maybe i should just write down the crdt stuff

crdt/gcounter.go

@@ -0,0 +1,60 @@
+package crdt
+
+import (
+	"fmt"
+	"constraints"
+)
+
+func max[N constraints.Ordered](a, b N) N {
+	if a >= b {
+		return a
+	}
+	return b
+}
+
+func NewGCounter[K comparable]() GCounter[K] {
+	return GCounter[K]{slots: make(map[K]int)}
+}
+
+type GCounter[K comparable] struct {
+	slots map[K]int `json:"slots"`
+}
+
+func (g GCounter[K]) Incr(slot K) error {
+	var zero K
+	if slot == zero {
+		return fmt.Errorf("gcounter refuses incr on the zero-value of its key")
+	}
+
+	g.slots[slot]++
+	return nil
+}
+
+func (g GCounter[K]) Add(slot K, delta int) error {
+	var zero K
+	if slot == zero {
+		return fmt.Errorf("gcounter refuses add on the zero-value of its key")
+	}
+
+	if delta < 0 {
+		return fmt.Errorf("gcounters cannot go down, use a pncounter instead")
+	}
+	g.slots[slot] += delta
+	return nil
+}
+
+func (g *GCounter[K]) Merge(dest *GCounter[K]) {
+	for slot, count := range g.slots {
+		v := max(count, dest.slots[slot])
+		dest.slots[slot] = v
+		g.slots[slot] = v
+	}
+}
+
+func (g *GCounter[K]) Total() int {
+	var n int
+	for _, count := range g.slots {
+		n += count
+	}
+	return n
+}

crdt/gcounter_test.go

@@ -0,0 +1,65 @@
+package crdt
+
+import (
+	"testing"
+)
+
+func TestGCounter(t *testing.T) {
+	t.Run("incr", func(t *testing.T) {
+		g := NewGCounter[string]()
+		if n := g.Total(); n != 0 {
+			t.Fatalf("new gcounter has count of %d, should be 0", n)
+		}
+
+		if err := g.Incr("jordan"); err != nil {
+			t.Fatalf("gcounter failed incr: %v", err)
+		}
+
+		if n := g.Total(); n != 1 {
+			t.Fatalf("new gcounter has count of %d, should be 1", n)
+		}
+
+		if err := g.Incr(""); err == nil {
+			t.Fatalf("incrementing the zero value succeeded, should have failed")
+		}
+
+		if err := g.Incr("jordan"); err != nil {
+			t.Fatalf("gcounter failed incr: %v", err)
+		}
+
+		if n := g.Total(); n != 2 {
+			t.Fatalf("new gcounter has count of %d, should be 2", n)
+		}
+	})
+
+	t.Run("add", func(t *testing.T) {
+		g := NewGCounter[string]()
+		if n := g.Total(); n != 0 {
+			t.Fatalf("new gcounter has count of %d, should be 0", n)
+		}
+
+		if err := g.Add("jordan", 4); err != nil {
+			t.Fatalf("gcounter failed incr: %v", err)
+		}
+
+		if n := g.Total(); n != 4 {
+			t.Fatalf("new gcounter has count of %d, should be 1", n)
+		}
+
+		if err := g.Add("", 10); err == nil {
+			t.Fatalf("adding to zero key succeeded, should have failed")
+		}
+
+		if err := g.Add("jordan", -3); err == nil {
+			t.Fatalf("adding negatively to the gcounter succeeded, should have failed")
+		}
+
+		if err := g.Add("jordan", 3); err != nil {
+			t.Fatalf("gcounter failed add: %v", err)
+		}
+
+		if n := g.Total(); n != 7 {
+			t.Fatalf("new gcounter has count of %d, should be 7", n)
+		}
+	})
+}