ReimuA Desync

Specifications

• Versions: 1.00a
• Difficulty: Easy - Normal - Hard - Lunatic - Extra
• Mode: Main game - Practice mode
• Shottype: ReimuA - ReimuB - ReimuC - MarisaA - MarisaB - MarisaC

What happens

Rarely, a replay desyncs on Stage 4. This only happens as ReimuA, and also only on the difficulty Easy mode. For the desync to happen, the replay can be started from any stage. This also means that Practice mode replays may desync. Though, the desync seemingly happens at random.

How it happens

For the desync to happen, the player must do the following:

1. Play as ReimuA
2. Play on Easy mode
3. Play on Stage 4, and survive until Satori's 3rd spell-card.

The desync sometimes happens on Satori's 3rd spell-card or later.

Why it happens

Satori's boss-fight is unique in the sense that every shottype faces three different spell-cards. The code provided in this section is from the stage4c00a.ecl file, which is the code that is called when playing as ReimuA. The code was decompiled using thecl.

The code

The relevant code of ReimuA's 3rd spell-card Recollection "Flying Insects' Nest" is as follows:

void BossCard3_00A() {
    [...]
    @BossCard3_00Aat3() async;
    [...]
    @BossCard3_00Aat() async;
    [...]
    @BossCard3_00Aat2() async;
    [...]
}

In this case, Satori calls for three different subroutines: BossCard3_00Aat3, BossCard3_00Aat and BossCard3_00Aat2. In particular, we want to look at the subroutine BossCard3_00Aat2.

The full code of subroutine BossCard3_00Aat2 is pasted below:

void BossCard3_00Aat2() {
    var A, B, C, D, E;
    etNew(0);
    etOffsetAbs(0, 0.0f, 128.0f);
    etEx(0, 0, 0, 4, 30, -999999, -0.3f, -999999.0f);
    %A = _f(-192) + (%RANDF2 * _f(8));
    %B = _f(2);
    goto BossCard3_00Aat2_1496 @ 0;
BossCard3_00Aat2_308:
    playSound(16);
    %D = %RANDF2 * _f(32);
    etOffset(0, %A + (%RANDF2 * _f(0)), %D);
    unless ((($E % 2) == 0) || ($DIFF >= 1)) goto BossCard3_00Aat2_948 @ 0;
    unless (%PLAYER_Y2 < (%D + _f(128))) goto BossCard3_00Aat2_900 @ 0;
    laserOnA2(3, 4, -1.5707964f, 12.0f, 0, 128.0f, 0.0f, 16.0f);
    goto BossCard3_00Aat2_948 @ 0;
BossCard3_00Aat2_900:
    laserOnA2(3, 4, 1.5707964f, 12.0f, 0, 128.0f, 0.0f, 16.0f);
BossCard3_00Aat2_948:
    %C = %B / 2.8f;
    %A = %A + (%C * %C);
    unless (%A > _f(192)) goto BossCard3_00Aat2_1232 @ 0;
    return;
BossCard3_00Aat2_1232:
    unless (%B < _f(128)) goto BossCard3_00Aat2_1388 @ 0;
    %B = %B + _f(1);
BossCard3_00Aat2_1388:
    $E = $E + 1;
    diffWait(6, 6, 6, 6);
BossCard3_00Aat2_1496:
    if (1) goto BossCard3_00Aat2_308 @ 0;
    return;
}

The first line var A, B, C, D, E; declares the variables A, B, C, D, and E. The variables aren't too useful in this stage - the variables need to be initialised first before usage, which is done by assigning the variables a value. All variables are assigned a value before they are read, except for variable E. Its value is read in the line unless ((($E % 2) == 0) || [...], but E is not assigned a value prior to this. This means that E is an uninitialised variable. The uninitialised variable E will have some value, but its value is not predictable.

Easy-exclusivity

The line that is affected is as follows:

unless ((($E % 2) == 0) || ($DIFF >= 1)) goto BossCard3_00Aat2_948 @ 0;

The variable DIFF is an integer that returns the current difficulty (Easy = 0, Normal = 1, Hard = 2, Lunatic = 3, Extra = 4).

The line ($DIFF >= 1) returns true if played on Normal/Hard/Lunatic, and it returns false if played on Easy.

The line ($E % 2) == 0) returns true if E is an even number, and it returns false if E is an odd number. However, since E is uninitialised, E has some value that is unpredictable. It is likely that E may not even be an integer value, which results in the condition having a higher chance to return false than to return true.

The logical OR operator || returns true if one of the operands is true and returns false otherwise. This means that the statement ($E % 2) == 0) || ($DIFF >= 1) always returns true if played on a difficulty other than Easy mode.

If the statement is true, then the game goes to the next line: unless (%PLAYER_Y2 < (%D + _f(128))) goto BossCard3_00Aat2_900 @ 0;
If the statement is false, then the game jumps to the label BossCard3_00Aat2_948, skipping code responsible for spawning lasers that otherwise would have run if the statement were true.

Conclusion

Links

Replays

Any ReimuA replay of Stage 4 on Easy mode suffices, so none are provided.

Videos

• Nylilsa. 「September 8, 2022」. "東方地霊殿～霊夢Aリプレイバグ" https://www.youtube.com/watch?v=e9yNY1ZXTRk