Question

Given the following direction enum:

typedef enum {
    DirectionNorth = 0,
    DirectionNorthEast,
    DirectionEast,
    DirectionSouthEast,
    DirectionSouth,
    DirectionSouthWest,
    DirectionWest,
    DirectionNorthWest
} Direction;

And number matrix similar to the numeric pad:

7 8 9
4 5 6
1 2 3

How would you write a function to return the direction between adjacent numbers from the matrix? Say:

1, 2 => DirectionEast
2, 1 => DirectionWest
4, 8 => DirectionNorthEast
1, 7 => undef

You may change the numeric values of the enum if you want to. Readable solutions preferred. (Not a homework, just an algorithm for an app I am working on. I have a working version, but I’m interested in more elegant takes.)

Answer 1

int direction_code(int a, int b)
{
    assert(a >= 1 && a <= 9 && b >= 1 && b <= 9);
    int ax = (a - 1) % 3, ay = (a - 1) / 3,
        bx = (b - 1) % 3, by = (b - 1) / 3,
        deltax = bx - ax, deltay = by - ay;
    if (abs(deltax) < 2 && abs(deltay) < 2)
        return 1 + (deltay + 1)*3 + (deltax + 1);
    return 5;
}

resulting codes are

1 south-west
2 south
3 south-east
4 west
5 invalid
6 east
7 north-west
8 north
9 north-east