Add Deterministically Seed A Random Number Generator as a Go TIL

go/deterministically-seed-a-random-number-generator.md

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+# Deterministically Seed A Random Number Generator
+
+If you need a random number in Go, you can always reach for the various
+functions in the `rand` package.
+
+```go
+package main
+
+import (
+	"fmt"
+	"math/rand"
+)
+
+func main() {
+	for range 5 {
+		roll := rand.Intn(6) + 1
+		fmt.Printf("- %d\n", roll)
+	}
+}
+```
+
+Each time I run that, I get a random set of values. Often in programming, we
+want some control over the randomness. We want to _seed_ the randomness so that
+it is deterministic. We want random, but the kind of random where we know how
+we got there.
+
+```go
+package main
+
+import (
+	"fmt"
+	"math/rand"
+)
+
+func main() {
+	seed := int64(123)
+	src := rand.NewSource(seed)
+	rng := rand.New(src)
+
+	for range 5 {
+		roll := rng.Intn(6) + 1
+		fmt.Printf("- %d\n", roll)
+	}
+}
+```
+
+In this second snippet, we create a `Source` with a specific seed value that we
+can use with a custom `Rand` struct. We can then deterministically get random
+numbers from it.