# This Source Code Form is subject to the terms of the Mozilla Public License,
# v. 2.0. If a copy of the MPL was not distributed with this file, You can
# obtain one at https://mozilla.org/MPL/2.0/.

# Stage4 compiler.
#
# - Taking value of local and global variables. Variables that doesn't begin
#   with "v" are considered global.
# - Simple variable assignment, e.g. v0 := 5 or v0 := global_variable;
#   7 words on the stack, 28 - 56, are reversed for procedure arguments (caller side).
# - Take address unary operation "@".

.section .rodata

.type keyword_section, @object
keyword_section: .ascii ".section"

.type keyword_type, @object
keyword_type: .ascii ".type"

.type keyword_ret, @object
keyword_ret: .ascii "ret"

.type keyword_global, @object
keyword_global: .ascii ".globl"

.type keyword_proc, @object
keyword_proc: .ascii "proc "

.type keyword_end, @object
keyword_end: .ascii "end"

.type keyword_begin, @object
keyword_begin: .ascii "begin"

.type keyword_var, @object
keyword_var: .ascii "var"

.type asm_prologue, @object
asm_prologue: .string "\taddi sp, sp, -64\n\tsw ra, 60(sp)\n\tsw s0, 56(sp)\n\taddi s0, sp, 64\n"

.type asm_epilogue, @object
asm_epilogue: .string "\tlw ra, 60(sp)\n\tlw s0, 56(sp)\n\taddi sp, sp, 64\n\tret\n"

.type asm_type_directive, @object
asm_type_directive: .string ".type "

.type asm_type_function, @object
asm_type_function: .string ", @function\n"

.type asm_colon, @object
asm_colon: .string ":\n"

.type asm_call, @object
asm_call: .string "\tcall "

.type asm_j, @object
asm_j: .string "\tj "

.type asm_li, @object
asm_li: .string "\tli "

.type asm_lw, @object
asm_lw: .string "\tlw "

.type asm_la, @object
asm_la: .string "\tla "

.type asm_sw, @object
asm_sw: .string "\tsw "

.type asm_addi, @object
asm_addi: .string "\taddi "

.type asm_t0, @object
asm_t0: .string "t0"

.type asm_t1, @object
asm_t1: .string "t1"

.type asm_comma, @object
asm_comma: .string ", "

.type asm_sp, @object
asm_sp: .string "sp"

.section .bss

# When modifiying also change the read size in the entry point procedure.
.type source_code, @object
source_code: .zero 81920

.section .data

.type source_code_position, @object
source_code_position: .word source_code

.section .text

# Reads standard input into a buffer.
# a0 - Buffer pointer.
# a1 - Buffer size.
#
# Returns the amount of bytes written in a0.
proc _read_file();
begin
	mv a2, a1
	mv a1, a0
	# STDIN.
	li a0, 0
	li a7, 63 # SYS_READ.
	ecall
end;

# Writes to the standard output.
#
# Parameters:
# a0 - Buffer.
# a1 - Buffer length.
proc _write_s();
begin
	mv a2, a1
	mv a1, a0
	# STDOUT.
	li a0, 1
	li a7, 64 # SYS_WRITE.
	ecall
end;

# Writes a number to a string buffer.
#
# t0 - Local buffer.
# t1 - Constant 10.
# t2 - Current character.
# t3 - Whether the number is negative.
#
# Parameters:
# a0 - Whole number.
# a1 - Buffer pointer.
#
# Sets a0 to the length of the written number.
proc _print_i();
begin
	li t1, 10
	addi t0, s0, -9

	li t3, 0
	bgez a0, .print_i_digit10
	li t3, 1
	neg a0, a0

.print_i_digit10:
	rem t2, a0, t1
	addi t2, t2, '0'
	sb t2, 0(t0)
	div a0, a0, t1
	addi t0, t0, -1
	bne zero, a0, .print_i_digit10

	beq zero, t3, .print_i_write_call
	addi t2, zero, '-'
	sb t2, 0(t0)
	addi t0, t0, -1

.print_i_write_call:
	mv a0, a1
	addi a1, t0, 1
	sub a2, s0, t0
	addi a2, a2, -9
	sw a2, 0(sp)

	_memcpy();

	lw a0, 0(sp)
end;

# Writes a number to the standard output.
#
# Parameters:
# a0 - Whole number.
proc _write_i();
begin
	addi a1, sp, 0
	_print_i();

	mv a1, a0
	addi a0, sp, 0
	_write_s();

end;

# Writes a character from a0 into the standard output.
proc _write_c();
begin
	sb a0, 0(sp)
	addi a0, sp, 0
	li a1, 1
	_write_s();
end;

# Write null terminated string.
#
# Parameters:
# a0 - String.
proc _write_z();
begin
	sw a0, 0(sp)

.write_z_loop:
	# Check for 0 character.
	lb a0, (a0)
	beqz a0, .write_z_end

	# Print a character.
	lw a0, 0(sp)
	lb a0, (a0)
	_write_c();

	# Advance the input string by one byte.
	lw a0, 0(sp)
	addi a0, a0, 1
	sw a0, 0(sp)

	goto .write_z_loop;

.write_z_end:
end;

# Detects if a0 is an uppercase character. Sets a0 to 1 if so, otherwise to 0.
proc _is_upper();
begin
	li t0, 'A' - 1
	sltu t1, t0, a0 # t1 = a0 >= 'A'

	sltiu t2, a0, 'Z' + 1 # t2 = a0 <= 'Z'
	and a0, t1, t2 # t1 = a0 >= 'A' & a0 <= 'Z'
end;

# Detects if a0 is an lowercase character. Sets a0 to 1 if so, otherwise to 0.
proc _is_lower();
begin
	li t0, 'a' - 1
	sltu t2, t0, a0 # t2 = a0 >= 'a'

	sltiu t3, a0, 'z' + 1 # t3 = a0 <= 'z'
	and a0, t2, t3 # t2 = a0 >= 'a' & a0 <= 'z'
end;

# Detects if the passed character is a 7-bit alpha character or an underscore.
#
# Paramters:
# a0 - Tested character.
#
# Sets a0 to 1 if the character is an alpha character or underscore, sets it to 0 otherwise.
proc _is_alpha();
begin
	sw a0, 0(sp)

	_is_upper();
	sw a0, 4(sp)

	_is_lower(v0);

	lw t0, 0(sp)
	xori t1, t0, '_'
	seqz t1, t1

	lw t0, 4(sp)
	or a0, a0, t0
	or a0, a0, t1
end;

# Detects whether the passed character is a digit
# (a value between 0 and 9).
#
# Parameters:
# a0 - Exemined value.
#
# Sets a0 to 1 if it is a digit, to 0 otherwise.
proc _is_digit();
begin
	li t0, '0' - 1
	sltu t1, t0, a0 # t1 = a0 >= '0'

	sltiu t2, a0, '9' + 1 # t2 = a0 <= '9'

	and a0, t1, t2
end;

proc _is_alnum();
begin
	sw a0, 4(sp)

	_is_alpha();
	sw a0, 0(sp)

	_is_digit(v4);

	lw a1, 0(sp)
	or a0, a0, a1
end;

# Reads the next token.
#
# Returns token length in a0.
proc _read_token();
begin
	la t0, source_code_position # Token pointer.
	lw t0, (t0)
	sw t0, 0(sp) # Current token position.
	sw zero, 4(sp) # Token length.

.read_token_loop:
	lb t0, (t0) # Current character.

	# First we try to read a derictive.
	# A derictive can contain a dot and characters.
	li t1, '.'
	beq t0, t1, .read_token_next

	lw a0, 0(sp)
	lb a0, (a0)
	_is_alnum();
	bnez a0, .read_token_next

	goto .read_token_end;

.read_token_next:
	# Advance the source code position and token length.
	lw t0, 4(sp)
	addi t0, t0, 1
	sw t0, 4(sp)

	lw t0, 0(sp)
	addi t0, t0, 1
	sw t0, 0(sp)

	goto .read_token_loop;

.read_token_end:
	lw a0, 4(sp)
end;

# a0 - First pointer.
# a1 - Second pointer.
# a2 - The length to compare.
#
# Returns 0 in a0 if memory regions are equal.
proc _memcmp();
begin
	mv t0, a0
	li a0, 0

.memcmp_loop:
	beqz a2, .memcmp_end

	lbu t1, (t0)
	lbu t2, (a1)
	sub a0, t1, t2

	bnez a0, .memcmp_end

	addi t0, t0, 1
	addi a1, a1, 1
	addi a2, a2, -1

	goto .memcmp_loop;

.memcmp_end:
end;

# Copies memory.
#
# Parameters:
# a0 - Destination.
# a1 - Source.
# a2 - Size.
#
# Preserves a0.
proc _memcpy();
begin
	mv t0, a0

.memcpy_loop:
	beqz a2, .memcpy_end

	lbu t1, (a1)
	sb t1, (a0)

	addi a0, a0, 1
	addi a1, a1, 1
	addi a2, a2, -1

	goto .memcpy_loop

.memcpy_end:
	mv a0, t0
end;

# Advances the token stream by a0 bytes.
proc _advance_token();
begin
	la t0, source_code_position
	lw t1, (t0)
	add t1, t1, a0
	sw t1, (t0)
end;

# Prints the current token.
#
# Parameters:
# a0 - Token length.
#
# Returns a0 unchanged.
proc _write_token();
begin
	sw a0, 0(sp)

	la a0, source_code_position
	lw a0, (a0)
	lw a1, 0(sp)
	_write_s();

	lw a0, 0(sp)
end;

proc _compile_section();
begin
	# Print and skip the ".section" (8 characters) directive and a space after it.
	_write_token(9);
	_advance_token();

	# Read the section name.
	_read_token();
	addi a0, a0, 1

	_write_token();
	_advance_token();
end;

# Prints and skips a line.
proc _skip_comment();
begin
	la t0, source_code_position
	lw t1, (t0)

.skip_comment_loop:
	# Check for newline character.
	lb t2, (t1)
	li t3, '\n'
	beq t2, t3, .skip_comment_end

	# Advance the input string by one byte.
	addi t1, t1, 1
	sw t1, (t0)

	goto .skip_comment_loop;

.skip_comment_end:
	# Skip the newline.
	addi t1, t1, 1
	sw t1, (t0)
end;

# Prints and skips a line.
proc _compile_line();
begin
.compile_line_loop:
	la a0, source_code_position
	lw a1, (a0)

	lb t0, (a1)
	li t1, '\n'
	beq t0, t1, .compile_line_end

	# Print a character.
	lw a0, (a1)
	_write_c();

	# Advance the input string by one byte.
	_advance_token(1);

	goto .compile_line_loop;

.compile_line_end:
	_write_c('\n');

	_advance_token(1);
end;

proc _compile_integer_literal();
begin
	la a0, asm_li
	_write_z();

	la a0, asm_t0
	_write_z();

	la a0, asm_comma
	_write_z();

	_read_token();
	_write_token();
	_advance_token();

	_write_c('\n');
end;

proc _compile_character_literal();
begin
	la a0, asm_li
	_write_z();

	la a0, asm_t0
	_write_z();

	la a0, asm_comma
	_write_z();

	_write_c('\'');
	_advance_token(1);

	la t0, source_code_position
	lw t0, (t0)
	lb a0, (t0)
	li t1, '\\'
	bne a0, t1, .compile_character_literal_end
	
	_write_c('\\');
	_advance_token(1);

.compile_character_literal_end:
	la t0, source_code_position
	lw t0, (t0)
	lb a0, (t0)
	_write_c();

	_write_c('\'');
	_write_c('\n');

	_advance_token(2);

end;

proc _compile_variable_expression();
begin
	_compile_designator();

	la a0, asm_lw
	_write_z();

	la a0, asm_t0
	_write_z();

	la a0, asm_comma
	_write_z();

	_write_c('(');
	la a0, asm_t0
	_write_z();

	_write_c(')');
	_write_c('\n');

end;

proc _compile_address_expression();
begin
	# Skip the "@" sign.
	_advance_token(1);
	_compile_designator();

end;

proc _compile_expression();
begin
	la t0, source_code_position
	lw t0, (t0)
	lb a0, (t0)
	sw a0, 0(sp)

	li t1, '\''
	beq a0, t1, .compile_expression_character_literal

	li t1, '@'
	beq a0, t1, .compile_expression_address

	_is_digit(v0);
	bnez a0, .compile_expression_integer_literal

	goto .compile_expression_variable;

.compile_expression_character_literal:
	_compile_character_literal();
	goto .compile_expression_end;

.compile_expression_integer_literal:
	_compile_integer_literal();
	goto .compile_expression_end;

.compile_expression_address:
	_compile_address_expression();
	goto .compile_expression_end;

.compile_expression_variable:
	_compile_variable_expression();
	goto .compile_expression_end;

.compile_expression_end:
end;

proc _compile_call();
begin
	# Stack variables:
	# v0 - Procedure name length.
	# v4 - Procedure name pointer. 
	# v8 - Argument count.

	_read_token();
	sw a0, 0(sp)
	la t0, source_code_position
	lw t0, (t0)
	sw t0, 4(sp)

	sw zero, 8(sp)

	# Skip the identifier and left paren.
	addi a0, a0, 1
	_advance_token();

	la t0, source_code_position
	lw t0, (t0)
	lb t0, (t0)

	li t1, ')'
	beq t0, t1, .compile_call_finalize

.compile_call_loop:
	_compile_expression();

	# Save the argument on the stack.
	la a0, asm_sw
	_write_z();

	la a0, asm_t0
	_write_z();

	la a0, asm_comma
	_write_z();

	# Calculate the stack offset: 52 - (4 * argument_counter)
	lw t0, 8(sp)
	li t1, 4
	mul t0, t0, t1
	li t1, 52
	sub a0, t1, t0
	_write_i();

	_write_c('(')
	la a0, asm_sp
	_write_z();
	_write_c(')')

	_write_c('\n');

	# Add one to the argument counter.
	lw t0, 8(sp)
	addi t0, t0, 1
	sw t0, 8(sp)

	la t0, source_code_position
	lw t0, (t0)
	lb t0, (t0)

	li t1, ','
	bne t0, t1, .compile_call_finalize

	_advance_token(2);
	goto .compile_call_loop;

.compile_call_finalize:
	# Load the argument from the stack.

	lw t0, 8(sp)
	beqz t0, .compile_call_end

	# Decrement the argument counter.
	lw t0, 8(sp)
	addi t0, t0, -1
	sw t0, 8(sp)

	la a0, asm_lw
	_write_z();

	_write_c('a');
	lw a0, 8(sp)
	_write_i();

	la a0, asm_comma
	_write_z();

	# Calculate the stack offset: 52 - (4 * argument_counter)
	lw t0, 8(sp)
	li t1, 4
	mul t0, t0, t1
	li t1, 52
	sub a0, t1, t0
	_write_i();

	_write_c('(');
	la a0, asm_sp
	_write_z();

	_write_c(')');
	_write_c('\n');

	goto .compile_call_finalize;

.compile_call_end:
	la a0, asm_call
	_write_z();

	_write_s(v4, v0);

	# Skip the right paren.
	_advance_token(1);
end;

proc _compile_goto();
begin
	_advance_token(5);

	_read_token();
	sw a0, 0(sp)

	la a0, asm_j
	_write_z();

	_write_token(v0);
	_advance_token();
end;

proc _compile_local_designator();
begin
	# Skip "v" in the local variable name.
	_advance_token(1);

	la a0, asm_addi
	_write_z();

	la a0, asm_t0
	_write_z();

	la a0, asm_comma
	_write_z();

	la a0, asm_sp
	_write_z();

	la a0, asm_comma
	_write_z();

	# Read local variable stack offset and save it.
	la t0, source_code_position
	lw t0, (t0)
	sw t0, 0(sp)

	_read_token();
	sw a0, 4(sp)

	_write_token();
	_advance_token();

	_write_c('\n');

end;

proc _compile_global_designator();
begin
	la a0, asm_la
	_write_z();

	la a0, asm_t0
	_write_z();

	la a0, asm_comma
	_write_z();

	_read_token();
	_write_token();
	_advance_token();

	_write_c('\n');

end;

proc _compile_designator();
begin
	la t0, source_code_position
	lw t0, (t0)
	lb a0, (t0)

	li t1, 'v'
	beq a0, t1, .compile_designator_local

	goto .compile_designator_global;

.compile_designator_local:
	_compile_local_designator();
	goto .compile_designator_end;

.compile_designator_global:
	_compile_global_designator();
	goto .compile_designator_end;

.compile_designator_end:
end;

proc _compile_assignment();
begin
	_compile_designator();

	# Save the assignee address on the stack.
	la a0, asm_sw
	_write_z();

	la a0, asm_t0
	_write_z();

	la a0, asm_comma
	_write_z();

	_write_i(20);
	_write_c('(');
	la a0, asm_sp
	_write_z();
	_write_c(')');
	_write_c('\n');
	# Skip the assignment sign (:=) with surrounding whitespaces.
	_advance_token(4);

	# Compile the assignment.
	_compile_expression();

	la a0, asm_lw
	_write_z();

	la a0, asm_t1
	_write_z();

	la a0, asm_comma
	_write_z();

	_write_i(20);
	_write_c('(');
	la a0, asm_sp
	_write_z();
	_write_c(')');
	_write_c('\n');

	la a0, asm_sw
	_write_z();

	la a0, asm_t0
	_write_z();

	la a0, asm_comma
	_write_z();

	_write_c('(');
	la a0, asm_t1
	_write_z();
	_write_c(')');
end;

proc _compile_statement();
begin
	# This is a call if the statement starts with an underscore.
	la t0, source_code_position
	lw t0, (t0)
	# First character after alignment tab.
	addi t0, t0, 1
	lb t0, (t0)
	
	li t1, '_'
	beq t0, t1, .compile_statement_call

	li t1, 'g'
	beq t0, t1, .compile_statement_goto

	li t1, 'v'
	beq t0, t1, .compile_statement_assignment

	_compile_line();
	goto .compile_statement_end;

.compile_statement_call:
	_advance_token(1);
	_compile_call();

	goto .compile_statement_semicolon;

.compile_statement_goto:
	_advance_token(1);
	_compile_goto();

	goto .compile_statement_semicolon;

.compile_statement_assignment:
	_advance_token(1);
	_compile_assignment();

	goto .compile_statement_semicolon;

.compile_statement_semicolon:
	_advance_token(2);

	_write_c('\n');

.compile_statement_end:
end;

proc _compile_procedure_body();
begin
.compile_procedure_body_loop:
	la a0, source_code_position
	lw a0, (a0)
	la a1, keyword_end
	li a2, 3 # "end" length.
	_memcmp();

	beqz a0, .compile_procedure_body_epilogue

	_compile_statement();
	goto .compile_procedure_body_loop;

.compile_procedure_body_epilogue:
end;

proc _compile_procedure();
begin
	# Skip "proc ".
	_advance_token(5);

	_read_token();
	sw a0, 0(sp) # Save the procedure name length.

	# Write .type _procedure_name, @function.
	la a0, asm_type_directive
	_write_z();

	_write_token(v0);

	la a0, asm_type_function
	_write_z();

	# Write procedure label, _procedure_name:
	_write_token(v0);

	la a0, asm_colon
	_write_z();

	# Skip the function name and trailing parens, semicolon, "begin" and newline.
	lw a0, 0(sp)
	addi a0, a0, 10
	_advance_token();

	la a0, asm_prologue
	_write_z();

	_compile_procedure_body();

	# Write the epilogue.
	la a0, asm_epilogue
	_write_z();

	# Skip the "end" keyword, semicolon and newline.
	_advance_token(5);
end;

proc _compile_type();
begin
	# Print and skip the ".type" (5 characters) directive and a space after it.
	_write_token(6);
	_advance_token();

	# Read and print the symbol name.
	_read_token();

	# Print and skip the symbol name, comma, space and @.
	addi a0, a0, 3
	_write_token();
	_advance_token();

	# Read the symbol type.
	_read_token();
	la t0, source_code_position
	lw t0, (t0)
	sw t0, 12(sp)

	# Print the symbol type and newline.
	addi a0, a0, 1
	_write_token();
	_advance_token();

	# Write the object definition itself.
	_compile_line();

.compile_type_end:
end;

proc _skip_newlines();
begin
	# Skip newlines.
	la t0, source_code_position
	lw t1, (t0)

.skip_newlines_loop:
	lb t2, (t1)
	li t3, '\n'
	bne t2, t3, .skip_newlines_end
	beqz t2, .skip_newlines_end

	addi t1, t1, 1
	sw t1, (t0)

	goto .skip_newlines_loop;

.skip_newlines_end:
end;

# Process the source code and print the generated code.
proc _compile();
begin
.compile_loop:
	_skip_newlines();

	la t0, source_code_position
	lw t0, (t0)
	lb t0, (t0)
	beqz t0, .compile_end
	li t1, '#'
	beq t0, t1, .compile_comment

	la a0, source_code_position
	lw a0, (a0)
	la a1, keyword_section
	li a2, 8 # ".section" length.
	_memcmp();

	beqz a0, .compile_section

	la a0, source_code_position
	lw a0, (a0)
	la a1, keyword_type
	li a2, 5 # ".type" length.
	_memcmp();

	beqz a0, .compile_type

	la a0, source_code_position
	lw a0, (a0)
	la a1, keyword_proc
	li a2, 5 # "proc " length. Space is needed to distinguish from "procedure".
	_memcmp();

	beqz a0, .compile_procedure

	la a0, source_code_position
	lw a0, (a0)
	la a1, keyword_global
	li a2, 6 # ".globl" length.
	_memcmp();

	beqz a0, .compile_global
	# Not a known token, exit.
	goto .compile_end;

.compile_section:
	_compile_section();

	goto .compile_loop;

.compile_type:
	_compile_type();

	goto .compile_loop;

.compile_global:
	_compile_line();

	goto .compile_loop;

.compile_comment:
	_skip_comment();

	goto .compile_loop;

.compile_procedure:
	_compile_procedure();

	goto .compile_loop;

.compile_end:
end;

# Terminates the program. a0 contains the return code.
#
# Parameters:
# a0 - Status code.
proc _exit();
begin
	li a7, 93 # SYS_EXIT
	ecall
end;

# Entry point.
.globl _start
proc _start();
begin
	# Read the source from the standard input.
	la a0, source_code
	li a1, 81920 # Buffer size.
	_read_file();
	_compile();

	_exit(0);

end;