gems/opal-task/internal/sync/strategy.go

package sync

import (
	"fmt"
	"os"
	"time"

	"git.jnss.me/joakim/opal/internal/engine"
	"github.com/google/uuid"
)

// ConflictResolution defines how to handle conflicts
type ConflictResolution int

const (
	LastWriteWins ConflictResolution = iota // Use modified timestamp
	ServerWins                              // Server always preferred
	ClientWins                              // Client always preferred
	Manual                                  // Prompt user (future)
)

// MergeTasks merges local and remote task lists
func MergeTasks(local, remote []*engine.Task, strategy ConflictResolution) ([]*engine.Task, int, error) {
	// Create maps for quick lookup
	localMap := make(map[uuid.UUID]*engine.Task)
	remoteMap := make(map[uuid.UUID]*engine.Task)

	for _, task := range local {
		localMap[task.UUID] = task
	}

	for _, task := range remote {
		remoteMap[task.UUID] = task
	}

	result := []*engine.Task{}
	conflicts := 0

	// Process all unique UUIDs
	seen := make(map[uuid.UUID]bool)

	// Add all local tasks
	for _, task := range local {
		if seen[task.UUID] {
			continue
		}
		seen[task.UUID] = true

		remoteTask, existsRemote := remoteMap[task.UUID]
		if !existsRemote {
			// Local only - add it
			result = append(result, task)
			continue
		}

		// Exists in both - resolve conflict
		if DetectConflict(task, remoteTask) {
			conflicts++
			winner := resolveConflict(task, remoteTask, strategy)
			logConflict(task, remoteTask, winner)
			result = append(result, winner)
		} else {
			// No conflict - use either (same content)
			result = append(result, task)
		}
	}

	// Add remote-only tasks
	for _, task := range remote {
		if seen[task.UUID] {
			continue
		}
		seen[task.UUID] = true
		result = append(result, task)
	}

	return result, conflicts, nil
}

// DetectConflict checks if two tasks with same UUID have conflicting changes
func DetectConflict(local, remote *engine.Task) bool {
	// If modified times are the same, no conflict
	if local.Modified.Unix() == remote.Modified.Unix() {
		return false
	}

	// If one is significantly older (e.g., > 1 second difference), consider it a conflict
	return true
}

// resolveConflict determines which version wins
func resolveConflict(local, remote *engine.Task, strategy ConflictResolution) *engine.Task {
	switch strategy {
	case ServerWins:
		return remote
	case ClientWins:
		return local
	case LastWriteWins:
		if remote.Modified.After(local.Modified) {
			return remote
		}
		return local
	default:
		// Default to last-write-wins
		if remote.Modified.After(local.Modified) {
			return remote
		}
		return local
	}
}

// logConflict writes conflict information to log file
func logConflict(local, remote *engine.Task, winner *engine.Task) {
	logPath, err := engine.GetSyncConflictLogPath()
	if err != nil {
		return
	}

	winnerLabel := "local"
	if winner.UUID == remote.UUID && winner.Modified.Equal(remote.Modified) {
		winnerLabel = "remote"
	}

	entry := fmt.Sprintf(
		"[%s] Conflict on task %s\n"+
			"  Local:  modified %s - %s\n"+
			"  Remote: modified %s - %s\n"+
			"  Winner: %s\n\n",
		time.Now().Format(time.RFC3339),
		local.UUID,
		local.Modified.Format(time.RFC3339),
		local.Description,
		remote.Modified.Format(time.RFC3339),
		remote.Description,
		winnerLabel,
	)

	f, _ := os.OpenFile(logPath, os.O_APPEND|os.O_CREATE|os.O_WRONLY, 0644)
	if f != nil {
		defer f.Close()
		f.WriteString(entry)
	}
}

// ParseStrategy converts string to ConflictResolution
func ParseStrategy(s string) ConflictResolution {
	switch s {
	case "server-wins":
		return ServerWins
	case "client-wins":
		return ClientWins
	case "last-write-wins":
		return LastWriteWins
	default:
		return LastWriteWins
	}
}

// StrategyString converts ConflictResolution to string
func StrategyString(strategy ConflictResolution) string {
	switch strategy {
	case ServerWins:
		return "server-wins"
	case ClientWins:
		return "client-wins"
	case LastWriteWins:
		return "last-write-wins"
	default:
		return "last-write-wins"
	}
}