spiral.c

inline bool contains(int x, int y, Bounds rect) {
    return (x <= rect.right) && (y <= rect.bottom) && (x >= rect.left) && (y >= rect.top);
}

void BuildStringFromMatrix(int* Matrix, int NumRows, int NumColumns, char* OutBuffer) {
    Bounds rect = { NumColumns - 1 , NumRows - 1, 0, 0 };

    int dir = 0;
    int count = 0;
    int x = 0;
    int y = 0;

    while (++count < NumRows * NumColumns) {
        OutBuffer += sprintf(OutBuffer, "%d, ", Matrix[x + NumColumns * y]);

        // move
        switch (dir % 4) {
            case 0: if (contains(++x,y,rect)) break; else x--; dir++; rect.top++;    // right
            case 1: if (contains(x,++y,rect)) break; else y--; dir++; rect.right--;  // down
            case 2: if (contains(--x,y,rect)) break; else x++; dir++; rect.bottom--; // left
            case 3: if (contains(x,--y,rect)) break; else y++; dir++; rect.left++;   // up
            default: ++x;
        };
    } 

    OutBuffer += sprintf(OutBuffer, "%d", Matrix[x + NumColumns * y]);
}