Help - How to limit my camera movement ?

Hi guys, i needing your help.

I have code for moving my camera with my touch, but i have some trouble now, how can i limit my camera movement within my terrain?

Thanks

this is my code :

Code (csharp):

1. using UnityEngine;
2. using System.Collections;
3.  
4. public class MovingCamera: MonoBehaviour {
5. Vector3 hit_position = Vector3.zero;
6. Vector3 current_position = Vector3.zero;
7. Vector3 camera_position = Vector3.zero;
8. float z = 0.0f;
9.  
10. // Use this for initialization
11. void Start () {
12.  
13. }
14.  
15. void Update(){
16.    
17.     if(Input.GetMouseButtonDown(0)){
18.         hit_position = Input.mousePosition;
19.         camera_position = transform.position;
20.     }
21.     if(Input.GetMouseButton(0)){
22.         current_position = Input.mousePosition;
23.         LeftMouseDrag();
24.     }
25.    
26. }
27.  
28. void LeftMouseDrag(){
29.     // From the Unity3D docs: "The z position is in world units from the camera."  In my case I'm using the y-axis as height
30.     // with my camera facing back down the y-axis.  You can ignore this when the camera is orthograhic.
31.     current_position.z = hit_position.z = camera_position.y;
32.  
33.     // Get direction of movement.  (Note: Don't normalize, the magnitude of change is going to be Vector3.Distance(current_position-hit_position)
34.     // anyways.  
35.     Vector3 direction = Camera.main.ScreenToWorldPoint(current_position) - Camera.main.ScreenToWorldPoint(hit_position);
36.  
37.     // Invert direction to that terrain appears to move with the mouse.
38.     direction = direction * -0.4f;
39.  
40.     Vector3 position = camera_position + direction;
41.  
42.     transform.position = position;
43. }
44. }