a generic algorithm that handles \m{a}\u{0301}\m{b}\u{0301} (which will be unchanged) should handle the above case also.

Thinking alound here.. (fyi @mcdurdin ):

(yes this calls the _segment() api but it's illustrative)

  {
    marker_map map;
    std::cout << __FILE__ << ":" << __LINE__ << " - support 2-segment markers " << std::endl;
    // e\m{1}`\m{2}_E\m{3}`\m{4}_
    const std::u32string src =
        U"e\uFFFF\u0008\u0001\u0300\uFFFF\u0008\u0002\u0320E\uFFFF\u0008\u0003\u0300\uFFFF\u0008\u0004\u0320";
    // e\m{2}_\m{1}`E\m{4}_\m{3}`
    const std::u32string expect =
        U"e\uFFFF\u0008\u0002\u0320\uFFFF\u0008\u0001\u0300E\uFFFF\u0008\u0004\u0320\uFFFF\u0008\u0003\u0300";
    std::u32string dst = src;
    assert(normalize_nfd_markers_segment(dst, map));
    if (dst != expect) {
      std::cout << "dst: " << Debug_UnicodeString(dst) << std::endl;
      std::cout << "exp: " << Debug_UnicodeString(expect) << std::endl;
    }
    zassert_string_equal(dst, expect);
    marker_map expm({{0x300, 0x1L}, {0x320, 0x2L}, {0x300, 0x3L}, {0x320, 0x4L}});
    assert_marker_map_equal(map, expm);
  }

the stream of markers may be parsed as shown on the expm (expected markers) stream. However, the text is reordered by normalization from:

e U+300 U+320 E U+300 U+320

to:

e U+320 U+300 E U+320 U+300

so then how can the 'add back markers' operate?

expm = {{0x300, 0x1L}, {0x320, 0x2L}, {0x300, 0x3L}, {0x320, 0x4L}}

so going from the right, it will first hit the U+300.
It could search and find {0x300, 0x3L} entry and use that and remove it, ok
That leaves the list as {{0x300, 0x1L}, {0x320, 0x2L}, /*deleted*/, {0x320, 0x4L}}
now when we hit the U+320 how do we know which one applies? Both of them? If we keep the 'deleted' state, then /*deleted*/ could be a boundary then we consume only the {0x320,0x4L}
That leaves the list as {{0x300, 0x1L}, {0x320, 0x2L}, /*deleted*/, /*deleted*/}
next we hit the 0x300, and repeat..

OK. Might work for this case.

But what about this one as an input string:

e \m{1} U+300 \m{2} U+320 a U+300 U+320 E \m{3} U+300 \m{4} U+320

The problem is that, when adding back markers, how do we know that \m{1} and \m{2} apply to e’s NSM, and not a’s? There’s no information to distinguish.

So, I think we need segmentation.

Then the problem looks like this: (Character boundaries shown by |)

e \m{1} U+300 \m{2} U+320 | a U+300 U+320 | E \m{3} U+300 \m{4} U+320

The middle segment has no markers. So now processed in three segments, each one will already work.

The branch I started on with the 'linear' marker_map above will handle duplicates and doubled markers well. But with the segment boundaries splitting up the problem, we don't have to worry about the marker being on the wrong segment.

Uh oh!

feat(core): segmented marker normalization 🙀 #10369

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