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    <p>Let me try - how about:</p> <pre><code>class bitIterator{ unsigned char* farray; int foffset; public: bitIterator(unsigned char* array, int offset) :farray(array), foffset(offset) {} bool operator*() const { return (farray[foffset/8] &gt;&gt; (7 - foffset%8)) &amp; 1; } bitIterator&amp; operator++(){ foffset++; return *this; } int offset() const { return foffset; } }; // Just to demonstrate how to call it int find_first_n(unsigned char* buffer, int length, int n){ return std::search_n(bitIterator(buffer, 0), bitIterator(buffer, length*8), n, 0).offset(); } </code></pre> <p>That was just for fun...</p> <p>Now if you really want to squeeze some performance out of it, I will suggset</p> <pre><code>int find_first_n(unsigned char* buffer, int length, int n){ int prev_trail = 0; unsigned int* buf = reinterpret_cast&lt;unsigned int*&gt;(buffer); int len = length/sizeof(int); // Last few bytes will need special treatment if your array is not multple of int-size length. // However last bytes should not influence overall performance of code - assuming it is used on rather long arrays. // If you plan using rather short arrays (20-50 bytes?) you will probably be better off just using plain char. for (int i=0; i&lt;len; i++){ if (!buf[i]) return i*sizeof(int)*8-prev_trail; // __builtin_clz and __builtin_ctz are undefined on zero ; // EDIT: if (!~buf[i]){ prev_trail = 0; continue; } // END EDIT! int shft = __builtin_clz(buf[i]); if (shft + prev_trail &gt;= n) return i*sizeof(int)*8-prev_trail; // Leading + previous trailing &lt;= n // Not enough leading zeros, continue search. prev_trail = __builtin_ctz(buf[i]); // Store it to use for next int in array unsigned int tmp =0; while(shft &lt; sizeof(int)*8-prev_trail){ // While we haven't got to trailing zeros in this uint tmp = buf[i] &lt;&lt; shft; // Shift-out leading zeros; shft += (__builtin_clz(~tmp)); tmp = buf[i] &lt;&lt; shft; // Shift-out leading ones; lead_zeros = __builtin_clz(tmp); if (lead_zeros &gt;= n) // and see if have enough leading zeros. return i*sizeof(int)*8+shft; shft += lead_zeros; } return length*8; // Not found :( } </code></pre> <p>This is rather difficult to read, but concept is simple - just iterate over every int-sized block, and for each see if leading zeros + trailing zeros from previous one are >= n. If not just repeatedly shift-out leading zeros and following ones (set bits), and check again for trailing zeroes >= n, as long as we didn't got to trailing bytes.</p> <p>And just one more idea:</p> <pre><code>int find_first_n(unsigned char* buffer, int length, int n){ union { unsigned char chr[2]; unsigned int uit; }; unsigned int onemask = 0x8000; for (int i = 1 ; i&lt;n; i++) onemask = onemask | (onemask &gt;&gt; 1); int* masks = new int[8]; for (int i = 0; i&lt;8; i++){ masks[i]=onemask &gt;&gt; i; } // generating masks - eg. for n == 3 would be: // [ 1110 0000 0000 0000 ] // [ 0111 0000 0000 0000 ] // [ 0011 1000 0000 0000 ] // [ 0001 1100 0000 0000 ] // [ ... ] // [ 0000 0001 1100 0000 ] uit = 0; for (int i=0; i&lt;length-1; i++){ chr[0] = buffer[i]; chr[1] = buffer[i+1]; for (int j=0; j&lt;8; j++){ if (!(uit &amp; masks[j])) return i*8+j; } } chr[0] = buffer[length-1]; chr[1] = 0xff; // Fill with ones at the end. for (int j=0; j&lt;8; j++){ if (!(uit &amp; masks[j])) return (length-1)*8+j; } return length*8; // Not found :( } </code></pre> <p>You might be tempted to cast pointer to variables still in buffer directly to word (<code>reinterpret_cast&lt;unsigned short int*&gt;(buffer[i])</code>) and apply masks without using union. Note however that half of such operations would use unaligned variables, which can degrade performance, and on some platforms even generate exceptions.</p>
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    1. POFastest way to find up to 6 consecutive 0 bits in a char array
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    1. COsizeof(int) is probably 4, do you mean sizeof(int)*CHAR_BIT? What if buf[i]==0xFFFFFFFF, wouldn't __builtin_clz(~tmp) be undefined?
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    2. CO@aka.nice To be exact it should be `sizeof(int)*8`. Size of character is irrelevant; we are counting in bytes (as `sizeof` does). I also edited condition for `buf[i]==0xFFFFFFFF`.
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