Refactored version directory names

3.0 and 2.0 do not work, there are multiple versions and anything with a period in it sucks from my point of view, v2 and v3 feel much better from a DX perspective.

datamodel/low/v3/callback.go

@@ -0,0 +1,52 @@
+// Copyright 2022 Princess B33f Heavy Industries / Dave Shanley
+// SPDX-License-Identifier: MIT
+
+package v3
+
+import (
+	"github.com/pb33f/libopenapi/datamodel/low"
+	"github.com/pb33f/libopenapi/index"
+	"gopkg.in/yaml.v3"
+)
+
+type Callback struct {
+	Expression low.ValueReference[map[low.KeyReference[string]]low.ValueReference[*PathItem]]
+	Extensions map[low.KeyReference[string]]low.ValueReference[any]
+}
+
+func (cb *Callback) FindExpression(exp string) *low.ValueReference[*PathItem] {
+	return low.FindItemInMap[*PathItem](exp, cb.Expression.Value)
+}
+
+func (cb *Callback) Build(root *yaml.Node, idx *index.SpecIndex) error {
+	cb.Extensions = low.ExtractExtensions(root)
+
+	// handle callback
+	var currentCB *yaml.Node
+	callbacks := make(map[low.KeyReference[string]]low.ValueReference[*PathItem])
+
+	for i, callbackNode := range root.Content {
+		if i%2 == 0 {
+			currentCB = callbackNode
+			continue
+		}
+		callback, eErr := low.ExtractObjectRaw[*PathItem](callbackNode, idx)
+		if eErr != nil {
+			return eErr
+		}
+		callbacks[low.KeyReference[string]{
+			Value:   currentCB.Value,
+			KeyNode: currentCB,
+		}] = low.ValueReference[*PathItem]{
+			Value:     callback,
+			ValueNode: callbackNode,
+		}
+	}
+	if len(callbacks) > 0 {
+		cb.Expression = low.ValueReference[map[low.KeyReference[string]]low.ValueReference[*PathItem]]{
+			Value:     callbacks,
+			ValueNode: root,
+		}
+	}
+	return nil
+}