.global _start

.data

v_elf_header:
	.byte 0x7f       # Magic Number
	.ascii "ELF"     #
	.byte 0x2        # 64-Bit
	.byte 0x1        # little-endian
	.byte 0x1        # ELFv1
	.byte 0x0        # System V ABI
	.quad 0x0        #
	.hword 0x02      # Executable
	.hword 0x3e      # AMD x86-64
	.int 0x1         # ELFv1
	.quad 0x401000   # Entry
	.quad 0x40       # Program Header Offset
	.quad 0x100      # TODO: Section Header Offset
	.int 0x0         #
	.hword 0x40      # ELF Header Size
	.hword 0x38      # Program Header Table Entry Size
	.hword 0x3       # TODO: Number Of Entries in Program Header Table
	.hword 0x40      # Section Header Table Entry Size
	.hword 0x3       # TODO: Number Of Entries in Section Header Table
	.hword 0x2       # TODO: Index of Section Header Table Entry containing section names

v_usage_message:
	.asciz "bfc [input file] [output file]\n"

v_input_fd:
	.int 0x0

v_output_fd:
	.int 0x0

v_buffer_start:
	.quad 0x0

v_buffer_end:
	.quad 0x0

v_buffer_cursor:
	.quad 0x0


.text
_start:
	mov %rsp, %rbp
	call f_check_args
	call f_open_files
	call f_init_buffer

	mov $v_buffer_cursor, %rbx
	mov (%rbx), %rsi
	add $0x300, %rsi
	mov %rsi, (%rbx)
	call f_check_buffer

	mov $v_buffer_cursor, %rsi
	mov (%rbx), %rsi
	add $0x300, %rsi
	mov %rsi, (%rbx)
	call f_check_buffer

	# ----

	mov $1, %rax
	mov %rcx, %rdi
	mov $v_elf_header, %rsi
	mov $0x40, %rdx
	syscall

	# ----

	call f_close_files
	mov $0, %rdi
	jmp f_exit

f_exit:
	mov $60, %rax
	syscall

f_check_args:
	mov (%rbp), %rax
	cmp $3, %rax
	je l_check_args_success

	mov $1, %rax
	mov $2, %rdi
	mov $v_usage_message, %rsi
	mov $32, %rdx
	syscall

	mov $-1, %rdi
	jmp f_exit

l_check_args_success:
	ret

f_open_files:
	mov $2, %rax
	mov 16(%rbp), %rdi
	mov $0, %rsi
	mov $0, %rdx
	syscall
	mov $v_input_fd, %rbx
	mov %eax, (%rbx)

	mov $2, %rax
	mov 24(%rbp), %rdi
	mov $0x41, %rsi
	mov $0744, %rdx
	syscall
	mov $v_output_fd, %rbx
	mov %eax, (%rbx)
	ret

f_close_files:
	mov $3, %rax
	mov %rbx, %rsi
	syscall

	mov $3, %rax
	mov %rcx, %rsi
	syscall
	ret

f_init_buffer:
	mov $12, %rax
	mov $0, %rdi
	syscall
	mov $v_buffer_start, %rbx
	mov %rax, (%rbx)
	mov $v_buffer_end, %rbx
	mov %rax, (%rbx)
	mov $v_buffer_cursor, %rbx
	mov %rax, (%rbx)
	/*
	 * We directly skip to f_resize_buffer
	 * since we want to resize it to a usable amount
	 */
f_resize_buffer:
	mov $12, %rax
	mov $v_buffer_end, %rbx
	mov (%rbx), %rdi
	add $0x400, %rdi
	syscall
	mov %rax, (%rbx)
	ret

/* %rsi = needed end */
f_check_buffer:
	mov $v_buffer_end, %rbx
	mov (%rbx), %rax
	cmp %rax, %rsi
	jl l_check_buffer_large_enough
	call f_resize_buffer
l_check_buffer_large_enough:
	ret