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use std::f64;
use activation::Activation;

#[derive(Copy, Clone)]
pub struct SoftPlus;

impl SoftPlus {
    pub fn new() -> SoftPlus {
        return SoftPlus;
    }
}

impl Activation for SoftPlus {
    /// Calculates the SoftPlus of input `x`
    fn calc(&self, x: f64) -> f64 {
        (1f64 + x.exp()).ln()
    }

    /// Calculates the Derivative SoftPlus of input `x`
    fn derivative(&self, x: f64) -> f64 {
        1f64 / (1f64 + (-x).exp())
    }
}

#[cfg(test)]
mod tests {
    use super::Activation;
    use super::SoftPlus;

    #[test]
    fn softplus_test() {
        let activation = SoftPlus::new();
        assert_approx_eq!(activation.calc(-1f64), 0.3132616875f64);
    }

    #[test]
    fn softplus_derivative_test() {
        let activation = SoftPlus::new();
        assert_approx_eq!(activation.derivative(-1f64), 0.2689414214f64);
    }
}