// Copyright 2025 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package mcp

import (
	"context"
	_ "embed"
	"fmt"
	"io"
	"log"
	"net"
	"net/http"
	"os"
	"sync"

	"github.com/modelcontextprotocol/go-sdk/mcp"
	"golang.org/x/tools/gopls/internal/cache"
	"golang.org/x/tools/gopls/internal/cache/metadata"
	"golang.org/x/tools/gopls/internal/file"
	"golang.org/x/tools/gopls/internal/protocol"
	"golang.org/x/tools/gopls/internal/settings"
	"golang.org/x/tools/gopls/internal/util/moremaps"
)

//go:embed instructions.md
var Instructions string

// A handler implements various MCP tools for an LSP session.
type handler struct {
	session   *cache.Session
	lspServer protocol.Server
}

// Sessions is the interface used to access gopls sessions.
type Sessions interface {
	Session(id string) (*cache.Session, protocol.Server)
	FirstSession() (*cache.Session, protocol.Server)
	SetSessionExitFunc(func(string))
}

// Serve starts an MCP server serving at the input address.
// The server receives LSP session events on the specified channel, which the
// caller is responsible for closing. The server runs until the context is
// canceled.
func Serve(ctx context.Context, address string, sessions Sessions, isDaemon bool) error {
	log.Printf("Gopls MCP server: starting up on http")
	listener, err := net.Listen("tcp", address)
	if err != nil {
		return err
	}
	defer listener.Close()

	// TODO(hxjiang): expose the MCP server address to the LSP client.
	if isDaemon {
		log.Printf("Gopls MCP daemon: listening on address %s...", listener.Addr())
	}
	defer log.Printf("Gopls MCP server: exiting")

	svr := http.Server{
		Handler: HTTPHandler(sessions, isDaemon),
		BaseContext: func(net.Listener) context.Context {
			return ctx
		},
	}

	// Run the server until cancellation.
	go func() {
		<-ctx.Done()
		svr.Close() // ignore error
	}()
	log.Printf("mcp http server listening")
	return svr.Serve(listener)
}

// StartStdIO starts an MCP server over stdio.
func StartStdIO(ctx context.Context, session *cache.Session, server protocol.Server, rpcLog io.Writer) error {
	s := newServer(session, server)
	if rpcLog != nil {
		return s.Run(ctx, &mcp.LoggingTransport{
			Transport: &mcp.StdioTransport{},
			Writer:    rpcLog,
		})
	} else {
		return s.Run(ctx, &mcp.StdioTransport{})
	}

}

func HTTPHandler(sessions Sessions, isDaemon bool) http.Handler {
	var (
		mu          sync.Mutex                         // lock for mcpHandlers.
		mcpHandlers = make(map[string]*mcp.SSEHandler) // map from lsp session ids to MCP sse handlers.
	)
	mux := http.NewServeMux()

	// In daemon mode, gopls serves mcp server at ADDRESS/sessions/$SESSIONID.
	// Otherwise, gopls serves mcp server at ADDRESS.
	if isDaemon {
		mux.HandleFunc("/sessions/{id}", func(w http.ResponseWriter, r *http.Request) {
			sessionID := r.PathValue("id")

			mu.Lock()
			handler, ok := mcpHandlers[sessionID]
			if !ok {
				if s, svr := sessions.Session(sessionID); s != nil {
					handler = mcp.NewSSEHandler(func(request *http.Request) *mcp.Server {
						return newServer(s, svr)
					}, nil)
					mcpHandlers[sessionID] = handler
				}
			}
			mu.Unlock()

			if handler == nil {
				http.Error(w, fmt.Sprintf("session %s not established", sessionID), http.StatusNotFound)
				return
			}

			handler.ServeHTTP(w, r)
		})
	} else {
		// TODO(hxjiang): should gopls serve only at a specific path?
		mux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
			mu.Lock()
			// When not in daemon mode, gopls has at most one LSP session.
			_, handler, ok := moremaps.Arbitrary(mcpHandlers)
			if !ok {
				s, svr := sessions.FirstSession()
				handler = mcp.NewSSEHandler(func(request *http.Request) *mcp.Server {
					return newServer(s, svr)
				}, nil)
				mcpHandlers[s.ID()] = handler
			}
			mu.Unlock()

			if handler == nil {
				http.Error(w, "session not established", http.StatusNotFound)
				return
			}

			handler.ServeHTTP(w, r)
		})
	}
	sessions.SetSessionExitFunc(func(sessionID string) {
		mu.Lock()
		defer mu.Unlock()
		// TODO(rfindley): add a way to close SSE handlers (and therefore
		// close their transports). Otherwise, we leak JSON-RPC goroutines.
		delete(mcpHandlers, sessionID)
	})
	return mux
}

func newServer(session *cache.Session, lspServer protocol.Server) *mcp.Server {
	h := handler{
		session:   session,
		lspServer: lspServer,
	}
	opts := &mcp.ServerOptions{
		Instructions: Instructions,
	}
	mcpServer := mcp.NewServer(&mcp.Implementation{Name: "gopls", Version: "v1.0.0"}, opts)

	defaultTools := []string{
		"go_workspace",
		"go_package_api",
		"go_diagnostics",
		"go_rename_symbol",
		"go_symbol_references",
		"go_search",
		"go_file_context",
		"go_vulncheck"}
	disabledTools := append(defaultTools,
		// The fileMetadata tool is redundant with fileContext.
		[]string{"go_file_metadata",
			// The context tool returns context for all imports, which can consume a
			// lot of tokens. Conservatively, rely on the model selecting the imports
			// to summarize using the outline tool.
			"go_context",
			// The fileDiagnosticsTool only returns diagnostics for the current file,
			// but often changes will cause breakages in other tools. The
			// workspaceDiagnosticsTool always returns breakages, and supports running
			// deeper diagnostics in selected files.
			"go_file_diagnostics",
			// The references tool requires a location, which models tend to get wrong.
			// The symbolic variant seems to be easier to get right, albeit less
			// powerful.
			"go_references",
		}...)
	var toolConfig map[string]bool // non-default settings
	// For testing, poke through to the gopls server to access its options,
	// and enable some of the disabled tools.
	if hasOpts, ok := lspServer.(interface{ Options() *settings.Options }); ok {
		toolConfig = hasOpts.Options().MCPTools
	}
	var tools []string
	for _, tool := range defaultTools {
		if enabled, ok := toolConfig[tool]; !ok || enabled {
			tools = append(tools, tool)
		}
	}
	// Disabled tools must be explicitly enabled.
	for _, tool := range disabledTools {
		if toolConfig[tool] {
			tools = append(tools, tool)
		}
	}
	for _, tool := range tools {
		addToolByName(mcpServer, h, tool)
	}
	return mcpServer
}

func addToolByName(mcpServer *mcp.Server, h handler, name string) {
	switch name {
	case "go_context":
		mcp.AddTool(mcpServer, &mcp.Tool{
			Name:        "go_context",
			Description: "Provide context for a region within a Go file",
		}, h.contextHandler)
	case "go_diagnostics":
		mcp.AddTool(mcpServer, &mcp.Tool{
			Name: "go_diagnostics",
			Description: `Provides Go workspace diagnostics.

Checks for parse and build errors across the entire Go workspace. If provided,
"files" holds absolute paths for active files, on which additional linting is
performed.
`,
		}, h.workspaceDiagnosticsHandler)
	case "go_file_context":
		mcp.AddTool(mcpServer, &mcp.Tool{
			Name:        "go_file_context",
			Description: "Summarizes a file's cross-file dependencies",
		}, h.fileContextHandler)
	case "go_file_diagnostics":
		mcp.AddTool(mcpServer, &mcp.Tool{
			Name:        "go_file_diagnostics",
			Description: "Provides diagnostics for a Go file",
		}, h.fileDiagnosticsHandler)
	case "go_file_metadata":
		mcp.AddTool(mcpServer, &mcp.Tool{
			Name:        "go_file_metadata",
			Description: "Provides metadata about the Go package containing the file",
		}, h.fileMetadataHandler)
	case "go_package_api":
		mcp.AddTool(mcpServer, &mcp.Tool{
			Name:        "go_package_api",
			Description: "Provides a summary of a Go package API",
		}, h.outlineHandler)
	case "go_references":
		mcp.AddTool(mcpServer, &mcp.Tool{
			Name:        "go_references",
			Description: "Provide the locations of references to a given object",
		}, h.referencesHandler)
	case "go_rename_symbol":
		mcp.AddTool(mcpServer, &mcp.Tool{
			Name: "go_rename_symbol",
			Description: `Renames a symbol in the Go workspace

For example, given arguments {"file": "/path/to/foo.go", "symbol": "Foo", "new_name": "Bar"},
go_rename_symbol returns the edits necessary to rename the symbol "Foo" (located in the file foo.go) to
"Bar" across the Go workspace.`,
		}, h.renameSymbolHandler)
	case "go_search":
		mcp.AddTool(mcpServer, &mcp.Tool{
			Name: "go_search",
			Description: `Search for symbols in the Go workspace.

Search for symbols using case-insensitive fuzzy search, which may match all or
part of the fully qualified symbol name. For example, the query 'foo' matches
Go symbols 'Foo', 'fooBar', 'futils.Oboe', 'github.com/foo/bar.Baz'.

Results are limited to 100 symbols.
`,
		}, h.searchHandler)
	case "go_symbol_references":
		mcp.AddTool(mcpServer, &mcp.Tool{
			Name: "go_symbol_references",
			Description: `Provides the locations of references to a (possibly qualified)
package-level Go symbol referenced from the current file.

For example, given arguments {"file": "/path/to/foo.go", "name": "Foo"},
go_symbol_references returns references to the symbol "Foo" declared
in the current package.

Similarly, given arguments {"file": "/path/to/foo.go", "name": "lib.Bar"},
go_symbol_references returns references to the symbol "Bar" in the imported lib
package.

Finally, symbol references supporting querying fields and methods: symbol
"T.M" selects the "M" field or method of the "T" type (or value), and "lib.T.M"
does the same for a symbol in the imported package "lib".
`,
		}, h.symbolReferencesHandler)
	case "go_workspace":
		mcp.AddTool(mcpServer, &mcp.Tool{
			Name:        "go_workspace",
			Description: "Summarize the Go programming language workspace",
		}, h.workspaceHandler)
	case "go_vulncheck":
		mcp.AddTool(mcpServer, &mcp.Tool{
			Name: "go_vulncheck",
			Description: `Runs a vulnerability check on the Go workspace.

	The check is performed on a given package pattern within a specified directory.
	If no directory is provided, it defaults to the workspace root.
	If no pattern is provided, it defaults to "./...".`,
		}, h.vulncheckHandler)
	}
}

// snapshot returns the best default snapshot to use for workspace queries.
func (h *handler) snapshot() (*cache.Snapshot, func(), error) {
	views := h.session.Views()
	if len(views) == 0 {
		return nil, nil, fmt.Errorf("No active builds.")
	}
	return views[0].Snapshot()
}

// fileOf is like [cache.Session.FileOf], but does a sanity check for file
// changes. Currently, it checks for modified files in the transitive closure
// of the file's narrowest package.
//
// This helps avoid stale packages, but is not a substitute for real file
// watching, as it misses things like files being added to a package.
func (h *handler) fileOf(ctx context.Context, file string) (file.Handle, *cache.Snapshot, func(), error) {
	uri := protocol.URIFromPath(file)
	fh, snapshot, release, err := h.session.FileOf(ctx, uri)
	if err != nil {
		return nil, nil, nil, err
	}
	md, err := snapshot.NarrowestMetadataForFile(ctx, uri)
	if err != nil {
		release()
		return nil, nil, nil, err
	}
	fileEvents, err := checkForFileChanges(ctx, snapshot, md.ID)
	if err != nil {
		release()
		return nil, nil, nil, err
	}
	if len(fileEvents) == 0 {
		return fh, snapshot, release, nil
	}
	release() // snapshot is not latest

	// We detect changed files: process them before getting the snapshot.
	if err := h.lspServer.DidChangeWatchedFiles(ctx, &protocol.DidChangeWatchedFilesParams{
		Changes: fileEvents,
	}); err != nil {
		return nil, nil, nil, err
	}
	return h.session.FileOf(ctx, uri)
}

// checkForFileChanges checks for file changes in the transitive closure of
// the given package, by checking file modification time. Since it does not
// actually read file contents, it may miss changes that occur within the mtime
// resolution of the current file system (on some operating systems, this may
// be as much as a second).
//
// It also doesn't catch package changes that occur due to added files or
// changes to the go.mod file.
func checkForFileChanges(ctx context.Context, snapshot *cache.Snapshot, id metadata.PackageID) ([]protocol.FileEvent, error) {
	var events []protocol.FileEvent

	seen := make(map[metadata.PackageID]struct{})
	var checkPkg func(id metadata.PackageID) error
	checkPkg = func(id metadata.PackageID) error {
		if _, ok := seen[id]; ok {
			return nil
		}
		seen[id] = struct{}{}

		mp := snapshot.Metadata(id)
		for _, uri := range mp.CompiledGoFiles {
			fh, err := snapshot.ReadFile(ctx, uri)
			if err != nil {
				return err // context cancelled
			}

			mtime, mtimeErr := fh.ModTime()
			fi, err := os.Stat(uri.Path())
			switch {
			case err != nil:
				if mtimeErr == nil {
					// file existed, and doesn't anymore, so the file was deleted
					events = append(events, protocol.FileEvent{URI: uri, Type: protocol.Deleted})
				}
			case mtimeErr != nil:
				// err == nil (from above), so the file was created
				events = append(events, protocol.FileEvent{URI: uri, Type: protocol.Created})
			case !mtime.IsZero() && fi.ModTime().After(mtime):
				events = append(events, protocol.FileEvent{URI: uri, Type: protocol.Changed})
			}
		}
		for _, depID := range mp.DepsByPkgPath {
			if err := checkPkg(depID); err != nil {
				return err
			}
		}
		return nil
	}
	return events, checkPkg(id)
}

func textResult(text string) *mcp.CallToolResult {
	return &mcp.CallToolResult{
		Content: []mcp.Content{&mcp.TextContent{Text: text}},
	}
}