random: add NewSource

math.Rand is the canonical way to generate pseudo-random numbers in Go.
Allow integrating it with Antithesis randomness.

Author: lmb

random/random.go

@@ -12,10 +12,37 @@
 package random
 
 import (
+	"math"
+	"math/rand"
+
 	"github.com/antithesishq/antithesis-sdk-go/internal"
 )
 
 // GetRandom returns a uint64 value chosen by Antithesis. You should not store this value or use it to seed a PRNG, but should use it immediately.
 func GetRandom() uint64 {
 	return internal.Get_random()
 }
+
+var _ rand.Source64 = source{}
+
+type source struct{}
+
+func (source) Seed(int64) {}
+
+func (source) Int63() int64 {
+	return int64(internal.Get_random() & (math.MaxUint64 >> 1))
+}
+
+func (source) Uint64() uint64 {
+	return internal.Get_random()
+}
+
+// NewSource initialises a source of pseudo-random data.
+//
+// Use this function to create a [math/rand.Rand] which provides feedback to the Antithesis platform.
+// Outside of Antithesis this is equivalent to calling [math/rand.NewSource].
+//
+// The returned source implements [math/rand.Source64].
+func NewSource(seed int64) rand.Source {
+	return source{}
+}

random/random_local.go

@@ -9,3 +9,7 @@ import (
 func GetRandom() uint64 {
 	return rand.Uint64()
 }
+
+func NewSource(seed int64) rand.Source {
+	return rand.NewSource(seed)
+}