; Fibonacci number generator using AMD64 assembly with Windows syscalls
; Uses lookup table for O(1) access
; Author: Assistant

global _main
extern _ExitProcess@4
extern _GetStdHandle@4
extern _WriteConsoleA@16

section .data
    ; Precomputed Fibonacci numbers (first 50)
    fib_table dd 1, 1, 2, 3, 5, 8, 13, 21, 34, 55
    fib_table dd 89, 144, 233, 377, 610, 987, 1597, 2584, 4181, 6765
    fib_table dd 10946, 17711, 28657, 46368, 75025, 121393, 196418, 317811, 514229, 832040
    fib_table dd 1346269, 2178309, 3524578, 5702887, 9227465, 14930352, 24157817, 39088169, 63245986, 102334155
    fib_table dd 165580141, 267914296, 433494437, 701408733, 1134903170, 1836311903
    
    ; Console handles
    hStdOut dd 0
    
    ; Error messages
    msg_error db 'Error: Index out of range', 0
    msg_success db 'Fibonacci number: ', 0

section .text
; Function to get Fibonacci number at index n (0-based)
; Input: RDI = index
; Output: RAX = Fibonacci number
get_fibonacci:
    ; Validate input
    cmp rdi, 0
    jl invalid_input
    
    ; Check if index is within range (max 50)
    cmp rdi, 49
    jg invalid_input
    
    ; Calculate address in table
    mov rax, rdi
    shl rax, 2          ; Multiply by 4 (DWORD size)
    
    ; Load Fibonacci number from table
    mov eax, [fib_table + rax]
    ret
    
invalid_input:
    xor rax, rax        ; Return 0 for invalid input
    ret

; Function to print a number to console
print_number:
    ; Input: RAX = number to print
    push rax
    push rbx
    push rcx
    push rdx
    
    ; Convert number to string and print
    mov rbx, 10
    mov rcx, 0          ; Digit count
    
    ; Special case for zero
    cmp rax, 0
    jne convert_loop
    mov rcx, 1
    jmp print_digits
    
convert_loop:
    cmp rax, 0
    je print_digits
    
    xor rdx, rdx        ; Clear high bits
    div rbx             ; Divide by 10
    push rdx            ; Push remainder (digit)
    inc rcx             ; Increment digit count
    jmp convert_loop
    
print_digits:
    cmp rcx, 0
    je done_printing
    
    pop rax             ; Get digit back
    add rax, '0'        ; Convert to ASCII
    push rax
    
    ; Write character to console
    mov edx, esp
    push 1              ; Number of characters
    push edx            ; Address of character
    push dword [hStdOut] ; Handle
    call _WriteConsoleA@16
    
    add rsp, 12         ; Clean up stack
    
    dec rcx
    jmp print_digits
    
done_printing:
    pop rdx
    pop rcx
    pop rbx
    pop rax
    ret

; Main program entry point
_main:
    ; Get console handle
    mov rdi, -11        ; STD_OUTPUT_HANDLE
    call _GetStdHandle@4
    mov [hStdOut], eax
    
    ; Test Fibonacci numbers
    mov rdi, 10         ; Get 11th Fibonacci number (0-indexed)
    call get_fibonacci
    
    ; Print result
    push rax
    call print_number
    add rsp, 8
    
    ; Exit program
    mov rdi, 0
    call _ExitProcess@4