feat(cardinal): deterministic wrapper around math/rand so users can u…

…se it. (Argus-Labs#782)

cardinal/cardinal_test.go

@@ -759,6 +759,29 @@ func TestCanQueryInsideSystem(t *testing.T) {
 	assert.Equal(t, gotNumOfEntities, wantNumOfEntities)
 }
 
+func TestRandomNumberGenerator(t *testing.T) {
+	tf := cardinal.NewTestFixture(t, nil)
+	world := tf.World
+	testAmount := 50
+	numbers1 := make([]int64, 0, testAmount)
+	err := cardinal.RegisterSystems(world, func(context cardinal.WorldContext) error {
+		time.Sleep(5 * time.Millisecond)
+		numbers1 = append(numbers1, context.Rand().Int63())
+		return nil
+	})
+	assert.NilError(t, err)
+	tf.StartWorld()
+	for i := 0; i < testAmount; i++ {
+		tf.DoTick()
+	}
+	mapOfNums := make(map[int64]bool)
+	for _, num := range numbers1 {
+		_, ok := mapOfNums[num]
+		assert.Assert(t, ok == false)
+		mapOfNums[num] = true
+	}
+}
+
 func TestCanGetTimestampFromWorldContext(t *testing.T) {
 	var ts uint64
 	tf := cardinal.NewTestFixture(t, nil)

cardinal/world_context.go

@@ -1,8 +1,10 @@
 package cardinal
 
 import (
+	"math/rand"
 	"reflect"
 
+	"github.com/rotisserie/eris"
 	"github.com/rs/zerolog"
 	"github.com/rs/zerolog/log"
 
@@ -34,6 +36,8 @@ type WorldContext interface {
 	EmitStringEvent(string) error
 	// Namespace returns the namespace of the world.
 	Namespace() string
+	// Rand returns a random number generator that is seeded specifically for a current tick.
+	Rand() *rand.Rand
 
 	// For internal use.
 
@@ -60,6 +64,7 @@ type worldContext struct {
 	txPool   *txpool.TxPool
 	logger   *zerolog.Logger
 	readOnly bool
+	rand     *rand.Rand
 }
 
 func newWorldContextForTick(world *World, txPool *txpool.TxPool) WorldContext {
@@ -68,6 +73,8 @@ func newWorldContextForTick(world *World, txPool *txpool.TxPool) WorldContext {
 		txPool:   txPool,
 		logger:   &log.Logger,
 		readOnly: false,
+		//nolint:gosec // we require manual in the rng which crypto/rand doesn't have, but math/rand does.
+		rand: rand.New(rand.NewSource(int64(world.timestamp.Load()))),
 	}
 }
 
@@ -77,6 +84,7 @@ func NewWorldContext(world *World) WorldContext {
 		txPool:   nil,
 		logger:   &log.Logger,
 		readOnly: false,
+		rand:     nil,
 	}
 }
 
@@ -86,6 +94,7 @@ func NewReadOnlyWorldContext(world *World) WorldContext {
 		txPool:   nil,
 		logger:   &log.Logger,
 		readOnly: true,
+		rand:     nil,
 	}
 }
 
@@ -102,6 +111,15 @@ func (ctx *worldContext) Logger() *zerolog.Logger {
 	return ctx.logger
 }
 
+func (ctx *worldContext) Rand() *rand.Rand {
+	if ctx.rand == nil {
+		// a panic is thrown here instead of returning an error to maintain method chaining.
+		// ex: wCtx.rand().Int63()
+		panic(eris.New("rand is only useable on a context generated by newWorldContextForTick"))
+	}
+	return ctx.rand
+}
+
 func (ctx *worldContext) getMessageByType(mType reflect.Type) (types.Message, bool) {
 	return ctx.world.GetMessageByType(mType)
 }