// Package agent provides a unified interface for executing prompts via
// coding agents (Claude Code, Codex, OpenCode). It mirrors the happy-cli AgentBackend
// pattern, translated to idiomatic Go.
package agent

import (
	"context"
	"fmt"
	"log/slog"
	"time"
)

// Backend is the unified interface for executing prompts via coding agents.
type Backend interface {
	// Execute runs a prompt and returns a Session for streaming results.
	// The caller should read from Session.Messages (optional) and wait on
	// Session.Result for the final outcome.
	Execute(ctx context.Context, prompt string, opts ExecOptions) (*Session, error)
}

// ExecOptions configures a single execution.
type ExecOptions struct {
	Cwd             string
	Model           string
	SystemPrompt    string
	MaxTurns        int
	Timeout         time.Duration
	ResumeSessionID string // if non-empty, resume a previous agent session
}

// Session represents a running agent execution.
type Session struct {
	// Messages streams events as the agent works. The channel is closed
	// when the agent finishes (before Result is sent).
	Messages <-chan Message
	// Result receives exactly one value — the final outcome — then closes.
	Result <-chan Result
}

// MessageType identifies the kind of Message.
type MessageType string

const (
	MessageText       MessageType = "text"
	MessageThinking   MessageType = "thinking"
	MessageToolUse    MessageType = "tool-use"
	MessageToolResult MessageType = "tool-result"
	MessageStatus     MessageType = "status"
	MessageError      MessageType = "error"
	MessageLog        MessageType = "log"
)

// Message is a unified event emitted by an agent during execution.
type Message struct {
	Type    MessageType
	Content string         // text content (Text, Error, Log)
	Tool    string         // tool name (ToolUse, ToolResult)
	CallID  string         // tool call ID (ToolUse, ToolResult)
	Input   map[string]any // tool input (ToolUse)
	Output  string         // tool output (ToolResult)
	Status  string         // agent status string (Status)
	Level   string         // log level (Log)
}

// Result is the final outcome after an agent session completes.
type Result struct {
	Status     string // "completed", "failed", "aborted", "timeout"
	Output     string // accumulated text output
	Error      string // error message if failed
	DurationMs int64
	SessionID  string
}

// Config configures a Backend instance.
type Config struct {
	ExecutablePath string            // path to CLI binary (claude, codex, or opencode)
	Env            map[string]string // extra environment variables
	Logger         *slog.Logger
}

// New creates a Backend for the given agent type.
// Supported types: "claude", "codex", "opencode".
func New(agentType string, cfg Config) (Backend, error) {
	if cfg.Logger == nil {
		cfg.Logger = slog.Default()
	}

	switch agentType {
	case "claude":
		return &claudeBackend{cfg: cfg}, nil
	case "codex":
		return &codexBackend{cfg: cfg}, nil
	case "opencode":
		return &opencodeBackend{cfg: cfg}, nil
	default:
		return nil, fmt.Errorf("unknown agent type: %q (supported: claude, codex, opencode)", agentType)
	}
}

// DetectVersion runs the agent CLI with --version and returns the output.
func DetectVersion(ctx context.Context, executablePath string) (string, error) {
	return detectCLIVersion(ctx, executablePath)
}