Partially implemented BitStream class - only write() so far.

libuavcan/src/marshalling/bit_array_copy.c

@@ -0,0 +1,119 @@
+/*
+ * Fast bit array copy algorithm.
+ * Source: http://stackoverflow.com/questions/3534535/whats-a-time-efficient-algorithm-to-copy-unaligned-bit-arrays
+ */
+
+#include <limits.h>
+#include <string.h>
+#include <stddef.h>
+
+#define PREPARE_FIRST_COPY()                                      \
+    do {                                                          \
+    if (src_len >= (CHAR_BIT - dst_offset_modulo)) {              \
+        *dst     &= reverse_mask[dst_offset_modulo];              \
+        src_len -= CHAR_BIT - dst_offset_modulo;                  \
+    } else {                                                      \
+        *dst     &= reverse_mask[dst_offset_modulo]               \
+              | reverse_mask_xor[dst_offset_modulo + src_len + 1];\
+         c       &= reverse_mask[dst_offset_modulo + src_len    ];\
+        src_len = 0;                                              \
+    } } while (0)
+
+
+void
+bitarray_copy(const unsigned char *src_org, int src_offset, int src_len,
+                    unsigned char *dst_org, int dst_offset)
+{
+    static const unsigned char reverse_mask[] =
+        { 0x55, 0x80, 0xc0, 0xe0, 0xf0, 0xf8, 0xfc, 0xfe, 0xff };
+    static const unsigned char reverse_mask_xor[] =
+        { 0xff, 0x7f, 0x3f, 0x1f, 0x0f, 0x07, 0x03, 0x01, 0x00 };
+
+    if (src_len > 0) {
+        const unsigned char *src;
+              unsigned char *dst;
+        int                  src_offset_modulo,
+                             dst_offset_modulo;
+
+        src = src_org + (src_offset / CHAR_BIT);
+        dst = dst_org + (dst_offset / CHAR_BIT);
+
+        src_offset_modulo = src_offset % CHAR_BIT;
+        dst_offset_modulo = dst_offset % CHAR_BIT;
+
+        if (src_offset_modulo == dst_offset_modulo) {
+            int              byte_len;
+            int              src_len_modulo;
+            if (src_offset_modulo) {
+                unsigned char   c;
+
+                c = reverse_mask_xor[dst_offset_modulo]     & *src++;
+
+                PREPARE_FIRST_COPY();
+                *dst++ |= c;
+            }
+
+            byte_len = src_len / CHAR_BIT;
+            src_len_modulo = src_len % CHAR_BIT;
+
+            if (byte_len) {
+                memcpy(dst, src, byte_len);
+                src += byte_len;
+                dst += byte_len;
+            }
+            if (src_len_modulo) {
+                *dst     &= reverse_mask_xor[src_len_modulo];
+                *dst |= reverse_mask[src_len_modulo]     & *src;
+            }
+        } else {
+            int             bit_diff_ls,
+                            bit_diff_rs;
+            int             byte_len;
+            int             src_len_modulo;
+            unsigned char   c;
+            /*
+             * Begin: Line things up on destination.
+             */
+            if (src_offset_modulo > dst_offset_modulo) {
+                bit_diff_ls = src_offset_modulo - dst_offset_modulo;
+                bit_diff_rs = CHAR_BIT - bit_diff_ls;
+
+                c = *src++ << bit_diff_ls;
+                c |= *src >> bit_diff_rs;
+                c     &= reverse_mask_xor[dst_offset_modulo];
+            } else {
+                bit_diff_rs = dst_offset_modulo - src_offset_modulo;
+                bit_diff_ls = CHAR_BIT - bit_diff_rs;
+
+                c = *src >> bit_diff_rs     &
+                    reverse_mask_xor[dst_offset_modulo];
+            }
+            PREPARE_FIRST_COPY();
+            *dst++ |= c;
+
+            /*
+             * Middle: copy with only shifting the source.
+             */
+            byte_len = src_len / CHAR_BIT;
+
+            while (--byte_len >= 0) {
+                c = *src++ << bit_diff_ls;
+                c |= *src >> bit_diff_rs;
+                *dst++ = c;
+            }
+
+            /*
+             * End: copy the remaing bits;
+             */
+            src_len_modulo = src_len % CHAR_BIT;
+            if (src_len_modulo) {
+                c = *src++ << bit_diff_ls;
+                c |= *src >> bit_diff_rs;
+                c     &= reverse_mask[src_len_modulo];
+
+                *dst     &= reverse_mask_xor[src_len_modulo];
+                *dst |= c;
+            }
+        }
+    }
+}

libuavcan/src/marshalling/bit_stream.cpp

@@ -0,0 +1,62 @@
+/*
+ * Copyright (C) 2014 Pavel Kirienko <pavel.kirienko@gmail.com>
+ */
+
+#include <sstream>
+#include <uavcan/internal/marshalling/bit_stream.hpp>
+
+namespace uavcan
+{
+
+int BitStream::write(const uint8_t* bytes, const int bitlen)
+{
+    // Temporary buffer is needed to merge new bits with cached unaligned bits from the last write() (see byte_cache_)
+    uint8_t tmp[MAX_BYTES_PER_RW + 1];
+
+    // Tmp space must be large enough to accomodate new bits AND unaligned bits from the last write()
+    const int bytelen = bitlenToBytelen(bitlen + (bit_offset_ % 8));
+    assert(MAX_BYTES_PER_RW >= bytelen);
+    tmp[0] = tmp[bytelen - 1] = 0;
+
+    copyBitArray(bytes, 0, bitlen, tmp, bit_offset_ % 8);
+
+    const int new_bit_offset = bit_offset_ + bitlen;
+
+    // Bitcopy algorithm resets skipped bits in the first byte. Restore them back.
+    tmp[0] |= byte_cache_;
+
+    // (new_bit_offset % 8 == 0) means that this write was perfectly aligned.
+    byte_cache_ = (new_bit_offset % 8) ? tmp[bytelen - 1] : 0;
+
+    /*
+     * Dump the data into the destination buffer.
+     * Note that if this write was unaligned, last written byte in the buffer will be rewritten with updated value
+     * within the next write() operation.
+     */
+    const int write_res = buf_.write(bit_offset_ / 8, tmp, bytelen);
+    bit_offset_ = new_bit_offset;
+
+    if (write_res < 0)
+        return write_res;
+    return (write_res == bytelen) ? 0 : -1;
+}
+
+std::string BitStream::toString() const
+{
+    std::ostringstream os;
+    for (int offset = 0; true; offset++)
+    {
+        uint8_t byte = 0;
+        if (1 != buf_.read(offset, &byte, 1))
+            break;
+        for (int i = 7; i >= 0; i--)     // Most significant goes first
+            os << !!(byte & (1 << i));
+        os << " ";
+    }
+    std::string output = os.str();
+    if (output.length())
+        output.erase(output.length() - 1, 1);
+    return output;
+}
+
+}