initialize support for versioning as requested, per route -- not finished yet

Former-commit-id: ade66610125f06a0b5ce3e90bcafe349f216a616

versioning/versioning.go

@@ -0,0 +1,87 @@
+package versioning
+
+import (
+	"github.com/kataras/iris/context"
+
+	"github.com/hashicorp/go-version"
+)
+
+type Map map[string]context.Handler
+
+func Handler(versions Map) context.Handler {
+	constraintsHandlers, notFoundHandler := buildConstraints(versions)
+
+	return func(ctx context.Context) {
+		versionString := GetVersion(ctx)
+		if versionString == NotFound {
+			notFoundHandler(ctx)
+			return
+		}
+
+		version, err := version.NewVersion(versionString)
+		if err != nil {
+			notFoundHandler(ctx)
+			return
+		}
+
+		for _, ch := range constraintsHandlers {
+			if ch.constraints.Check(version) {
+				ctx.Header("X-API-Version", version.String())
+				ch.handler(ctx)
+				return
+			}
+		}
+
+		notFoundHandler(ctx)
+	}
+}
+
+type constraintsHandler struct {
+	constraints version.Constraints
+	handler     context.Handler
+}
+
+func buildConstraints(versionsHandler Map) (constraintsHandlers []*constraintsHandler, notfoundHandler context.Handler) {
+	for v, h := range versionsHandler {
+		if v == NotFound {
+			notfoundHandler = h
+			continue
+		}
+
+		constraints, err := version.NewConstraint(v)
+		if err != nil {
+			panic(err)
+		}
+
+		constraintsHandlers = append(constraintsHandlers, &constraintsHandler{
+			constraints: constraints,
+			handler:     h,
+		})
+	}
+
+	if notfoundHandler == nil {
+		notfoundHandler = NotFoundHandler
+	}
+
+	// no sort, the end-dev should declare
+	// all version constraint, i.e < 4.0 may be catch 1.0 if not something like
+	// >= 3.0, < 4.0.
+	// I can make it ordered but I do NOT like the final API of it:
+	/*
+		app.Get("/api/user", Handler( // accepts an array, ordered, see last elem.
+			V("1.0", vHandler("v1 here")),
+			V("2.0", vHandler("v2 here")),
+			V("< 4.0", vHandler("v3.x here")),
+		))
+		instead we have:
+
+		app.Get("/api/user", Handler(Map{ // accepts a map, unordered, see last elem.
+			"1.0":           Deprecated(vHandler("v1 here")),
+			"2.0":           vHandler("v2 here"),
+			">= 3.0, < 4.0": vHandler("v3.x here"),
+			VersionUnknown: customHandlerForNotMatchingVersion,
+		}))
+	*/
+
+	return
+}