diff --git a/zfec/__init__.py b/zfec/__init__.py
index b218b92..35fc709 100644
--- a/zfec/__init__.py
+++ b/zfec/__init__.py
@@ -9,6 +9,9 @@
from . import _version
__version__ = _version.get_versions()['version']
+OPTION_POWER_SEQUENCE = 0
+OPTION_SEQUENTIAL_INTEGERS = 1
+
from ._fec import Encoder, Decoder, Error
from . import easyfec, filefec, cmdline_zfec, cmdline_zunfec
diff --git a/zfec/_fecmodule.c b/zfec/_fecmodule.c
index 846101f..5dc3dc9 100644
--- a/zfec/_fecmodule.c
+++ b/zfec/_fecmodule.c
@@ -73,10 +73,11 @@ Encoder_init(Encoder *self, PyObject *args, PyObject *kwdict) {
static char *kwlist[] = {
"k",
"m",
+ "option",
NULL
};
- int ink, inm;
- if (!PyArg_ParseTupleAndKeywords(args, kwdict, "ii:Encoder.__init__", kwlist, &ink, &inm))
+ int ink, inm, option = FEC_OPTION_POWER_SEQUENCE;
+ if (!PyArg_ParseTupleAndKeywords(args, kwdict, "ii|i:Encoder.__init__", kwlist, &ink, &inm, &option))
return -1;
if (ink < 1) {
@@ -95,11 +96,15 @@ Encoder_init(Encoder *self, PyObject *args, PyObject *kwdict) {
PyErr_Format(py_fec_error, "Precondition violation: first argument is required to be less than or equal to the second argument, but they were %d and %d respectively", ink, inm);
return -1;
}
+ if (option != FEC_OPTION_POWER_SEQUENCE && option != FEC_OPTION_SEQUENTIAL_INTEGERS) {
+ PyErr_Format(py_fec_error, "Precondition violation: option third argument must be one of (%d,%d)", FEC_OPTION_POWER_SEQUENCE, FEC_OPTION_SEQUENTIAL_INTEGERS);
+ return -1;
+ }
self->kk = (unsigned short)ink;
self->mm = (unsigned short)inm;
Py_BEGIN_ALLOW_THREADS
- self->fec_matrix = fec_new(self->kk, self->mm);
+ self->fec_matrix = fec_new2(self->kk, self->mm, option);
Py_END_ALLOW_THREADS
return 0;
@@ -340,11 +345,12 @@ Decoder_init(Encoder *self, PyObject *args, PyObject *kwdict) {
static char *kwlist[] = {
"k",
"m",
+ "option",
NULL
};
- int ink, inm;
- if (!PyArg_ParseTupleAndKeywords(args, kwdict, "ii:Decoder.__init__", kwlist, &ink, &inm))
+ int ink, inm, option = FEC_OPTION_POWER_SEQUENCE;
+ if (!PyArg_ParseTupleAndKeywords(args, kwdict, "ii|i:Decoder.__init__", kwlist, &ink, &inm, &option))
return -1;
if (ink < 1) {
@@ -367,7 +373,7 @@ Decoder_init(Encoder *self, PyObject *args, PyObject *kwdict) {
self->mm = (unsigned short)inm;
Py_BEGIN_ALLOW_THREADS
- self->fec_matrix = fec_new(self->kk, self->mm);
+ self->fec_matrix = fec_new2(self->kk, self->mm, option);
Py_END_ALLOW_THREADS
return 0;
diff --git a/zfec/easyfec.py b/zfec/easyfec.py
index c97bf34..b334e21 100644
--- a/zfec/easyfec.py
+++ b/zfec/easyfec.py
@@ -22,8 +22,8 @@ def ab(x): # debuggery
return "%s:%s" % (len(x), "--empty--",)
class Encoder(object):
- def __init__(self, k, m):
- self.fec = zfec.Encoder(k, m)
+ def __init__(self, k, m, option=0):
+ self.fec = zfec.Encoder(k, m, option=option)
def encode(self, data):
"""
@@ -39,8 +39,8 @@ def encode(self, data):
return self.fec.encode(l)
class Decoder(object):
- def __init__(self, k, m):
- self.fec = zfec.Decoder(k, m)
+ def __init__(self, k, m, option=0):
+ self.fec = zfec.Decoder(k, m, option=option)
def decode(self, blocks, sharenums, padlen):
"""
diff --git a/zfec/fec.c b/zfec/fec.c
index 993f22a..24032b1 100644
--- a/zfec/fec.c
+++ b/zfec/fec.c
@@ -429,6 +429,11 @@ fec_free (fec_t *p) {
fec_t *
fec_new(unsigned short k, unsigned short n) {
+ return fec_new2(k, n, FEC_OPTION_POWER_SEQUENCE);
+}
+
+fec_t *
+fec_new2(unsigned short k, unsigned short n, fec_option_t option) {
unsigned row, col;
gf *p, *tmp_m;
@@ -456,16 +461,24 @@ fec_new(unsigned short k, unsigned short n) {
tmp_m[0] = 1;
for (col = 1; col < k; col++)
tmp_m[col] = 0;
- for (p = tmp_m + k, row = 0; row + 1 < n; row++, p += k)
- for (col = 0; col < k; col++)
- p[col] = gf_exp[modnn (row * col)];
+ if (option == FEC_OPTION_POWER_SEQUENCE) {
+ for (p = tmp_m + k, row = 0; row + 1 < n; row++, p += k)
+ for (col = 0; col < k; col++)
+ p[col] = gf_exp[modnn (row * col)];
+ } else if (option == FEC_OPTION_SEQUENTIAL_INTEGERS) {
+ for (p = tmp_m + k, row = 1; row < n; row++, p += k)
+ for (col = 0; col < k; col++)
+ p[col] = gf_exp[modnn (gf_log[row] * col)];
+ } else {
+ assert(false);
+ }
/*
* quick code to build systematic matrix: invert the top
* k*k vandermonde matrix, multiply right the bottom n-k rows
* by the inverse, and construct the identity matrix at the top.
*/
- _invert_vdm (tmp_m, k); /* much faster than _invert_mat */
+ _invert_mat (tmp_m, k); /* much faster than _invert_mat */
_matmul(tmp_m + k * k, tmp_m, retval->enc_matrix + k * k, n - k, k, k);
/*
* the upper matrix is I so do not bother with a slow multiply
diff --git a/zfec/fec.h b/zfec/fec.h
index aaa4257..fc823bd 100644
--- a/zfec/fec.h
+++ b/zfec/fec.h
@@ -8,6 +8,25 @@
typedef unsigned char gf;
+typedef enum {
+ /**
+ * This is the default option if you do not specify one.
+ * This instanciates the vandermonde matrix over a vector of
+ * powers of the primitive element. It ensures maximum
+ * separation between elements in the field, which enhances
+ * error correction performance.
+ */
+ FEC_OPTION_POWER_SEQUENCE = 0,
+ /**
+ * This instanciates the vandermonde matrix over a vector of
+ * sequential integers. This choice is common among libraries
+ * that implement reed solomon erasure coding.
+ * Known libraries that do this:
+ * - klauspost/reedsolomon
+ */
+ FEC_OPTION_SEQUENTIAL_INTEGERS = 1,
+} fec_option_t;
+
typedef struct {
unsigned long magic;
unsigned short k, n; /* parameters of the code */
@@ -37,6 +56,12 @@ void fec_init(void);
* param m the total number of blocks created
*/
fec_t* fec_new(unsigned short k, unsigned short m);
+/**
+ * param k the number of blocks required to reconstruct
+ * param m the total number of blocks created
+ * param option options that control how the encoding/decoding matrix is built.
+ */
+fec_t* fec_new2(unsigned short k, unsigned short m, fec_option_t option);
void fec_free(fec_t* p);
/**