Nightshade: Routine at BEDF

Used by the routine at GameOver.

Works out how much of the game has been completed and stores it as a percentage in PercentageHundreds (the hundreds digit) and PercentageTensAndUnits (the tens and units, as BCD).

Every bit set in the 80 bytes at VisitedCells is a cell of the village that has been visited. Every monster that has been killed counts as three more cells. The total is multiplied by 2830 and divided by 10000, so that visiting about 027D cells (or a few less when monsters have been killed) gives 64 percent.

Used by the routine at GameOver. Works out how much of the game has been completed and stores it as a percentage in PercentageHundreds (the hundreds digit) and PercentageTensAndUnits (the tens and units, as BCD). Every bit set in the 80 bytes at VisitedCells is a cell of the village that has been visited. Every monster that has been killed counts as three more cells. The total is multiplied by 2830 and divided by 10000, so that visiting about 027D cells (or a few less when monsters have been killed) gives 64 percent.
Count the visited cells.
CalculatePercentage	BEDF	LD HL,$0000	Start the count of cells at zero.
	BEE2	LD DE,$BC0E	Point at the bitmap of visited cells.
	BEE5	LD C,$80	Set the number of bytes in the bitmap to 80.
CalculatePercentage_ByteLoop	BEE7	LD A,(DE)	Fetch a byte of the bitmap and move on to the next.
	BEE8	INC DE	Fetch a byte of the bitmap and move on to the next.
	BEE9	LD B,$08	Set a counter for the eight bits in the byte.
CalculatePercentage_BitLoop	BEEB	RRCA	Check the next bit of the bitmap, and move it into the carry flag.
	BEEC	JR NC,CalculatePercentage_NextBit
	BEEE	INC HL	Count the visited cell.
CalculatePercentage_NextBit	BEEF	DJNZ CalculatePercentage_BitLoop	Loop back until all eight bits have been counted.
	BEF1	DEC C	Loop back until every byte of the bitmap has been counted.
	BEF2	JR NZ,CalculatePercentage_ByteLoop	Loop back until every byte of the bitmap has been counted.
Add three for every monster that has been killed.
	BEF4	LD IY,$BD5E	Start with the first of the four monsters.
	BEF8	LD B,$04	Set a counter for the four monsters.
CalculatePercentage_MonsterLoop	BEFA	LD A,(IY+$00)	Fetch the type of the monster and subtract 60.
	BEFD	SUB $60	Fetch the type of the monster and subtract 60.
	BEFF	CP $10	Jump to CalculatePercentage_NextMonster if the monster is still alive (its type is 60 to 6F).
	BF01	JR C,CalculatePercentage_NextMonster
	BF03	LD DE,$0003	Add three to the count of cells.
	BF06	ADD HL,DE	Add three to the count of cells.
CalculatePercentage_NextMonster	BF07	LD DE,$0010	Set the size of a monster's entry in the object table to 10 bytes.
	BF0A	ADD IY,DE	Move on to the next monster.
	BF0C	DJNZ CalculatePercentage_MonsterLoop	Loop back until all four monsters have been counted.
Multiply the count by 2830 and divide by 10000, using BCD arithmetic for the high byte.
	BF0E	LD BC,$2830	Set the value to add on each pass.
	BF11	XOR A	Start the BCD result at zero.
	BF12	EX DE,HL	Make the count of cells the number of times to add.
	BF13	LD HL,$0000	Start the running total at zero.
CalculatePercentage_MultiplyLoop	BF16	ADD HL,BC	Add the value to the total.
	BF17	ADC A,$00	Add any carry to the BCD result.
	BF19	DAA	Adjust the BCD result.
	BF1A	PUSH AF	Stash the result on the stack.
	BF1B	DEC DE	Decrease the count.
	BF1C	LD A,E	Jump to CalculatePercentage_Finish if the count has reached zero.
	BF1D	OR D
	BF1E	JR Z,CalculatePercentage_Finish
	BF20	POP AF	Restore the result from the stack.
	BF21	JR CalculatePercentage_MultiplyLoop	Jump to CalculatePercentage_MultiplyLoop.
CalculatePercentage_Finish	BF23	POP AF	Restore the result from the stack.
	BF24	LD BC,$0090	Add 90 to the total, which rounds the percentage.
	BF27	ADD HL,BC	Add any carry to the BCD result.
	BF28	ADC A,$00
	BF2A	DAA
	BF2B	LD ($BBFE),A	Set the tens and units of the percentage.
	BF2E	LD A,$00	Set the hundreds digit to one if the percentage has overflowed.
	BF30	ADC A,$00
	BF32	LD ($BBFD),A	Set the hundreds digit of the percentage.
	BF35	RET	Return.