Move changes from exp2 to main

This is largely a move to simplify a lot of the readers
Also further breaks out functions.

internal/data/reader.go

@@ -0,0 +1,77 @@
+package data
+
+import (
+	"bytes"
+	"io"
+
+	"github.com/CalebQ42/squashfs/internal/decompress"
+)
+
+type Reader struct {
+	master     io.Reader
+	cur        io.Reader
+	fragRdr    io.Reader
+	d          decompress.Decompressor
+	blockSizes []uint32
+	blockSize  uint32
+}
+
+func NewReader(r io.Reader, d decompress.Decompressor, blockSizes []uint32, blockSize uint32) (*Reader, error) {
+	var out Reader
+	out.d = d
+	out.master = r
+	out.blockSizes = blockSizes
+	out.blockSize = blockSize
+	err := out.advance()
+	return &out, err
+}
+
+func (r *Reader) AddFragment(rdr io.Reader) {
+	r.fragRdr = rdr
+}
+
+func realSize(siz uint32) uint32 {
+	return siz &^ (1 << 24)
+}
+
+func (r *Reader) advance() (err error) {
+	if clr, ok := r.cur.(io.Closer); ok {
+		clr.Close()
+	}
+	if len(r.blockSizes) == 0 {
+		return io.EOF
+	}
+	if len(r.blockSizes) == 1 && r.fragRdr != nil {
+		r.cur = r.fragRdr
+	} else {
+		size := realSize(r.blockSizes[0])
+		if size == 0 {
+			r.cur = bytes.NewReader(make([]byte, r.blockSize))
+		} else {
+			r.cur = io.LimitReader(r.master, int64(size))
+			if size == r.blockSizes[0] {
+				r.cur, err = r.d.Reader(r.cur)
+			}
+		}
+	}
+	r.blockSizes = r.blockSizes[1:]
+	return
+}
+
+func (r *Reader) Read(p []byte) (n int, err error) {
+	n, err = r.cur.Read(p)
+	if err == io.EOF {
+		err = r.advance()
+		if err != nil {
+			return
+		}
+		var tmpN int
+		tmp := make([]byte, len(p)-n)
+		tmpN, err = r.Read(tmp)
+		for i := range tmp {
+			p[n+i] = tmp[i]
+		}
+		n += tmpN
+	}
+	return
+}