From 99425eb87e07f5f42f98fe6e6bcb6c2f3bd67d8d Mon Sep 17 00:00:00 2001
From: Vince Weaver <vincent.weaver@maine.edu>
Date: Thu, 4 Mar 2021 17:06:46 -0500
Subject: [PATCH] entropy_tiny: not small enough

---
 basic/two-liners/entropy_tiny.s | 183 +++++++-------------------------
 1 file changed, 36 insertions(+), 147 deletions(-)

diff --git a/basic/two-liners/entropy_tiny.s b/basic/two-liners/entropy_tiny.s
index 8dd1aab2..ba5a893a 100644
--- a/basic/two-liners/entropy_tiny.s
+++ b/basic/two-liners/entropy_tiny.s
@@ -14,56 +14,33 @@
 ;	NEXT: NEXT: NEXT: NEXT
 
 
+	; SCALE=(RND(1)<E)*RND(1)*E*20+1
+	;
+	; Equivalent to IF RND(1)<E THEN SCALE=RND(1)*E*20+1
+	;		ELSE SCALE=1
+
+	; What this does:
+	;	if EPOS is 8,9 then value is either 1 or 2
+	;	if EPOS is 10,11,12,13,14 then value is either 1, 2, or 3
+
+
+
 ; Optimization
-;	144 bytes:	first working version (including DOS33 4-byte size/addr)
-;	141 bytes:	nextx: cache XPOS in X register
-;	140 bytes:	nexty: we know state of carry flag
-;	139 bytes:	change jmp to bcs
-;	138 bytes:	jmp at end now fits into a bcs
-;	136 bytes:	store YPOS on stack
-;	135 bytes:	store X to HGR_SCALE rather than TXA+STA
-;	131 bytes:	some fancy branch elimination by noticing X=1
-;	126 bytes:	nextx: simplify by using knowledge of possible x/y vals
-;	124 bytes:	qkumba noticed we can bump yloop up to include the
-;			pha, letting us remove two now unneeded stack ops
-;	123 bytes:	qkumba noticed XDRAW0 always exits with X==0 so
-;			we can move some things to use X instead and
-;			can get a "1" value simply by using INX
-;	122 bytes:	Nick Westgate noticed that we could save a byte
-;			in eloop by pushing the stx ELOOP to the beginning
-;			rather than the end.
+;	88 bytes -- hack up the entropy code
+;	83 bytes -- count down YPOS instead of up
+;	73 bytes -- XPOS only up to 256
 
 ;BLT=BCC, BGE=BCS
 
 ; zero page locations
 HGR_SHAPE	=	$1A
-FAC_EXP		=	$9D
-FAC_HO		=	$9E
-FAC_MOH		=	$9F
-FAC_MO		=	$A0
-FAC_LO		=	$A1
-FAC_SGN		=	$A2
-RND_EXP		=	$C9
-RND_HO		=	$CA
-RND_MOH		=	$CB
-RND_MO		=	$CC
-RND_LO		=	$CD
-RND_SGN		=	$CE
 HGR_SCALE	=	$E7
 HGR_ROTATION	=	$F9
-EPOS		=	$FC
+FRAME		=	$FC
 XPOS		=	$FD
-XPOSH		=	$FE
 YPOS		=	$FF
 
 ; ROM calls
-CONINT		=	$E6FB
-FMULT		=	$E97F
-MUL10		=	$EA39
-DIV10		=	$EA55
-MOVAF		=	$EB63
-FLOAT		=	$EB93
-RND		=	$EFAE
 HGR2		=	$F3D8
 HPOSN		=	$F411
 XDRAW0		=	$F65D
@@ -72,97 +49,28 @@ XDRAW0		=	$F65D
 entropy:
 
 	jsr	HGR2		; Hi-res graphics, no text at bottom
-				; Y=0, A=$60 after this call
+				; Y=0, A=0 after this call
 
-	ldx	#8		; Unlike the BASIC, our loop is *100
-				; 8 to 15 rather than .08 to .15
 eloop:
-	stx	EPOS		; EPOS was temporarily in X
-
-
-	lda	#4		; FOR Y=4 to 189 STEP 6
+	lda	#180		; FOR Y=4 to 189 STEP 6
+	sta	YPOS
 yloop:
-	pha			; YPOS stored on stack
+
 	lda	#4		; FOR X=4 to 278 STEP 6
 	sta	XPOS
-	ldx	#0		; can't fit 278 in one byte, need overflow byte
-	stx	XPOSH
 xloop:
 
-
-	; SCALE=(RND(1)<E)*RND(1)*E*20+1
-	;
-	; Equivalent to IF RND(1)<E THEN SCALE=RND(1)*E*20+1
-	;		ELSE SCALE=1
-
-	; Note the Apple II generates a seed based on keypresses
-	; but by default RND is never seeded from there.
-	; Someone actually wrote an entire academic paper complaining about
-	; this
-	;
-	; J.W. Aldridge.  "Cautions regarding random number generation
-	;	on the Apple II", Behavior Research Methods, Instruments,
-	;	& Computers, 1987, 19 (4), 397-399.
-
-	; Many of these values are in Applesoft 5-byte floating point
-
-				; get random value in FAC
-				;    (floating point accumlator)
-
-	inx			; X is always 0 coming in, increment to 1
-
-				; RND(1), Force 1
-				; returns "random" value between 0 and 1
-	jsr	RND+6		; we skip passing the argument
-				; as a floating point value
-				; as that would be a pain
-
- 	; Compare to E
-
-	jsr	MUL10		; EPOS is E*100
-	jsr	MUL10		; so multiply rand*100 before compare
-	jsr	CONINT		; now convert to int, result in X
-				; X is now RND(1)*100
-
-	cpx	EPOS		; compare E*100 to RND*100
-
-	ldx	#1		; load 1 into X (this is clever)
-
-	bcs	done		; if EPOS>=RND then SCALE=1, skip ahead
-
-	; SCALE=RND(1)*E*20+1
-	; EPOS is E*100, so RND(1)*(EPOS/10)*2+1
-
-	; What this does:
-	;	if EPOS is 8,9 then value is either 1 or 2
-	;	if EPOS is 10,11,12,13,14 then value is either 1, 2, or 3
-
-
-				; put random value in FAC
-;	ldx	#1		; RND(1), Force 1, this set from earlier
-	jsr	RND+6		; skip arg parsing in RND
-
-	lda	EPOS
-	jsr	FLOAT		; convert value in A to float in FAC
-	jsr	DIV10		; FAC=FAC/10
-
-	ldy	#>RND_EXP	; point (Y,A) to RND value
-	lda	#<RND_EXP
-	jsr	FMULT		; multiply FAC by RND in (Y,A)
-
-	inc	FAC_EXP		; multiply by 2
-
-	jsr	CONINT		; convert to int (in X)
-
-	inx			; add 1
-
-done:
+	ldx	#1
+	ldy	FRAME
+	lda	$D000,y
+	bmi	no_add
+	inx
+no_add:
 	stx	HGR_SCALE	; set scale value
 
-	ldy	XPOSH		; setup X and Y co-ords
+	ldy	#0		; setup X and Y co-ords
 	ldx	XPOS
-	pla			; YPOS is on stack
-	pha
+	lda	YPOS
 	jsr	HPOSN		; X= (y,x) Y=(a)
 
 
@@ -175,41 +83,22 @@ done:
 				; Both A and X are 0 at exit
 
 nextx:				; NEXT X
+	inc	FRAME
+
 	lda	XPOS							; 2
 	clc								; 1
 	adc	#6		; x+=6					; 2
 	sta	XPOS							; 2
 
-	; we know that the X=4 to 278 STEP 6 loop passes through exactly 256
-	; so we can check for that by looking for overflow to zero
-
-	bne	skip							; 2
-	inc	XPOSH							; 2
-skip:
-	; the X=4 to 278 STEP 6 loop actually ends when X is at 280, which
-	; is 256+24.  The lower part of the loop does not hit 24, so we
-	; can check for the end by looking for the low byte at 24.
-
-	cmp	#24		; see if less than 278			; 2
 	bne	xloop		; if so, loop				; 2
-								;============
-								;	 15
-nexty:				; NEXT Y
-	pla			; YPOS on stack
-	adc	#5		; y+=6
-				; carry always set coming in, so only add 5
-	cmp	#189		; see if less than 189
-	bcc	yloop		; if so, loop
 
-nexte:				; NEXT E
-	ldx	EPOS
-	inx			; EPOS saved at beginning of eloop
-	cpx	#15
-	bcc	eloop		; branch if <15
-
-	bcs	entropy
+nexty:
+	sec
+	lda	YPOS
+	sbc	#6		; y+=6
+	sta	YPOS
+	bne	yloop		; if so, loop
+	beq	eloop
 
 shape_table:
-;	.byte 1,0				; 1 shape
-;	.byte 4,0				; offset at 4 bytes
 	.byte 18,63,36,36,45,45,54,54,63,0	; data