  We investigate the collision dynamics of discrete soliton (DS) pair in a
realistic semi-infinite nonlinear waveguide array (WA) in the context of
\textit{diffractive resonant radiation} (DifRR). Depending on the initial
amplitude ($A_0$) and wavenumber ($k_0$), a co-moving pair of identical DSs
either collide elastically or merge to form a discrete \textit{breather}. For
large amplitude and small wavenumber, DSs form a bound state and do not
interact .
  We map the domain of interaction by iterative simulation and present a phase
plot in ($A_0$-$k_0$) parameter space.
  A variational technique is developed where the interaction term is considered
as perturbation. A proper choice of Lagrangian density and the
\textit{ans\"{a}tz} function followed by the Ritz optimization leads us to a
set of ordinary differential equations that describe the collision mechanism.
The analytical result corroborate well with numerical data.
  We further investigate the role of initial phase detuning between two
identical DS and observe a periodic energy exchange during their interaction.
  Varied degree of energy exchange occurs when two DS with different
wavenumbers collide which results in three distinct output states accompanied
by DifRR generation.
  Extending our investigation to a more generalized condition by taking
different amplitudes and wavenumber of DS pair, we find an unique secondary
radiation in $k$-space which is originated due to the collision of solitons.
  The nature of this collision mediated secondary radiation is found to be
different from usual DifRR.
  Our results shed light on the interesting aspects of the collision dynamics
of DS pair in nonlinear WA and useful in understanding the complex mechanism.
