From db18f631485eea0b1e7d6c8cf853ff4a281c2aba Mon Sep 17 00:00:00 2001
From: "B. Watson" <yalhcru@gmail.com>
Date: Thu, 27 May 2021 17:12:46 -0400
Subject: cash_dirty, change random seeding, now 9006 bytes

---
 rand.s | 104 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 103 insertions(+), 1 deletion(-)

(limited to 'rand.s')

diff --git a/rand.s b/rand.s
index 6efcabd..f1b2c72 100644
--- a/rand.s
+++ b/rand.s
@@ -1,6 +1,5 @@
 
  .export _randl, _rand1to3
- .import _rand
 ; .export _randb, _randi, _randl
 ; .export _rand1in5
 ; .export _randbit
@@ -37,6 +36,109 @@ _rand1to3:
 	ldx #0         ; now A is 1..3, but we have to force X to 0...
 	rts
 
+;;; This rand() function copied from cc65-2.19's libsrc/common/rand.s
+;;; and modified for my nefarious purposes.
+;;; srand() is not present (we don't use it).
+;
+; Random number generator
+;
+; Written and donated by Sidney Cadot - sidney@ch.twi.tudelft.nl
+; 2016-11-07, modified by Brad Smith
+; 2019-10-07, modified by Lewis "LRFLEW" Fox
+;
+; May be distributed with the cc65 runtime using the same license.
+;
+;
+; int rand (void);
+; void srand (unsigned seed);
+;
+;  Uses 4-byte state.
+;  Multiplier must be 1 (mod 4)
+;  Added value must be 1 (mod 2)
+;  This guarantees max. period (2**32)
+;  The lowest bits have poor entropy and
+;  exhibit easily detectable patterns, so
+;  only the upper bits 16-22 and 24-31 of the
+;  4-byte state are returned.
+;
+;  The best 8 bits, 24-31 are returned in the
+;  low byte A to provide the best entropy in the
+;  most commonly used part of the return value.
+;
+;  Uses the following LCG values for ax + c (mod m)
+;  a = $01010101
+;  c = $B3B3B3B3
+;  m = $100000000 (32-bit truncation)
+;
+;  The multiplier was carefully chosen such that it can
+;  be computed with 3 adc instructions, and the increment
+;  was chosen to have the same value in each byte to allow
+;  the addition to be performed in conjunction with the
+;  multiplication, adding only 1 additional adc instruction.
+;
+
+ .export _rand, _randseed, _randseedl, _randseedh, _initrand, _addrandbits
+
+.bss
+
+; The seed. Not ANSI C compliant: we default to 0 rather than 1.
+_randseedl:
+_randseed:   .res 4
+_randseedh = _randseed+2
+
+.code
+
+_rand:  clc
+        lda     _randseed+0
+        adc     #$B3
+        sta     _randseed+0
+        adc     _randseed+1
+        sta     _randseed+1
+        adc     _randseed+2
+        sta     _randseed+2
+        and     #$7f            ; Suppress sign bit (make it positive)
+        tax
+        lda     _randseed+2
+        adc     _randseed+3
+        sta     _randseed+3
+        rts                     ; return bit (16-22,24-31) in (X,A)
+
+; Initially the seed comes from sequential reads of POKEY's random
+; register. It never returns 0 so we're guaranteed to have a usable
+; seed.
+_initrand:
+ ldx #3
+@l:
+ lda RANDOM
+ sta _randseed,x
+ dex
+ bpl @l
+ rts
+
+; Caller passes us a user keystroke as ATASCII, in A. We take bits 0
+; to 2, wait that many scanlines, get a random number from POKEY, and
+; EOR it into the _randseed byte pointed to by the low 2 bits of the
+; frame counter (RTCLOK+2).
+; Note that agetc() is still calling rand() on odd frames while all
+; this is going on.
+_addrandbits:
+ and #$07
+ tax
+@l1:
+ sta WSYNC
+ dex
+ bpl @l1
+ lda RTCLOK+2
+ and #$03
+ tax
+ lda RANDOM
+@e:
+ eor _randseed,x
+ beq @e ; if the result is 0, undo the eor.
+ sta _randseed,x
+ rts
+
+
  ;;; rest of file is commented out
 
 ; RANDOM is the POKEY LFSR read address. According to the POKEY data
-- 
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