I've spent some time on looking for an appropriate way to solve my problem with the specific rectangles placement.

Lets get directly to the problem.

I've got two rectangles, which are fixed size. One of them is locked, and cannot be moved, and the other one can be move freely.

I've got two of them placed on the fixed width and height canvas. Lets say it's 1000x1000.

I need to place the second, moveable rectangle in the horizontal or vertical alignment with the other, locked one, while the moveable one cannot get outside the range of 1000x1000 canvas.

What I understand under the definition of horizontal/vertical alignment is that if you look only at X (that's for horizontal) or Y (that's for vertical) axis, you have the locked one inside the moveable one or the other way.

I know that I can get through my problem with a hack, but I want to find any smarter solution to it.

Any suggestions?

EDIT: All the informations about two rectangles are known. Corners, widths/heights.

Given a coordinate plane over some X[-inf, +inf] axis and a nominal Y[-inf, +inf] axis, two points can be said to be in vertical alignment if their coordinates on the Horizontal axis (we Choose X) are equal. They can be said to be in horizontal alignment if their coordinates on the Vertical axis (Y) are equal.

A rectangle can be described by two pairs of coordinates (x1, y1) and (x2, y2), such that x1 <= x2, and y1 <= y2. (If your pairs come mixed, you can actually rearrange them to use this format - it'll make your life easier.)

A rectangle (r1) can be said to reside INSIDE another rectangle(r2) if and ONLY if r1.x1 >= r2.x1, r1.y1 >= r2.y1, r1.x2 <= r2.x2, and r1.y2 <= r2.y2 all hold true. Note that this holds true no matter which quadrant you're in, or in which direction your axes run.

A rectangle (r1) can be said to reside OUTSIDE another rectangle(r2) if and ONLY if both r1.x1 and r1.x2 fall outside the range (r2.x1, r2.x2) and both r1.y1 and r1.y2 fall outside the range (r2.y1, r2.y2). Note that this includes the case in which r2 is INSIDE r1.

A rectangle (r1) can be said to be SEPARATE from another rectangle (r2) if and only if it is OUTSIDE that rectangle, and that rectangle is not INSIDE it. (that is: r1 OUTSIDE r2 && NOT r2 INSIDE r1)

A rectangle (r1) can be said to OVERLAP another rectangle(r2) if and only if it is neither INSIDE nor OUTSIDE that rectangle. (Alternatively, you may choose to claim that a rectangle wholly inside another is a valid overlap, and say that OVERLAP = !SEPARATE. Depends on your applications.)

Alignment is much harder for rectangles. Rectangles of the SAME EXACT size can be said to be aligned in the same way as points (using (x1, y1) of each rectangle as your points of reference. For Rectangles of Differing size, I suggest you use the center point as a reference: Rectangles are VERTICALLY aligned if (r1.x1+r1.x2)/2 = (r2.x1+r2.x2)/2, and horizontally aligned if (r1.y1+r1.y2)/2 = (r2.y1+r2.y2)/2.

I'd write a class that encompasses these rules, and use that to check each movement, using three rectangles - the two you mentioned, and the bounding box as the third.

CLASS rectangle
    x1
    x2
    y1
    y2
    
    CONSTRUCT(xx1, yy1, xx2, yy2):
        Ensure xx1 <= xx2 and yy1 <= yy2 - swap them if you like
        x1 = xx1, y1 = yy1, x2 = xx2, y2 = yy2

    IS_INSIDE(rectangle r2):
        IF r2.x1<=x1 && r2.y1<=y1 && y2 <= r2.y2 && x2 <= r2.x2:
            RETURN TRUE
        RETURN FALSE
   
    IS_OUTSIDE(rectangle r2):
        IF r2.x1 <= x1 <= r2.x2 || r2.x1 <= x2 <= r2.x2 ||  r2.y1 <= y1 <= r2.y2 || r2.y1 <= y2 <= r2.y2 :
            RETURN FALSE
         RETURN TRUE
   
    IS_SEPARATE(rectangle r2):
        IF IS_OUTSIDE(r2) && ! r2.IS_INSIDE(me):
            RETURN TRUE
        RETURN FALSE

    IS_OVERLAPPED(rectangle r2):
        IF ! IS_OUTSIDE(r2) && ! IS_INSIDE(r2):
            RETURN TRUE
        RETURN FALSE

    IS_VERTICALLY_ALIGNED(rectangle r2):
        IF (x1+x2)/2 = (r2.x1+r2.x2)/2:
            RETURN TRUE
        RETURN FALSE

    IS_HORIZONTALLY_ALIGNED(rectangle r2):
        IF (y1+y2)/2 = (r2.y1+r2.y2)/2:
            RETURN TRUE
        RETURN FALSE

Then you can write a very simple function to see if r2 is validly placed, assuming r2 is the moveable rectangle, r1 is the fixed one, and box is an undisplayed rectangle representing the bounding box of the canvas:

IS_VALID_PLACEMENT(r2, r1, box):
    //In the bounding box
    IF r2.IS_OUTSIDE(box):
        RETURN FALSE
    
    //Aligned with r1
    IF ! r2.IS_VERTICALLY_ALIGNED(r1) && ! r2.IS_HORIZONTALLY_ALIGNED(r1):
        RETURN FALSE

    //Not overlapping r1
    IF ! r2.IS_SEPARATE(r1):
        RETURN FALSE

    RETURN TRUE

And just run that every time the box moves. If it returns false, undo the move, or use bounding logic to do abutment.

Edit

I was rereading your question, and I noticed your definition of vertical alignment. Totally doable:

    IS_VERTICALLY_ALIGNED(rectangle r2):
        IF x1 <= r2.x1 <= r2.x2 <= x2 || r2.x1 <= x1 <= x2 <= r2.x2:
            RETURN TRUE
        RETURN FALSE 

    IS_HORIZONTALLY_ALIGNED(rectangle r2):
        IF y1 <= r2.y1 <= r2.y2 <= y2 || r2.y1 <= y1 <= y2 <= r2.y2:
            RETURN TRUE
        RETURN FALSE