From: zooko <zooko@zooko.com>
Date: Thu, 25 Jan 2007 21:25:19 +0000 (+0530)
Subject: add utility method file_fec
X-Git-Url: https://git.rkrishnan.org/specifications/provisioning?a=commitdiff_plain;h=2594d395923dd945cd62c836f34c0688ba6adfd7;p=tahoe-lafs%2Fzfec.git

add utility method file_fec

darcs-hash:96fa282d9bf90f4817029dc6d3f4de3f4ee5a6fb
---

diff --git a/pyfec/fec/filefec.py b/pyfec/fec/filefec.py
new file mode 100644
index 0000000..a2c036e
--- /dev/null
+++ b/pyfec/fec/filefec.py
@@ -0,0 +1,107 @@
+import fec
+
+import array
+
+def encode_file(inf, cb, k, m, chunksize=4096):
+    """
+    Read in the contents of inf, encode, and call cb with the results.
+
+    First, k "input shares" will be read from inf, each input share being of 
+    size chunksize.  Then these k shares will be encoded into m "result 
+    shares".  Then cb will be invoked, passing a list of the m result shares 
+    as its first argument, and the length of the encoded data as its second 
+    argument.  (The length of the encoded data is always equal to k*chunksize, 
+    until the last iteration, when the end of the file has been reached and 
+    less than k*chunksize bytes could be read from the file.)  This procedure 
+    is iterated until the end of the file is reached, in which case the space 
+    of the input shares that is unused is filled with zeroes before encoding.
+
+    Note that the sequence passed in calls to cb() contains mutable array
+    objects in its first k elements whose contents will be overwritten when 
+    the next segment is read from the input file.  Therefore the 
+    implementation of cb() has to either be finished with those first k arrays 
+    before returning, or if it wants to keep the contents of those arrays for 
+    subsequent use after it has returned then it must make a copy of them to 
+    keep.
+
+    @param inf the file object from which to read the data
+    @param cb the callback to be invoked with the results
+    @param k the number of shares required to reconstruct the file
+    @param m the total number of shares created
+    @param chunksize how much data to read from inf for each of the k input 
+        shares
+    """
+    enc = fec.Encoder(k, m)
+    l = tuple([ array.array('c') for i in range(k) ])
+    indatasize = k*chunksize # will be reset to shorter upon EOF
+    ZEROES=array.array('c', ['\x00'])*chunksize
+    while indatasize == k*chunksize:
+        # This loop body executes once per segment.
+        i = 0
+        while (i<len(l)):
+            # This loop body executes once per chunk.
+            a = l[i]
+            i += 1
+            del a[:]
+            try:
+                a.fromfile(inf, chunksize)
+            except EOFError:
+                indatasize = i*chunksize + len(a)
+                
+                # padding
+                a.fromstring("\x00" * (chunksize-len(a)))
+                while (i<len(l)):
+                    a[:] = ZEROES
+                    i += 1
+
+        # print "about to encode()... len(l[0]): %s, l[0]: %s" % (len(l[0]), type(l[0]),),
+        res = enc.encode(l)
+        # print "...finished to encode()"
+        cb(res, indatasize)
+
+def encode_file_stringy(inf, cb, k, m, chunksize=4096):
+    """
+    Read in the contents of inf, encode, and call cb with the results.
+
+    First, k "input shares" will be read from inf, each input share being of 
+    size chunksize.  Then these k shares will be encoded into m "result 
+    shares".  Then cb will be invoked, passing a list of the m result shares 
+    as its first argument, and the length of the encoded data as its second 
+    argument.  (The length of the encoded data is always equal to k*chunksize, 
+    until the last iteration, when the end of the file has been reached and 
+    less than k*chunksize bytes could be read from the file.)  This procedure 
+    is iterated until the end of the file is reached, in which case the space 
+    of the input shares that is unused is filled with zeroes before encoding.
+
+    @param inf the file object from which to read the data
+    @param cb the callback to be invoked with the results
+    @param k the number of shares required to reconstruct the file
+    @param m the total number of shares created
+    @param chunksize how much data to read from inf for each of the k input 
+        shares
+    """
+    enc = fec.Encoder(k, m)
+    indatasize = k*chunksize # will be reset to shorter upon EOF
+    while indatasize == k*chunksize:
+        # This loop body executes once per segment.
+        i = 0
+	l = []
+        ZEROES = '\x00'*chunksize
+        while i<k:
+            # This loop body executes once per chunk.
+            i += 1
+            l.append(inf.read(chunksize))
+            if len(l[-1]) < chunksize:
+                indatasize = i*chunksize + len(l[-1])
+                
+                # padding
+                l[-1] = l[-1] + "\x00" * (chunksize-len(l[-1]))
+                while i<k:
+                    l.append(ZEROES)
+                    i += 1
+
+        # print "about to encode()... len(l[0]): %s, l[0]: %s" % (len(l[0]), type(l[0]),),
+        res = enc.encode(l)
+        # print "...finished to encode()"
+        cb(res, indatasize)
+