// nearest block edge where 0 <= n < blockSize.
func blockPadding(offset int64) (n int64) {
	return -offset & (blockSize - 1)
}

var zeroBlock block

type block [blockSize]byte

// Convert block to any number of formats.
func (b *block) toV7() *headerV7       { return (*headerV7)(b) }
func (b *block) toGNU() *headerGNU     { return (*headerGNU)(b) }

	    and ends at the end of the innermost containing block.</li>

	<li>The scope of a type identifier declared inside a function
	    begins at the identifier in the TypeSpec
	    and ends at the end of the innermost containing block.</li>
</ol>

<p>
An identifier declared in a block may be redeclared in an inner block.
While the identifier of the inner declaration is in scope, it denotes

			b.Write(buf[:1])
		}
	}
}

func BenchmarkBufferWriteBlock(b *testing.B) {
	block := make([]byte, 1024)
	for _, n := range []int{1 << 12, 1 << 16, 1 << 20} {
		b.Run(fmt.Sprintf("N%d", n), func(b *testing.B) {
			b.ReportAllocs()
			for i := 0; i < b.N; i++ {
				var bb Buffer
				for bb.Len() < n {
					bb.Write(block)
				}
			}
		})
	}
}

func BenchmarkBufferAppendNoCopy(b *testing.B) {

					sa.IsExtended()[0] = 1
				}
				return sp
			}
			sp2 := formatSPD(spd, blk.GNU().Sparse())
			for len(sp2) > 0 {
				var spHdr block
				sp2 = formatSPD(sp2, spHdr.Sparse())
				spb = append(spb, spHdr[:]...)
			}

			// Update size fields in the header block.
			realSize := hdr.Size
			hdr.Size = 0 // Encoded size; does not account for encoded sparse map
			for _, s := range spd {
				hdr.Size += s.Length

		}
		pre = cur
	}
	return true
}

// alignSparseEntries mutates src and returns dst where each fragment's
// starting offset is aligned up to the nearest block edge, and each
// ending offset is aligned down to the nearest block edge.
//
// Even though the Go tar Reader and the BSD tar utility can handle entries
// with arbitrary offsets and lengths, the GNU tar utility can only handle

	return paxHdrs, nil
}

// readHeader reads the next block header and assumes that the underlying reader
// is already aligned to a block boundary. It returns the raw block of the
// header in case further processing is required.
//
// The err will be set to io.EOF only when one of the following occurs:
//   - Exactly 0 bytes are read and EOF is hit.
//   - Exactly 1 block of zeros is read and EOF is hit.

	    and ends at the end of the innermost containing block.</li>

	<li>The scope of a type identifier declared inside a function
	    begins at the identifier in the TypeSpec
	    and ends at the end of the innermost containing block.</li>
</ol>

<p>
An identifier declared in a block may be redeclared in an inner block.
While the identifier of the inner declaration is in scope, it denotes

		// Write the initial GNU header.
		var blk block
		gnu := blk.toGNU()
		sparse := gnu.sparse()
		copy(gnu.realSize(), size)
		sps = populateSparseMap(sparse, sps)
		if format != FormatUnknown {
			blk.setFormat(format)
		}
		out = append(out, blk[:]...)

		// Write extended sparse blocks.
		for len(sps) > 0 {
			var blk block
			sps = populateSparseMap(blk.toSparse(), sps)

	// callback(x) calls goCallback(x)
	callbackMutex.Lock()
	callbackToken++
	i := callbackToken
	callbackFuncs[i] = f
	callbackMutex.Unlock()

	// Pass the address of i because the C function was written to
	// take a pointer.  We could pass an int if we felt like
	// rewriting the C code.
	C.callback(unsafe.Pointer(&i))

	callbackMutex.Lock()
	delete(callbackFuncs, i)
	callbackMutex.Unlock()
}

		if len(extra) < fieldSize {
			break
		}
		fieldBuf := extra.sub(fieldSize)

		switch fieldTag {
		case zip64ExtraID:
			f.zip64 = true

			// update directory values from the zip64 extra block.
			// They should only be consulted if the sizes read earlier
			// are maxed out.
			// See golang.org/issue/13367.
			if needUSize {
				needUSize = false
				if len(fieldBuf) < 8 {
					return ErrFormat