Is there an appoximate bias in AUC calculation

[tswsxk]

Notice the way AUC is calculated is by approximating to the true one where the code is

for i,p in ipairs(allPredictions) do
		if(p['truth'] == 1) then
			truePositives = truePositives + 1
		else
			falsePositives = falsePositives + 1
		end

		local guess = 0
		if(p['pred'] > 0.5) then guess = 1 end
		if(guess == p['truth']) then correct = correct + 1 end

		local fpr = falsePositives / totalNegatives
		local tpr = truePositives / totalPositives
		if(i % 500 == 0) then
			if lastFpr ~= nil then
				local trapezoid = (tpr + lastTpr) * (fpr - lastFpr) *.5
				auc = auc + trapezoid
			end	
			lastFpr = fpr
			lastTpr = tpr
		end
		if(recall == 1) then break end
	end

Wondering whether this appoximating calculation may affect the auc value.