nyanimedb/modules/backend/handlers/cursor.go

package handlers

import (
	"encoding/base64"
	"encoding/json"
	"fmt"
	"reflect"
	"strconv"
)

// ParseCursorInto parses an opaque base64 cursor and injects values into target struct.
//
// Supported sort types:
//   - "id"     → sets CursorID (must be *int64)
//   - "year"   → sets CursorID (*int64) + CursorYear (*int32)
//   - "rating" → sets CursorID (*int64) + CursorRating (*float64)
//
// Target struct may have any subset of these fields (e.g. only CursorID).
// Unknown fields are ignored. Missing fields → values are dropped (safe).
//
// Returns error if cursor is invalid or inconsistent with sort_by.
func ParseCursorInto(sortBy, cursorStr string, target any) error {
	if cursorStr == "" {
		return nil // no cursor → nothing to do
	}

	// 1. Decode cursor
	payload, err := decodeCursor(cursorStr)
	if err != nil {
		return err
	}

	// 2. Extract ID (required for all types)
	id, err := extractInt64(payload, "id")
	if err != nil {
		return fmt.Errorf("cursor: %v", err)
	}

	// 3. Get reflect value of target (must be ptr to struct)
	v := reflect.ValueOf(target)
	if v.Kind() != reflect.Ptr || v.IsNil() {
		return fmt.Errorf("target must be non-nil pointer to struct")
	}
	v = v.Elem()
	if v.Kind() != reflect.Struct {
		return fmt.Errorf("target must be pointer to struct")
	}

	// 4. Helper: set field if exists and compatible
	setField := func(fieldName string, value any) {
		f := v.FieldByName(fieldName)
		if !f.IsValid() || !f.CanSet() {
			return // field not found or unexported
		}
		ft := f.Type()
		vv := reflect.ValueOf(value)

		// Case: field is *T, value is T → wrap in pointer
		if ft.Kind() == reflect.Ptr {
			elemType := ft.Elem()
			if vv.Type().AssignableTo(elemType) {
				ptr := reflect.New(elemType)
				ptr.Elem().Set(vv)
				f.Set(ptr)
			}
			// nil → leave as zero (nil pointer)
		} else if vv.Type().AssignableTo(ft) {
			f.Set(vv)
		}
		// else: type mismatch → silently skip (safe)
	}

	// 5. Dispatch by sort type
	switch sortBy {
	case "id":
		setField("CursorID", id)

	case "year":
		setField("CursorID", id)
		param, err := extractString(payload, "param")
		if err != nil {
			return fmt.Errorf("cursor year: %w", err)
		}
		year, err := strconv.Atoi(param)
		if err != nil {
			return fmt.Errorf("cursor year: param must be integer, got %q", param)
		}
		setField("CursorYear", int32(year)) // or int, depending on your schema

	case "rating":
		setField("CursorID", id)
		param, err := extractString(payload, "param")
		if err != nil {
			return fmt.Errorf("cursor rating: %w", err)
		}
		rating, err := strconv.ParseFloat(param, 64)
		if err != nil {
			return fmt.Errorf("cursor rating: param must be float, got %q", param)
		}
		setField("CursorRating", rating)

	default:
		return fmt.Errorf("unsupported sort_by: %q", sortBy)
	}

	return nil
}

// --- helpers ---
func decodeCursor(cursorStr string) (map[string]any, error) {
	data, err := base64.RawURLEncoding.DecodeString(cursorStr)
	if err != nil {
		data, err = base64.StdEncoding.DecodeString(cursorStr)
		if err != nil {
			return nil, fmt.Errorf("invalid base64 cursor")
		}
	}
	var m map[string]any
	if err := json.Unmarshal(data, &m); err != nil {
		return nil, fmt.Errorf("invalid cursor JSON: %w", err)
	}
	return m, nil
}

func extractInt64(m map[string]any, key string) (int64, error) {
	v, ok := m[key]
	if !ok {
		return 0, fmt.Errorf("missing %q", key)
	}
	switch x := v.(type) {
	case float64:
		if x == float64(int64(x)) {
			return int64(x), nil
		}
	case string:
		i, err := strconv.ParseInt(x, 10, 64)
		if err == nil {
			return i, nil
		}
	case int64, int, int32:
		return reflect.ValueOf(x).Int(), nil
	}
	return 0, fmt.Errorf("%q must be integer", key)
}

func extractString(m map[string]interface{}, key string) (string, error) {
	v, ok := m[key]
	if !ok {
		return "", fmt.Errorf("missing %q", key)
	}
	s, ok := v.(string)
	if !ok {
		return "", fmt.Errorf("%q must be string", key)
	}
	return s, nil
}