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pagination.go
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pagination.go
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// Copyright 2020 the Exposure Notifications Verification Server authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
package database
import (
"math"
"github.com/google/exposure-notifications-verification-server/pkg/pagination"
"github.com/jinzhu/gorm"
)
const (
// maxPrevPages and maxNextPages control the "window" to show.
maxPrevPages = 5
maxNextPages = 5
)
// Paginate is a helper that paginates a gorm query into the given result. In
// addition to reflecting into the provided result, it returns a pagination
// struct.
//
// If page is 0, it defaults to 1. If limit is 0, it defaults to the global
// pagination default limit.
func Paginate(query *gorm.DB, result interface{}, page, limit uint64) (*pagination.Paginator, error) {
if page < 1 {
page = 1
}
if limit < 1 {
limit = pagination.DefaultLimit
}
return PaginateFn(query, page, limit, func(query *gorm.DB, offset uint64) error {
return query.
Limit(limit).
Offset(offset).
Find(result).Error
})
}
// PaginateFn paginates with a custom function for returning results.
func PaginateFn(query *gorm.DB, page, limit uint64, populateFn func(query *gorm.DB, offset uint64) error) (*pagination.Paginator, error) {
var total uint64
if err := query.
Count(&total).
Error; err != nil {
return nil, err
}
// Calculate offset.
if page < 1 {
page = 1 // pages start at 1
}
offset := (page - 1) * limit
// Get the list of records, limiting and offsetting as needed.
if err := populateFn(query, offset); err != nil {
return nil, err
}
// Calculate how many future pages exist.
var nextPages uint64
if current := offset + limit; current < total {
remaining := total - current
nextPages = uint64(math.Ceil(float64(remaining) / float64(limit)))
if nextPages > maxNextPages {
nextPages = maxNextPages
}
}
prevPages := page - 1
if prevPages > maxPrevPages {
prevPages = maxPrevPages
}
var paginator pagination.Paginator
paginator.CurrPage = &pagination.Page{
Number: page,
}
for i := prevPages; i > 0; i-- {
paginator.PrevPages = append(paginator.PrevPages, &pagination.Page{
Number: page - i,
})
}
if prevPages > 0 {
paginator.PrevPage = &pagination.Page{
Number: page - 1,
}
}
for i := uint64(1); i <= nextPages; i++ {
paginator.NextPages = append(paginator.NextPages, &pagination.Page{
Number: page + i,
})
}
if nextPages > 0 {
paginator.NextPage = &pagination.Page{
Number: page + 1,
}
}
return &paginator, nil
}