// EncodeRing takes a Sector and encodes it as a Ring: a set of common.QuorumSize Segments that include redundancy.
// The encoding parameters are stored in params.
// k is the number of non-redundant segments, and b is the size of each segment. b is calculated from k.
// The erasure-coding algorithm requires that the original data must be k*b in size, so it is padded here as needed.
//
// The return value is a Ring.
// The first k Segments of the Ring are the original data split up.
// The remaining Segments are newly generated redundant data.
func EncodeRing(sec *common.Sector, params *common.EncodingParams) (ring [common.QuorumSize]common.Segment, err error) {
k, b, length := params.GetValues()
// check for legal size of k
if k <= 0 || k >= common.QuorumSize {
err = fmt.Errorf("k must be greater than 0 and smaller than %v", common.QuorumSize)
return
}
// check for legal size of b
if b < common.MinSegmentSize || b > common.MaxSegmentSize {
err = fmt.Errorf("b must be greater than %v and smaller than %v", common.MinSegmentSize, common.MaxSegmentSize)
return
}
// check for legal size of length
if length != len(sec.Data) {
err = fmt.Errorf("length mismatch: sector length %v != parameter length %v", len(sec.Data), length)
return
} else if length > common.MaxSegmentSize*common.QuorumSize {
err = fmt.Errorf("length must be smaller than %v", common.MaxSegmentSize*common.QuorumSize)
}
// pad data as needed
padding := k*b - len(sec.Data)
paddedData := append(sec.Data, bytes.Repeat([]byte{0x00}, padding)...)
// call the encoding function
m := common.QuorumSize - k
redundantChunk := C.encodeRedundancy(C.int(k), C.int(m), C.int(b), (*C.char)(unsafe.Pointer(&paddedData[0])))
redundantBytes := C.GoBytes(unsafe.Pointer(redundantChunk), C.int(m*b))
// split paddedData into ring
for i := 0; i < k; i++ {
ring[i] = common.Segment{
paddedData[i*b : (i+1)*b],
uint8(i),
}
}
// split redundantString into ring
for i := k; i < common.QuorumSize; i++ {
ring[i] = common.Segment{
redundantBytes[(i-k)*b : (i-k+1)*b],
uint8(i),
}
}
// free the memory allocated by the C file
C.free(unsafe.Pointer(redundantChunk))
return
}
// RebuildSector takes a Ring and returns a Sector containing the original data.
// The encoding parameters are stored in params.
// k must be equal to the number of non-redundant segments when the file was originally built.
// Because recovery is just a bunch of matrix operations, there is no way to tell if the data has been corrupted
// or if an incorrect value of k has been chosen. This error checking must happen before calling RebuildSector.
// Each Segment's Data must have the correct Index from when it was encoded.
func RebuildSector(ring []common.Segment, params *common.EncodingParams) (sec *common.Sector, err error) {
k, b, length := params.GetValues()
if k == 0 && b == 0 {
err = fmt.Errorf("could not rebuild using uninitialized encoding parameters")
return
}
// check for legal size of k
if k > common.QuorumSize || k < 1 {
err = fmt.Errorf("k must be greater than 0 but smaller than %v", common.QuorumSize)
return
}
// check for legal size of b
if b < common.MinSegmentSize || b > common.MaxSegmentSize {
err = fmt.Errorf("b must be greater than %v and smaller than %v", common.MinSegmentSize, common.MaxSegmentSize)
return
}
// check for legal size of length
if length > common.MaxSegmentSize*common.QuorumSize {
err = fmt.Errorf("length must be smaller than %v", common.MaxSegmentSize*common.QuorumSize)
}
// check for correct number of segments
if len(ring) < k {
err = fmt.Errorf("insufficient segments: expected at least %v, got %v", k, len(ring))
return
}
// move all data into a single slice
var segmentData []byte
var segmentIndices []uint8
for i := 0; i < k; i++ {
// verify that each segment is the correct length
// TODO: skip bad segments and continue rebuilding if possible
if len(ring[i].Data) != b {
err = fmt.Errorf("at least 1 Segment's Data field is the wrong length")
return
}
segmentData = append(segmentData, ring[i].Data...)
segmentIndices = append(segmentIndices, ring[i].Index)
}
// call the recovery function
C.recoverData(C.int(k), C.int(common.QuorumSize-k), C.int(b), (*C.uchar)(unsafe.Pointer(&segmentData[0])), (*C.uchar)(unsafe.Pointer(&segmentIndices[0])))
// remove padding introduced by EncodeRing()
sec, err = common.NewSector(segmentData[:length])
return
}