package pipeline

import (
	"errors"
	"io"
)

const cdnBodyLimit = 8 << 20

// ErrCDNBodyTooLarge reports a CDN response larger than the 8 MiB decoded-body limit.
var ErrCDNBodyTooLarge = errors.New("pipeline: cdn response body exceeds 8 MiB")

type limitedCDNBody struct {
	io.ReadCloser
	remaining int64
}

func (b *limitedCDNBody) Read(p []byte) (int, error) {
	if len(p) == 0 {
		return 0, nil
	}
	if b.remaining == 0 {
		var one [1]byte
		n, err := b.ReadCloser.Read(one[:])
		if n > 0 {
			return 0, ErrCDNBodyTooLarge
		}
		return 0, err
	}
	if int64(len(p)) > b.remaining {
		p = p[:int(b.remaining)]
	}
	n, err := b.ReadCloser.Read(p)
	b.remaining -= int64(n)
	return n, err
}

// ReadCDNBody reads a CDN response and rejects decoded bodies above 8 MiB.
// The extra byte distinguishes an exact-limit response from a truncated one.
func ReadCDNBody(body io.Reader) ([]byte, error) {
	data, err := io.ReadAll(io.LimitReader(body, cdnBodyLimit+1))
	if err != nil {
		return nil, err
	}
	if len(data) > cdnBodyLimit {
		return nil, ErrCDNBodyTooLarge
	}
	return data, nil
}