package mcp

import (
	"bufio"
	"bytes"
	"context"
	"encoding/json"
	"fmt"
	"io"
	"log/slog"
	"os/exec"
	"strconv"
	"strings"
	"sync"
)

// stdioTransport runs an MCP server as a child process and speaks
// newline-delimited JSON-RPC 2.0 to its stdin/stdout.
type stdioTransport struct {
	cfg       MCPConfig
	log       *slog.Logger
	line      *lineRPC
	cmd       *exec.Cmd
	stdin     io.WriteCloser
	closeOnce sync.Once
	closed    bool
}

// newStdioTransport builds a stdio transport for cfg.
func newStdioTransport(cfg MCPConfig, log *slog.Logger) *stdioTransport {
	return &stdioTransport{cfg: cfg, log: log}
}

// Start spawns the subprocess and wires up the request/response streams.
func (s *stdioTransport) Start(ctx context.Context) error {
	if err := ctx.Err(); err != nil {
		return fmt.Errorf("mcp stdio start: %w", err)
	}
	if s.cfg.Command == "" {
		return fmt.Errorf("mcp stdio: %w", ErrNoTransport)
	}
	cmd := exec.Command(s.cfg.Command, s.cfg.Args...)
	stdin, err := cmd.StdinPipe()
	if err != nil {
		return fmt.Errorf("mcp stdio stdin: %w", err)
	}
	stdout, err := cmd.StdoutPipe()
	if err != nil {
		return fmt.Errorf("mcp stdio stdout: %w", err)
	}
	stderr, err := cmd.StderrPipe()
	if err != nil {
		return fmt.Errorf("mcp stdio stderr: %w", err)
	}
	if err := cmd.Start(); err != nil {
		return fmt.Errorf("mcp stdio start %q: %w", s.cfg.Command, err)
	}
	s.cmd = cmd
	s.stdin = stdin
	s.line = &lineRPC{in: stdin, out: bufio.NewReader(stdout), log: s.log}
	go s.drainStderr(stderr)
	return nil
}

// Call sends a request over stdio and decodes the response.
func (s *stdioTransport) Call(ctx context.Context, method string, params any, result any) error {
	if s.line == nil {
		return fmt.Errorf("mcp stdio %s: %w", method, ErrNotConnected)
	}
	return s.line.call(ctx, method, params, result)
}

// Close terminates the child process and closes its streams.
func (s *stdioTransport) Close() error {
	s.closeOnce.Do(func() {
		s.closed = true
		if s.stdin != nil {
			_ = s.stdin.Close()
		}
		if s.cmd != nil && s.cmd.Process != nil {
			_ = s.cmd.Process.Kill()
		}
		if s.cmd != nil {
			_ = s.cmd.Wait()
		}
	})
	return nil
}

// drainStderr reads and logs the child's standard error at debug level so server
// diagnostics are visible without blocking the process.
func (s *stdioTransport) drainStderr(r io.Reader) {
	buf := make([]byte, 4096)
	for {
		n, err := r.Read(buf)
		if n > 0 {
			s.log.Debug("mcp server stderr", "name", s.cfg.Name, "msg", strings.TrimSpace(string(buf[:n])))
		}
		if err != nil {
			return
		}
	}
}

// lineRPC carries newline-delimited JSON-RPC 2.0 over a stream pair. Calls are
// serialized so at most one request is in flight at a time, which keeps id
// matching simple and deterministic.
type lineRPC struct {
	in     io.Writer
	out    *bufio.Reader
	log    *slog.Logger
	mu     sync.Mutex
	nextID int
}

// call writes a request and reads until the matching response arrives.
func (l *lineRPC) call(ctx context.Context, method string, params any, result any) error {
	l.mu.Lock()
	defer l.mu.Unlock()
	if err := ctx.Err(); err != nil {
		return fmt.Errorf("mcp %s: %w", method, err)
	}
	l.nextID++
	id := strconv.Itoa(l.nextID)
	req, err := encodeRequest(id, method, params)
	if err != nil {
		return fmt.Errorf("mcp %s: %w", method, err)
	}
	req = append(req, '\n')
	if _, err := l.in.Write(req); err != nil {
		return fmt.Errorf("mcp %s write: %w", method, err)
	}
	return l.readResponse(id, method, result)
}

// readResponse reads lines until a response matching id arrives, skipping blank
// lines, malformed frames and notifications (no id).
func (l *lineRPC) readResponse(id, method string, result any) error {
	for {
		line, err := l.out.ReadBytes('\n')
		if err != nil {
			return fmt.Errorf("mcp %s read: %w", method, err)
		}
		line = bytes.TrimSpace(line)
		if len(line) == 0 {
			continue
		}
		var resp rpcResponse
		if err := json.Unmarshal(line, &resp); err != nil {
			l.log.Debug("mcp ignoring malformed frame", "line", string(line))
			continue
		}
		if resp.ID == nil {
			continue
		}
		if !decodeIDString(resp, id) {
			continue
		}
		if resp.Error != nil {
			return fmt.Errorf("mcp %s: %w: %w", method, ErrRPC, resp.Error)
		}
		if result != nil {
			if err := json.Unmarshal(resp.Result, result); err != nil {
				return fmt.Errorf("mcp %s decode: %w", method, err)
			}
		}
		return nil
	}
}

// notify writes a fire-and-forget JSON-RPC notification (no id).
func (l *lineRPC) notify(ctx context.Context, method string, params any) error {
	l.mu.Lock()
	defer l.mu.Unlock()
	req, err := encodeRequest(nil, method, params)
	if err != nil {
		return fmt.Errorf("mcp notify %s: %w", method, err)
	}
	req = append(req, '\n')
	if _, err := l.in.Write(req); err != nil {
		return fmt.Errorf("mcp notify %s write: %w", method, err)
	}
	return nil
}