Two-phase and Multiphase linear Regression

Function MultiLineFit(w, x) : fitfunc
    Wave w
    Variable x
   
    Variable nSegs = NumPnts(w)/2
    // format of coefficient wave is
    // w[0] = a1
    // w[1] = b1
    // w[2] = x1
    // w[3] = b2
    // w[4] = x2    (if necessary)
    // w[5] = b3    (only used if x2 is present
   
    Variable result = w[0]
    if (x < w[2])
        return result + w[1]*x
    endif
   
    Variable i
    Variable a2 = w[0]
    for (i = 1; i < nSegs; i += 1)
        Variable xIndex = 2*i
        a2 = (w[xIndex-1] - w[xIndex + 1])*w[xIndex] + a2
        if ( (i == nSegs-1) || (x < w[2*(i+1)]) )
            return a2 + w[xIndex+1]*x
        endif
    endfor
   
    // if we get here, something went wrong
    return NaN
end