Consider exposing functionality to retrieve the size of compressed streams

[burnedskies]

Currently there's no way to get the compressed size of a packed stream, and for some use cases it can be useful.

Here is an example.

package main

import (
	"fmt"

	"github.com/bodgit/sevenzip"
)

func main() {
	o("testdata/t0.7z")
	o("testdata/t4.7z")
}

func o(name string) {
	r, err := sevenzip.OpenReader(name)
	if err != nil {
		panic(err)
	}

	fmt.Println("Archive contents for:", name)
	for _, file := range r.File {
		stream, err := r.PackedStreamSize(file.Stream)
		if err != nil {
			panic(err)
		}

		fmt.Printf(
			"%-20s | Checksum: %08X | Stream: %02d | Uncompressed Size: %-9d | Compressed Stream Size: %d\n",
			file.Name,
			file.CRC32,
			file.Stream,
			file.UncompressedSize,
			stream,
		)
	}

}

Expected output

Archive contents for: testdata/t0.7z
bar                  | Checksum: 04A2B3E9 | Stream: 00 | Uncompressed Size: 4         | Compressed Stream Size: 4
foo                  | Checksum: 7E3265A8 | Stream: 01 | Uncompressed Size: 4         | Compressed Stream Size: 4
Archive contents for: testdata/t4.7z
bar                  | Checksum: 04A2B3E9 | Stream: 00 | Uncompressed Size: 4         | Compressed Stream Size: 16
foo                  | Checksum: 7E3265A8 | Stream: 00 | Uncompressed Size: 4         | Compressed Stream Size: 16

Here is a patch for reference.

diff --git a/reader.go b/reader.go
index 8ff32ef..ca42fbe 100644
--- a/reader.go
+++ b/reader.go
@@ -801,6 +801,14 @@ func (z *Reader) openReadDir(dir string) []fileListEntry {
 	return files[i:j]
 }
 
+func (z *Reader) PackedStreamSize(stream int) (uint64, error) {
+	if 0 <= stream && len(z.si.packInfo.size) >= stream {
+		return z.si.packInfo.size[stream], nil
+	}
+
+	return 0, fmt.Errorf("stream `%d` is out of range", stream)
+}
+
 type openDir struct {
 	e      *fileListEntry
 	files  []fileListEntry