// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
//
// SPDX-License-Identifier: MIT

package mail

import (
	"io"
)

// Reader is a type that implements the io.Reader interface for a Msg
type Reader struct {
	buffer []byte // contents are the bytes buffer[offset : len(buffer)]
	offset int    // read at &buffer[offset], write at &buffer[len(buffer)]
	err    error  // initialization error
}

// Error returns an error if the Reader err field is not nil
func (r *Reader) Error() error {
	return r.err
}

// Read reads the length of p of the Msg buffer to satisfy the io.Reader interface
func (r *Reader) Read(payload []byte) (n int, err error) {
	if r.err != nil {
		return 0, r.err
	}
	if r.empty() || r.buffer == nil {
		r.Reset()
		if len(payload) == 0 {
			return 0, nil
		}
		return 0, io.EOF
	}
	n = copy(payload, r.buffer[r.offset:])
	r.offset += n
	return n, err
}

// Reset resets the Reader buffer to be empty, but it retains the underlying storage
// for use by future writes.
func (r *Reader) Reset() {
	r.buffer = r.buffer[:0]
	r.offset = 0
}

// empty reports whether the unread portion of the Reader buffer is empty.
func (r *Reader) empty() bool { return len(r.buffer) <= r.offset }