ident: system identifier generator

This is pretty fast and guarantees uniqueness when initialised correctly.

Signed-off-by: Yonah <contrib@gensokyo.uk>

ident/ident.go

@@ -6,7 +6,10 @@ import (
 	"encoding"
 	"errors"
 	"fmt"
+	"math/rand/v2"
 	"strconv"
+	"sync"
+	"time"
 	"unsafe"
 )
 
@@ -97,3 +100,47 @@ func (full *F[V, T]) UnmarshalText(data []byte) (err error) {
 	}
 	return full.Remote.UnmarshalText(data[sz+1:])
 }
+
+// PartG is machine-specific state of [Generator].
+type PartG struct {
+	// Deployment site, typically denoting a datacenter servicing a region.
+	Site uint32
+	// Servicing host behind load balancer, unique within its Site.
+	Host uint32
+}
+
+// A Generator emits unique instances of [S].
+type Generator struct {
+	PartG
+	mu sync.Mutex
+	r  rand.Source
+}
+
+// S populates the value pointed to by p with a new unique value.
+func (g *Generator) S(p *S) {
+	g.mu.Lock()
+	*p = S{PartG: g.PartG, Time: uint64(time.Now().UnixNano()), ID: g.r.Uint64()}
+	g.mu.Unlock()
+}
+
+// NewS creates a unique instance of [S] and returns its address.
+func (g *Generator) NewS() *S {
+	var sid S
+	g.S(&sid)
+	return &sid
+}
+
+// M populates the value pointed to by p with a new unique value.
+func (g *Generator) M(p *PartM, serial uint64) {
+	g.mu.Lock()
+	*p = PartM{Serial: serial, Time: uint64(time.Now().UnixNano()), ID: g.r.Uint64()}
+	g.mu.Unlock()
+}
+
+// New initialises a new instance of [Generator] and returns its address.
+func New(site, host uint32) *Generator {
+	return &Generator{PartG{site, host}, sync.Mutex{}, rand.NewPCG(
+		uint64(site)<<32|uint64(host),
+		uint64(time.Now().UnixNano()),
+	)}
+}