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use libm::{
  exp  as libm_exp,
  tanh as libm_tanh,
  fabs as libm_abs,
  sqrt as libm_sqrt
};

pub const fn linear(x: f64)        -> f64 { x }
pub       fn threshold(x: f64)     -> f64 { if x > 0.0 { 1.0 } else { 0.0 } }
pub       fn relu(x: f64)          -> f64 { if x > 0.0 { x } else { 0.0 } } 
pub       fn sign(x: f64)          -> f64 { if x > 0.0 { 1.0 } else if x == 0.0 { 0.0 } else { -1.0 } }
pub       fn sigmoid(x: f64)       -> f64 { 1.0 / (1.0 + libm_exp(-x)) }
pub       fn tanh(x: f64)          -> f64 { libm_tanh(x) }
pub       fn soft_sign(x: f64)     -> f64 { x / (1.0 + libm_abs(x)) }
pub       fn bent_identity(x: f64) -> f64 { ((libm_sqrt((x * x) + 1.0) - 1.0) / 2.0) + x }