Rework caching_paged_reader into Table[T]

This *should* fix some issues with extraction due to race conditions

low/table.go

@@ -0,0 +1,78 @@
+package squashfslow
+
+import (
+	"encoding/binary"
+	"errors"
+	"sync"
+
+	"github.com/CalebQ42/squashfs/internal/metadata"
+	"github.com/CalebQ42/squashfs/internal/toreader"
+)
+
+var errOutOfBounds = errors.New("out of bounds")
+var errUnexpectedOutOfBounds = errors.New("unexpected out of bounds")
+var errNilCollection = errors.New("nil collection")
+
+type Table[T any] struct {
+	totalItems    uint32
+	itemsPerBlock uint32
+	offset        uint64
+	mut           sync.RWMutex
+	currentItems  []T
+	rdr           *Reader
+}
+
+func NewTable[T any](rdr *Reader, start uint64, totalItems uint32) *Table[T] {
+	var zero T
+	return &Table[T]{
+		totalItems:    totalItems,
+		itemsPerBlock: 8192 / uint32(binary.Size(zero)),
+		offset:        start,
+		mut:           sync.RWMutex{},
+		rdr:           rdr,
+	}
+}
+
+func (t *Table[T]) Get(requestedItemIndex uint32) (T, error) {
+	t.mut.RLock()
+	if requestedItemIndex >= t.totalItems {
+		t.mut.RUnlock()
+		var zero T
+		return zero, errOutOfBounds
+	}
+	if uint32(len(t.currentItems)) > requestedItemIndex {
+		t.mut.RUnlock()
+		return t.currentItems[requestedItemIndex], nil
+	}
+	t.mut.RUnlock()
+	return t.fillAndGet(requestedItemIndex)
+}
+
+func (t *Table[T]) fillAndGet(requestedItemIndex uint32) (T, error) {
+	t.mut.Lock()
+	defer t.mut.Unlock()
+	var offset uint64
+	var toRead uint32
+	var rdr *toreader.Reader
+	var metaRdr metadata.Reader
+	var err error
+	for uint32(len(t.currentItems)) <= requestedItemIndex {
+		rdr = toreader.NewReader(t.rdr.r, int64(t.offset))
+		err = binary.Read(rdr, binary.LittleEndian, &offset)
+		if err != nil {
+			var zero T
+			return zero, err
+		}
+		t.offset += 8
+		toRead = min(t.itemsPerBlock, t.totalItems-uint32(len(t.currentItems)))
+		new := make([]T, toRead)
+		metaRdr = metadata.NewReader(toreader.NewReader(t.rdr.r, int64(offset)), t.rdr.d)
+		err = binary.Read(&metaRdr, binary.LittleEndian, new)
+		if err != nil {
+			var zero T
+			return zero, err
+		}
+		t.currentItems = append(t.currentItems, new...)
+	}
+	return t.currentItems[requestedItemIndex], nil
+}