trace_line()
I never used it but it looks like it "shoots" a line from start to end and if it hits something it returns it, also I think it returns the origin that it hit the ent in 4th param
You should also check if the ent returned is a player... with if(1 <= ent <= 32) (replace 32 with your maxplayers var)
1st it's obvious, ignore an ent... I'm not sure about this but if you set it to 0 it might go through walls xD, -1 I think is the "don't ignore anything" setting... but as I said, never used it so I'm not sure, just assuming
You can also look at
http://forums.alliedmods.net/showthread.php?p=663892 or
http://forums.alliedmods.net/showthread.php?t=66564 for their laser break detecting method
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