TileTextureManager.cs

using UnityEngine;
using System.Collections;

public class TileTextureManager : MonoBehaviour {

    // Sprites to use
    public Sprite fullBlock;
    public Sprite straightBlock;
    public Sprite endBlock;
    public Sprite emptyBlock;
    public Sprite cornerBlock;
    public Sprite tBlock;

    // Occupied sides
    private bool top = false;
    private bool bottom = false;
    private bool left = false;
    private bool right = false;

    // Other
    public float blockSize;
    public string Tag;
    public int layerToMask = 8;

    // Initialization
    void Start ()
    {
        blockSize /= 1.5f;

        int layerMask = ~(1 << layerToMask);

        // Find objects
        RaycastHit2D topHit = Physics2D.Raycast (transform.position + Vector3.up * blockSize,
                                                 Vector2.up, blockSize, layerMask);
        RaycastHit2D bottomHit = Physics2D.Raycast (transform.position + -Vector3.up * blockSize,
                                                    -Vector2.up, blockSize, layerMask);
        RaycastHit2D leftHit = Physics2D.Raycast (transform.position + -Vector3.right * blockSize,
                                                  -Vector2.right, blockSize, layerMask);
        RaycastHit2D rightHit = Physics2D.Raycast (transform.position + Vector3.right * blockSize,
                                                   Vector2.right, blockSize, layerMask);

        // Determine what was found
        if (topHit.collider != null)
            if (topHit.collider.tag == Tag)
                top = true;
        if (bottomHit.collider != null)
            if (bottomHit.collider.tag == Tag)
                bottom = true;
        if (leftHit.collider != null)
            if (leftHit.collider.tag == Tag)
                left = true;
        if (rightHit.collider != null)
            if (rightHit.collider.tag == Tag)
                right = true;

        SpriteRenderer renderer = GetComponent<SpriteRenderer> ();

        // Change sprite to fit based on found objects
        if (top)
        {
            if (bottom)
            {
                if (left)
                {
                    if (right)
                    {
                        // All sides occupied
                        renderer.sprite = emptyBlock;
                    }
                    else
                    {
                        // Top, bottom, left, occupied
                        renderer.sprite = tBlock;
                        transform.Rotate (new Vector3 (0, 0, 90));
                    }
                }
                else if (right)
                {
                    // Top, bottom, right occupied
                    renderer.sprite = tBlock;
                    transform.Rotate (new Vector3 (0, 0, -90));
                }
                else
                {
                    // Top, bottom occupied
                    renderer.sprite = straightBlock;
                    transform.Rotate (new Vector3 (0, 0, 90));
                }
            }
            else if (left)
            {
                if (right)
                {
                    renderer.sprite = tBlock;
                }
                else
                {
                    renderer.sprite = cornerBlock;
                }
            }
            else if (right)
            {
                // Top, right occupied
                renderer.sprite = cornerBlock;
                transform.Rotate (new Vector3 (0, 0, -90));
            }
            else
            {
                // Top occupied
                renderer.sprite = endBlock;
                transform.Rotate (new Vector3 (0, 0, -90));
            }
        }
        else if (bottom)
        {
            if (left)
            {
                if (right)
                {
                    // Bottom, left, right occupied
                    renderer.sprite = tBlock;
                    transform.Rotate (new Vector3 (0, 0, 180));
                }
                else
                {
                    // Bottom, left occupied
                    renderer.sprite = cornerBlock;
                    transform.Rotate (new Vector3 (0, 0, 90));
                }
            }
            else if (right)
            {
                // Bottom, right occupied
                renderer.sprite = cornerBlock;
                transform.Rotate (new Vector3 (0, 0, 180));
            }
            else
            {
                // Bottom occupied
                renderer.sprite = endBlock;
                transform.Rotate (new Vector3 (0, 0, 90));
            }
        }
        else if (left)
        {
            if (right)
            {
                // Left, right occupied
                renderer.sprite = straightBlock;
            }
            else
            {
                // Left occupied
                renderer.sprite = endBlock;
            }
        }
        else if (right)
        {
            // Right occupied
            renderer.sprite = endBlock;
            transform.Rotate (new Vector3 (0, 0, 180));
        }
        else
        {
            // None occupied
        }
    }
}