Smooth lighting no longer works properly in certain block configurations

NOTE: the following fix pertains to 1.7, an updated version will be needed for 1.16+

Let me first off say that I know this is a duplicate, but I have code-fixes for it.

As you can see in the 2 images, the first one (The bugged version), has inconsistent lighting travelling from one block to the next, the corners don't match up. This is primarily noticeable in 1 wide staircases underground, and for good reason.

Basically what is happening is that the render code is checking the transparency of blocks with an incorrect offset. When you get your coordinates wrong you're bound to get some weird results.

This code is drawing the bottom face.

if (this.renderMinY <= 0.0D)
{
     --y;
}

this.aoBrightnessXYNN = par1Block.getMixedBrightnessForBlock(this.blockAccess, x - 1, y, z);
this.aoBrightnessYZNN = par1Block.getMixedBrightnessForBlock(this.blockAccess, x, y, z - 1);
this.aoBrightnessYZNP = par1Block.getMixedBrightnessForBlock(this.blockAccess, x, y, z + 1);
this.aoBrightnessXYPN = par1Block.getMixedBrightnessForBlock(this.blockAccess, x + 1, y, z);
this.aoLightValueScratchXYNN = par1Block.getAmbientOcclusionLightValue(this.blockAccess, x - 1, y, z);
this.aoLightValueScratchYZNN = par1Block.getAmbientOcclusionLightValue(this.blockAccess, x, y, z - 1);
this.aoLightValueScratchYZNP = par1Block.getAmbientOcclusionLightValue(this.blockAccess, x, y, z + 1);
this.aoLightValueScratchXYPN = par1Block.getAmbientOcclusionLightValue(this.blockAccess, x + 1, y, z);
transparencyXYPN = Block.canBlockGrass[this.blockAccess.getBlockId(x + 1, y - 1, z)];
transparencyXYNN = Block.canBlockGrass[this.blockAccess.getBlockId(x - 1, y - 1, z)];
transparencyYZNP = Block.canBlockGrass[this.blockAccess.getBlockId(x, y - 1, z + 1)];
transparencyYZNN = Block.canBlockGrass[this.blockAccess.getBlockId(x, y - 1, z - 1)];

if (!transparencyYZNN && !transparencyXYNN)
{
     this.aoLightValueScratchXYZNNN = this.aoLightValueScratchXYNN;
     this.aoBrightnessXYZNNN = this.aoBrightnessXYNN;
}
else
{
     this.aoLightValueScratchXYZNNN = par1Block.getAmbientOcclusionLightValue(this.blockAccess, x - 1, y, z - 1);
     this.aoBrightnessXYZNNN = par1Block.getMixedBrightnessForBlock(this.blockAccess, x - 1, y, z - 1);
}

What it does is decrements the y value to save a bunch of subtractions since the whole next section uses "y - 1". It appears as if in the case of every face, a part was left as "y - 1" (Or any other respective coordinate changes such as "y + 1", "x - 1", "x + 1", "z - 1", "z + 1").

transparencyXYPN = Block.canBlockGrass[this.blockAccess.getBlockId(x + 1, y - 1, z)];
transparencyXYNN = Block.canBlockGrass[this.blockAccess.getBlockId(x - 1, y - 1, z)];
transparencyYZNP = Block.canBlockGrass[this.blockAccess.getBlockId(x, y - 1, z + 1)];
transparencyYZNN = Block.canBlockGrass[this.blockAccess.getBlockId(x, y - 1, z - 1)];

The way I was able to fix this was by adding 3 ints at the top of the method called xConst, yConst, zConst (or more appropriate names), set those to the coordinates passed into the function and changed the code to

transparencyXYPN = Block.canBlockGrass[this.blockAccess.getBlockId(x + 1, yConst - 1, z)];
transparencyXYNN = Block.canBlockGrass[this.blockAccess.getBlockId(x - 1, yConst - 1, z)];
transparencyYZNP = Block.canBlockGrass[this.blockAccess.getBlockId(x, yConst - 1, z + 1)];
transparencyYZNN = Block.canBlockGrass[this.blockAccess.getBlockId(x, yConst - 1, z - 1)];

This bug is both aesthetically unpleasing, but also causes the "Lava light leaking through corners" bug.

Also, while I was looking through the rendering code, I found the source of the upside-down half-slab lighting bug. A half-slab gets lit using the highest of the neighbour's light values, but it doesn't check the light value of the block below.

topBrightness = this.getSpecialBlockBrightness(par1EnumSkyBlock, par2, par3 + 1, par4);
eastBrightness = this.getSpecialBlockBrightness(par1EnumSkyBlock, par2 + 1, par3, par4);
int westBrightness = this.getSpecialBlockBrightness(par1EnumSkyBlock, par2 - 1, par3, par4);
int northBrightness = this.getSpecialBlockBrightness(par1EnumSkyBlock, par2, par3, par4 + 1);
int southBrightness = this.getSpecialBlockBrightness(par1EnumSkyBlock, par2, par3, par4 - 1);

if (eastBrightness > topBrightness)
{
	topBrightness = eastBrightness;
}

if (westBrightness > topBrightness)
{
	topBrightness = westBrightness;
}

if (northBrightness > topBrightness)
{
	topBrightness = northBrightness;
}

if (southBrightness > topBrightness)
{
	topBrightness = southBrightness;
}

return topBrightness;

The first idea that comes to mind is to push this code into the Block class so that it can be overloaded in child classes such as slabs.