Customized World Preset Biome ID >= 8 adds 2

Yep. Not fixed in 14w25b.

14w26c

Description: Updating screen events

java.lang.NullPointerException: Updating screen events
	at bov.b(SourceFile:500)
	at bov.a(SourceFile:456)
	at bng.e(SourceFile:53)
	at bng.<init>(SourceFile:25)
	at bnt.a(SourceFile:163)
	at bnt.a(SourceFile:137)
	at bnt.r(SourceFile:42)

14w32d

java.lang.NullPointerException: Updating screen events
	at bwa.b(SourceFile:501)
	at bwa.a(SourceFile:457)
	at buf.e(SourceFile:52)
	at buf.<init>(SourceFile:24)
	at bus.a(SourceFile:162)
	at bus.a(SourceFile:136)
	at bus.r(SourceFile:41)
	at bus.<init>(SourceFile:31)
	at bwa.f(SourceFile:364)
	at bwa.b(SourceFile:221)
	at bww.b(SourceFile:368)

Confirmed for 1.8-pre1. But for me, it's actually not crashing. It's showing "Biome: ?" in the button. The world generates normally with that option.

The game does not crash either in 1.8pre2. The main issue is fixed.

Confirmed for 1.8.1-pre2: MC-73434

One of the problems of the biome setting in the custom world settings menu still exists in 1.8.1-pre2. Set it to a certain biome, exit the menu, re-enter the menu, and it will be a different biome. I set it to DesertHills, then exited and re-entered, and it said TaigaHills.

The key of the problem is the fact that the biome 8 and 9, respectively the hell and end biome, can't be selected in customized. So for the biome 8 and all the above, there is a difference of 2 between the real biome id and the id of the biome on the slider in customized. Some menu give the wrong id so when you either either; go to "Presets" and press "Use Presets "; or; exit the customized menu ("Done") than come back ("Customize"); the slider move forward of 2, changing the biome.

This part of the problem is still in 1.8.1.

This bug is caused from dealing with the Hell and Sky biomes in a bad manner. The intended behavior was apparently to remove those biomes from the list in the biome selector. However, this offset in biomeIDs is dealt with in the wrong bit of code: in the preset string interpretation, which is often executed multiple times (e.g. when the "Customize World Settings" are opened). Instead, the offset calculation should be done immediately where the value of the slider is evaluated.
There are two easy fixes that could be done here. The first option is the simplest and my favorite, which is simply to keep the Hell and Sky in the biome selector. They don't really harm anyone. The second option would be to do the offset management right at the slider read out, just as I mentioned.

I will also try to explain this bug and fixes in terms of code. I got the original code through MCP, but I will only show the key bits here, with a bunch of variables renamed, to help convey the information (also so I'm not posting actual source code).

Current code:

ChunkProviderSettings.java

// Line ~526
fixedBiomeID = JsonUtils.getJsonObjectIntegerFieldValueOrDefault(preset, "fixedBiome", fixedBiomeID);

if (fixedBiomeID < 38 && fixedBiomeID >= -1)
{
    if (fixedBiomeID >= BiomeGenBase.hell.biomeID) fixedBiomeID += 2;
}
else
{
    fixedBiomeID = -1;
}

As I mentioned, this bit of code is in totally the wrong place! Nothing in this class should deal with any sort of offset, as it may be called multiple times.

GuiCustomizeWorldScreen.java

// Line ~220
String getSliderLabel(int sliderID, float sliderValue)
{
    switch(sliderID)
    // ...
    case BIOME_SELECTOR: // = 162
        if (sliderValue < 0.0F)
        {
            // return "All"
        }
        else
        {
            if ((int)sliderValue >= BiomeGenBase.hell.biomeID)
            {
                // return name for biomeID = (int)sliderValue + 2
            }
            else
            {
                // return name for biomeID = (int)sliderValue
            }
        }
    // ...
}

// Line ~370
void evaluateSlider(int sliderID, float sliderValue)
{
    switch(sliderID)
    // ...
    case BIOME_SELECTOR:
        chunkProviderSettings.fixedBiomeID = (int)sliderValue;
        break;
    // ...
}

An implementation of the the first fix would be:

ChunkProviderSettings.java

// Line ~526: Remove  all the unnecessary code
fixedBiomeID = JsonUtils.getJsonObjectIntegerFieldValueOrDefault(preset, "fixedBiome", fixedBiomeID);
// ...

GuiCustomizeWorldScreen.java

// Line ~220
String getSliderLabel(int sliderID, float sliderValue)
{
    switch(sliderID)
    case BIOME_SELECTOR:
        if ((int)sliderValue < 0)
        {
            // return "All"
        }
        else
        {
            // return name for biomeID = (int)sliderValue
        }
    // ...
}

// Line ~370: Unchanged
void evaluateSlider(int sliderID, float sliderValue)
{
    switch(sliderID)
    // ...
    case BIOME_SELECTOR:
        chunkProviderSettings.fixedBiomeID = (int)sliderValue;
        break;
    // ...
}

An implementation of the the second fix would be:

ChunkProviderSettings.java

// Line ~526: Remove  all the unnessesary code
fixedBiomeID = JsonUtils.getJsonObjectIntegerFieldValueOrDefault(preset, "fixedBiome", fixedBiomeID);
// ...

GuiCustomizeWorldScreen.java

// Line ~220: Unchanged, although one could play around with (int) casts or ceiling/floor values.
String getSliderLabel(int sliderID, float sliderValue)
{
    switch(sliderID)
    case BIOME_SELECTOR:
        if (sliderValue < 0.0F)
        {
            // return "All"
        }
        else
        {
            if ((int)sliderValue >= BiomeGenBase.hell.biomeID)
            {
                // return name for biomeID = (int)sliderValue + 2
            }
            else
            {
                // return name for biomeID = (int)sliderValue
            }
        }
    // ...
}

// Line ~370: Do the offset management here
void evaluateSlider(int sliderID, float sliderValue)
{
    switch(sliderID)
    // ...
    case BIOME_SELECTOR:
        biomeID = (int)sliderValue;
        if (biomeID >= BiomeGenBase.hell.biomeID) 
        {
            biomeID -= 2;
        }
        chunkProviderSettings.fixedBiomeID = biomeID;
        break;
    // ...
}

Also, in order to get the right position of the slider upon reload, one has to change the default value of the slider, to revert the offset.
After "createWorld.customize.custom.fixedBiome" in GuiCustomizeWorldScreen.java, change

(float)(this.field_175336_F.field_177869_G)

to

(float)((this.field_175336_F.field_177869_G>=BiomeGenBase.hell.biomeID) ? this.field_175336_F.field_177869_G-2 : this.field_175336_F.field_177869_G

After this overkill of a comment I should mention that I can confirm this bug (not the crash, but the offsets) in versions 1.8, 1.8.8 and 15w49b 😛
P.S.
I tested the all the fixes in MCP for 1.8.

Confirmed for the 1.9 release.

Game doesn't crash anymore in 1.10.2.
Instead, this happens:
screenshot is attached

Please link in the description to @unknown's code analysis

Confirmed for 16w38a

Still present in 16w40a

Confirmed for 16w44a

Still in 1.11-pre1 😞

Confirmed for 16w50a