package main

import (
	"bytes"
	"crypto/rand"
	"encoding/hex"
	"encoding/json"
	"errors"
	"fmt"
	"io"
	"os"
	"path/filepath"
	"slices"
	"strings"

	"golang.org/x/sys/unix"
)

const (
	// agentInstructionsFileName is the file of the user's instructions in
	// the directory of an agent.
	agentInstructionsFileName = "instructions.md"

	// agentIDBytes is the number of random bytes of an agent ID.
	agentIDBytes = 16
)

// openAgent reads the config.json of the agent in dir. When the file does not
// exist, it makes the directory and writes the file with the defaults and a new
// ID. A missing field of an existing file takes the default, but the ID must be
// there. An unknown field is an error, so a typo does not become a silent
// default.
func openAgent(dir string) (*Agent, error) {
	err := os.MkdirAll(dir, 0o700)
	if err != nil {
		return nil, fmt.Errorf("create agent directory: %w", err)
	}

	path := filepath.Join(dir, "config.json")
	data, err := os.ReadFile(path)
	if errors.Is(err, os.ErrNotExist) {
		return createAgent(dir)
	}

	if err != nil {
		return nil, fmt.Errorf("read agent config: %w", err)
	}

	config, err := defaultAgentConfig()
	if err != nil {
		return nil, err
	}

	decoder := json.NewDecoder(bytes.NewReader(data))
	decoder.DisallowUnknownFields()

	err = decoder.Decode(&config)
	if err == nil && !errors.Is(decoder.Decode(&struct{}{}), io.EOF) {
		err = errors.New("data after the JSON object")
	}

	if err == nil {
		err = config.validate()
	}

	if err != nil {
		return nil, fmt.Errorf("%s: %w", path, err)
	}

	return &Agent{
		dir:    dir,
		config: config,
	}, nil
}

// Agent is one agent and its directory. The directory holds config.json, the
// user's instructions in instructions.md, the chat, and the usage file.
type Agent struct {
	dir    string
	config AgentConfig
}

// Images returns the shared data-directory image store, also for a child.
func (a *Agent) Images() *ImageStore {
	root := a.dir
	if a.config.Parent != "" {
		root = filepath.Dir(filepath.Dir(root))
	}

	return &ImageStore{dir: filepath.Join(filepath.Dir(root), imageDirName)}
}

// CacheKey returns the prompt cache key of every model call of the agent.
func (a *Agent) CacheKey() string {
	return "onechat-" + a.config.ID
}

// Prompt returns the system prompt of the agent with the user's instructions
// from instructions.md. It reads the file on each call, so a change applies at
// the next call. A missing file means no instructions.
func (a *Agent) Prompt() (string, error) {
	instructions, err := a.instructions()
	if err != nil {
		return "", fmt.Errorf("read prompt instructions: %w", err)
	}

	path, err := filepath.Abs(filepath.Join(a.dir, agentInstructionsFileName))
	if err != nil {
		return "", fmt.Errorf("find instructions path: %w", err)
	}

	if a.config.Parent != "" {
		return promptChild(a.config.Name, a.config.Parent, a.config.Responsibility, path, instructions), nil
	}

	return prompt(a.config.Name, path, instructions), nil
}

// Tools returns the tools shared by the turns and compactions of this agent. A
// child has no image output, WhatsApp, child creation, or route tools.
func (a *Agent) Tools() []ModelTool {
	tools := promptTools()
	if a.config.Parent != "" {
		tools = slices.DeleteFunc(tools, func(tool ModelTool) bool {
			return strings.HasPrefix(tool.Name, "whatsapp_") || tool.Name == "image_show" || tool.Name == "agent_create" || tool.Name == "route" || tool.Name == "unroute" || tool.Name == "routes"
		})
	}

	return tools
}

// Children reads the direct children in directory order. Each child needs an
// existing config with its directory name and this agent as parent. Children
// cannot have children. A missing children directory means no children.
func (a *Agent) Children() ([]*Agent, error) {
	dir := filepath.Join(a.dir, "children")
	entries, err := os.ReadDir(dir)
	if errors.Is(err, os.ErrNotExist) {
		return nil, nil
	}

	if err != nil {
		return nil, fmt.Errorf("read children: %w", err)
	}

	if a.config.Parent != "" && len(entries) != 0 {
		return nil, errors.New("children cannot have children")
	}

	var children []*Agent

	keys := map[string]bool{a.CacheKey(): true}

	for _, entry := range entries {
		name := entry.Name()
		if !entry.IsDir() {
			return nil, fmt.Errorf("child %q must be a directory", name)
		}

		path := filepath.Join(dir, name)
		_, err = os.Stat(filepath.Join(path, "config.json"))
		if err != nil {
			return nil, fmt.Errorf("child %q config: %w", name, err)
		}

		child, err := openAgent(path)
		if err != nil {
			return nil, fmt.Errorf("open child %q: %w", name, err)
		}

		if child.config.Name != name || child.config.Parent != a.config.Name {
			return nil, fmt.Errorf("child %q needs name %q and parent %q", name, name, a.config.Name)
		}

		if child.config.Name == a.config.Name || keys[child.CacheKey()] {
			return nil, fmt.Errorf("child %q must have its own name and ID", name)
		}

		keys[child.CacheKey()] = true
		_, err = child.Children()
		if err != nil {
			return nil, fmt.Errorf("read child %q: %w", name, err)
		}

		children = append(children, child)
	}

	return children, nil
}

// CreateChild writes a new direct child with its own ID and the model, effort,
// and working directory of the root. Child and subagent names share a
// namespace. The config is complete before an atomic rename publishes the
// child. Preparation stays outside children, so an interrupted write cannot
// stop startup.
func (a *Agent) CreateChild(name, responsibility string, subagents *Subagents) (_ *Agent, err error) {
	if a.config.Parent != "" {
		return nil, errors.New("only the root can create children")
	}

	if !agentChildNameValid(name) {
		return nil, errors.New("name must have 2 to 32 ASCII letters and start with a capital letter")
	}

	if name == a.config.Name {
		return nil, errors.New("the name is already used by the root")
	}

	responsibility = strings.TrimSpace(responsibility)
	if responsibility == "" {
		return nil, errors.New("responsibility must not be empty")
	}

	err = subagents.Reserve(name)
	if err != nil {
		return nil, fmt.Errorf("reserve child name: %w", err)
	}

	published := false
	defer func() {
		if !published {
			subagents.Release(name)
		}
	}()

	staged, err := os.MkdirTemp(a.dir, ".child-*")
	if err != nil {
		return nil, fmt.Errorf("prepare child directory: %w", err)
	}

	defer func() {
		cleanupErr := os.RemoveAll(staged)
		if cleanupErr != nil {
			err = errors.Join(err, fmt.Errorf("remove prepared child: %w", cleanupErr))
		}
	}()

	config := a.config
	config.Name = name
	config.ID = ""
	config.Parent = a.config.Name
	config.Responsibility = responsibility
	child, err := writeAgent(staged, config)
	if err != nil {
		return nil, fmt.Errorf("write child: %w", err)
	}

	dir := filepath.Join(a.dir, "children")
	err = os.MkdirAll(dir, 0o700)
	if err != nil {
		return nil, fmt.Errorf("create children directory: %w", err)
	}

	dir = filepath.Join(dir, name)
	err = unix.Renameat2(unix.AT_FDCWD, staged, unix.AT_FDCWD, dir, unix.RENAME_NOREPLACE)
	if err != nil {
		return nil, fmt.Errorf("publish child directory: %w", err)
	}

	published = true
	child.dir = dir

	return child, nil
}

// SubagentPrompt returns the system prompt of a subagent of the agent, with the
// user's instructions as Prompt reads them.
func (a *Agent) SubagentPrompt() (string, error) {
	instructions, err := a.instructions()
	if err != nil {
		return "", err
	}

	return promptSubagent(a.config.Name, instructions), nil
}

// instructions reads the user's instructions from instructions.md. A missing
// file means no instructions.
func (a *Agent) instructions() (string, error) {
	data, err := os.ReadFile(filepath.Join(a.dir, agentInstructionsFileName))
	if err != nil && !errors.Is(err, os.ErrNotExist) {
		return "", fmt.Errorf("read instructions: %w", err)
	}

	return string(data), nil
}

// AgentConfig holds the settings of the config.json of an agent. ID is random,
// made once when the file is made, and never changes. The cache key comes from
// the ID, so a rename keeps the prompt cache and two agents with one name do
// not share a key. Workdir is the directory where the commands of the agent
// run. Responsibility and Parent are set together for a child agent.
type AgentConfig struct {
	Name           string `json:"name"`
	ID             string `json:"id"`
	Model          string `json:"model"`
	Effort         string `json:"effort"`
	Workdir        string `json:"workdir"`
	Responsibility string `json:"responsibility,omitempty"`
	Parent         string `json:"parent,omitempty"`
}

// validate makes sure that each setting has a valid value.
func (c AgentConfig) validate() error {
	if c.Name == "" || strings.ContainsAny(c.Name, "\r\n") {
		return errors.New("name must be one line of text")
	}

	id, err := hex.DecodeString(c.ID)
	if err != nil || len(id) != agentIDBytes {
		return fmt.Errorf("id must be %d hexadecimal digits", 2*agentIDBytes)
	}

	if c.Model == "" || c.Effort == "" {
		return errors.New("agent needs a model and an effort")
	}

	if (strings.TrimSpace(c.Responsibility) == "") != (c.Parent == "") {
		return errors.New("a child needs both responsibility and parent")
	}

	if strings.ContainsAny(c.Parent, "\r\n") {
		return errors.New("parent must be one line of text")
	}

	if !filepath.IsAbs(c.Workdir) {
		return fmt.Errorf("workdir %q is not an absolute path", c.Workdir)
	}

	return nil
}

// agentChildNameValid accepts one word of 2 to 32 ASCII letters with an initial
// capital letter. Child names describe their area.
func agentChildNameValid(name string) bool {
	if len(name) < 2 || len(name) > 32 || name[0] < 'A' || name[0] > 'Z' {
		return false
	}

	for i := 1; i < len(name); i++ {
		letter := name[i]
		if (letter < 'A' || letter > 'Z') && (letter < 'a' || letter > 'z') {
			return false
		}
	}

	return true
}

// createAgent writes the config.json of a new agent in dir with the defaults
// and a new ID.
func createAgent(dir string) (*Agent, error) {
	config, err := defaultAgentConfig()
	if err != nil {
		return nil, err
	}

	return writeAgent(dir, config)
}

// writeAgent gives config a new ID and writes it in dir.
func writeAgent(dir string, config AgentConfig) (*Agent, error) {
	id := make([]byte, agentIDBytes)
	_, err := rand.Read(id)
	if err != nil {
		return nil, fmt.Errorf("make agent ID: %w", err)
	}

	config.ID = hex.EncodeToString(id)
	data, err := json.MarshalIndent(config, "", "  ")
	if err != nil {
		return nil, fmt.Errorf("encode agent config: %w", err)
	}

	err = writeFileAtomic(filepath.Join(dir, "config.json"), append(data, '\n'))
	if err != nil {
		return nil, fmt.Errorf("write agent config: %w", err)
	}

	return &Agent{
		dir:    dir,
		config: config,
	}, nil
}

// defaultAgentConfig returns the settings of a new agent, without an ID. The
// commands run in the home directory of the user.
func defaultAgentConfig() (AgentConfig, error) {
	home, err := os.UserHomeDir()
	if err != nil {
		return AgentConfig{}, fmt.Errorf("find working directory: %w", err)
	}

	return AgentConfig{
		Name:    "OneChat",
		Model:   "gpt-6.1-sol",
		Effort:  "medium",
		Workdir: home,
	}, nil
}