GameMaker: Beautifying/pretty-printing JSON

Partially related to an earlier blog post about minifying JSON (which I had just updated), but adressing a different problem - sometimes, rather than trying to make your JSON more compact (and, consequently, less readable), you may want to make it more readable - be that for your own debugging purposes, or not to scare players away from files that they are allowed to edit.

Take Nuclear Throne, for example. The save file is a big nested JSON structure, and, needless to say, you aren't getting around passing it through an external JSON beautifier if you want to be able to make sense of it:

With a bit of string processing, however, you can have it printed out nicely readable:

So this post is about that.

The idea

The general outline of process is as following:

  • Trim any unnecessary whitespace (in case it's inconsistent).
  • Cut trailing zeroes from numeric values (4.500000 -> 4.5, 1.000000 -> 1).
  • Add a space after each colon if there isn't one ("key": "value", "key": [...], ...).
  • After each opening bracket not immediately followed by a closing bracket, increase indentation level and add a new line (if followed, use compact format - [], {}).
  • Add a new line (with indentation level in mind) after each comma.
  • Reduce indentation level and add a new line before each closing bracket (except for those preceded by an opening bracket).
  • Make sure not to reformat contents of strings.

The code

The code is largely based on my 2018 version of minifier script.

First, you'll need a helper script called buffer_write_slice:

/// buffer_write_slice(buffer, data_buffer, data_start, data_end)
var start = argument2;
var next = argument3 - start;
if (next <= 0) exit;
var buf = argument0;
var data = argument1;
var size = buffer_get_size(buf);
var pos = buffer_tell(buf);
var need = pos + next;
if (size < need) {
    do size *= 2 until (size >= need);
    buffer_resize(buf, size);
}
buffer_copy(data, start, next, buf, pos);
buffer_seek(buf, buffer_seek_relative, next);

This writes a section of one buffer to other buffer, and is used extensively here.

Then you can add the script itself, called json_beautify:

/// json_beautify(json_string)
// initialization
// in old versions of GMS, you'd have this ran separately instead.
// in GMS2 it'd need to be @"..." instead of just "..."
gml_pragma("global", "
global.g_json_beautify_fb = buffer_create(1024, buffer_fast, 1);
global.g_json_beautify_rb = buffer_create(1024, buffer_grow, 1);
");
var src = argument0;
// copy text to string buffer:
var rb = global.g_json_beautify_rb;
buffer_seek(rb, buffer_seek_start, 0);
buffer_write(rb, buffer_string, src);
var size = buffer_tell(rb) - 1;
var rbsize = buffer_get_size(rb);
// then copy it to "fast" input buffer for peeking:
var fb = global.g_json_beautify_fb;
if (buffer_get_size(fb) < size) buffer_resize(fb, size);
buffer_copy(rb, 0, size, fb, 0);
buffer_seek(rb, buffer_seek_start, 0);
//
var rbpos = 0; // writing position in output buffer
var start = 0; // start offset in input buffer
var pos = 0; // reading position in input buffer
var next; // number of bytes to be copied
var need;
var nest = 0;
while (pos < size) {
    var c = buffer_peek(fb, pos++, buffer_u8);
    switch (c) {
        case 9: case 10: case 13: case 32: // `\t\n\r `
            buffer_write_slice(rb, fb, start, pos - 1);
            // skip over trailing whitespace:
            while (pos < size) {
                switch (buffer_peek(fb, pos, buffer_u8)) {
                    case 9: case 10: case 13: case 32: pos += 1; continue;
                    // default -> break
                } break;
            }
            start = pos;
            break;
        case 34: // `"`
            while (pos < size) {
                switch (buffer_peek(fb, pos++, buffer_u8)) {
                    case 92: pos++; continue; // `\"`
                    case 34: break; // `"` -> break
                    default: continue; // else
                } break;
            }
            break;
        case ord("["): case ord("{"):
            buffer_write_slice(rb, fb, start, pos);
            // skip over trailing whitespace:
            while (pos < size) {
                switch (buffer_peek(fb, pos, buffer_u8)) {
                    case 9: case 10: case 13: case 32: pos += 1; continue;
                    // default -> break
                } break;
            }
            // indent or contract `[]`/`{}`
            c = buffer_peek(fb, pos, buffer_u8);
            switch (c) {
                case ord("]"): case ord("}"): // `[]` or `{}`
                    buffer_write(rb, buffer_u8, c);
                    pos += 1;
                    break;
                default: // `[\r\n\t
                    buffer_write(rb, buffer_u16, 2573); // `\r\n`
                    repeat (++nest) buffer_write(rb, buffer_u8, 9); // `\t`
            }
            start = pos;
            break;
        case ord("]"): case ord("}"):
            buffer_write_slice(rb, fb, start, pos - 1);
            buffer_write(rb, buffer_u16, 2573); // `\r\n`
            repeat (--nest) buffer_write(rb, buffer_u8, 9); // `\t`
            buffer_write(rb, buffer_u8, c);
            start = pos;
            break;
        case ord(","):
            buffer_write_slice(rb, fb, start, pos);
            buffer_write(rb, buffer_u16, 2573); // `\r\n`
            repeat (nest) buffer_write(rb, buffer_u8, 9); // `\t`
            start = pos;
            break;
        case ord(":"):
            if (buffer_peek(fb, pos, buffer_u8) != ord(" ")) {
                buffer_write_slice(rb, fb, start, pos);
                buffer_write(rb, buffer_u8, ord(" "));
                start = pos;
            } else pos += 1;
            break;
        default:
            if (c >= ord("0") && c <= ord("9")) { // `0`..`9`
                var pre = true; // whether reading pre-dot or not
                var till = pos - 1; // index at which meaningful part of the number ends
                while (pos < size) {
                    c = buffer_peek(fb, pos, buffer_u8);
                    if (c == ord(".")) {
                        pre = false; // whether reading pre-dot or not
                        pos += 1; // index at which meaningful part of the number ends
                    } else if (c >= ord("0") && c <= ord("9")) {
                        // write all pre-dot, and till the last non-zero after dot:
                        if (pre || c != ord("0")) till = pos;
                        pos += 1;
                    } else break;
                }
                if (till < pos) { // flush if number can be shortened
                    buffer_write_slice(rb, fb, start, till + 1);
                    start = pos;
                }
            }
    }
}
if (start == 0) return src; // source string was unchanged
buffer_write_slice(rb, fb, start, pos);
buffer_write(rb, buffer_u8, 0); // terminating byte
buffer_seek(rb, buffer_seek_start, 0);
return buffer_read(rb, buffer_string);

and then use it on JSON strings you get from json_encode or wherever else.

As an additional note, this uses tabs for indentation, but you can have it to use spaces by changing buffer_write(rb, buffer_u8, 9); to buffer_write(rb, buffer_text, " "), for example.

Have fun !

Related posts:

One thought on “GameMaker: Beautifying/pretty-printing JSON

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.