package zw

import (
	"context"
	"io"
	"io/fs"
	"sync"
	"sync/atomic"
)

type downloadReader struct {
	recv   func() ([]byte, error)
	cancel context.CancelFunc

	pending  []byte
	terminal error

	cancelOnce sync.Once
	closed     atomic.Bool
}

func (r *downloadReader) Read(destination []byte) (int, error) {
	if len(destination) == 0 {
		return 0, nil
	}

	if r.closed.Load() {
		return 0, fs.ErrClosed
	}

	if len(r.pending) > 0 {
		n := copy(destination, r.pending)
		r.pending = r.pending[n:]
		return n, nil
	}

	if r.terminal != nil {
		return 0, r.terminal
	}

	for {
		chunk, err := r.recv()
		if err != nil {
			if r.closed.Load() {
				return 0, fs.ErrClosed
			}

			if err == io.EOF {
				r.terminal = io.EOF
			} else {
				r.terminal = convertRPCError(err)
			}
			r.cancelOnce.Do(r.cancel)
			return 0, r.terminal
		}

		if len(chunk) == 0 {
			continue
		}

		n := copy(destination, chunk)
		r.pending = chunk[n:]
		return n, nil
	}
}

func (r *downloadReader) Close() error {
	r.closed.Store(true)
	r.cancelOnce.Do(r.cancel)
	return nil
}