From b79fb00f2e856d9a5f675b05791b74d828644c56 Mon Sep 17 00:00:00 2001
From: Jordan Orelli <jordanorelli@gmail.com>
Date: Sun, 28 Nov 2021 23:03:38 +0000
Subject: [PATCH] bags

---
 bag/bag.go      | 77 +++++++++++++++++++++++++++++++++++++++++
 bag/bag_test.go | 92 +++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 169 insertions(+)
 create mode 100644 bag/bag.go
 create mode 100644 bag/bag_test.go

diff --git a/bag/bag.go b/bag/bag.go
new file mode 100644
index 0000000..e6183ec
--- /dev/null
+++ b/bag/bag.go
@@ -0,0 +1,77 @@
+package bag
+
+import (
+	"errors"
+)
+
+var errNotFound = errors.New("not found")
+var errTypeError = errors.New("type error")
+
+// Bag is a read-only collection of values. Consumer can add elements to the
+// bag if and only no element has been added for that key in the past. Elements
+// can be added to the bag either as values or as pointers.
+type Bag map[string]bagged
+
+// Add adds a value to a bag. The provided value can be retrieved from the bag
+// directly. There's really no reason to call this with a pointer but I don't
+// know how to prevent that.
+func Add(b Bag, k string, v interface{}) bool {
+	if b.Has(k) {
+		return false
+	}
+	b[k] = bagged{val: v}
+	return true
+}
+
+// Ref adds a reference to a bag. The provided value must be a pointer. Once
+// added, the pointer is never retrievable from the bag; reading this key from
+// the bag dereferences the pointer at the time of reading.
+func Ref[V any](b Bag, k string, v *V) bool {
+	if b.Has(k) {
+		return false
+	}
+
+	if v == nil {
+		return false
+	}
+
+	b[k] = bagged{val: v, ref: true}
+	return true
+}
+
+// Get retrieves a value from a bag. Whether a value was added or a ref was
+// added, you always get a value out.
+func Get[V any](b Bag, k string) (V, error) {
+	bv, ok := b[k]
+	if !ok {
+		var zero V
+		return zero, errNotFound
+	}
+
+	if bv.ref {
+		ptr, ok := bv.val.(*V)
+		if !ok {
+			var zero V
+			return zero, errTypeError
+		}
+		return *ptr, nil
+	}
+
+	v, ok := bv.val.(V)
+	if !ok {
+		var zero V
+		return zero, errTypeError
+	}
+	return v, nil
+}
+
+// Has describes whether or not the bag contains the given key
+func (b Bag) Has(k string) bool {
+	_, ok := b[k]
+	return ok
+}
+
+type bagged struct {
+	val interface{}
+	ref bool
+}
diff --git a/bag/bag_test.go b/bag/bag_test.go
new file mode 100644
index 0000000..c0c569b
--- /dev/null
+++ b/bag/bag_test.go
@@ -0,0 +1,92 @@
+package bag
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestEmpty(t *testing.T) {
+	b := make(Bag)
+
+	_, err := Get[string](b, "foo")
+	if !errors.Is(err, errNotFound) {
+		t.Fatalf("expected not found error, saw %v", err)
+	}
+}
+
+func TestAdd(t *testing.T) {
+	b := make(Bag)
+
+	if !Add(b, "foo", "bar") {
+		t.Fatalf("weird add failure")
+	}
+
+	foo, err := Get[string](b, "foo")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if foo != "bar" {
+		t.Fatalf("unexpected value: %v", foo)
+	}
+
+	if Add(b, "foo", "again") {
+		t.Fatalf("weird add success")
+	}
+
+	_, err = Get[int](b, "foo")
+	if !errors.Is(err, errTypeError) {
+		t.Fatalf("unexpected error: %v", err)
+	}
+}
+
+func TestRef(t *testing.T) {
+	b := make(Bag)
+
+	name := "Jordan"
+	if !Ref(b, "name", &name) {
+		t.Fatal("ref failed")
+	}
+
+	_, err := Get[*string](b, "name")
+	if !errors.Is(err, errTypeError) {
+		t.Fatal("retrieving pointer for ref did not fail")
+	}
+
+	_, err = Get[int](b, "name")
+	if !errors.Is(err, errTypeError) {
+		t.Fatal("retrieving value of differing type for ref did not fail")
+	}
+
+	readName, err := Get[string](b, "name")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if readName != "Jordan" {
+		t.Fatalf("unexpected value: %v", readName)
+	}
+
+	name = "Jordan Orelli"
+	if readName != "Jordan" {
+		t.Fatalf("unexpected value: %v", readName)
+	}
+	readName, err = Get[string](b, "name")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if readName != "Jordan Orelli" {
+		t.Fatalf("unexpected value: %v", readName)
+	}
+
+	fn := func(s *string) {
+		*s = "mute"
+	}
+	fn(&name)
+
+	readName, err = Get[string](b, "name")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if readName != "mute" {
+		t.Fatalf("unexpected value: %v", readName)
+	}
+}