Add WIM parser

This new code supports parsing WIM files. Currently, only WIM files that
were created with no compression or LZX compression are supported. Xpress
compression is not.

wim/decompress.go

@@ -0,0 +1,138 @@
+package wim
+
+import (
+	"encoding/binary"
+	"io"
+	"io/ioutil"
+
+	"github.com/Microsoft/go-winio/wim/lzx"
+)
+
+const chunkSize = 32768 // Compressed resource chunk size
+
+type compressedReader struct {
+	r            *io.SectionReader
+	d            io.ReadCloser
+	chunks       []int64
+	curChunk     int
+	originalSize int64
+}
+
+func newCompressedReader(r *io.SectionReader, originalSize int64, offset int64) (*compressedReader, error) {
+	nchunks := (originalSize + chunkSize - 1) / chunkSize
+	var base int64
+	chunks := make([]int64, nchunks)
+	if originalSize <= 0xffffffff {
+		// 32-bit chunk offsets
+		base = (nchunks - 1) * 4
+		chunks32 := make([]uint32, nchunks-1)
+		err := binary.Read(r, binary.LittleEndian, chunks32)
+		if err != nil {
+			return nil, err
+		}
+		for i, n := range chunks32 {
+			chunks[i+1] = int64(n)
+		}
+
+	} else {
+		// 64-bit chunk offsets
+		base = (nchunks - 1) * 8
+		err := binary.Read(r, binary.LittleEndian, chunks[1:])
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	for i, c := range chunks {
+		chunks[i] = c + base
+	}
+
+	cr := &compressedReader{
+		r:            r,
+		chunks:       chunks,
+		originalSize: originalSize,
+	}
+
+	err := cr.reset(int(offset / chunkSize))
+	if err != nil {
+		return nil, err
+	}
+
+	suboff := offset % chunkSize
+	if suboff != 0 {
+		_, err := io.CopyN(ioutil.Discard, cr.d, suboff)
+		if err != nil {
+			return nil, err
+		}
+	}
+	return cr, nil
+}
+
+func (r *compressedReader) chunkOffset(n int) int64 {
+	if n == len(r.chunks) {
+		return r.r.Size()
+	}
+	return r.chunks[n]
+}
+
+func (r *compressedReader) chunkSize(n int) int {
+	return int(r.chunkOffset(n+1) - r.chunkOffset(n))
+}
+
+func (r *compressedReader) uncompressedSize(n int) int {
+	if n < len(r.chunks)-1 {
+		return chunkSize
+	}
+	size := int(r.originalSize % chunkSize)
+	if size == 0 {
+		size = chunkSize
+	}
+	return size
+}
+
+func (r *compressedReader) reset(n int) error {
+	if n >= len(r.chunks) {
+		return io.EOF
+	}
+	if r.d != nil {
+		r.d.Close()
+	}
+	r.curChunk = n
+	size := r.chunkSize(n)
+	uncompressedSize := r.uncompressedSize(n)
+	section := io.NewSectionReader(r.r, r.chunkOffset(n), int64(size))
+	if size != uncompressedSize {
+		d, err := lzx.NewReader(section, uncompressedSize)
+		if err != nil {
+			return err
+		}
+		r.d = d
+	} else {
+		r.d = ioutil.NopCloser(section)
+	}
+
+	return nil
+}
+
+func (r *compressedReader) Read(b []byte) (int, error) {
+	for {
+		n, err := r.d.Read(b)
+		if err != io.EOF {
+			return n, err
+		}
+
+		err = r.reset(r.curChunk + 1)
+		if err != nil {
+			return n, err
+		}
+	}
+}
+
+func (r *compressedReader) Close() error {
+	var err error
+	if r.d != nil {
+		err = r.d.Close()
+		r.d = nil
+	}
+	return err
+}