upto(n) = {m = Map();
mapput(m, 1, 0); 
endOfChain = 1; 
for(i = 2, n, 
	\\print(maptovec()); 
	c = findsum(i);
	if(c == -1,
		setNewEndOfChain(i)
	, 
		insertAt(c)	
	) 
); known(n)
}

findsum(n) = {
my(key = endOfChain); 
my(val = mapget(m, key)); 
while(key != 1, \\ the beginning of the chain is always 0. If beginning is non-constant, you might want to use mapisdefined. 
	if(val + key == n,
		return(key); 
	); 
	key = val;
	val = mapget(m, key); 
);  return(-1)
}

insertAt(key) = {
my(val = mapget(m, key), s = val + key);
	mapput(m, key, s); 
	mapput(m, s, val); 
}

setNewEndOfChain(i) = {
	mapput(m, i, endOfChain);
	endOfChain = i;
}

maptovec() = {my(res = List(), c = endOfChain);
while(c != 0, 
	listput(res, c); 
	c = mapget(m, c);
); listput(res, 0); Vecrev(Vec(res))
}

known(n) = {my(i = 1);
v = maptovec();  
while(i < #v, 
	if(v[i] + v[i+1] > n,
		return(vector(i, j, v[j]))
	); i++
); return(v)
}