mdw@git.distorted.org.uk Git - ssr/blob - StraySrc/Libraries/Sapphire/s/rand

   1 ;
   2 ; rand.s
   3 ;
   4 ; Generating random numbers
   5 ;
   6 ; © 1994-1998 Straylight
   7 ;
   8
   9 ;----- Licensing note -------------------------------------------------------
  10 ;
  11 ; This file is part of Straylight's Sapphire library.
  12 ;
  13 ; Sapphire is free software; you can redistribute it and/or modify
  14 ; it under the terms of the GNU General Public License as published by
  15 ; the Free Software Foundation; either version 2, or (at your option)
  16 ; any later version.
  17 ;
  18 ; Sapphire is distributed in the hope that it will be useful,
  19 ; but WITHOUT ANY WARRANTY; without even the implied warranty of
  20 ; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  21 ; GNU General Public License for more details.
  22 ;
  23 ; You should have received a copy of the GNU General Public License
  24 ; along with Sapphire.  If not, write to the Free Software Foundation,
  25 ; 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  26
  27 ;----- Standard header ------------------------------------------------------
  28
  29                 GET     libs:header
  30                 GET     libs:swis
  31
  32 ;----- External dependencies ------------------------------------------------
  33
  34                 GET     sapphire:divide
  35                 GET     sapphire:sapphire
  36
  37 ;----- Main code ------------------------------------------------------------
  38
  39                 AREA    |Sapphire$$Code|,CODE,READONLY
  40
  41 ; --- rand ---
  42 ;
  43 ; On entry:     --
  44 ;
  45 ; On exit:      R0 == a pseudorandom number between 0 and &7FFFFFFF
  46 ;
  47 ; Use:          Returns a pseudorandom number.  The algorithm used is the
  48 ;               additive generator found in Knuth.  The table is generated
  49 ;               from the current monotonic time using a linear congruential
  50 ;               generator.  Randomness is fairly good, and it's very quick.
  51
  52                 EXPORT  rand
  53 rand            ROUT
  54
  55                 STMFD   R13!,{R1-R3,R12,R14}    ;Save some registers
  56                 WSPACE  rand__wSpace            ;Find my workspace address
  57                 LDMIB   R12,{R1,R2}             ;Load the table indices
  58                 ADR     R3,rand__table          ;Find the ring buffer
  59                 LDR     R0,[R3,R1,LSL #2]       ;Load the value x[n-24]
  60                 LDR     R14,[R3,R2,LSL #2]      ;Load the value x[n-55]
  61                 ADD     R0,R0,R14               ;And form the new number
  62                 STR     R0,[R3,R2,LSL #2]       ;Store new number in buffer
  63                 SUBS    R1,R1,#1                ;Decrement x-24 index
  64                 MOVLT   R1,#54                  ;If too small, wrap it round
  65                 SUBS    R2,R2,#1                ;Decrement x-55 index
  66                 MOVLT   R2,#54                  ;If too small, wrap it round
  67                 STMIB   R12,{R1,R2}             ;Save the indices back again
  68                 BIC     R0,R0,#&80000000        ;Clear the top bit (positive)
  69                 LDMFD   R13!,{R1-R3,R12,PC}^    ;Return to caller
  70
  71                 LTORG
  72
  73 ; --- rand_setSeed ---
  74 ;
  75 ; On entry:     R0 == a pseudorandom seed
  76 ;
  77 ; On exit:      --
  78 ;
  79 ; Use:          Sets up the random number generator (rand) to the given start
  80 ;               position.  The table of values is initialised from the seed
  81 ;               in a psuedorandom manner, using a linear congruential
  82 ;               generator.
  83
  84                 EXPORT  rand_setSeed
  85 rand_setSeed    ROUT
  86
  87                 STMFD   R13!,{R0-R2,R12,R14}    ;Save some registers
  88                 WSPACE  rand__wSpace            ;Find my workspace address
  89
  90                 ; --- Reset the indices ---
  91
  92                 MOV     R1,#23                  ;Start 24-index at 24-1
  93                 MOV     R2,#54                  ;Start 55-index at 55-1
  94                 STMIB   R12,{R1,R2}             ;Save them away
  95
  96                 ; --- Now start populating the table ---
  97
  98                 ADR     R1,rand__table          ;Point to the ring buffer
  99                 MOV     R2,#55                  ;Counter for the items
 100
 101 00rand_setSeed  ADD     R14,R0,R0,LSR #16       ;Mangle the seed a little
 102                 STR     R14,[R1],#4             ;Save it in the table
 103
 104                 ADD     R14,R0,R0,LSL #2        ;Do multiply op in generator
 105                 RSB     R14,R14,R14,LSL #4
 106                 RSB     R0,R0,R14,LSL #2
 107                 RSB     R0,R0,R0,LSL #3
 108                 ADD     R0,R0,R0,LSL #5
 109
 110                 ADD     R0,R0,#&66000000        ;Do add op in generator
 111                 ADD     R0,R0,#&00D60000
 112                 ADD     R0,R0,#&00001900
 113                 ADD     R0,R0,#&000000E1
 114
 115                 SUBS    R2,R2,#1                ;Decrement the counter
 116                 BGT     %00rand_setSeed         ;If not finished, go again
 117
 118                 LDMFD   R13!,{R0-R2,R12,PC}^    ;Return to caller
 119
 120                 LTORG
 121
 122 ; --- rnd ---
 123 ;
 124 ; On entry:     R0 == start value (inclusive)
 125 ;               R1 == end value (inclusive)
 126 ;
 127 ; On exit:      R0 == random number between the boundaries given
 128 ;
 129 ; Use:          Returns a random integer between the boundaries given.
 130 ;               The distribution is slightly skewed towards lower numbers,
 131 ;               but there's not a lot I can do about this, folks.
 132
 133                 EXPORT  rnd
 134 rnd             ROUT
 135
 136                 STMFD   R13!,{R1,R2,R14}        ;Save a bunch of registers
 137                 MOV     R2,R0                   ;Look after the start value
 138                 SUB     R1,R1,R0                ;Subtract the initial value
 139                 ADD     R1,R1,#1                ;Make it inclusive nicely
 140                 BL      rand                    ;Get a random number
 141                 BL      divide                  ;Force it to be in range
 142                 ADD     R0,R1,R2                ;And add the start value
 143                 LDMFD   R13!,{R1,R2,PC}^        ;Return to caller
 144
 145                 LTORG
 146
 147 ; --- rand_init ---
 148 ;
 149 ; On entry:     --
 150 ;
 151 ; On exit:      --
 152 ;
 153 ; Use:          Initialise the random number table.
 154
 155                 EXPORT  rand_init
 156 rand_init       ROUT
 157
 158                 STMFD   R13!,{R12,R14}          ;Save a register or two
 159                 WSPACE  rand__wSpace            ;Find my workspace
 160                 LDR     R14,rand__flags         ;Load my flags word
 161                 TST     R14,#rFlag__inited      ;Am I initialised yet?
 162                 LDMNEFD R13!,{R12,PC}^          ;Yes -- don't do it again
 163
 164                 ORR     R14,R14,#rFlag__inited  ;Set the flag then
 165                 STR     R14,rand__flags         ;And save back the new flags
 166                 STMFD   R13!,{R0}               ;Save another register
 167                 SWI     OS_ReadMonotonicTime    ;Load the current time
 168                 BL      rand_setSeed            ;Set up the random table
 169                 LDMFD   R13!,{R0,R12,PC}^       ;And return to caller
 170
 171                 LTORG
 172
 173 rand__wSpace    DCD     0
 174
 175 ;------ Workspace -----------------------------------------------------------
 176
 177                 ^       0,R12
 178 rand__wStart    #       0
 179
 180 rand__flags     #       4                       ;Various flaglike objects
 181
 182 rand__24        #       4                       ;Offset to item 24 in buffer
 183 rand__55        #       4                       ;Offset to item 55 in buffer
 184
 185 rand__table     #       55*4                    ;The random number table
 186
 187 rand__wSize     EQU     {VAR}-rand__wStart
 188
 189 rFlag__inited   EQU     (1<<0)                  ;Am I initialised yet?
 190
 191                 AREA    |Sapphire$$LibData|,CODE,READONLY
 192
 193                 DCD     rand__wSize
 194                 DCD     rand__wSpace
 195                 DCD     0
 196                 DCD     rand_init
 197
 198 ;----- That's all, folks ----------------------------------------------------
 199
 200                 END