Transpose the encoding matrix
Stripe together the encodings from each of the 16 codewords, so that
if a 512-byte DRAM line is knocked out, it affects 256 bits from each
codeword rather than 4096 bits from a single encoded codeword.
Rather than using std::bitset, we directly set and read bits in
the std::vector<uint8_t>, because the striping means that copying it
will now cost not4k in allocation but 64k.
Decode directly to a word, without using list decoding. It seems
we don't need list decoding for the error rates that matter here,
and we never completed the implementation of it anyway.
Declare and test only the full interface, now that it doesn't decompose
quite so neatly.
Bug: 63928581
Test: atest HadamardTest
Change-Id: If022d3f4a8d6fccdf68119d4666f83ce5005bccb
diff --git a/rebootescrow/aidl/default/HadamardUtils.h b/rebootescrow/aidl/default/HadamardUtils.h
index 21cfc78..85e635f 100644
--- a/rebootescrow/aidl/default/HadamardUtils.h
+++ b/rebootescrow/aidl/default/HadamardUtils.h
@@ -18,9 +18,6 @@
#include <stdint.h>
-#include <bitset>
-#include <queue>
-#include <utility>
#include <vector>
namespace aidl {
@@ -29,18 +26,14 @@
namespace rebootescrow {
namespace hadamard {
-constexpr uint32_t CODE_K = 15;
+constexpr auto BYTE_LENGTH = 8u;
+constexpr auto CODEWORD_BYTES = 2u; // uint16_t
+constexpr auto CODEWORD_BITS = CODEWORD_BYTES * BYTE_LENGTH;
+constexpr uint32_t CODE_K = CODEWORD_BITS - 1;
constexpr uint32_t ENCODE_LENGTH = 1u << CODE_K;
-constexpr auto KEY_SIZE_IN_BYTES = 32u;
-
-// Encodes a 2 bytes word with hadamard code. The encoding expands a word of k+1 bits to a 2^k
-// bitset. Returns the encoded bitset.
-std::bitset<ENCODE_LENGTH> EncodeWord(uint16_t word);
-
-// Decodes the input bitset, and returns a sorted list of pair with (score, value). The value with
-// a higher score indicates a greater likehood.
-std::priority_queue<std::pair<int32_t, uint16_t>> DecodeWord(
- const std::bitset<ENCODE_LENGTH>& encoded);
+constexpr auto KEY_CODEWORDS = 16u;
+constexpr auto KEY_SIZE_IN_BYTES = KEY_CODEWORDS * CODEWORD_BYTES;
+constexpr auto OUTPUT_SIZE_BYTES = KEY_CODEWORDS * ENCODE_LENGTH / BYTE_LENGTH;
// Encodes a key that has a size of KEY_SIZE_IN_BYTES. Returns a byte array representation of the
// encoded bitset. So a 32 bytes key will expand to 16*(2^15) bits = 64KiB.
@@ -49,12 +42,6 @@
// Given a byte array representation of the encoded keys, decodes it and return the result.
std::vector<uint8_t> DecodeKey(const std::vector<uint8_t>& encoded);
-// Converts a bitset of length |ENCODE_LENGTH| to a byte array.
-std::vector<uint8_t> BitsetToBytes(const std::bitset<ENCODE_LENGTH>& encoded_bits);
-
-// Converts a byte array of encoded words back to the bitset.
-std::bitset<ENCODE_LENGTH> BytesToBitset(const std::vector<uint8_t>& encoded);
-
} // namespace hadamard
} // namespace rebootescrow
} // namespace hardware