointer`
 *
 * @param frame_pointer a pointer to a stack frame.
 */
struct pso_pointer fetch_env( struct pso_pointer frame_pointer ) {
    return stackp( frame_pointer ) ?
        pointer_to_pso4( frame_pointer )->payload.stack_frame.env : nil;
}

/**
 * Push a binding (and therefore a reference) for this `local` onto the 
 * stack_frame indicated by this `frame_pointer`, thereby protecting the
 * `local` from garbage collection until the frame itself is disposed of.
 *
 * This is a hack. For Lisp functions, where the stack frames are set up
 * and torn down by eval/apply, it shouldn't be necessary.
 */
struct pso_pointer push_local( struct pso_pointer frame_pointer,
                               struct pso_pointer local ) {
    if ( stackp( frame_pointer ) ) {
        struct pso4 *frame = pointer_to_pso4( frame_pointer );

        struct pso_pointer l = make_cons( frame_pointer, local,
                                          frame->payload.stack_frame.locals );
        frame->payload.stack_frame.locals = l;
    }

    return local;
}