Refactor random tier function for clarity. Add simulations.

cmd/microservice-ethereum-create-transaction-lootboxes/pick-random-tier.go

@@ -9,45 +9,45 @@ import (
 	"math/rand"
 )
 
-// PayoutChances to pick the random number between
+// PayoutChances to pick the random number between 0 and this value
 const PayoutChances = 1_000_000_000
 
 const (
-	// PayoutTier1 that the random number must fall below to win
-	PayoutTier1 = 400_000_000
 
-	// PayoutTier2 that the payout must be below, but above PayoutTier1, to win
-	PayoutTier2 = 500_000_000
+	// The probabilities of a Tier being drawn (out of PayoutChances draws)
+	Tier1Prob = 400_000_000 // this is 400_000_000 / 1_000_000_000
+	Tier2Prob = 100_000_000 // this is 100_000_000 / 1_000_000_000
+	Tier3Prob = 50_000_000  //etc...
+	Tier4Prob = 10_000_000
+	Tier5Prob = 100
+	//The probability of NoBottle is: (PayoutChances - (The Sum of all TierXProb values above)) / PayoutChances
+
+	// Thresholds that the random number must fall below to win a tier
+	PayoutTier1 = Tier1Prob
+	PayoutTier2 = Tier2Prob + PayoutTier1
+	PayoutTier3 = Tier3Prob + PayoutTier2
+	PayoutTier4 = Tier4Prob + PayoutTier3
+	PayoutTier5 = Tier5Prob + PayoutTier4
+	//Anything above PayoutTier5 is NoBottle
 
-	// PayoutTier3
-	PayoutTier3 = 550_000_000
-
-	// PayoutTier4
-	PayoutTier4 = 560_000_000
-
-	// PayoutNoBottle is awarded when no bottle is sent to the user
-	PayoutNoBottle = 999_999_000
-
-	// PayoutTier5 (very low probability)
-	PayoutTier5 = 1_000_000_000
 )
 
-// pickRandomReward, 0 being no rewards.
+// pickRandomReward. Returns Lootbox Tier 0-5, 0 being no rewards.
 func pickRandomNumber() int {
 	n := rand.Int31n(PayoutChances)
 	switch {
-	case n >= PayoutNoBottle && n <= PayoutTier5:
-		return 5
-	case n >= PayoutTier4 && n <= PayoutNoBottle:
-		return 0
-	case n >= PayoutTier3 && n <= PayoutTier4:
-		return 4
-	case n >= PayoutTier2 && n <= PayoutTier3:
-		return 3
-	case n >= PayoutTier1 &&n  <= PayoutTier2:
-		return 2
 	case n <= PayoutTier1:
 		return 1
+	case n <= PayoutTier2:
+		return 2
+	case n <= PayoutTier3:
+		return 3
+	case n <= PayoutTier4:
+		return 4
+	case n <= PayoutTier5:
+		return 5
+	case n > PayoutTier5 && n <= PayoutChances:
+		return 0 //NoBottle
 	default:
 		panic(fmt.Sprintf("bad pickRandomNumber impl: %v", n))
 	}

cmd/microservice-ethereum-create-transaction-lootboxes/pick-random-tier_test.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// TestPickRandomTier. Simulate a large number of draws and compare to the expected probabilities.
+func TestPickRandomTier(t *testing.T) {
+	simIterations := 10_000_000
+
+	expectedResults := getExpectedTierPcts()
+
+	//Prepare result data structure
+	simResults := make(map[int]int)
+	for i := 0; i < 6; i++ {
+		simResults[i] = 0
+	}
+
+	//Run Simulations
+	for i := 0; i < simIterations; i++ {
+		lootboxRewardTier := pickRandomNumber()
+		simResults[lootboxRewardTier] += 1
+	}
+
+	//Calculate the Simulated Result Outcome Percentages for each Tier
+	simResultPcts := make(map[int]float64)
+	for i := 0; i < 6; i++ {
+		simResultPcts[i] = (float64(simResults[i]) / float64(simIterations)) * 100
+	}
+
+	//This is "how close" the simulated outcome percentages must be to the expected probabilities to pass the test
+	testTolerancePct := float64(0.5) //Percentage, so 0.5 = 0.5%  or  1 = 1%
+
+	//Actual testing & asserts.
+	//Compare the Simulated Result Outcome Pcts to an "acceptable" value range around the Expected Result
+	for i := 0; i < 6; i++ {
+
+		//Define an acceptable range of [Expected Result Pct] + or - TestTolerancePct
+		acceptableHigh := expectedResults[i] + testTolerancePct //expectedResults[i] is a pct. so we just +/- the tolerance
+		acceptableLow := expectedResults[i] - testTolerancePct
+		if acceptableLow < 0 {
+			acceptableLow = 0
+		}
+
+		thisSimResultPct := simResultPcts[i] // The simulated outcome pct
+
+		//Assert: Simulated Output Pct must be within the range of acceptable expected values.
+		assert.True(t, thisSimResultPct >= float64(acceptableLow) && thisSimResultPct <= float64(acceptableHigh), fmt.Sprintf("Tier %v value is not within expected range", i))
+	}
+}
+
+// getExpectedTierPcts. Returns the expected percentage chance of being drawn for all tiers
+func getExpectedTierPcts() map[int]float64 {
+	expectedMap := make(map[int]float64)
+	expectedMap[0] = float64((PayoutChances - PayoutTier5)) / float64(PayoutChances) * 100
+	expectedMap[1] = float64(Tier1Prob) / float64(PayoutChances) * 100
+	expectedMap[2] = float64(Tier2Prob) / float64(PayoutChances) * 100
+	expectedMap[3] = float64(Tier3Prob) / float64(PayoutChances) * 100
+	expectedMap[4] = float64(Tier4Prob) / float64(PayoutChances) * 100
+	expectedMap[5] = float64(Tier5Prob) / float64(PayoutChances) * 100
+	return expectedMap
+}