Flat indexing

[LeandroMartinsdS]

Using flat indexing might improve performance and make it less convoluted.

Example

#define ROWS 100
#define COLS 100

for (int k = 0; k < ROWS * COLS; k++) {
    int i = k / COLS;
    int j = k % COLS;
    do_something(i, j);
}