package depager_test

import (
	"fmt"
	"testing"

	dp "idio.link/go/depager/v2"
)

type NoopClient[T any] struct {
	err   error
	pages []dp.Page[T]
	m     int
	cnt   uint64
}

func (c *NoopClient[T]) NextPage(
	_offset uint64,
) (page dp.Page[T], cnt uint64, err error) {
	if len(c.pages) == 0 {
		return
	}
	if c.m >= len(c.pages) {
		err = fmt.Errorf("client: next page: exceeded max pages")
		return
	}
	page = c.pages[c.m]
	cnt = c.cnt
	err = c.err
	c.m++
	return
}

func NewNoopClient[T any](
	cnt int,
	err error,
	pages []dp.Page[T],
) dp.Client[T] {
	return &NoopClient[T]{
		cnt:   uint64(cnt),
		err:   err,
		pages: pages,
	}
}

type Aggr[T any] []T

func (a Aggr[T]) Elems() []T {
	return []T(a)
}

func TestUsingNoopClient(t *testing.T) {
	client := NewNoopClient[any](0, nil,
		[]dp.Page[any]{
			Aggr[any]{},
		},
	)
	pager := dp.NewPager(client, 0)
	for range pager.Iter() {
	}
	if err := pager.LastErr(); err != nil {
		t.Errorf("unexpected error in pager with noop client: %v", err)
	}
}

func TestNoopClientReturnsError(t *testing.T) {
	client := NewNoopClient[any](0, fmt.Errorf("whomp"),
		[]dp.Page[any]{
			Aggr[any]{},
		},
	)
	pager := dp.NewPager(client, 0)
	for range pager.Iter() {
	}
	if err := pager.LastErr(); err == nil {
		t.Errorf("unexpected success: %v", err)
	}
}

func TestClientReturnsNonemptyPage(t *testing.T) {
	pageSize := 2
	itemCount := 3
	client := NewNoopClient[any](itemCount, nil,
		[]dp.Page[any]{
			Aggr[any]{1, 2},
			Aggr[any]{3},
		},
	)
	pager := dp.NewPager(client, uint64(pageSize))
	var elem int
	for e := range pager.Iter() {
		elem = e.(int)
	}
	if err := pager.LastErr(); err != nil {
		t.Errorf("unexpected error in pager: %v", err)
	}
	if elem != 3 {
		t.Errorf("unexpected value: '%v'", elem)
	}
}

func TestClientReturnsFewerPagesThanExpected(t *testing.T) {
	pageSize := 1
	itemCount := pageSize + 1
	client := NewNoopClient[any](itemCount, nil,
		[]dp.Page[any]{
			Aggr[any]{0},
		},
	)
	pager := dp.NewPager(client, uint64(pageSize))
	for range pager.Iter() {
	}
	if err := pager.LastErr(); err == nil {
		t.Errorf("unexpected success in pager: %v", err)
	}
}