Multiple arrays in gmem[]

[SaschArt]

I'm trying to make a set of functions that will merge several arrays into one. We have shared a single gmem[] global array and many times more arrays are needed. Here's what I did:

function alloc(sz) global() (
  memset((this.top+=sz)-sz,0,sz);
);
alloc.top=0; 
arr0=alloc(22); // array index 0 - maximum allocated dimension, we never exceed it with setArrLen()
arr1=alloc(22); // array index 1
arr2=alloc(22); // array index 2

function getI(i_arr) local(i_a, j) (
  i_a=0; j=1;
  while(i_a<i_arr) ( // scan all arrays length to get the position of index
    j+=gmem[j]+1;
    i_a+=1;
  );
  j;
);
function getArr(i_arr, i) (
  gmem[getI(i_arr)+i+1];
);
function setArr(i_arr, i, val) (
  gmem[getI(i_arr)+i+1]=val;
);
function setArrLen(i_arr, val) local(i, j) (
  i=getI(i_arr);
  last=gmem[i];
  val!=last && last ? (
    val<last ? (
      j=i+last;
      while(j<=gmem[0]) ( // move the old data to new position
        gmem[j]=j+last-val<=gmem[0] ? gmem[j+last-val] : 0;
        j+=1;
      );
    ) : (
      j=gmem[0];
      while(j>=i+last) (
        gmem[j+val-last]=j<i+val ? 0:gmem[j];
        j-=1;
      );
    );  
  );
  gmem[i]=val; // store array length on the first position for each array
  gmem[0]+=val-last; // store all array length
);
gmem[0]=3; // alloc arrays number
setArrLen(0, 5);
setArrLen(1, 7);
setArrLen(2, 7);


arr0[1]+=1;
arr1[2]+=2;
arr2[0]+=2;
arr2[1]+=3;
arr2[6]+=4;
setArr(0, 1, arr0[1]);
setArr(1, 2, arr1[2]);
setArr(2, 0, arr2[0]);
setArr(2, 1, arr2[1]);
setArr(2, 6, arr2[6]);

res=getArr(0, 1); // good result
res=getArr(1, 2); // good result
res=getArr(2, 1); // good result
setArrLen(0, gmem[getI(0)]+1);
res=getArr(0, 1); // good result
res=getArr(1, 2); // good result
res=getArr(2, 1); // good result
setArrLen(0, gmem[getI(1)]+1);
res=getArr(1, 2); // wrong result

As you can see in the code, I want to be able to change the size of the arrays. It only works if I increment or decrement the size of first array by one. How to improve to can I set any size I want to any array (up to maximum allocated dimension)?