Question

I am trying to understand Double-Array Trie implementation from http://linux.thai.net/~thep/datrie/datrie.html
But I do not understand following part.

check[base[s] + c] = s
base[s] + c = t 

What does adding c means here.

Can anybody explain algorithm in simple language.

Answer 1

Imagine you collaps a 2D array into a 1D array:

int arr2D[2][3];
int arr1D[2 * 3]; // # of rows * # of columns

// access element [1][2] of 2D array, i.e., the last element
int elem2D = arr2D[1][2];
// access element [1][2] of 1D array, i.e., the last element
int elem1D = arr1D[1 * 3 + 2];
// =========================================================
lets visualize the array access:
arr2D => x/y 0  1  2
          0 [N][N][N]
+1 on x > 1 [N][N][N]
+2 on y ---------- ^
y_len  =>   ^-------^ 3 elements
so the access happens with x * y_len + y
                           1 *   3   + 2
now for the 1D array
we want the second row, so we go with 1 * 3
(0-based access, y_len = 3)
and then we want the 3rd element, so + 2
(again, 0-based access)
arr1D =>  x  0  1  2 
            [N][N][N]
             3  4  5
1 * 3 = 3 > [N][N][N]
      + 2 = 5 ---- ^

I hope I didn’t make this too complicated (even though I think I did…). 🙂