Add Deterministically Seed A Random Number Generator as a Go TIL

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://crafty-builder-6996.ck.page/e169c61186).
 
-_1545 TILs and counting..._
+_1546 TILs and counting..._
 
 ---
 
@@ -406,6 +406,7 @@ _1545 TILs and counting..._
 - [Check If Cobra Flag Was Set](go/check-if-cobra-flag-was-set.md)
 - [Combine Two Slices](go/combine-two-slices.md)
 - [Detect If Stdin Comes From A Redirect](go/detect-if-stdin-comes-from-a-redirect.md)
+- [Deterministically Seed A Random Number Generator](go/deterministically-seed-a-random-number-generator.md)
 - [Do Something N Times](go/do-something-n-times.md)
 - [Find Executables Installed By Go](go/find-executables-installed-by-go.md)
 - [Format Date And Time With Time Constants](go/format-date-and-time-with-time-constants.md)

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

@@ -0,0 +1,49 @@
+# 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.