// Package keepalive provides agent activity signaling via file touch.
//
// This package uses a best-effort design: all write operations silently ignore
// errors. This is intentional because:
//
//  1. Keepalive signals are non-critical - the system works without them
//  2. Failures (disk full, permissions, etc.) should not interrupt gt commands
//  3. The daemon tolerates missing signals by using timeouts
//
// Functions in this package write JSON files to .runtime/ or daemon/ directories.
// These files are used by the daemon to detect agent activity and implement
// features like exponential backoff during idle periods.
//
// # Sentinel Pattern
//
// This package uses the nil sentinel pattern for graceful degradation:
//
//   - [Read] returns nil when the keepalive file doesn't exist or can't be parsed,
//     rather than returning an error. This allows callers to treat "no signal"
//     and "stale signal" uniformly.
//
//   - [State.Age] accepts nil receivers and returns a sentinel duration of 365 days,
//     which is guaranteed to exceed any reasonable staleness threshold. This enables
//     simple threshold checks without nil guards:
//
//     state := keepalive.Read(root)
//     if state.Age() > 5*time.Minute {
//     // Agent is idle or keepalive missing - both handled the same way
//     }
//
// The sentinel approach simplifies daemon logic by eliminating error-handling
// branches for the common case of missing or stale keepalives.
package keepalive

import (
	"encoding/json"
	"os"
	"path/filepath"
	"strings"
	"time"

	"github.com/steveyegge/gastown/internal/workspace"
)

// State represents the keepalive file contents.
type State struct {
	LastCommand string    `json:"last_command"`
	Timestamp   time.Time `json:"timestamp"`
}

// Touch updates the keepalive file in the workspace's .runtime directory.
// It silently ignores errors (best-effort signaling).
func Touch(command string) {
	TouchWithArgs(command, nil)
}

// TouchWithArgs updates the keepalive file with the full command including args.
// It silently ignores errors (best-effort signaling).
func TouchWithArgs(command string, args []string) {
	root, err := workspace.FindFromCwd()
	if err != nil || root == "" {
		return // Not in a workspace, nothing to do
	}

	// Build full command string
	fullCmd := command
	if len(args) > 0 {
		fullCmd = command + " " + strings.Join(args, " ")
	}

	TouchInWorkspace(root, fullCmd)
}

// TouchInWorkspace updates the keepalive file in a specific workspace.
// It silently ignores errors (best-effort signaling).
func TouchInWorkspace(workspaceRoot, command string) {
	runtimeDir := filepath.Join(workspaceRoot, ".runtime")

	// Ensure .runtime directory exists
	if err := os.MkdirAll(runtimeDir, 0755); err != nil {
		return
	}

	state := State{
		LastCommand: command,
		Timestamp:   time.Now().UTC(),
	}

	data, err := json.Marshal(state)
	if err != nil {
		return
	}

	keepalivePath := filepath.Join(runtimeDir, "keepalive.json")
	_ = os.WriteFile(keepalivePath, data, 0644) // non-fatal: status file for debugging
}

// Read returns the current keepalive state for the workspace.
//
// This function uses the nil sentinel pattern: it returns nil (not an error)
// when the keepalive file doesn't exist, can't be read, or contains invalid JSON.
// Callers can safely pass the result to [State.Age] without nil checks.
func Read(workspaceRoot string) *State {
	keepalivePath := filepath.Join(workspaceRoot, ".runtime", "keepalive.json")

	data, err := os.ReadFile(keepalivePath)
	if err != nil {
		return nil
	}

	var state State
	if err := json.Unmarshal(data, &state); err != nil {
		return nil
	}

	return &state
}

// Age returns how old the keepalive signal is.
//
// This method implements the sentinel pattern by accepting nil receivers.
// When s is nil (indicating no keepalive exists), it returns 365 days—a value
// guaranteed to exceed any reasonable staleness threshold. This allows callers
// to write simple threshold checks without nil guards:
//
//	if keepalive.Read(root).Age() > 5*time.Minute { ... }
//
// The 365-day sentinel was chosen because:
//   - It exceeds any practical idle timeout (typically seconds to minutes)
//   - It's semantically "infinitely old" for activity detection purposes
//   - It avoids magic values like MaxInt64 that could cause overflow issues
func (s *State) Age() time.Duration {
	if s == nil {
		return 24 * time.Hour * 365 // Sentinel: treat missing keepalive as maximally stale
	}
	return time.Since(s.Timestamp)
}