Source code in the 8080 programming language

	org	100h
	jmp	demo
	;;;	Calculate the amount of water a row of towers will hold
	;;;	Note: this will destroy the input array.
	;;;	Input: DE = tower array, BC = length of array
	;;;	Output: A = amount of water 
water:	xra	a	; Start with no water
	sta	w_out+1
wscanr:	mov	h,d	; HL = right edge
	mov	l,e
	dad	b
wscrlp:	dcx	h
	call	cmp16	; Reached beginning?
	jnc	w_out	; Then stop
	mov	a,m	; Otherwise, if current tower is zero
	ora	a
	jz	wscrlp	; Then keep scanning
	push	b	; Keep length
	push	d	; Keep array begin
	mvi	b,0	; No blocks yet
	xchg		; HL = left scanning edge, DE = right
wscanl:	mov	a,m	; Get current column
	ora	a	; Is zero?
	jz	wunit	; Then see if an unit of water must be added
	dcr	m	; Otherwise, decrease column
	inr	b	; Increase blocks
	jmp	wnext
wunit:	mov	a,b	; Any blocks?
	ora	a
	jz	wnext
	lda	w_out+1	; If so, add water
	inr	a
	sta	w_out+1
wnext:	inx	h	; Next column
	call	cmp16
	jnc	wscanl	; Until right edge reached
	mov	a,b
	cmc		; Check if more than 1 block left
	rar
	ora	a
	pop 	d	; Restore array begin
	pop	b	; and length
	jnz	wscanr	; If more than 1 block, keep scanning
w_out:	mvi	a,0	; Load water into A
	ret 	
	;;;	16-bit compare DE to HL 
cmp16:	mov	a,d
	cmp	h
	rnz
	mov	a,e
	cmp 	l
	ret
	;;;	Calculate and print the amount of water for each input
demo:	lxi	h,series
load:	mov	e,m	; Load pointer
	inx	h
	mov	d,m
	inx	h
	mov	c,m	; Load length
	inx	h
	mov	b,m
	inx	h
	mov	a,d	; If pointer is zero,
	ora	e
	rz		; stop.
	push 	h	; Otherwise, save the series pointer
	call	water	; Calculate amount of water
	call	printa	; Output amount of water
	pop	h	; Restore series pointer
	jmp	load	; Load next example 
	;;;	Print A as integer value
printa:	lxi	d,num	; Pointer to number string
	mvi	c,10	; Divisor
digit:	mvi	b,-1	; Quotient
dloop:	inr	b	; Divide (by trial subtraction)
	sub	c
	jnc	dloop
	adi	'0'+10	; ASCII digit from remainder
	dcx	d	; Store ASCII digit
	stax	d
	mov	a,b	; Continue with quotient
	ana	a	; If not zero
	jnz	digit
	mvi	c,9	; 9 = CP/M print string syscall
	jmp	5	; Print number string
	db	'***'	; Output number placeholder
num:	db	' $'
	;;;	Series
t1:	db	1,5,3,7,2
t2:	db	5,3,7,2,6,4,5,9,1,2
t3:	db	2,6,3,5,2,8,1,4,2,2,5,3,5,7,4,1
t4:    	db	5,5,5,5
t5:	db	5,6,7,8
t6:    	db	8,7,7,6
t7:	db    	6,7,10,7,6
t_end:	equ	$
	;;;	Lengths and pointers
series:	dw	t1,t2-t1
	dw	t2,t3-t2 
	dw	t3,t4-t3 
	dw	t4,t5-t4
	dw	t5,t6-t5
	dw	t6,t7-t6
	dw	t7,t_end-t7
	dw	0