/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at https://mozilla.org/MPL/2.0/.
 */

package depager

import (
	"context"
	"fmt"
)

/*
The `Page` interface must wrap server responses. This
allows pagers to calculate page sizes and iterate over
response aggregates.

If the underlying value of this interface is `nil` (e.g. a
nil pointer to a struct or a nil slice), `Elems()` will
panic.
*/
type Page[T any] interface {
	// Elems must return the items from this page
	Elems() []T

	// URI must return the URI associated with this page
	URI() string

	// Count must return the total number of items being paged
	Count() uint64
}

// Exposes the part of the client that depager understands.
type Client[T any] interface {
	// NextPage returns the next page or it returns an error
	NextPage(
		page Page[T],
		offset uint64, // item offset at which to start page
	) (err error)
}

type Pager[T any] interface {
	// Iter is intended to be used in a for-range loop
	Iter() <-chan T

	// IterPages iterates over whole pages rather than items
	IterPages() <-chan Page[T]

	// LastErr must return the first error encountered, if any
	LastErr() error

	// Abort causes the pager to relinquish all pages back to
	// the page pool and stop all running goroutines.
	Abort() error
}

func NewPager[T any](
	ctx context.Context,
	c Client[T],
	pagePool chan Page[T],
) Pager[T] {
	if len(pagePool) == 0 {
		panic("new pager: provided page pool is empty")
	}
	var pageSize uint64
	pg := <-pagePool
	pageSize = uint64(cap(pg.Elems()))
	pagePool <- pg

	ctx2, cancel := context.WithCancel(ctx)
	done := make(chan struct{})

	return &pager[T]{
		pctx:     ctx,
		ctx:      ctx2,
		cancel:   cancel,
		done:     done,
		client:   c,
		n:        pageSize,
		pagePool: pagePool,
		poolSize: len(pagePool),
	}
}

/*
Retrieve n items in the range [m*n, m*n + n - 1], inclusive.
We keep len(pagePool) pages buffered.
*/
type pager[T any] struct {
	pctx     context.Context // Parent context.
	ctx      context.Context
	cancel   context.CancelFunc
	done     chan struct{} // Notify Abort when finished.
	client   Client[T]
	m        uint64
	n        uint64
	err      error
	pagePool chan Page[T]
	poolSize int
	cnt      uint64
}

func (p *pager[T]) iteratePages() <-chan Page[T] {
	ch := make(chan Page[T], len(p.pagePool))
	go func() {
		defer close(ch)
		var page Page[T]
		for {
			if p.pctx.Err() != nil {
				break
			}
			page = <-p.pagePool
			err := p.client.NextPage(page, p.m*p.n)
			if err != nil {
				p.err = err
				p.pagePool <- page
				return
			}
			if p.cnt == 0 {
				p.cnt = page.Count()
			}
			// When page.Count() is zero, we must rely on the
			// absence of returned results to know when to stop.
			if p.cnt == 0 && len(page.Elems()) == 0 {
				p.pagePool <- page
				return
			}
			ch <- page

			if (p.m*p.n + p.n) >= p.cnt {
				return
			}
			p.m++
		}
	}()
	return ch
}

func (p *pager[T]) IterPages() <-chan Page[T] {
	ch := make(chan Page[T], p.n)
	go func() {
		defer close(p.done)
		defer close(ch)
		for page := range p.iteratePages() {
			if p.pctx.Err() != nil {
				p.pagePool <- page
				break
			}
			if p.err != nil {
				p.err = fmt.Errorf("pager: iterate pages: %w", p.err)
				p.pagePool <- page
				return
			}
			ch <- page
		}
	}()
	return ch
}

func (p *pager[T]) Iter() <-chan T {
	ch := make(chan T, p.n)
	go func() {
		defer close(p.done)
		defer close(ch)
		for page := range p.iteratePages() {
			if p.pctx.Err() != nil {
				p.pagePool <- page
				break
			}
			for _, i := range page.Elems() {
				ch <- i
			}
			p.pagePool <- page
			if p.err != nil {
				p.err = fmt.Errorf("pager: iterate items: %w", p.err)
				return
			}
		}
	}()
	return ch
}

func (p *pager[T]) LastErr() error {
	return p.err
}

func (p *pager[T]) reset() error {
	err := p.err
	p.ctx, p.cancel = context.WithCancel(p.pctx)
	p.done = make(chan struct{})
	p.m, p.cnt = 0, 0
	p.err = nil
	return err
}

func (p *pager[T]) Abort() error {
	p.cancel()
	<-p.done
	return p.reset()
}