Author: William Gerbert
Are 2 files identical? Do a 32bit CRC-sum..
Answer:
To determine, whether 2 files are identical, I first would compare their sizes.
If they are identical, I would calculate a 32bit CRC sum for each of them and
compare the results.
It is extremely unlikely that the files are different but have the same CRC.
Someone asked me:
"Are you sure? Take for example two files, where the only difference would be that
in one of them, I turned a "hello" into a "ehllo". I guess the checksum would be
the same, don't you think?"
Answer: No, a CRC might even tell you where the difference was (if the files are
really that short).
The following function does compute such a 32bit CRC (cyclic redundancy check)
value:
1
2 function ComputeFileCRC32(const FileName: string): LongInt;
3
4
5 to compare, use something like this:
6
7
8 function Ident (const fn1, fn2 : string) : boolean;
9 begin
10 Ident := ComputeFileCRC32(fn1) = ComputeFileCRC32(fn2);
11 end;
12
13
14 unit Crc32;
15
16 interface
17
18 uses
19 Dialogs,
20 SysUtils,
21 WinTypes,
22 WinProcs;
23
24 function UpdateCRC32(InitCRC: LongInt; Buffer: Pointer; BufferLength: WORD):
25 LongInt;
26 function ComputeFileCRC32(const FileName: string): LongInt;
27
28 implementation
29
30 type
31 CRCTable = array[0..255] of LongInt;
32
33 const
34 BufferLength = 16384;
35 CRC32Table: CRCTable = (
36 $000000000, $077073096, $0ee0e612c, $0990951ba,
37 $0076dc419, $0706af48f, $0e963a535, $09e6495a3,
38 $00edb8832, $079dcb8a4, $0e0d5e91e, $097d2d988,
39 $009b64c2b, $07eb17cbd, $0e7b82d07, $090bf1d91,
40
41 $01db71064, $06ab020f2, $0f3b97148, $084be41de,
42 $01adad47d, $06ddde4eb, $0f4d4b551, $083d385c7,
43 $0136c9856, $0646ba8c0, $0fd62f97a, $08a65c9ec,
44 $014015c4f, $063066cd9, $0fa0f3d63, $08d080df5,
45
46 $03b6e20c8, $04c69105e, $0d56041e4, $0a2677172,
47 $03c03e4d1, $04b04d447, $0d20d85fd, $0a50ab56b,
48 $035b5a8fa, $042b2986c, $0dbbbc9d6, $0acbcf940,
49 $032d86ce3, $045df5c75, $0dcd60dcf, $0abd13d59,
50
51 $026d930ac, $051de003a, $0c8d75180, $0bfd06116,
52 $021b4f4b5, $056b3c423, $0cfba9599, $0b8bda50f,
53 $02802b89e, $05f058808, $0c60cd9b2, $0b10be924,
54 $02f6f7c87, $058684c11, $0c1611dab, $0b6662d3d,
55
56 $076dc4190, $001db7106, $098d220bc, $0efd5102a,
57 $071b18589, $006b6b51f, $09fbfe4a5, $0e8b8d433,
58 $07807c9a2, $00f00f934, $09609a88e, $0e10e9818,
59 $07f6a0dbb, $0086d3d2d, $091646c97, $0e6635c01,
60
61 $06b6b51f4, $01c6c6162, $0856530d8, $0f262004e,
62 $06c0695ed, $01b01a57b, $08208f4c1, $0f50fc457,
63 $065b0d9c6, $012b7e950, $08bbeb8ea, $0fcb9887c,
64 $062dd1ddf, $015da2d49, $08cd37cf3, $0fbd44c65,
65
66 $04db26158, $03ab551ce, $0a3bc0074, $0d4bb30e2,
67 $04adfa541, $03dd895d7, $0a4d1c46d, $0d3d6f4fb,
68 $04369e96a, $0346ed9fc, $0ad678846, $0da60b8d0,
69 $044042d73, $033031de5, $0aa0a4c5f, $0dd0d7cc9,
70
71 $05005713c, $0270241aa, $0be0b1010, $0c90c2086,
72 $05768b525, $0206f85b3, $0b966d409, $0ce61e49f,
73 $05edef90e, $029d9c998, $0b0d09822, $0c7d7a8b4,
74 $059b33d17, $02eb40d81, $0b7bd5c3b, $0c0ba6cad,
75
76 $0edb88320, $09abfb3b6, $003b6e20c, $074b1d29a,
77 $0ead54739, $09dd277af, $004db2615, $073dc1683,
78 $0e3630b12, $094643b84, $00d6d6a3e, $07a6a5aa8,
79 $0e40ecf0b, $09309ff9d, $00a00ae27, $07d079eb1,
80
81 $0f00f9344, $08708a3d2, $01e01f268, $06906c2fe,
82 $0f762575d, $0806567cb, $0196c3671, $06e6b06e7,
83 $0fed41b76, $089d32be0, $010da7a5a, $067dd4acc,
84 $0f9b9df6f, $08ebeeff9, $017b7be43, $060b08ed5,
85
86 $0d6d6a3e8, $0a1d1937e, $038d8c2c4, $04fdff252,
87 $0d1bb67f1, $0a6bc5767, $03fb506dd, $048b2364b,
88 $0d80d2bda, $0af0a1b4c, $036034af6, $041047a60,
89 $0df60efc3, $0a867df55, $0316e8eef, $04669be79,
90
91 $0cb61b38c, $0bc66831a, $0256fd2a0, $05268e236,
92 $0cc0c7795, $0bb0b4703, $0220216b9, $05505262f,
93 $0c5ba3bbe, $0b2bd0b28, $02bb45a92, $05cb36a04,
94 $0c2d7ffa7, $0b5d0cf31, $02cd99e8b, $05bdeae1d,
95
96 $09b64c2b0, $0ec63f226, $0756aa39c, $0026d930a,
97 $09c0906a9, $0eb0e363f, $072076785, $005005713,
98 $095bf4a82, $0e2b87a14, $07bb12bae, $00cb61b38,
99 $092d28e9b, $0e5d5be0d, $07cdcefb7, $00bdbdf21,
100
101 $086d3d2d4, $0f1d4e242, $068ddb3f8, $01fda836e,
102 $081be16cd, $0f6b9265b, $06fb077e1, $018b74777,
103 $088085ae6, $0ff0f6a70, $066063bca, $011010b5c,
104 $08f659eff, $0f862ae69, $0616bffd3, $0166ccf45,
105
106 $0a00ae278, $0d70dd2ee, $04e048354, $03903b3c2,
107 $0a7672661, $0d06016f7, $04969474d, $03e6e77db,
108 $0aed16a4a, $0d9d65adc, $040df0b66, $037d83bf0,
109 $0a9bcae53, $0debb9ec5, $047b2cf7f, $030b5ffe9,
110
111 $0bdbdf21c, $0cabac28a, $053b39330, $024b4a3a6,
112 $0bad03605, $0cdd70693, $054de5729, $023d967bf,
113 $0b3667a2e, $0c4614ab8, $05d681b02, $02a6f2b94,
114 $0b40bbe37, $0c30c8ea1, $05a05df1b, $02d02ef8d);
115
116 var
117 Buffer: array[1..BufferLength] of Byte;
118
119 {$ifndef WIN32}
120 type
121 DWORD = longint;
122 {$endif}
123
124 function UpdateCRC32(InitCRC: LongInt; Buffer: Pointer; BufferLength: WORD):
125 LongInt;
126 var
127 crc: LongInt;
128 index: integer;
129 i: integer;
130 begin
131 crc := InitCRC;
132 for i := 0 to BufferLength-1 do
133 begin
134 index := (crc xor Integer(Pointer(DWORD(Buffer)+i)^)) and $000000FF;
135 crc := ((crc shr 8) and $00FFFFFF) xor CRC32Table[index];
136 end;
137 Result := crc;
138 end;
139
140 function ComputeFileCRC32(const FileName : string): LongInt;
141 var
142 InputFile: file;
143 Crc32: LongInt;
144 ResultLength: Integer;
145 BufPtr: Pointer;
146 begin
147 BufPtr := @Buffer;
148 Assign(InputFile, FileName);
149 Reset(InputFile, 1);
150 Crc32 := $FFFFFFFF; { 32 bit crc starts with all bits on }
151 repeat
152 BlockRead(InputFile, Buffer, BufferLength, ResultLength);
153 Crc32 := UpdateCrc32(Crc32, BufPtr, ResultLength);
154 until Eof(InputFile);
155 Close(InputFile);
156 Crc32 := not(Crc32); { Finish 32 bit crc by inverting all bits }
157 Result := CRC32;
158 end;
159
160 end.
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