internal/mcp: keep an ordered list of feature keys

This change is necessary to support pagination which will be implemented
in subsequent CLs.

Change-Id: I8bc616a57463ea569dc7cadf1d4f06c69602ddd5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/674135
Reviewed-by: Jonathan Amsterdam <jba@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

mcp/features_test.go

@@ -0,0 +1,75 @@
+// Copyright 2025 The Go MCP SDK Authors. All rights reserved.
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+
+package mcp
+
+import (
+	"context"
+	"slices"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+	"github.com/google/go-cmp/cmp/cmpopts"
+	"github.com/modelcontextprotocol/go-sdk/jsonschema"
+)
+
+type SayHiParams struct {
+	Name string `json:"name"`
+}
+
+func SayHi(ctx context.Context, cc *ServerSession, params *SayHiParams) ([]*Content, error) {
+	return []*Content{
+		NewTextContent("Hi " + params.Name),
+	}, nil
+}
+
+func TestFeatureSetOrder(t *testing.T) {
+	toolA := NewTool("apple", "apple tool", SayHi).Tool
+	toolB := NewTool("banana", "banana tool", SayHi).Tool
+	toolC := NewTool("cherry", "cherry tool", SayHi).Tool
+
+	testCases := []struct {
+		tools []*Tool
+		want  []*Tool
+	}{
+		{[]*Tool{toolA, toolB, toolC}, []*Tool{toolA, toolB, toolC}},
+		{[]*Tool{toolB, toolC, toolA}, []*Tool{toolA, toolB, toolC}},
+		{[]*Tool{toolA, toolC}, []*Tool{toolA, toolC}},
+		{[]*Tool{toolA, toolA, toolA}, []*Tool{toolA}},
+		{[]*Tool{}, nil},
+	}
+	for _, tc := range testCases {
+		fs := newFeatureSet(func(t *Tool) string { return t.Name })
+		fs.add(tc.tools...)
+		got := slices.Collect(fs.all())
+		if diff := cmp.Diff(got, tc.want, cmpopts.IgnoreUnexported(jsonschema.Schema{})); diff != "" {
+			t.Errorf("expected %v, got %v, (-want +got):\n%s", tc.want, got, diff)
+		}
+	}
+}
+
+func TestFeatureSetAbove(t *testing.T) {
+	toolA := NewTool("apple", "apple tool", SayHi).Tool
+	toolB := NewTool("banana", "banana tool", SayHi).Tool
+	toolC := NewTool("cherry", "cherry tool", SayHi).Tool
+
+	testCases := []struct {
+		tools []*Tool
+		above string
+		want  []*Tool
+	}{
+		{[]*Tool{toolA, toolB, toolC}, "apple", []*Tool{toolB, toolC}},
+		{[]*Tool{toolA, toolB, toolC}, "banana", []*Tool{toolC}},
+		{[]*Tool{toolA, toolB, toolC}, "cherry", nil},
+	}
+	for _, tc := range testCases {
+		fs := newFeatureSet(func(t *Tool) string { return t.Name })
+		fs.add(tc.tools...)
+		got := slices.Collect(fs.above(tc.above))
+		if diff := cmp.Diff(got, tc.want, cmpopts.IgnoreUnexported(jsonschema.Schema{})); diff != "" {
+			t.Errorf("expected %v, got %v, (-want +got):\n%s", tc.want, got, diff)
+
+		}
+	}
+}