comparison toolboxes/distance_learning/mlr/separationOracle/separationOraclePrecAtK.m @ 0:e9a9cd732c1e tip

first hg version after svn

+function [Y, Loss] = separationOraclePrecAtK(q, D, pos, neg, k)
+%
+%   [Y,Loss]  = separationOraclePrecAtK(q, D, pos, neg, k)
+%
+%   q   = index of the query point
+%   D   = the current distance matrix
+%   pos = indices of relevant results for q
+%   neg = indices of irrelevant results for q
+%   k   = length of the list to consider
+%
+%   Y is a permutation 1:n corresponding to the maximally
+%   violated constraint
+%
+%   Loss is the loss for Y, in this case, 1-Prec@k(Y)
+
+
+    % First, sort the documents in descending order of W'Phi(q,x)
+    % Phi = - (X(q) - X(x)) * (X(q) - X(x))'
+    
+    % Sort the positive documents
+    ScorePos        = - D(pos,q);
+    [Vpos, Ipos]    = sort(full(ScorePos'), 'descend');
+    Ipos            = pos(Ipos);
+    
+    % Sort the negative documents
+    ScoreNeg        = -D(neg,q);
+    [Vneg, Ineg]    = sort(full(ScoreNeg'), 'descend');
+    Ineg            = neg(Ineg);
+
+    % Now, solve the DP for the interleaving
+
+    numPos  = length(pos);
+    numNeg  = length(neg);
+    n       = numPos + numNeg;
+
+    cVpos           = cumsum(Vpos);
+    cVneg           = cumsum(Vneg);
+
+
+    % If we don't have enough positive (or negative) examples, scale k down
+    k = min([k, numPos, numNeg]);
+
+    % Algorithm:
+    %   For each precision score in 0, 1/k, 2/k, ... 1
+    %       Calculate maximum discriminant score for that precision level
+    Precision       = (0:(1/k):1)';
+    Discriminant    = zeros(k+1, 1);
+    NegsBefore      = zeros(numPos, k+1);
+
+    % For 0 precision, all positives go after the first k negatives
+
+    NegsBefore(:,1) = k + binarysearch(Vpos, Vneg(k+1:end));
+
+    Discriminant(1) = Vpos * (numNeg - 2 * NegsBefore(:,1)) + numPos * cVneg(end) ...
+                    - 2 * sum(cVneg(NegsBefore((NegsBefore(:,1) > 0),1)));
+
+
+
+    % For precision (a-1)/k, swap the (a-1)'th positive doc
+    %   into the top (k-a) negative docs
+
+    for a = 2:(k+1)
+        NegsBefore(:,a) = NegsBefore(:,a-1);
+
+        % We have a-1 positives, and k - (a-1) negatives
+        NegsBefore(a-1, a) = binarysearch(Vpos(a-1), Vneg(1:(k-a+1)));
+
+        % There were NegsBefore(a-1,a-1) negatives before (a-1)
+        % Now there are NegsBefore(a,a-1)
+        Discriminant(a) = Discriminant(a-1) ...
+                        + 2 * (NegsBefore(a-1,a-1) - NegsBefore(a-1,a)) * Vpos(a-1);
+
+        if NegsBefore(a-1,a-1) > 0
+            Discriminant(a) = Discriminant(a) + 2 * cVneg(NegsBefore(a-1,a-1));
+        end
+        if NegsBefore(a-1,a) > 0
+            Discriminant(a) = Discriminant(a) - 2 * cVneg(NegsBefore(a-1,a));
+        end
+    end
+
+    % Normalize discriminant scores
+    Discriminant    = Discriminant / (numPos * numNeg);
+    [s, x]          = max(Discriminant - Precision);
+
+    % Now we know that there are x-1 relevant docs in the max ranking
+    % Construct Y from NegsBefore(x,:)
+
+    Y = nan * ones(n,1);
+    Y((1:numPos)' + NegsBefore(:,x)) = Ipos;
+    Y(isnan(Y))                     = Ineg;
+
+    % Compute loss for this list
+    Loss        = 1 - Precision(x);
+end
+