Krakout: Compose the level's brick field into the tile map

9731: Compose the level's brick field into the tile map

Composes the current level's brick field into the tile map, once at level start. Each slot code becomes a 2×2 group of tile codes and one attribute byte. The bricks reach the screen only through the tile-map render.
The grid is 9×9 slots, each 2×2 characters. It spans the playfield's full height, rows 4 to 21, and 18 of its columns. The field sits against the wall opposite the bat.
compose_brick_field	9731	LD A,(bat_side)	Read the bat side: 0 for the right, non-zero for the left.
	9734	OR A
	9735	JR Z,bat_on_right
	9737	LD HL,TILEMAP_BASE+$9C	Bat on the left: anchor the grid at tile-map row 4, column 28.
	973A	LD (compose_grid+$2),HL	Store the anchor as the grid's start cell, in the operand of the LD HL below.
	973D	LD HL,$2D2D	Patch the per-cell column step to DEC L,DEC L: cells advance 2 columns leftward.
	9740	LD (smc_grid_col_step),HL
	9743	JR seek_layout
bat_on_right	9745	LD HL,TILEMAP_BASE+$82	Bat on the right: anchor the grid at tile-map row 4, column 2.
	9748	LD (compose_grid+$2),HL	Store the anchor as the grid's start cell.
	974B	LD HL,$2C2C	Patch the per-cell column step to INC L,INC L: cells advance 2 columns rightward.
	974E	LD (smc_grid_col_step),HL
seek_layout	9751	LD A,(level)	Index this level's slot layout by the level index.
	9754	LD HL,level_layouts	Base of the slot-layout table.
	9757	OR A	Level 0 uses the table base unchanged.
	9758	JR Z,compose_grid	Level 0 uses the table base unchanged.
	975A	LD B,A	Layouts are 81 bytes each (a 9×9 grid of slot codes); skip the layouts of earlier levels.
	975B	LD DE,$0051
seek_layout_loop	975E	ADD HL,DE	Step past one layout per earlier level. HL then points at this level's layout.
	975F	DJNZ seek_layout_loop
compose_grid	9761	EX DE,HL	Point DE at the first slot code of this level's layout.
	9762	LD HL,$0000	Point HL at the anchor cell. The bat-side setup above patches the operand.
	9765	LD B,$09	Walk the grid's rows of bricks.
next_brick_row	9767	PUSH BC	Save the row counter and this row's start cell.
	9768	PUSH HL	Save the row counter and this row's start cell.
	9769	LD B,$09	Walk this row's bricks.
next_brick_cell	976B	PUSH BC	Save the slot counter, the layout pointer, and this cell's tile-map address.
	976C	PUSH DE
	976D	PUSH HL
	976E	LD A,(DE)	Read this cell's code; keep a copy in A' for the attribute lookup.
	976F	EX AF,AF'
	9770	LD A,(DE)	Look up the slot code's four tile codes in the brick tile-glyph groups, 4 bytes per code, and point DE at them. They are the slot's tiles in row order.
	9771	LD L,A
	9772	LD H,$00
	9774	ADD HL,HL
	9775	ADD HL,HL
	9776	LD DE,brick_tile_groups
	9779	ADD HL,DE
	977A	EX DE,HL
	977B	POP HL	Recover this cell's address (the top-left character of the 2×2 block).
	977C	PUSH HL
	977D	LD A,(DE)	Write the cell's top row: the top-left tile, then the top-right one in the next column. Then step down one tile-map row.
	977E	LD (HL),A
	977F	INC L
	9780	INC DE
	9781	LD A,(DE)
	9782	LD (HL),A
	9783	INC DE
	9784	DEC L
	9785	CALL advance_buffer_row
	9788	LD A,(DE)	Write the bottom row: the bottom-left tile, then the bottom-right one in the next column.
	9789	LD (HL),A
	978A	INC L
	978B	INC DE
	978C	LD A,(DE)
	978D	LD (HL),A
	978E	POP HL	Recover the cell address for the attribute pass.
	978F	PUSH HL	Recover the cell address for the attribute pass.
	9790	LD A,H	Map the tile-map page to the attribute page of the same cell; the shared low byte keeps the same cell.
	9791	AND $03
	9793	ADD A,$58
	9795	LD H,A
	9796	PUSH HL	Look up the cell's attribute by code in the brick attributes, into C.
	9797	EX AF,AF'
	9798	LD E,A
	9799	LD D,$00
	979B	LD HL,brick_attrs
	979E	ADD HL,DE
	979F	LD C,(HL)
	97A0	POP HL
	97A1	LD (HL),C	Paint all four attribute cells of the 2×2 block with C.
	97A2	INC L
	97A3	LD (HL),C
	97A4	CALL advance_buffer_row
	97A7	LD (HL),C
	97A8	DEC L
	97A9	LD (HL),C
	97AA	POP HL	Recover the cell address.
smc_grid_col_step	97AB	INC L	Self-modified per-cell column step, this byte and the next, patched by the bat-side setup. Both are INC L to advance 2 columns right, or both DEC L to advance left.
	97AC	INC L
	97AD	POP DE	Restore the layout pointer and step to the next slot code.
	97AE	INC DE	Restore the layout pointer and step to the next slot code.
	97AF	POP BC	Loop for the 9 cells across this row.
	97B0	DJNZ next_brick_cell	Loop for the 9 cells across this row.
	97B2	POP HL	Recover this row's start cell.
	97B3	CALL advance_buffer_row	Step down 2 tile-map rows to the next cell row (each cell is 2 rows tall).
	97B6	CALL advance_buffer_row
	97B9	POP BC	Loop for the 9 cell rows.
	97BA	DJNZ next_brick_row	Loop for the 9 cell rows.
	97BC	RET

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Routines