Saving, restoring, restarting and verifying the program from within itself.

• §1. Environment
• §2. Verification
• §3. Save, restore, restart
• §4. Undo

§1. Environment. The language "interpreter" here supposes that the eventual program is running in a VM which is being interpreted, and that may not be the case, but it's traditional.

[ VM_ReportOnInterpreter ix;
    @gestalt 1 0 ix;
    print "Interpreter version ", ix / $10000, ".", (ix & $FF00) / $100,
    ".", ix & $FF, " / ";
    @gestalt 0 0 ix;
    print "VM ", ix / $10000, ".", (ix & $FF00) / $100, ".", ix & $FF, "^";
];

§2. Verification. This verifies that the current story file is intact.

[ VM_Verify res;
    @verify res;
    return res;
];

§3. Save, restore, restart. Restart does what it says: restarts the program as if it had just loaded for the first time.

VM_Save() attempts to save the current state of the program to a file, and returns 0 if this fails, 1 if this succeeds, or 2 if in fact a restore has just succeeded. (A successful restoration should resume execution where the save succeeded, but we want to distinguish those cases.)

VM_Restore() pretends to return true or false according to whether or not it succeeds, but in fact it can only return false to indicate failure, because a successful restoration means that execution has transferred to VM_Save instead.

[ VM_Restart;
    @restart;
];

[ VM_Restore res fref;
    fref = glk_fileref_create_by_prompt($01, $02, 0);
    if (fref == 0) jump RFailed;
    gg_savestr = glk_stream_open_file(fref, $02, GG_SAVESTR_ROCK);
    glk_fileref_destroy(fref);
    if (gg_savestr == 0) jump RFailed;
    @restore gg_savestr res;
    glk_stream_close(gg_savestr, 0);
    gg_savestr = 0;
    .RFailed;
    rfalse;
];

[ VM_Save fref len res;
    fref = glk_fileref_create_by_prompt(fileusage_SavedGame, filemode_Write, 0);
    if (fref) {
        gg_savestr = glk_stream_open_file(fref, filemode_Write, GG_SAVESTR_ROCK);
        if (gg_savestr) {
            @save gg_savestr res;
            if (res == -1) {
                The player actually just typed "restore". We first have to recover
                all the Glk objects; the values in our global variables are all wrong.
                GGRecoverObjects();
                glk_stream_close(gg_savestr, GLK_NULL);
                gg_savestr = 0;
                return 2;
            }
            glk_stream_close(gg_savestr, GLK_NULL);
            gg_savestr = 0;
            if (res == 0) {
                Check that the savefile was actually written - this is mostly to account for browser limits in Parchment
                if (glk_fileref_does_file_exist(fref)) {
                    gg_savestr = glk_stream_open_file(fref, filemode_Read, GG_SAVESTR_ROCK);
                    if (gg_savestr) {
                        glk_stream_set_position(gg_savestr, 0, seekmode_End);
                        len = glk_stream_get_position(gg_savestr);
                        glk_stream_close(gg_savestr, GLK_NULL);
                        gg_savestr = 0;
                        if (len) {
                            We've confirmed the file exists and has content, which is about all we can do
                            glk_fileref_destroy(fref);
                            return 1;
                        }
                        Cleanup the empty file
                        glk_fileref_delete_file(fref);
                    }
                }
            }
        }
        glk_fileref_destroy(fref);
    }
    return 0;
];

§4. Undo. These are really emulations of the Z-machine's conventions on UNDO.

[ VM_Undo result_code;
    @restoreundo result_code;
    return (~~result_code);
];

[ VM_Save_Undo result_code;
    @saveundo result_code;
    if (result_code == -1) { GGRecoverObjects(); return 2; }
    return (~~result_code);
];