From 67adc6fcfcb53a9dbbc90f8e6f8f45b2cdbbb35f Mon Sep 17 00:00:00 2001
From: Clanmaster21 <asalukehopkins@gmail.com>
Date: Sun, 20 Oct 2019 22:26:07 +0100
Subject: [PATCH] Removed skip leading zeroes, added skip first multiply

Now instead of skipping leading zeroes, the first digit is loaded directly into hl without first multiplying by 10. This means the first loop is skipped in the overhead, making the method 2-3 times faster overall, and is now faster for the more common fewer digit cases too. The number of bytes is exactly the same, and the inner loop is slightly faster too thanks to no longer needing to load a into c.
To be more precise about the speed increase over the current code, for decimals of length 1 it'll be 3.18x faster, for decimals of length 2, 2.50x faster, for length 3, 2.31x faster, for length 4, 2.22x faster, and for length 5 and above, at least 2.03x faster. In terms of cycles, this is around 100+(132*length) cycles saved per decimal.
---
 apps/lib/parse.asm | 47 +++++++++++++++++++++-------------------------
 1 file changed, 21 insertions(+), 26 deletions(-)

diff --git a/apps/lib/parse.asm b/apps/lib/parse.asm
index bf59f4e..c129dd5 100644
--- a/apps/lib/parse.asm
+++ b/apps/lib/parse.asm
@@ -17,29 +17,32 @@
 ; 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
+; 55 bytes
+; 107 cycles overhead + up to 45 cycles if length >= 5
+; 136 cycles in loop
+; first digit is skipped in overhead
 parseDecimal:
 	push 	hl
 
-.skip:	; Skips leading zeroes
-	ld	a, (hl)
-	inc	hl
-	cp	'0'
-	jr	z, .skip
-
+	ld 	a, (hl)
+	add	a, 0xc6	; converts '0'-'9' to 0-9
+	sub	0xf6	; carries if out of range
 	exx		; preserve bc, hl, de
-	ld	hl, 0
-	ld	b, 5	; Carries can only occur for decimals >=5 in length
-	jr	.start
+	ld	h, 0
+	ld	l, a	; load first digit in without multiplying
+	ld	b, 4	; Carries can only occur for decimals >=5 in length
+	jr	c, .error
 
 .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
+	inc hl
+	ld a, (hl)
+	exx
+
+	add	a, 0xc6	; converts '0'-'9' to 0-9
+	sub	0xf6	; carries if out of range
+	jr	c, .error
+	
 	add	hl, hl	; x2
 	ld	d, h
 	ld	e, l		; de is x2
@@ -47,16 +50,9 @@ parseDecimal:
 	add	hl, hl	; x8
 	add	hl, de	; x10
 	ld	d, 0
-	ld	e, c
+	ld	e, a
 	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
 
 
@@ -71,8 +67,7 @@ parseDecimal:
 	sub 	0xd0	; if a is null, set Z
 			; a is checked for null before any errors
 .end:
-	push	hl
-	pop	ix
+	push	hl \ pop ix
 	exx	; restore original de and bc
 	pop	hl	
 	ret