refactored

nbt.js

const TAG_End = 0;
const TAG_Byte = 1;
const TAG_Short = 2;
const TAG_Int = 3;
const TAG_Long = 4;
const TAG_Float = 5;
const TAG_Double = 6;
const TAG_Byte_Array = 7;
const TAG_String = 8;
const TAG_List = 9;
const TAG_Compound = 10;
const TAG_Int_Array = 11;
const TAG_Long_Array = 12;

const processNBT = [];

processNBT[TAG_End] = (u8,i)=>{
    throw new Error("unexpected TAG_END");
};

processNBT[TAG_Byte] = (u8,i)=>{
    return [u8[i],i+1];
};

processNBT[TAG_Short] = (u8,i)=>{
    const val = (u8[i++]<<8)|u8[i++];
    return [val,i];
};

processNBT[TAG_Int] = (u8,i)=>{
    const val = (u8[i++]<<24)|(u8[i++]<<16)|(u8[i++]<<8)|u8[i++];
    return [val,i];
};

processNBT[TAG_Long] = (u8,i)=>{
    const v1 = (u8[i++]<<24)|(u8[i++]<<16)|(u8[i++]<<8)|u8[i++];
    const v2 = (u8[i++]<<24)|(u8[i++]<<16)|(u8[i++]<<8)|u8[i++];
    const val = BigInt(v1)*4294967296n+BigInt(v2);
    return [val,i];
};

processNBT[TAG_Float] = (u8,i)=>{
    const val = bytesToFloat(u8[i++],u8[i++],u8[i++],u8[i++]);
    return [val,i];
};

processNBT[TAG_Double] = (u8,i)=>{
    const val = bytesToDouble(u8[i++],u8[i++],u8[i++],u8[i++],
    u8[i++],u8[i++],u8[i++],u8[i++]);
    return [val,i];
};

processNBT[TAG_Byte_Array] = (u8,i)=>{
    const len = (u8[i++]<<24)|(u8[i++]<<16)|(u8[i++]<<8)|u8[i++];
    const val = new Int8Array(len);
    for(let j = 0; j < len; j++){
        val[j] = u8[i++];
    }
    return [val,i];
};

processNBT[TAG_String] = (u8,i)=>{
    const len = (u8[i++]<<8)|u8[i++];
    const val = decodeJavaUTF8(u8,i,len);
    i += len;
    return [val,i];
};

processNBT[TAG_List] = (u8,i)=>{
    const typeid = u8[i++];
    const len = (u8[i++]<<24)|(u8[i++]<<16)|(u8[i++]<<8)|u8[i++];
    const val = [];
    for(let j = 0; j < len; j++){
        let v;
        [v,i] = processNBT[typeid](u8,i);
        val.push(v);
    }
    return [val,i];
};

processNBT[TAG_Compound] = (u8,i)=>{
    const val = Object.create(null);
    while(i < u8.length){
        const typeid = u8[i++];
        if(typeid === 0/*TAG_END*/)break;
        let name,v;
        [name,i] = processNBT[8/*TAG_STRING*/](u8,i);
        [v,i] = processNBT[typeid](u8,i);
        val[name] = v;
    }
    return [val,i];
};

processNBT[TAG_Int_Array] = (u8,i)=>{
    const len = (u8[i++]<<24)|(u8[i++]<<16)|(u8[i++]<<8)|u8[i++];
    const val = new Int32Array(len);
    for(let j = 0; j < len; j++){
        val[j] = (u8[i++]<<24)|(u8[i++]<<16)|(u8[i++]<<8)|u8[i++];
    }
    return [val,i];
};

processNBT[TAG_Long_Array] = (u8,i)=>{
    const len = (u8[i++]<<24)|(u8[i++]<<16)|(u8[i++]<<8)|u8[i++];
    const val = new BigInt64Array(len);
    for(let j = 0; j < len; j++){
        const v1 = (u8[i++]<<24)|(u8[i++]<<16)|(u8[i++]<<8)|u8[i++];
        const v2 = (u8[i++]<<24)|(u8[i++]<<16)|(u8[i++]<<8)|u8[i++];
        val[j] = BigInt(v1)*4294967296n+BigInt(v2);
    }
    return [val,i];
};