public PairRankedItems previous; // automatically construct linked list
#endregion Fields
#region Constructors
public PairRankedItems(RankedItem r1, RankedItem r2)
{
item1 = r1;
item2 = r2;
previous = mostRecent;
mostRecent = this;
}
/// <summary>
/// Find every pair, compute alpha, and store the pair in pairRankedItems. Items are assumed already ranked.
/// In fact the convex version (limited to [0,1]) is equivalent to the non-convex version (allowing infinite steps)
/// (just rescale by 1/(1-alpha)). However the latter is useful because (1) we can easily control step size and (2)
/// the old weights are left unchanged.
/// </summary>
/// <param name="rankedItems"></param>
/// <param name="convex"></param>
/// <param name="maxStep">max step to use for non-convex version.</param>
/// <param name="ctr"></param>
/// <returns></returns>
public static PairRankedItems FillPairRankedItems(RankedItem[] rankedItems, bool convex, bool alphaPos, double maxStep, ref int ctr)
{
PairRankedItems pri = null;
for(int i = 0; i < rankedItems.Length - 1; ++i)
{
RankedItem x = rankedItems[i];
for(int j = i + 1; j < rankedItems.Length; ++j)
{
RankedItem y = rankedItems[j];
// See boostingNotes.docx.
if(convex)
{
// The convex combination version: (1-alpha)*score1 + alpha*score2. Disadvantage: previously computed weights
// keep changing.
if(x.score2 - x.score1 + y.score1 - y.score2 != 0)
{
// Note ranks go inversely with score
double alpha = ( y.score1 - x.score1 ) / ( x.score2 - x.score1 + y.score1 - y.score2 );
if(alpha >= 0.0 && alpha <= 1.0)
{
pri = new PairRankedItems(x, y);
pri.alpha = alpha;
}
}
}
else
{
// The score1 + alpha*score2 version. Advantage: leaves previously computed weights the same.
if(x.score2 != y.score2)
{
double alpha = ( x.score1 - y.score1 ) / ( y.score2 - x.score2 );
// alpha=0 corresponds to original rank order for both convex and non-convex combinations
if( (alphaPos && alpha >= 0.0 && alpha <= maxStep) ||
(!alphaPos && alpha <= 0.0 && alpha >= -maxStep) )
{
pri = new PairRankedItems(x, y);
pri.alpha = alpha;
}
}
}
}
}
return pri;
}
/// <summary>
/// Add jitter to those items and only those items with duplicate scores, to reduce single crossing points
/// as alpha sweeps. However, although this helps a lot, it is not sufficient: consider the case where scores1 = {1,2,3}
/// and scores2 = {3,2,1}. They all meet in the middle. This is fixed in FindBestAlpha. Algorithmically speaking
/// this is not needed, but we add jitter here to significantly reduce overall computational cost when the amount of
/// degeneracy is high.
///
/// Returns with rankedItems sorted by the (jittered) score1's.
/// </summary>
/// <param name="rankedItems"></param>
/// <param name="ran"></param>
public static void SortNJitterOld(RankedItem[] rankedItems, Random ran)
{
// We only have to sort score2 to add jitter, and if needed, we don't need to re-sort.
double scale = 1e-6;
for (int i = 0; i < rankedItems.Length; ++i)
{
rankedItems[i].score = rankedItems[i].score2;
}
Array.Sort(rankedItems);
for (int i = 0, j = 1; i < rankedItems.Length - 1; ++i, ++j)
{
if (rankedItems[i].score2 == rankedItems[j].score2)
{
rankedItems[i].score2 += scale * (ran.NextDouble() - 0.5); // leave 'j' alone for next comparison
}
}
bool needResorting = false;
for (int i = 0; i < rankedItems.Length; ++i)
{
rankedItems[i].score = rankedItems[i].score1;
}
Array.Sort(rankedItems);
for (int i = 0, j = 1; i < rankedItems.Length - 1; ++i, ++j)
{
if (rankedItems[i].score1 == rankedItems[j].score1)
{
rankedItems[i].score1 += scale * (ran.NextDouble() - 0.5);
rankedItems[i].score = rankedItems[i].score1;
needResorting = true;
}
}
if (needResorting)
Array.Sort(rankedItems);
}
/// <summary>
/// This version jitters everything, in an attempt to prevent any degeneracy. This also effectively picks a particular ranking
/// for the NDCG baseline, if there is degeneracy. The only case where this returns null is when all the scores (before or after or
/// both) are the same, in which case we simply should return 1.0 for the optimal alpha, since the lines will never cross (and if
/// all initial scores are zero, chances are we've started with an empty model and should just take the scores of the first trained model,
/// which here corresponds to scores2).
/// </summary>
/// <param name="rankedItems"></param>
/// <param name="ran"></param>
public static void SortNJitter(RankedItem[] rankedItems, Random rand)
{
double scale = 1e-6;
double val;
double max1 = double.NegativeInfinity, min1 = double.PositiveInfinity, max2 = double.NegativeInfinity, min2 = double.PositiveInfinity;
for (int i = 0; i < rankedItems.Length; ++i)
{
val = rankedItems[i].score1;
if(val > max1)
max1 = val;
if(val < min1)
min1 = val;
val = rankedItems[i].score2;
if(val > max2)
max2 = val;
if(val < min2)
min2 = val;
}
for (int i = 0; i < rankedItems.Length; ++i)
{
val = rankedItems[i].score1;
double ranVal = (2.0 * rand.NextDouble() - 1.0) * scale;
double mult = (max1 == min1) ? 1.0 : (max1 - min1); // A very degenerate case, but we must handle it
if (val == 0.0)
{
rankedItems[i].score1 = mult * ranVal;
}
else
{
rankedItems[i].score1 = val * (1.0 + ranVal);
}
val = rankedItems[i].score2;
ranVal = (2.0 * rand.NextDouble() - 1.0) * scale;
mult = (max2 == min2) ? 1.0 : (max2 - min2); // A very degenerate case, but we must handle it
if (val == 0.0)
{
rankedItems[i].score2 = mult * ranVal;
}
else
{
rankedItems[i].score2 = val * (1.0 + ranVal);
}
rankedItems[i].score = rankedItems[i].score1;
}
Array.Sort(rankedItems);
}
public static void FillRanks(RankedItem[] rankedItems)
{
for(int i = 0; i < rankedItems.Length; ++i)
{
rankedItems[i].rank = i;
}
}
/// <summary>
/// query1 and query2 must be the same query (but with different scores). The urls must be in the same order. However they need not be sorted.
/// </summary>
/// <param name="query1"></param>
/// <param name="query2"></param>
/// <param name="dcg"></param>
/// <param name="truncLevel"></param>
/// <returns>Null if this query has zero maxDCG. Else, a rankedItem array, sorted by the scores in query1.</returns>
public static RankedItem[] FillRankedItems(Query query1, Query query2, DCGScorer scorer, Random ran)
{
if(query1.Length != query2.Length)
throw new Exception("Query length mismatch.");
if(query1.QueryID != query2.QueryID)
throw new Exception("Queries have differnt IDs.");
int length = query1.Length;
double maxDCG = query1.MaxNonTruncDCG;
if(maxDCG == 0.0)
return null;
RankedItem[] rankedItems = new RankedItem[length];
double[] scores1 = query1.scores;
double[] scores2 = query2.scores;
string QID = query1.QueryID;
for(int i = 0; i < length; ++i)
{
float label = query1.Labels[i];
if(label != query2.Labels[i])
throw new Exception("FillRankedItems: label mismatch.");
rankedItems[i] = new RankedItem((double)DCGScorer.scoresMap[(int)label] / maxDCG, scores1[i], scores2[i], label);//, QueryID);
}
if (rankedItems != null)
{
SortNJitter(rankedItems, ran);
}
return rankedItems;
}
public int CompareTo(RankedItem other)
{
return score > other.score ? -1 : ( score < other.score ? 1 : 0 );
}