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POGet Integer From Bits Inside `std::vector<char>`
 

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  1. POGet Integer From Bits Inside `std::vector<char>`
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    <p>I have a <code>vector&lt;char&gt;</code> and I want to be able to get an unsigned integer from a range of bits within the vector. E.g.</p> <p><img src="https://i.stack.imgur.com/q7N7u.png" alt="visualisation of bitvalues"></p> <p>And I can't seem to be able to write the correct operations to get the desired output. My intended algorithm goes like this:</p> <ul> <li><code>&amp;</code> the first byte with <code>(0xff &gt;&gt; unused bits in byte on the left)</code></li> <li><code>&lt;&lt;</code> the result left the number of output bytes * number of bits in a byte</li> <li><code>|</code> this with the final output</li> <li>For each subsequent byte: <ul> <li><code>&lt;&lt;</code> left by the (byte width - index) * bits per byte</li> <li><code>|</code> this byte with the final output</li> </ul></li> <li><code>|</code> the final byte (not shifted) with the final output</li> <li><code>&gt;&gt;</code> the final output by the number of unused bits in the byte on the right</li> </ul> <p>And here is my attempt at coding it, which does not give the correct result:</p> <pre><code>#include &lt;vector&gt; #include &lt;iostream&gt; #include &lt;cstdint&gt; #include &lt;bitset&gt; template&lt;class byte_type = char&gt; class BitValues { private: std::vector&lt;byte_type&gt; bytes; public: static const auto bits_per_byte = 8; BitValues(std::vector&lt;byte_type&gt; bytes) : bytes(bytes) { } template&lt;class return_type&gt; return_type get_bits(int start, int end) { auto byte_start = (start - (start % bits_per_byte)) / bits_per_byte; auto byte_end = (end - (end % bits_per_byte)) / bits_per_byte; auto byte_width = byte_end - byte_start; return_type value = 0; unsigned char first = bytes[byte_start]; first &amp;= (0xff &gt;&gt; start % 8); return_type first_wide = first; first_wide &lt;&lt;= byte_width; value |= first_wide; for(auto byte_i = byte_start + 1; byte_i &lt;= byte_end; byte_i++) { auto byte_offset = (byte_width - byte_i) * bits_per_byte; unsigned char next_thin = bytes[byte_i]; return_type next_byte = next_thin; next_byte &lt;&lt;= byte_offset; value |= next_byte; } value &gt;&gt;= (((byte_end + 1) * bits_per_byte) - end) % bits_per_byte; return value; } }; int main() { BitValues&lt;char&gt; bits(std::vector&lt;char&gt;({'\x78', '\xDA', '\x05', '\x5F', '\x8A', '\xF1', '\x0F', '\xA0'})); std::cout &lt;&lt; bits.get_bits&lt;unsigned&gt;(15, 29) &lt;&lt; "\n"; return 0; } </code></pre> <p>(In action: <a href="http://coliru.stacked-crooked.com/a/261d32875fcf2dc0" rel="nofollow noreferrer">http://coliru.stacked-crooked.com/a/261d32875fcf2dc0</a>)</p> <p>I just can't seem to wrap my head around these bit manipulations, and I find debugging very difficult! If anyone can correct the above code, or help me in any way, it would be much appreciated! </p> <p>Edit:</p> <ul> <li>My bytes are 8 bits long</li> <li>The integer to return could be 8,16,32 or 64 bits wside</li> <li>The integer is stored in big endian</li> </ul>
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