# Some Binary Bit Extraction Utilities

Aug 8th, 2018
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1. // Function to extract a series of bits from a value.
2. // Params: 'num_bits' bits from 'init_pos' position (e.g., pos 8, 8 bits, pos 16, 8m bits, pos 24 8 bits.
3. // Returns the extracted value as int.
4. // Signed values may have undefined behaviour based on endian-ness, so we use abs(). -Z
5. int extract_bits(int base_val, int num_bits, int init_pos)
6. {
7.     unsigned int tempVal = abs(base_val); //Disregard sign.
8.     return ( ( (1 << num_bits) - 1 ) & ( tempVal >> (init_pos - 1) ) );
9. }
10.
11. //int foo 11110010001100111100010000110100; //11110010 00110011 11000100 00110100
12. //int lowestEight = bitExtract(foo,8,8); //00110100
13. //int secondEight = bitExtract(foo,8,16); //11000100
14.
15. // I don't know if any of these have specific merit beyond the above. -Z
16.
17. unsigned short extract(unsigned short value, int begin, int end)
18. {
19.     unsigned short mask = (1 << (end - begin)) - 1;
20.     return (value >> begin) & mask;
21. }
22.
23. int extractBit(int byte, int pos)
24. {
25.     if( !((pos >= 0) && (pos < 16)) )
26.     {
27.         return 0;
28.     }
29.     return ( ( byte & (1<<pos) ) >> pos);
30. }
31.
32. uint32_t getBits( uint32_t allbits, int lsb, int count )
33. {
34.     uint32_t mask = ~(0xffffffff << count) << lsb ;
35.     return (allbits & mask) >> lsb ;
36. }
37.
38. uint32_t getBits( uint32_t allbits, int lsb, int msb )
39. {
40.     uint32_t mask = ~(0xffffffff << (msb + 1 - lsb)) << lsb ;
41.     return (allbits & mask) >> lsb ;
42. }
43.
44.
45. int extractBitRange(int byte, int startingPos, int offset)
46. {
47.
48.
49.    if(  !(((startingPos + offset) >= 0) && ( (startingPos + offset) < 16)) )
50.    {
51.         return 0;
52.    }
53.    return ( byte >> startingPos ) & ~(0xff << (offset + 1));
54. }
55.
56. unsigned extract(unsigned num, unsigned hi, unsigned lo) {
57.     uint32_t range = (hi - lo + 1);  //number of bits to be extracted
58.     //shifting a number by the number of bits it has produces inconsistent
59.     //results across machines so we need a special case for extract(num, 31, 0)
60.     if(range == 32)
61.         return num;
62.     uint32_t result = 0;
63.     //following the rule above, ((1 << x) - 1) << y) makes the mask:
64.     uint32_t mask = ((1 << range) -1) << lo;
65.     //AND num and mask to get only the bits in our range
66.     result = num & mask;
67.     result = result >> lo; //gets rid of trailing 0s
68.     return result;
69. }
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