// The bytes read must match the number of remaining bytes in the current file.
//
// If the current file is sparse and r is an io.ReadSeeker,
// then readFrom uses Seek to skip past holes defined in Header.SparseHoles,
// assuming that skipped regions are all NULs.
// This always reads the last byte to ensure r is the right size.
//
// TODO(dsnet): Re-export this when adding sparse file support.

// deals with sparseDatas.
//
// However, the external API uses sparseHoles instead of sparseDatas because the
// zero value of sparseHoles logically represents a normal file (i.e., there are
// no holes in it). On the other hand, the zero value of sparseDatas implies
// that the file has no data in it, which is rather odd.
//
// As an example, if the underlying raw file contains the 10-byte data:
//

type Value struct {
	// A unique identifier for the value. For performance we allocate these IDs
	// densely starting at 1.  There is no guarantee that there won't be occasional holes, though.
	ID ID

	// The operation that computes this value. See op.go.
	Op Op

	// The type of this value. Normally this will be a Go type, but there
	// are a few other pseudo-types, see ../types/type.go.