local Index, parent = torch.class('nn.Index', 'nn.Module')

function Index:__init(dimension)
    parent.__init(self)
    self.dimension = dimension
    self.gradInput = {self.gradInput, self.gradInput.new()}
end

function Index:updateOutput(input)
    local t = input[1]
    local index = input[2]
    self.output:index(t, self.dimension, index)
    return self.output
end

function Index:updateGradInput(input, gradOutput)
    local t = input[1]
    local index = input[2]

    self.gradInput[2]:resize(index:size()):zero()
    local gradInput = self.gradInput[1] -- no gradient for the index variable
    gradInput:resizeAs(t):zero()
    gradInput:indexAdd(self.dimension, index, gradOutput)
    return self.gradInput
end

function Index:clearState()
    self.gradInput[1]:set()
    self.gradInput[2]:set()
    self.output:set()
    return self
end