function [K] = kernel(ker,x,y)
% Calculate kernel function.
%
% x: 输入样本,d×n1的矩阵,n1为样本个数,d为样本维数
% y: 输入样本,d×n2的矩阵,n2为样本个数,d为样本维数
%
% ker 核参数(结构体变量)
% the following fields:
% type - linear : k(x,y) = x'*y
% poly : k(x,y) = (x'*y+c)^d
% gauss : k(x,y) = exp(-0.5*(norm(x-y)/s)^2)
% tanh : k(x,y) = tanh(g*x'*y+c)
% degree - Degree d of polynomial kernel (positive scalar).
% offset - Offset c of polynomial and tanh kernel (scalar, negative for tanh).
% width - Width s of Gauss kernel (positive scalar).
% gamma - Slope g of the tanh kernel (positive scalar).
%
% ker = struct('type','linear');
% ker = struct('type','ploy','degree',d,'offset',c);
% ker = struct('type','gauss','width',s);
% ker = struct('type','tanh','gamma',g,'offset',c);
%
% K: 输出核参数,n1×n2的矩阵
%-------------------------------------------------------------%
switch ker.type
case 'linear'
K = x'*y;
case 'ploy'
d = ker.degree;
c = ker.offset;
K = (x'*y+c).^d;
case 'gauss'
s = ker.width;
rows = size(x,2);
cols = size(y,2);
tmp = zeros(rows,cols);
for i = 1:rows
for j = 1:cols
tmp(i,j) = norm(x(:,i)-y(:,j));
end
end
K = exp(-0.5*(tmp/s).^2);
case 'tanh'
g = ker.gamma;
c = ker.offset;
K = tanh(g*x'*y+c);
otherwise
K = 0;
end
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