String functions optimised (#86)

* String functions optimised

A few functions have been tweaked, but the biggest changes are in strlen, strskip and toWS, which take around two third of the cycles they used to (although strskip has more overhead). 10 bytes saved total.
toWS had two bytes added inlining the isWS call, and a jump to unsetZ was inlined too, saving a byte. This saved 29 cycles, with the original function being 90 cycles. I looked at other uses of isWS and it's difficult to inline it effectively in every situation, so I haven't inlined it elsewhere.
rdWS had a byte and two cycles saved by inlining a jump to unsetZ.
strskip is the same size, with the loop cut down from 35 cycles to 21 cycles, but 18 cycles are added outside the loop. I expect one character strings are in the minority, so this should save cycles overall.
strlen had 8 bytes saved, with the loop cut down from 38 cycles to 21 cycles, and 18 cycles removed outside the loop.

* Fixed strskip

Strskip wasn't preserving a properly. The new code uses the shadow af register, so whilst a byte and 4 cycles have been added outside the loop, it's safer and cleaner. The flags register isn't affected, but since the search goes for up to 64Kb I think it's safe to say the end of the string will always be reached.

* Remove inlining of isWS

apps/lib/util.asm

@@ -11,8 +11,8 @@ toWS:
 	ld	a, (hl)
 	call	isWS
 	ret	z
-	or	a
+	cp	0x01	; if a is null, carries and unsets z
-	jp	z, unsetZ
+	ret	c
 	inc	hl
 	jr	toWS
 
@@ -22,8 +22,8 @@ rdWS:
 	ld	a, (hl)
 	call	isWS
 	jr	nz, .ok
-	or	a
+	cp	0x01	; if a is null, carries and unsets z
-	jp	z, unsetZ
+	ret	c
 	inc	hl
 	jr	rdWS
 .ok:
@@ -61,13 +61,11 @@ strcmp:
 	ld	a, (de)
 	cp	(hl)
 	jr	nz, .end	; not equal? break early. NZ is carried out
-				; to the called
+				; to the caller
 	or	a		; If our chars are null, stop the cmp
-	jr	z, .end		; The positive result will be carried to the
-	                        ; caller
 	inc	hl
 	inc	de
-	jr	.loop
+	jr	nz, .loop	; Z is carried through
 
 .end:
 	pop	de
@@ -79,35 +77,31 @@ strcmp:
 
 ; Given a string at (HL), move HL until it points to the end of that string.
 strskip:
-	push	af
+	push	bc
+	ex	af, af'
 	xor	a	; look for null char
-.loop:
+	ld	b, a
-	cp	(hl)
+	ld	c, a
-	jp	z, .found
+	cpir	; advances HL regardless of comparison, so goes one too far
-	inc	hl
+	dec	hl
-	jr	.loop
+	ex	af, af'
-.found:
+	pop	bc
-	pop	af
 	ret
 
 ; Returns length of string at (HL) in A.
 ; Doesn't include null termination.
 strlen:
 	push	bc
-	push	hl
-	ld	bc, 0
 	xor	a	; look for null char
-.loop:
+	ld	b, a
-	cpi
+	ld	c, a
-	jp	z, .found
+	cpir		; advances HL to the char after the null
-	jr	.loop
 .found:
-	; How many char do we have? the (NEG BC)-1, which started at 0 and
+	; How many char do we have? We have strlen=(NEG BC)-1, since BC started 
-	; decreased at each CPI call. In this routine, we stay in the 8-bit
+	; at 0 and decreased at each CPIR loop. In this routine, 
-	; realm, so C only.
+	; we stay in the 8-bit realm, so C only.
-	ld	a, c
+	add 	hl, bc
-	neg
+	sub	c
 	dec	a
-	pop	hl
 	pop	bc
 	ret