/*
Example of mxlookup

	auto recs = MxRecord.lookup(args[1]);
	foreach(rec;recs){
		writefln("Got: %d %s", rec.weight, rec.name);
	}
*/


 



extern(C) {
	int __res_init ();
	int __res_search (char *, int, int, ubyte *, int);
	int *__h_errno_location ();
	ushort ntohs (ushort __netshort);
	int __dn_skipname (ubyte *, ubyte *);
	int __dn_expand (ubyte *, ubyte *, ubyte *, char *, int) ;
	

	
}
struct HEADER {
	short id ;
	ubyte td_tc_aa_opcode_qr;
	ubyte rcode_cd_ad_unused_ra;
	ushort qdcount ;
	ushort ancount ;
	ushort nscount ;
	ushort arcount ;
} 
const ns_c_in = 1;
const ns_t_mx = 15;


import std.stdio;

static this()
{
	__res_init();
}

class MxRecord
{
	this(int w, char[] n) 
	{
		this.weight = w;
		this.name = n;
	}
	int weight;
	char[] name;
	int opCmp( Object B) {
		
		return weight - (cast(MxRecord)B).weight;
    }
	
	static MxRecord[] lookup (char[] domain)
	
		 
	{
		
		
		HEADER *hp;
		ubyte *end;
		
		MxRecord[] ret;
		int qdc;
		ubyte *cp;
		ushort type;
		
		
		ubyte ans[8192];
		ushort weight = 0;
		int i, count;
		char buf[64];


		if((i = __res_search(std.string.toStringz(domain),
						 ns_c_in,
						 ns_t_mx,
						 ans.ptr,
						 ans.sizeof)) < 0){
					throw new Exception(nsError((*__h_errno_location ()), domain));
			 
		}
		if (i > ans.sizeof) {
			i = ans.sizeof;
		}
		
		hp = cast(HEADER *)(ans.ptr);
		
		cp = (ans.ptr + 12);
		end =(ans.ptr +i);
		
			
		for (qdc = ntohs(hp.qdcount);   qdc --; cp =  (cp + i + 4)) {
			
		
			if ((i = __dn_skipname(cp, end)) < 0 ) {
				throw new Exception("No Records found");
			}
		}
		
		
		count = ntohs(hp.ancount);
		
		while (--count >= 0 && cp < end) {
			 
			if ((i = __dn_skipname(cp, end)) < 0 ) {
				continue;
			}
			cp += i;
			type = (*cp << 8) | *(cp+1); 
			cp += 8; 
			i = (*cp << 8) | *(cp+1) ;
			cp += 2; 
			
			if (type != ns_t_mx) {
				//printf("not mx %d != %d", type, ns_t_mx);
				cp += i;
				continue;
			}
				
			weight = (*cp << 8) | *(cp+1) ; 
			cp += 2; 
			
			
			if ((i = __dn_expand(ans.ptr, end, cp, buf.ptr, cast(int)buf.sizeof-1)) < 0) {
				throw new Exception("Invalid data returned");
			}
			cp += i;
			ret ~= new MxRecord(weight, std.string.toString(buf.ptr).dup);
			//writefln("got %d, %s\n", weight, std.string.toString(buf.ptr));

		}
		return ret.sort;
		
		
	}
	static char[] nsError(int error, char[] domain) 
	{
		switch(error){
			case 1:
			  return  "Unknown domain: "~ domain;

			case 4:
			  return  "No NS records for " ~ domain;
			  
			case 2:
			  return "No response for NS query";

			default:
			  return "Unexpected error";
			  
		}
	}
		}