From 9797405789d0fb72bdabeebfd60027d31324ae80 Mon Sep 17 00:00:00 2001
From: Clanmaster21 <asalukehopkins@gmail.com>
Date: Sun, 13 Oct 2019 13:50:07 +0100
Subject: [PATCH] Major parsing optimisations

Totally reworked both parseDecimal and parseDecimalDigit
parseDecimalDigit no longer exists, as it could be replaced by an inline alternative in the 4 places it appeared. This saves one byte overall, as the inline version is 4 bytes, 1 byte more than a call, and removing the function saved 5 bytes. It has been reduced from between 52 and 35 cycles (35 on error, so we'd expect 52 cycles to be more common unless someone's really bad at programming) to 14 cycles, so 2-3 times faster.
parseDecimal has been reduced by a byte, and now the main loop is just about twice as fast, but with increased overhead. To put this into perspective, if we ignore error cases:
For decimals of length 1 it'll be 1.20x faster, for decimals of length 2, 1.41x faster, for length 3, 1.51x faster, for length 4, 1.57x faster, and for length 5 and above, at least 1.48x faster (even faster if there's leading zeroes or not the worst case scenario).
I believe there is still room for improvement, since the first iteration can be nearly replaced with "ld l, c" since 0*10=0, but when I tried this I could either add a zero check into the main loop, adding around 40 cycles and 10 bytes, or add 20 bytes to the overhead, and I don't think either of those options are worth it.
---
 apps/lib/parse.asm | 102 ++++++++++++++++++++++++++-------------------
 1 file changed, 58 insertions(+), 44 deletions(-)

diff --git a/apps/lib/parse.asm b/apps/lib/parse.asm
index 6ab6e16..bf59f4e 100644
--- a/apps/lib/parse.asm
+++ b/apps/lib/parse.asm
@@ -1,5 +1,5 @@
 ; *** Requirements ***
-; unsetZ
+; None
 ;
 ; *** Code ***
 
@@ -7,58 +7,72 @@
 ; result in A.
 ;
 ; On success, the carry flag is reset. On error, it is set.
-parseDecimalDigit:
-	; First, let's see if we have an easy 0-9 case
-	cp	'0'
-	ret	c	; if < '0', we have a problem
-	sub	'0'		; our value now is valid if it's < 10
-	cp	10		; on success, C is set, which is the opposite
-				; of what we want
-	ccf			; invert C flag
-	ret
+; Also, zero flag set if '0'
+; parseDecimalDigit has been replaced with the following code inline:
+;	add a, 0xc6	; Maps '0'-'9' onto 0xf6-0xff
+;	sub 0xf6	; Anything but 0xf6-0xff carries
+			; Maps 0xf6-0xff onto 0-9 
 
 ; Parse string at (HL) as a decimal value and return value in IX under the
 ; same conditions as parseLiteral.
 ; Sets Z on success, unset on error.
+
+; 55 bytes, 32 cycles in first loop
+; 90 cycles overhead + up to 69 cycles if length >= 5
+; 140 cycles in loop
 parseDecimal:
-	push	hl
-	push	de
+	push 	hl
 
-	ld	ix, 0
-.loop:
+.skip:	; Skips leading zeroes
 	ld	a, (hl)
-	or	a
-	jr	z, .end	; success!
-	call	parseDecimalDigit
-	jr	c, .error
-
-	; Now, let's add A to IX. First, multiply by 10.
-	push	ix \ pop de
-	add	ix, ix	; x2
-	jr	c, .error
-	add	ix, ix	; x4
-	jr	c, .error
-	add	ix, ix	; x8
-	jr	c, .error
-	add	ix, de	; x9
-	jr	c, .error
-	add	ix, de	; x10
-	jr	c, .error
-	ld	d, 0
-	ld	e, a
-	add	ix, de
-	jr	c, .error
-
 	inc	hl
-	jr	.loop
+	cp	'0'
+	jr	z, .skip
 
-	cp	a	; ensure Z
-	jr	.end
+	exx		; preserve bc, hl, de
+	ld	hl, 0
+	ld	b, 5	; Carries can only occur for decimals >=5 in length
+	jr	.start
+
+.loop:
+	ld	c, a		; c holds current digit
+	exx	;swap hl back in to get address
+	ld	a, (hl)	; a checks if following digit is null at end of loop
+	inc	hl
+	exx
+	add	hl, hl	; x2
+	ld	d, h
+	ld	e, l		; de is x2
+	add	hl, hl	; x4
+	add	hl, hl	; x8
+	add	hl, de	; x10
+	ld	d, 0
+	ld	e, c
+	add	hl, de
+	jr	c, .error	; if hl was 0x1999, it may carry here
+	; This check could be taken outside the loop, but at the cost of 6 bytes
+
+.start:
+	add	a, 0xc6	; converts '0'-'9' to 0-9
+	sub	0xf6	; carries if out of range
+	jr	c, .error
+
+	djnz	.loop
+
+
+	inc 	b	; so loop only executes once more
+	; only numbers >0x1999 can carry when multiplied by 10.
+	ld	de, 0xE666	
+	ex	de, hl
+	add	hl, de
+	ex	de, hl
+	jr	nc, .loop	; if it doesn't carry, it's small enough
 .error:
-	call	unsetZ
+	sub 	0xd0	; if a is null, set Z
+			; a is checked for null before any errors
 .end:
-	pop	de
-	pop	hl
+	push	hl
+	pop	ix
+	exx	; restore original de and bc
+	pop	hl	
 	ret
-
-