package handler

import (
	"context"
	"database/sql"
	"encoding/json"
	"errors"
	"net/http"
	"strconv"
	"time"

	"github.com/go-chi/chi/v5"
	"github.com/wotra/wotra/internal/domain"
	"github.com/wotra/wotra/internal/store"
)

// SyncHandler serves /api/sync routes.
type SyncHandler struct {
	sync        *store.SyncStore
	entries     *store.EntryStore
	adjustments *store.BalanceAdjustmentStore
	settings    *store.SettingsStore
}

func NewSyncHandler(
	sync *store.SyncStore,
	entries *store.EntryStore,
	adjustments *store.BalanceAdjustmentStore,
	settings *store.SettingsStore,
) *SyncHandler {
	return &SyncHandler{
		sync:        sync,
		entries:     entries,
		adjustments: adjustments,
		settings:    settings,
	}
}

func (h *SyncHandler) Routes(r chi.Router) {
	r.Get("/sync/pull", h.Pull)
	r.Post("/sync/push", h.Push)
}

// Pull GET /api/sync/pull?since=N
func (h *SyncHandler) Pull(w http.ResponseWriter, r *http.Request) {
	sinceStr := r.URL.Query().Get("since")
	var since int64
	if sinceStr != "" {
		var err error
		since, err = strconv.ParseInt(sinceStr, 10, 64)
		if err != nil {
			writeError(w, http.StatusBadRequest, "invalid since parameter")
			return
		}
	}

	changes, serverVersion, err := h.sync.Pull(r.Context(), since)
	if err != nil {
		if errors.Is(err, store.ErrSyncStale) {
			writeError(w, http.StatusGone, "sync_stale")
			return
		}
		writeError(w, http.StatusInternalServerError, err.Error())
		return
	}

	// Return empty array rather than null.
	if changes == nil {
		changes = []store.SyncChange{}
	}

	writeJSON(w, http.StatusOK, map[string]any{
		"changes":        changes,
		"server_version": serverVersion,
	})
}

// pushItem is a single change submitted by the client.
type pushItem struct {
	Entity   string          `json:"entity"`
	EntityID string          `json:"entity_id"`
	Op       string          `json:"op"` // "upsert" | "delete"
	Payload  json.RawMessage `json:"payload"`
}

// Push POST /api/sync/push
func (h *SyncHandler) Push(w http.ResponseWriter, r *http.Request) {
	var body struct {
		Changes []pushItem `json:"changes"`
	}
	if err := decodeJSON(r, &body); err != nil {
		writeError(w, http.StatusBadRequest, "invalid JSON")
		return
	}

	ctx := r.Context()
	var applied, skipped []string

	for _, item := range body.Changes {
		ok, err := h.applyPushItem(ctx, item)
		if err != nil {
			// Log and skip on unexpected errors; don't abort the whole push.
			skipped = append(skipped, item.EntityID)
			continue
		}
		if ok {
			applied = append(applied, item.EntityID)
		} else {
			skipped = append(skipped, item.EntityID)
		}
	}

	if applied == nil {
		applied = []string{}
	}
	if skipped == nil {
		skipped = []string{}
	}

	writeJSON(w, http.StatusOK, map[string]any{
		"applied": applied,
		"skipped": skipped,
	})
}

// applyPushItem applies one client change. Returns (true, nil) if applied,
// (false, nil) if skipped (e.g. server row is newer), (false, err) on error.
func (h *SyncHandler) applyPushItem(ctx context.Context, item pushItem) (bool, error) {
	switch item.Entity {
	case "entries":
		return h.applyEntry(ctx, item)
	case "balance_adjustments":
		return h.applyBalanceAdjustment(ctx, item)
	case "settings_history":
		return h.applySettings(ctx, item)
	default:
		// closed_days, closed_weeks — server-only mutations; skip silently.
		return false, nil
	}
}

// ── entries ───────────────────────────────────────────────────────────────────

func (h *SyncHandler) applyEntry(ctx context.Context, item pushItem) (bool, error) {
	if item.Op == "delete" {
		var payload struct {
			ID        string `json:"id"`
			UpdatedAt int64  `json:"updated_at"`
		}
		if err := json.Unmarshal(item.Payload, &payload); err != nil {
			return false, err
		}
		now := time.Now().UnixMilli()
		// Only soft-delete if server row is not newer.
		existing, err := h.entries.GetByID(ctx, item.EntityID)
		if err != nil {
			if errors.Is(err, sql.ErrNoRows) {
				return true, nil // already gone
			}
			return false, err
		}
		if existing.UpdatedAt > payload.UpdatedAt {
			return false, nil // server is newer
		}
		if err := h.entries.SoftDelete(ctx, item.EntityID, now); err != nil {
			return false, err
		}
		if err := h.sync.LogEntryDelete(ctx, item.EntityID); err != nil {
			return false, err
		}
		return true, nil
	}

	// upsert
	var e domain.Entry
	if err := json.Unmarshal(item.Payload, &e); err != nil {
		return false, err
	}
	existing, err := h.entries.GetByID(ctx, e.ID)
	if err != nil && !errors.Is(err, sql.ErrNoRows) {
		return false, err
	}
	if existing != nil && existing.UpdatedAt >= e.UpdatedAt {
		return false, nil // server is newer or equal
	}
	if existing == nil {
		if err := h.entries.Create(ctx, &e); err != nil {
			return false, err
		}
	} else {
		if err := h.entries.Update(ctx, &e); err != nil {
			return false, err
		}
	}
	if err := h.sync.LogEntry(ctx, &e); err != nil {
		return false, err
	}
	return true, nil
}

// ── balance_adjustments ───────────────────────────────────────────────────────

func (h *SyncHandler) applyBalanceAdjustment(ctx context.Context, item pushItem) (bool, error) {
	if item.Op == "delete" {
		var payload struct {
			UpdatedAt int64 `json:"updated_at"`
		}
		if err := json.Unmarshal(item.Payload, &payload); err != nil {
			return false, err
		}
		existing, err := h.adjustments.GetByID(ctx, item.EntityID)
		if err != nil {
			if errors.Is(err, store.ErrAdjustmentNotFound) {
				return true, nil // already gone
			}
			return false, err
		}
		if existing.UpdatedAt > payload.UpdatedAt {
			return false, nil // server is newer
		}
		if err := h.adjustments.Delete(ctx, item.EntityID); err != nil {
			return false, err
		}
		if err := h.sync.LogBalanceAdjustmentDelete(ctx, item.EntityID); err != nil {
			return false, err
		}
		return true, nil
	}

	// upsert
	var a domain.BalanceAdjustment
	if err := json.Unmarshal(item.Payload, &a); err != nil {
		return false, err
	}
	existing, err := h.adjustments.GetByID(ctx, a.ID)
	if err != nil && !errors.Is(err, store.ErrAdjustmentNotFound) {
		return false, err
	}
	if existing != nil && existing.UpdatedAt >= a.UpdatedAt {
		return false, nil
	}
	if existing == nil {
		if err := h.adjustments.Create(ctx, &a); err != nil {
			return false, err
		}
	} else {
		if err := h.adjustments.Update(ctx, &a); err != nil {
			return false, err
		}
	}
	if err := h.sync.LogBalanceAdjustment(ctx, &a); err != nil {
		return false, err
	}
	return true, nil
}

// ── settings_history ──────────────────────────────────────────────────────────

func (h *SyncHandler) applySettings(ctx context.Context, item pushItem) (bool, error) {
	if item.Op == "delete" {
		// Refuse to delete if it would leave zero rows (same rule as service).
		count, err := h.settings.Count(ctx)
		if err != nil {
			return false, err
		}
		if count <= 1 {
			return false, nil // skip silently
		}
		if err := h.settings.Delete(ctx, item.EntityID); err != nil {
			return false, err
		}
		if err := h.sync.LogSettingsDelete(ctx, item.EntityID); err != nil {
			return false, err
		}
		return true, nil
	}

	// upsert — last updated_at wins via store.Upsert
	var s domain.Settings
	if err := json.Unmarshal(item.Payload, &s); err != nil {
		return false, err
	}
	if err := h.settings.Upsert(ctx, &s); err != nil {
		return false, err
	}
	if err := h.sync.LogSettings(ctx, &s); err != nil {
		return false, err
	}
	return true, nil
}