river-raid: Routine at 6990

6990: Render one line of island terrain

Renders the left and right edges of an island on the current screen line. Islands are rendered as two diagonal edges (left and right) with solid terrain ($FF) filling the space from screen edge to each diagonal edge.
The left edge X position is calculated as: state_island_byte_2 + profile_value + $80. The $80 centers the coordinate system (screen center). The profile_value comes from data_terrain_profiles indexed by state_island_profile_idx and state_terrain_position. The right edge position depends on state_island_byte_3: 0=use byte_3 directly, 1=mirror around center (2*$3C - left), 2=offset from left ($3C + left). $3C (60) is the default river half-width.
render_island_line	6990	LD HL,$5EFD	Increment island line counter.
	6993	INC (HL)	Increment island line counter.
	6994	LD A,($5EFA)	Set up lookup: HL=data_terrain_profiles, DE=TERRAIN_PROFILE_SIZE.
	6997	LD HL,$8063
	699A	LD DE,$0010
	699D	OR A
	699E	SBC HL,DE
locate_island_profile	69A0	ADD HL,DE	Locate 16-byte profile entry by state_island_profile_idx.
	69A1	DEC A
	69A2	JR NZ,locate_island_profile
	69A4	LD A,($5F7D)	Index into profile by (state_terrain_position AND $0F) to get edge offset byte.
	69A7	AND $0F
	69A9	LD D,$00
	69AB	LD E,A
	69AC	ADD HL,DE
	69AD	LD A,($5EFB)	Left edge X = state_island_byte_2 + profile_byte + $80. Save X on stack.
	69B0	ADD A,(HL)
	69B1	PUSH AF
	69B2	LD B,$00
	69B4	ADD A,$80
	69B6	LD D,A
	69B7	LD HL,$89F2
	69BA	LD A,D	Look up 2-byte edge sprite from terrain_edge_left using (X AND $06) as sprite index.
	69BB	AND $07
	69BD	SRL A
	69BF	LD C,A
	69C0	SLA C
	69C2	LD A,D
	69C3	ADD HL,BC
	69C4	EX DE,HL
	69C5	LD C,A	Calculate screen address = screen_ptr + (X >> 3). Copy 2-byte edge sprite via LDIR.
	69C6	LD HL,($5F7B)
	69C9	LD B,$00
	69CB	SRL C
	69CD	SRL C
	69CF	SRL C
	69D1	ADD HL,BC
	69D2	EX DE,HL
	69D3	LD BC,$0002
	69D6	LDIR
	69D8	DEC DE	Calculate fill count = (X >> 3) - 16. This is number of solid tiles left of edge.
	69D9	DEC DE
	69DA	SRL A
	69DC	SRL A
	69DE	SRL A
	69E0	SUB $10
	69E2	AND $0F
	69E4	LD B,A
	69E5	CP $00
	69E7	JP Z,prepare_right_edge
	69EA	LD A,$FF	Fill leftward with $FF bytes (solid terrain) for fill count iterations.
fill_island_left_loop	69EC	DEC DE
	69ED	LD (DE),A
	69EE	DJNZ fill_island_left_loop
prepare_right_edge	69F0	POP AF	Restore left X from stack. Set D=left X, C=$3C (river half-width), B=state_island_byte_2.
	69F1	LD B,$00
	69F3	LD D,A
	69F4	LD C,$3C
	69F6	LD A,($5EFB)
	69F9	LD B,A
	69FA	LD A,($5EFC)	Dispatch based on state_island_byte_3: 1=calc_mirrored_edge, 2=calc_offset_edge, else use byte_3 as right X.
	69FD	CP $01
	69FF	JP Z,calc_mirrored_edge
	6A02	LD A,($5EFC)
	6A05	CP $02
	6A07	JP Z,calc_offset_edge
This entry point is used by the routines at calc_mirrored_edge and calc_offset_edge.
draw_right_edge	6A0A	LD D,A	Look up 2-byte edge sprite from terrain_edge_right using (right X AND $06) as index.
	6A0B	LD HL,$89FA
	6A0E	LD A,D
	6A0F	AND $06
	6A11	LD C,A
	6A12	LD B,$00
	6A14	LD A,D
	6A15	ADD HL,BC
	6A16	EX DE,HL
	6A17	LD C,A
	6A18	LD HL,($5F7B)	Calculate screen address = screen_ptr + (right X >> 3). Copy 2-byte edge sprite via LDIR.
	6A1B	SRL C
	6A1D	SRL C
	6A1F	SRL C
	6A21	ADD HL,BC
	6A22	EX DE,HL
	6A23	LD BC,$0002
	6A26	LDIR	Calculate fill count = 15 - (right X >> 3). This is number of solid tiles right of edge.
	6A28	LD B,A
	6A29	SRL B
	6A2B	SRL B
	6A2D	SRL B
	6A2F	LD A,$0F
	6A31	SUB B
	6A32	LD B,A
	6A33	AND $0F
	6A35	CP $00
	6A37	RET Z
	6A38	LD A,$FF	Load $FF (solid terrain byte) into A.
fill_island_right_loop	6A3A	LD (DE),A	Write A to (DE), increment DE, loop B times to fill rightward.
	6A3B	INC DE
	6A3C	DJNZ fill_island_right_loop
	6A3E	RET	Return after filling right terrain.