Sanae Frogs Not Working

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Specifications

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• Versions: 1.00a - 1.00b
• Difficulty: Easy - Normal - Hard - Lunatic - Extra
• Mode: Legacy mode - Point Device mode - Practice mode
• Shottype: Reimu - Marisa - Sanae - Reisen

What happens

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Occasionally, Sanae's frogs do not work in which case her bullets are not shot. This issue only affects Sanae using her frogs regardless of power level. This issue can be resolved by releasing Z for about half a second.

How it happens

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To activate this condition, the player must involve releasing and holding Z multiple times. In particular, the following needs to be done:

1. The player must press Z.
2. The player must press Z again anywhere between 104 and 119 frames after the initial Z input.
3. The player must hold Z 120 frames after the initial Z input.

This sequence of actions is very easy to do accidentally meaning that it isn't uncommon to encounter this problem.

Why it happens

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In this game, each player's shooter is controlled by two timers (short_timer and long_timer), which determine when bullets are fired by the player. short_timer operates on a 15-frame interval, and long_timer operates on a 120-frame interval.

The shooter thats use long_timer are from Sanae (unfocus shot); the other shooters use short_timer. The reason why Sanae's frogs are using long_timer is because those shooters have been manually assigned to long_timer by ZUN. In other words, ZUN made the decision to assign long_timer to these shooters himself.

Both timers have a default value of -1. Whenever the player presses the Z key, both timers increment by 1 every frame: short_timer increments to 15 and long_timer increments to 120. Once their treshold is reached, the game looks at the current Z input. If Z is held, the timer is set to 0, otherwise the timer is reset to -1. This explains why if you press Z even for one frame, the player continues to shoot bullets for 15 frames instead of immediately stopping.

The player can only shoot bullets only if short_timer is a value between 0 and 15. This means that the player bullets from long_timer are dependent on short_timer: if long_timer is ticking but short_timer is -1 then no bullets of long_timer are be shot.

For long_timer to go from -1 to 0, short_timer also needs to go from -1 to 0. In other words, to activate long_timer, short_timer also needs to be activated simultaneously. In pseudocode its the following:

if (short_timer < 0) {
    if (long_timer < 0) {
        [...] // logic that activates long_timer
    }
}

This proposes a slight issue. If short_timer is already active (i.e. its value is between 0 and 15), then long_timer cannot go from -1 to 0. We can achieve this problem by running both timers by pressing Z. Once both timers are active, we stop short_timer by releasing Z. This pause in short_timer creates a misalignment between the two timers' cycles. After this, we press Z again to restart short_timer while long_timer continues its cycle independently. We then release Z to fully reset long_timer to -1: long_timer stops completely. By holding Z once more, we ensure that short_timer stays active: we end up with a situation where short_timer runs while long_timer remains inactive.

Is Z button held?					…
short_timer	-1	0	1	2	…	-1	0	1	2	3
long_timer	-1	0	1	2	…	117	118	119	-1	-1

Because long_timer has stopped, it means that the shooters that use long_timer are inactive, thus those shooters do not produce any bullets. However, shooters that use short_timer still produce bullets.

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Links

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Replays

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Videos

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NNN_shooter. 「October 3, 2017」. "東方天空璋 ショット未発生バグ" https://www.nicovideo.jp/watch/sm32038719