Delphi compatible LCG Random: what changed

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Old	New		Line
1	1		### Delphi compatible random numbers
2	2
3	3		Many Free Pascal programmers also maintain sourcecode in Delphi.\
4	4		Even if you have moved to Free Pascal from Delphi you may have data that relies on Delphi's Random.\
5	5		Here are cross-platform functions that generate Delphi-identical pseudo-random numbers given the same RandSeed:
6	6
7	7		``` pascal
8	8		unit lcg_random;
9	9		// Delphi compatible LCG random number generator routines for Free Pascal.
10	10		// (c)2017, Thaddy de Koning. Use as you like
11	11		// Algorithm, Delphi multiplier and increment taken from:
12	12		// https://en.wikipedia.org/wiki/Linear_congruential_generator
13	13		// The default Delphi RandomSeed is determined as zero.
14	14		{$ifdef fpc}{$mode objfpc}{$endif}
15	15
16	16		interface
17	17
18	18		function LCGRandom: extended; overload;inline;
19	19		function LCGRandom(const range:longint):longint;overload;inline;
20	20
21	21		implementation
22	22
23	23		function IM:cardinal;inline;
24	24		begin
25	25		RandSeed := RandSeed * 134775813 + 1;
26	26		Result := RandSeed;
27	27		end;
28	28
29	29		function LCGRandom: extended; overload;inline;
30	30		begin
31	31		Result := IM * 2.32830643653870e-10;
32	32		end;
33	33
34	34		function LCGRandom(const range:longint):longint;overload;inline;
35	35		begin
36	36		Result := IM * range shr 32;
37	37		end;
38	38
39	39		end.
40	40		```
41	41
42		-	\
43	42		A small demo
44	43
45	44		``` pascal
46	45		program lcgdemo;
47	46		// Compile and run in both Delphi and FPC and compare the output.
48	47		// It is exactly the same
49	48		{$ifdef fpc}{$mode objfpc}
50	49		{$Macro on}{$define random := LCGRandom}
51	50		{$endif}
52	51		{$ifdef mswindows}{$apptype console}{$endif}
53	52		uses lcg_random;
54	53
55	54		var i:integer;
56	55		begin
57	56		RandSeed := 999; // default delphi randseed is zero
58	57		for i := 1 to 20 do
59	58		begin
60	59		write(Random(100):4); // Delphi: Random FPC:LCGRandom. See macro
61	60		if i mod 5 = 0 then writeln;
62	61		end;
63	62
64	63		RandSeed := 999; // default delphi randseed is zero
65	64		for i := 1 to 20 do
66	65		begin
67	66		write(Random:4:8,' ');// Delphi: Random FPC:LCGRandom
68	67		if i mod 5 = 0 then writeln;
69	68		end;
70	69		Readln;
71	70		end.
72	71		```
73	72
74	73		Here is another one with the randseed factored out. You can use that for non-delphi compatible purposes:
75	74
76	75		``` pascal
77	76		unit lcg_random2;
78	77		// Delphi compatible LCG random number generator routines for Freepascal.
79	78		// (c)2017, Thaddy de Koning. Use as you like
80	79		// Algorithm, Delphi multiplier and increment taken from:
81	80		// https://en.wikipedia.org/wiki/Linear_congruential_generator
82	81		// The default Delphi RandomSeed is determined as zero.
83	82		//
84	83		// In this version the system randseed is factored out.
85	84		{$ifdef fpc}{$mode objfpc}{$endif}{$J+}
86	85
87	86		interface
88	87		const
89	88		RandSd:cardinal = 0;
90	89
91	90		function LCGRandom: extended; overload;inline;
92	91		function LCGRandom(const range:longint):longint;overload;inline;
93	92
94	93		implementation
95	94
96	95		function IM:cardinal;inline;
97	96		begin
98	97		RandSd := RandSd * 134775813 + 1;
99	98		Result := RandSd;
100	99		end;
101	100
102	101		function LCGRandom: extended; overload;inline;
103	102		begin
104	103		Result := IM * 2.32830643653870e-10;
105	104		end;
106	105
107	106		function LCGRandom(const range:longint):longint;overload;inline;
108	107		begin
109	108		Result := IM * range shr 32;
110	109		end;
111	110
112	111		end.
113	112		```
114	113
115	114		You can use this for example for simple encryption and decryption based on the predictability and repeatability of a any Pseudo Random Number Generator:
116	115
117	116		``` pascal
118	117		program cryptrandom;
119	118		{$mode objfpc}{$modeswitch typehelpers}{$H+}
120	119		{ This is demo that you can actually use random to encrypt/decrypt.
121	120		This is due to the nature of a Pseudo Random Number Generator,
122	121		where given a certain random seed, the sequence is predictable.
123	122		Here I use the Random Seed as a key to encrypt/decrypt a string.
124	123		The same code would also work with the Default random from FreePascal,
125	124		but it is better not to touch system.randseed, So I used a different
126	125		Random.
127	126
128	127		Have fun, Thaddy}
129	128		uses
130	129		lcg_random2;
131	130
132	131
133	132		function Crypt(const value:AnsiString;const Key:Cardinal):AnsiString;
134	133		var
135	134		i:integer;
136	135		begin
137	136		RandSd := Key;
138	137		SetLength(Result,Length(Value));
139	138		for i := 1 to length(Value) do
140	139		Result[i] := Chr(Ord(Value[i]) xor lcgrandom(255));
141	140		end;
142	141
143	142		var
144	143		a:AnsiString;
145	144		begin
146	145		a:='This is the text to encrypt/decrypt.';
147	146		writeln(a);
148	147		writeln('Encrypting..');
149	148		a:=Crypt(a, 12345);
150	149		writeln(a);
151	150		writeln('Decrypting..');
152	151		a:=Crypt(a, 12345);
153	152		writeln(a);
154	153		end.
155	154		```
156	155
157	156		### See also
158	157
159	158		1. [LCG on wikipedia](https://en.wikipedia.org/wiki/Linear_congruential_generator)
160	159		2. [[Generating Random Numbers]]
161	160
162	161		## See also
163	162
164	163		- [Wikipedia article on the linear congruential generator algorithm](http://en.wikipedia.org/wiki/Linear_congruential_generator)
165	164
166	165		---
167	166		From the Free Pascal wiki, page "Delphi compatible LCG Random", as it stood on 2024-05-14 after a change by Alextp, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).
168	167		Category: Modelling and Simulation
169	168		Category: Statistical algorithms

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