/*
SeekableBuffer

Copyright (c) 2023 aler9
Copyright (c) 2025 Zervó Zadachin

Implementation based on code from "bluenviron/mediacommon".
See: https://github.com/bluenviron/mediacommon/blob/main/pkg/formats/fmp4/seekablebuffer/seekablebuffer.go

Original code was licensed under the MIT License.
See: https://github.com/bluenviron/mediacommon/blob/main/LICENSE
*/

package seekablebuffer

import (
	"bytes"
	"fmt"
	"io"
)

// Buffer is a bytes.Buffer with an additional Seek() method.
type Buffer struct {
	bytes.Buffer
	pos int64
}

// Write implements io.Writer.
func (b *Buffer) Write(p []byte) (int, error) {
	var n int

	if b.pos < int64(b.Len()) {
		n = copy(b.Bytes()[b.pos:], p)
		p = p[n:]
	}

	if len(p) > 0 {
		// Buffer.Write should never return an error here.
		nn, _ := b.Buffer.Write(p)
		n += nn
	}

	b.pos += int64(n)
	return n, nil
}

// Read (almost) implements io.Reader.
func (b *Buffer) Read(_ []byte) (int, error) {
	return 0, fmt.Errorf("read not implemented")
}

// Seek implements io.Seeker.
func (b *Buffer) Seek(offset int64, whence int) (int64, error) {
	var pos2 int64

	switch whence {
	case io.SeekStart:
		pos2 = offset

	case io.SeekCurrent:
		pos2 = b.pos + offset

	case io.SeekEnd:
		pos2 = int64(b.Len()) + offset

	default:
		return 0, fmt.Errorf("invalid seek whence")
	}

	if pos2 < 0 {
		return 0, fmt.Errorf("negative position")
	}

	b.pos = pos2

	// Expand buffer if needed
	diff := b.pos - int64(b.Len())
	if diff > 0 {
		// Buffer.Write should never return an error here.
		b.Buffer.Write(make([]byte, diff))
	}

	return pos2, nil
}

// Reset resets the buffer state.
func (b *Buffer) Reset() {
	b.Buffer.Reset()
	b.pos = 0
}