Fixed more errors, clearer choice of constants

apps/lib/parse.asm

@@ -9,28 +9,23 @@
 ; On success, the carry flag is reset. On error, it is set.
 ; 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 
+;	add	a, 0xff-'9'	; maps '0'-'9' onto 0xf6-0xff
+;	sub	0xff-9		; maps to 0-9 and carries if not a digit
 
 ; 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.
 
-; 61 bytes
-; 107 cycles overhead + up to 73 cycles if length >= 5
-; 136 cycles in loop
-; first digit is skipped in overhead
 parseDecimal:
 	push 	hl
 
 	ld 	a, (hl)
-	add	a, 0xc6	; converts '0'-'9' to 0-9
-	sub	0xf6	; carries if out of range
+	add	a, 0xff-'9'	; maps '0'-'9' onto 0xf6-0xff
+	sub	0xff-9		; maps to 0-9 and carries if not a digit
 	exx		; preserve bc, hl, de
 	ld	h, 0
 	ld	l, a	; load first digit in without multiplying
-	ld	b, 4	; Carries can only occur for decimals >=5 in length
+	ld	b, 3	; Carries can only occur for decimals >=5 in length
 	jr	c, .error
 
 .loop:
@@ -39,8 +34,8 @@ parseDecimal:
 	ld a, (hl)
 	exx
 
-	add	a, 0xc6	; converts '0'-'9' to 0-9
-	sub	0xf6	; carries if out of range
+	add	a, 0xff-'9'	; maps '0'-'9' onto 0xf6-0xff
+	sub	0xff-9		; maps to 0-9 and carries if not a digit
 	jr	c, .error
 	
 	add	hl, hl	; x2
@@ -68,7 +63,7 @@ parseDecimal:
 	inc 	hl
 	ld 	a, (hl)
 	exx
-	add 	a, 0xd0
+	add 	a, 0xd0	; the next line expects a null to be mapped to 0xd0
 .error:
 	sub 	0xd0	; if a is null, set Z
 			; a is checked for null before any errors