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1 | /* -*- C++ -*- */ |
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2 | |
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3 | /** |
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4 | * @file bitindex.h |
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5 | * |
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6 | * Bit-access operations, including msb and lsb. |
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7 | * @author Emery Berger (emery@cs.umass.edu) |
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8 | */ |
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9 | |
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10 | // lsb is due to Leiserson, Prokop, Randall (MIT). |
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11 | // msb is also a fast implementation of floor(lg_2). |
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12 | |
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13 | #ifndef _BITINDEX_H_ |
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14 | #define _BITINDEX_H_ |
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15 | |
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16 | #include <assert.h> |
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17 | |
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18 | namespace HL { |
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19 | |
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20 | class BitIndex { |
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21 | private: |
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22 | |
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23 | enum { debruijn32 = 0x077CB531UL }; |
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24 | static int index32[32]; |
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25 | static unsigned long lgtable[16]; |
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26 | static unsigned long on[32]; |
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27 | static unsigned long off[32]; |
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28 | |
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29 | void setup (void); |
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30 | |
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31 | public: |
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32 | |
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33 | BitIndex (void); |
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34 | ~BitIndex (void) {} |
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35 | |
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36 | // Set bit_index in b (to 1). |
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37 | static void set (unsigned long &b, int index) |
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38 | { |
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39 | assert (index >= 0); |
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40 | assert (index < 32); |
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41 | b |= on[index]; |
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42 | } |
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43 | |
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44 | // Reset bit_index in b (set it to 0). |
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45 | static void reset (unsigned long &b, int index) |
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46 | { |
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47 | assert (index >= 0); |
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48 | assert (index < 32); |
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49 | b &= off[index]; |
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50 | } |
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51 | |
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52 | // Find the least-significant bit. |
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53 | static int lsb (unsigned long b) |
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54 | { |
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55 | //#if 0 |
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56 | #if 0 // i386 |
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57 | // Intel x86 code. |
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58 | register unsigned long index = 0; |
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59 | if (b > 0) { |
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60 | asm ("bsfl %1, %0" : "=r" (index) : "r" (b)); |
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61 | } |
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62 | return index; |
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63 | #else |
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64 | b &= (unsigned long) -((signed long) b); |
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65 | b *= debruijn32; |
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66 | b >>= 27; |
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67 | return index32[b]; |
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68 | #endif |
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69 | } |
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70 | |
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71 | // Find the most-significant bit. |
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72 | static int msb (unsigned long b) |
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73 | { |
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74 | #if 0 // i386 |
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75 | // Intel x86 code. |
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76 | register unsigned long index = 0; |
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77 | if (b > 0) { |
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78 | asm ("bsrl %1, %0" : "=r" (index) : "r" (b)); |
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79 | } |
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80 | return index; |
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81 | #else |
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82 | int l = 0; |
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83 | // b < 2^32 |
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84 | if (b >= 65536) { |
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85 | l += 16; |
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86 | b >>= 16; |
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87 | } |
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88 | // b < 2^16 |
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89 | if (b >= 256) { |
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90 | l += 8; |
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91 | b >>= 8; |
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92 | } |
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93 | // b < 2^8 |
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94 | if (b >= 16) { |
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95 | l += 4; |
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96 | b >>= 4; |
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97 | } |
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98 | return l + lgtable[b]; |
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99 | #endif |
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100 | } |
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101 | |
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102 | |
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103 | }; |
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104 | |
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105 | }; |
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106 | |
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107 | #endif |
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