Backwards compatibility issue with C++ compilation

Issue Description

Some older versions of C++ compilers require that structs be initialized in the same order that they are declared. The current version of lv_font_conv generates .c font files that have some structs that do not meet his criteria.

Indicative Error

Compilation crashes with this error message: sorry, unimplemented: non-trivial designated initializers not supported

Issue example

Struct declaration from lv_font.h in the LittlevGL library:

{
    uint16_t adv_w; /**< The glyph needs this space. Draw the next glyph after this width. 8 bit integer, 4 bit fractional */
    uint8_t box_w;  /**< Width of the glyph's bounding box*/
    uint8_t box_h;  /**< Height of the glyph's bounding box*/
    int8_t ofs_x;   /**< x offset of the bounding box*/
    int8_t ofs_y;  /**< y offset of the bounding box*/
    uint8_t bpp;   /**< Bit-per-pixel: 1, 2, 4, 8*/
}lv_font_glyph_dsc_t;

An example of the same struct that is generated in a font file by the current version of lv_font_conv:

static const lv_font_fmt_txt_glyph_dsc_t glyph_dsc[] = {
    {.bitmap_index = 0, .adv_w = 0, .box_h = 0, .box_w = 0, .ofs_x = 0, .ofs_y = 0} /* id = 0 reserved */,
    {.bitmap_index = 0, .adv_w = 99, .box_h = 0, .box_w = 0, .ofs_x = 0, .ofs_y = 0},
    {.bitmap_index = 0, .adv_w = 70, .box_h = 15, .box_w = 3, .ofs_x = 1, .ofs_y = 0},
    {.bitmap_index = 6, .adv_w = 119, .box_h = 5, .box_w = 6, .ofs_x = 1, .ofs_y = 10},
    {.bitmap_index = 10, .adv_w = 255, .box_h = 15, .box_w = 16, .ofs_x = 0, .ofs_y = 0},
    {.bitmap_index = 40, .adv_w = 252, .box_h = 18, .box_w = 15, .ofs_x = 1, .ofs_y = -2},
    {.bitmap_index = 74, .adv_w = 309, .box_h = 15, .box_w = 17, .ofs_x = 1, .ofs_y = 0},
    {.bitmap_index = 106, .adv_w = 300, .box_h = 15, .box_w = 18, .ofs_x = 1, .ofs_y = 0},
   ..................
   ..................
}

Notice that: The .box_h and .box_w parameters in the generated struct are out of order with the struct declaration in lv_font.h

Suggested fix

There are 3 files in the lv_font_conv project that are responsible for generating the affected structs found in the font file:

1. lv_table_cmap.js
2. lv_table_glyf.js
3. lv_table_head.js

The affected structs are:

1. lv_font_fmt_txt_glyph_dsc_t
2. lv_font_fmt_txt_cmap_t
3. lv_font_fmt_txt_kern_pair_t
4. lv_font_fmt_txt_dsc_t
5. lv_font_t example_font

Fix example

From lv_table.glyf.js

  to_lv_glyph_dsc() {
    this.lv_compile();

    /* eslint-disable max-len */

    let result = [ '    {.bitmap_index = 0, .adv_w = 0, .box_h = 0, .box_w = 0, .ofs_x = 0, .ofs_y = 0} /* id = 0 reserved */' ];

    this.lv_data.forEach(d => {
      const idx = d.offset,
            adv_w = Math.round(d.glyph.advanceWidth * 16),
            h = d.glyph.bbox.height,
            w = d.glyph.bbox.width,
            x = d.glyph.bbox.x,
            y = d.glyph.bbox.y;
      result.push(`    {.bitmap_index = ${idx}, .adv_w = ${adv_w}, .box_h = ${h}, .box_w = ${w}, .ofs_x = ${x}, .ofs_y = ${y}}`);
    });

    return result.join(',\n');
  }

should become:

  to_lv_glyph_dsc() {
    this.lv_compile();

    /* eslint-disable max-len */

    let result = [ '    {.bitmap_index = 0, .adv_w = 0, .box_w = 0, .box_h = 0, .ofs_x = 0, .ofs_y = 0} /* id = 0 reserved */' ];

    this.lv_data.forEach(d => {
      const idx = d.offset,
            adv_w = Math.round(d.glyph.advanceWidth * 16),
            h = d.glyph.bbox.height,
            w = d.glyph.bbox.width,
            x = d.glyph.bbox.x,
            y = d.glyph.bbox.y;
      result.push(`    {.bitmap_index = ${idx}, .adv_w = ${adv_w}, .box_w = ${h}, .box_h = ${w}, .ofs_x = ${x}, .ofs_y = ${y}}`);
    });

    return result.join(',\n');
  }

Reproduction and conclusion

I am using g++ as a C++ compiler: Target: x86_64-linux-gnu gcc version: 7.4.0

Apparently newer versions of the g++ compiler allow out-of-order struct initialization. I seem to remember seeing version 8.0.0 and newer supports that. However, many hardware projects have version constraints: for example I can only use 7.4.0. It would be a good idea to make this fix for backwards-compatibility’s sake. I would do it myself but there are legal issues that prevent me from doing so. Hopefully I have given enough information for it to be an easy fix for whoever picks it up.

[puzrin]

done

[jared-nelsen]

I’ve built and used the changes you made on the dev branch. It works great! The changes you made are exactly the ones we needed. Thanks @puzrin !

I am on board for merging these changes to master.

Thanks for all the help! My team is really loving lvgl @kisvegabor !