UNITSCRIPT:

local attacking, parried, isparrying

function script.parry()
   if math.random()>0.33 then
      parried = true
   end
end

function script.HitByWeapon ( x, z, weaponDefID, damage )
   parried = false
   GG.CheckParriableWeapon(unitID, weaponDefID)
   if (parried and (z>0)) then
      if ((not attacking) and (not isparrying)) then
         isparrying = true      
         StartThread(MeleeAnimations)   
         return(0)
      end
   else
      return(damage)
   end   
end

function script.AimWeapon1(heading, pitch)
   randomsleeptime = math.random(50)
   Sleep(randomsleeptime)
   if isparring then
      return false
   end
   Sleep(50)
   Signal(SIG_AIM1)
   SetSignalMask(SIG_AIM1)
   return true
end

function script.Shot1()
   attacking=true
   StartThread(MeleeAnimations)
end

MeleeAnimations=function()
   if attacking then
      .......
   end
   if isparrying then
      .......
   end
   attacking=false
   isparrying = false
   return(1)   
end

GADGET:

if (not gadgetHandler:IsSyncedCode()) then
   return false
end

local parriableWeaponDefs = {}

local function CallUnitScript(unitID, funcName, ...)
   Spring.UnitScript.CallAsUnit(unitID, Spring.UnitScript.GetScriptEnv(unitID).script[funcName], ...)
end 


function gadget:Initialize()
   for i=1,#WeaponDefs do
      if (WeaponDefs[i].description =="Melee") then
         parriableWeaponDefs[i] = 0.98
      end
   end
end

function GG.CheckParriableWeapon(unitID, weaponDefID)
   if not parriableWeaponDefs[weaponDefID] then return end
    CallUnitScript(unitID,"parry")   
end