sms/pad: implement character selection mechanism

Still a bit rough around the edges, but still, works rather well.

kernel/sms/pad.asm

@@ -1,6 +1,6 @@
 ; pad - read input from MD controller
 ;
-; Conveniently expose an API to read the status of a MD pad. Moreover,
+; Conveniently expose an API to read the status of a MD pad A. Moreover,
 ; implement a mechanism to input arbitrary characters from it. It goes as
 ; follow:
 ;
@@ -16,26 +16,49 @@
 .equ	PAD_CTLPORT	0x3f
 .equ	PAD_D1PORT	0xdc
 
+.equ	PAD_UP		0
+.equ	PAD_DOWN	1
+.equ	PAD_LEFT	2
+.equ	PAD_RIGHT	3
+.equ	PAD_BUTB	4
+.equ	PAD_BUTC	5
+.equ	PAD_BUTA	6
+.equ	PAD_START	7
+
 ; *** Variables ***
 ;
-; Previous result of padStatusA
-.equ	PAD_PREVSTAT_A	PAD_RAMSTART
-.equ	PAD_RAMEND	PAD_PREVSTAT_A+1
+; Button status of last padUpdateSel call. Used for debouncing.
+.equ	PAD_SELSTAT	PAD_RAMSTART
+; Button status of last padGetC call.
+.equ	PAD_GETCSTAT	PAD_SELSTAT+1
+; Current selection "class". 0 = lowcase, 1 = upcase, 2 = num, 3 = symbols
+.equ	PAD_SELCLASS	PAD_GETCSTAT+1
+; Current selected character
+.equ	PAD_SELCHR	PAD_SELCLASS+1
+; Whether current sel is "new", that is, that its value has never been fetched
+; though padUpdateSel. This flag is set when "avancing" in GetC and also on
+; module init.
+.equ	PAD_SELNEW	PAD_SELCHR+1
+.equ	PAD_RAMEND	PAD_SELNEW+1
 
 ; *** Code ***
 
 padInit:
 	ld	a, 0xff
-	ld	(PAD_PREVSTAT_A), a
+	ld	(PAD_SELSTAT), a
+	ld	(PAD_GETCSTAT), a
+	ld	(PAD_SELNEW), a
+	xor	a
+	ld	(PAD_SELCLASS), a
+	ld	a, 'a'
+	ld	(PAD_SELCHR), a
 	ret
 
 ; Put status for port A in register A. Bits, from MSB to LSB:
 ; Start - A - C - B - Right - Left - Down - Up
 ; Each bit is high when button is unpressed and low if button is pressed. For
 ; example, when no button is pressed, 0xff is returned.
-; Moreover, sets Z according to whether the status changed since the last call.
-; Z is set if status is the same, unset if different.
-padStatusA:
+padStatus:
 	; This logic below is for the Genesis controller, which is modal. TH is
 	; an output pin that swiches the meaning of TL and TR. When TH is high
 	; (unselected), TL = Button B and TR = Button C. When TH is low
@@ -56,12 +79,88 @@ padStatusA:
 	sla	a
 	or	b
 	pop	bc
+	ret
 
-	; set Z according to whether status has change. Also, update saved
-	; status.
+; From a pad status in A, update current char selection and return it.
+; Returns the same Z as padStatus: set if unchanged, unset if changed
+padUpdateSel:
+	; special case: when selnew is set, always return current sel and
+	; disable selnew
+	ld	a, (PAD_SELNEW)
+	or	a
+	jr	z, .notnew
+	; selection is new, return it
+	xor	a
+	ld	(PAD_SELNEW), a
+	ld	a, (PAD_SELCHR)
+	jp	unsetZ
+.notnew:
+	call	padStatus
 	push	hl
-	ld	hl, PAD_PREVSTAT_A
-	cp	(hl)		; Sets Z to proper value
+	ld	hl, PAD_SELSTAT
+	cp	(hl)
 	ld	(hl), a
 	pop	hl
+	jr	z, .nothing	; nothing changed
+	bit	PAD_UP, a
+	jr	z, .up
+	bit	PAD_DOWN, a
+	jr	z, .down
+	bit	PAD_LEFT, a
+	jr	z, .left
+	bit	PAD_RIGHT, a
+	jr	z, .right
+	jr	.nothing
+.up:
+	ld	a, (PAD_SELCHR)
+	inc	a
+	jr	.setchr
+.down:
+	ld	a, (PAD_SELCHR)
+	dec	a
+	jr	.setchr
+.left:
+	ld	a, (PAD_SELCHR)
+	dec	a \ dec a \ dec a \ dec a \ dec a
+	jr	.setchr
+.right:
+	ld	a, (PAD_SELCHR)
+	inc	a \ inc a \ inc a \ inc a \ inc a
+	jr	.setchr
+.setchr:
+	ld	(PAD_SELCHR), a
+	jp	unsetZ
+.nothing:
+	cp	a		; ensure Z
+	ld	a, (PAD_SELCHR)
+	ret
+
+padGetC:
+	call	padStatus
+	push	hl
+	ld	hl, PAD_GETCSTAT
+	cp	(hl)
+	ld	(hl), a
+	pop	hl
+	jp	z, unsetZ	; nothing changed
+	bit	PAD_BUTC, a
+	jr	z, .advance
+	bit	PAD_BUTA, a
+	jr	z, .backspace
+	bit	PAD_START, a
+	jr	z, .return
+	jp	unsetZ
+.advance:
+	ld	a, 1
+	ld	(PAD_SELNEW), a
+	ld	a, (PAD_SELCHR)
+	cp	a
+	ret
+.backspace:
+	ld	a, ASCII_BS
+	cp	a
+	ret
+.return:
+	ld	a, ASCII_LF
+	cp	a
 	ret