package iolimit

import (
	"errors"
	"io"
)

// ErrExpectedLengthExceeded reports that a stream
// produced bytes beyond the expected length.
var ErrExpectedLengthExceeded = errors.New("iolimit: stream exceeded expected length")

// ExpectLengthReader wraps src and enforces an expected byte length.
//
// It returns io.ErrUnexpectedEOF
// if src ends before expected bytes are read.
// It returns ErrExpectedLengthExceeded
// if reads continue beyond the expected boundary
// and src still produces bytes.
//
// This reader does not drain src on close or at the expected boundary.
// As a result,
// overlength streams are detected only
// when a caller reads at or past the boundary.
func ExpectLengthReader(src io.Reader, expected int) io.Reader {
	return &expectLengthReader{
		src:       src,
		remaining: expected,
	}
}

type expectLengthReader struct {
	src       io.Reader
	remaining int
}

func (reader *expectLengthReader) Read(dst []byte) (int, error) {
	if len(dst) == 0 {
		return 0, nil
	}

	if reader.remaining == 0 {
		var probe [1]byte

		n, err := reader.src.Read(probe[:])
		if n > 0 {
			return 0, ErrExpectedLengthExceeded
		}

		if err == nil {
			return 0, nil
		}

		return 0, err
	}

	if len(dst) > reader.remaining {
		dst = dst[:reader.remaining]
	}

	n, err := reader.src.Read(dst)
	if n > 0 {
		reader.remaining -= n
	}

	if errors.Is(err, io.EOF) {
		if reader.remaining > 0 {
			return n, io.ErrUnexpectedEOF
		}

		if n > 0 {
			return n, nil
		}

		return 0, io.EOF
	}

	return n, err
}