Question

I have SQL request that returns (x,y,z) tuples with :

• x between 1 and 9
• y between 1 and 500
• z between 1 and 15

So I have 9 * 500 * 15 = 67500 possibilities maximum and 0 possibilities minimum if all my 67500 possibilities are stored into database.

I can have (4, 256, 1) for example in my lists but not (3, 410, 8).

I want to generate 3 linked lists. For example if I choose x=2 I want the second list to display all the available y for this x value and idem for z values with y choice.

Should I use Collection ar Array to retrieve easily values to generate mys lists ?

Answer 1

With the keys falling in small ranges like this, a simple 2D array of shorts would be sufficient:

short data[10][500];

The first index is the value of x; the second index is the value of y.

The values of z are stored as individual bits of the short at [x][y]. To access individual bits, use bit operations:

if ((data[x][y] & (1 << z)) != 0) // The element at x,y,x is present
    ...
data[x][y] != (1 << z); // Add an element at x,y,z
data[x][y] &= ~(1 << z); // Remove the element at x,y,z

This data structure is very efficient in space: it needs roughly 10KBytes of memory to store the entire data set.