backprop.swift

class Layer {
    
    ...
        
    var dW: Matrix!
    var db: Matrix!
    var dZ: Matrix!
    
    func backward(m: Double, y: Matrix? = nil) {
        if let nextLayer = nextLayer, let previousLayer = previousLayer {
            dZ = nextLayer.weights.T.dot(nextLayer.dZ) * activation.backward(Z)
            dW = (1.0 / m) * (dZ.dot(previousLayer.A.T))
            db = (1.0 / m) * (dZ.sum(direction: .rows))
        } else if let previousLayer = previousLayer, let Y = y {
            dZ = ((Y / A) - ((1 - Y) / (1 - A))).invertSign()
            dW = (1.0 / m) * (dZ.dot(previousLayer.A.T))
            db = (1.0 / m) * (dZ.sum(direction: .rows))
        }
    }
}