dnn: Allow LSTM layer to operate in reverse direction

This is useful for bidirectional LSTMs.

modules/dnn/test/test_layers.cpp

@@ -489,6 +489,55 @@ TEST(Layer_RNN_Test_Accuracy_with_, CaffeRecurrent)
     normAssert(h_ref, output[0]);
 }
 
+TEST(Layer_LSTM_Test_Accuracy_, Reverse)
+{
+    // This handcrafted setup calculates (approximately) the prefix sum of the
+    // input, assuming the inputs are suitably small.
+    cv::Mat input(2, 1, CV_32FC1);
+    input.at<float>(0, 0) = 1e-5f;
+    input.at<float>(1, 0) = 2e-5f;
+
+    cv::Mat Wx(4, 1, CV_32FC1);
+    Wx.at<float>(0, 0) = 0.f;  // Input gate
+    Wx.at<float>(1, 0) = 0.f;  // Forget gate
+    Wx.at<float>(2, 0) = 0.f;  // Output gate
+    Wx.at<float>(3, 0) = 1.f;  // Update signal
+
+    cv::Mat Wh(4, 1, CV_32FC1);
+    Wh.at<float>(0, 0) = 0.f;  // Input gate
+    Wh.at<float>(1, 0) = 0.f;  // Forget gate
+    Wh.at<float>(2, 0) = 0.f;  // Output gate
+    Wh.at<float>(3, 0) = 0.f;  // Update signal
+
+    cv::Mat bias(4, 1, CV_32FC1);
+    bias.at<float>(0, 0) = 1e10f;  // Input gate - always allows input to c
+    bias.at<float>(1, 0) = 1e10f;  // Forget gate - never forget anything on c
+    bias.at<float>(2, 0) = 1e10f;  // Output gate - always output everything
+    bias.at<float>(3, 0) = 0.f;  // Update signal
+
+    LayerParams lp;
+    lp.set("reverse", true);
+    lp.set("use_timestamp_dim", true);
+    lp.blobs.clear();
+    lp.blobs.push_back(Wh);
+    lp.blobs.push_back(Wx);
+    lp.blobs.push_back(bias);
+
+    cv::Ptr<cv::dnn::LSTMLayer> layer = LSTMLayer::create(lp);
+    std::vector<cv::Mat> outputs;
+    std::vector<cv::Mat> inputs;
+    inputs.push_back(input);
+    runLayer(layer, inputs, outputs);
+
+    ASSERT_EQ(1, outputs.size());
+    cv::Mat out = outputs[0];
+    ASSERT_EQ(3, out.dims);
+    ASSERT_EQ(shape(2, 1, 1), shape(out));
+    float* data = reinterpret_cast<float*>(out.data);
+    EXPECT_NEAR(std::tanh(1e-5f) + std::tanh(2e-5f), data[0], 1e-10);
+    EXPECT_NEAR(std::tanh(2e-5f), data[1], 1e-10);
+}
+
 
 class Layer_RNN_Test : public ::testing::Test
 {