From f7f5ae6f96b41d70d24cf0a4670164aadb73813f Mon Sep 17 00:00:00 2001
From: Tijmen van Nesselrooij <tijmen_222@hotmail.com>
Date: Mon, 5 Aug 2019 20:39:46 +0200
Subject: [PATCH] Added collatz homework assignment

---
 x86_64/4_collatz.asm | 125 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 125 insertions(+)
 create mode 100644 x86_64/4_collatz.asm

diff --git a/x86_64/4_collatz.asm b/x86_64/4_collatz.asm
new file mode 100644
index 0000000..8d4a614
--- /dev/null
+++ b/x86_64/4_collatz.asm
@@ -0,0 +1,125 @@
+global _start
+
+section .bss
+  NR_INPUT_BUFFER: resb 4
+  NR_INPUT_BUFFER_SIZE: equ 4
+
+section .data
+	RQ_NR_STR: db 'Please give me a number (0-999): '
+	RQ_NR_STR_SIZE: equ $ - RQ_NR_STR
+
+	NR_STR: db '8'
+	NR_STR_SIZE: equ 1
+
+	ZERO_ASCII_VALUE: equ 48
+	NINE_ASCII_VALUE: equ 57
+	INPUT_BASE: equ 10
+
+  STDOUT: equ 1
+
+section .text
+_start:
+  ; Write to STDOUT
+  mov rax, 1 ; = write
+  mov rdi, STDOUT
+  mov rsi, RQ_NR_STR
+  mov rdx, RQ_NR_STR_SIZE
+  syscall
+
+  ; Read from STDIN
+  xor rax, rax ; 0 = read
+  xor rdi, rdi ; 0 = stdin
+  mov rsi, NR_INPUT_BUFFER
+  mov rdx, NR_INPUT_BUFFER_SIZE
+  syscall
+
+	; Parse number
+	mov rdi, NR_INPUT_BUFFER ; array to loop
+	dec rax
+	mov rsi, rax ; loop limit
+	call parseNR
+
+	mov rdi, rax
+	call collatz
+
+	mov rdi, rax
+exit:
+	mov rax, 60
+	syscall
+
+; int ParseNumber(char * p, size_t length)
+; IN
+; - rdi = ptr to byte array
+; - rsi = length of said array (greater than 0)
+; OUT
+; - rax = resulting number => returns 0 if it cannot parse
+parseNR:
+	xor r8, r8 ; loop variable
+	xor rcx, rcx ; sum
+.loop_body:
+	mov rax, rcx
+	mov rcx, qword INPUT_BASE
+	mul rcx
+	mov rcx, rax
+
+	movzx rax, byte [rdi + r8]
+	; Check if the value is within ASCII numeric bounds
+	cmp al, byte ZERO_ASCII_VALUE
+	jb .exit_failure
+	cmp al, byte NINE_ASCII_VALUE
+	jnb .exit_failure
+
+	; Subtract zero to make it a number
+	sub al, byte ZERO_ASCII_VALUE
+
+	; Store the sum
+	add rcx, rax
+
+	inc r8
+
+	cmp r8, rsi
+	jb .loop_body
+
+	; Return the result
+	mov rax, rcx
+	jmp .exit
+
+.exit_failure:
+	xor rax, rax
+.exit:
+	ret
+
+; int Collatz(int n)
+; IN
+; - rdi = number to "collatz"
+; OUT
+; - rax = number of "collatz" iterations it took to reach 0
+collatz:
+	xor r9, r9 ; iteration count
+
+.loop_start:
+	cmp rdi, qword 1
+	jbe .loop_exit
+
+	inc r9
+
+	; Is rdi odd?
+	mov rax, rdi
+	and rax, qword 1
+	cmp rax, qword 1
+	je .uneven_routine
+
+;.even_routine:
+	shr rdi, 1 ; divide by two using right shift 1
+	jmp .loop_start
+
+.uneven_routine:
+	mov rax, rdi
+	add rdi, rdi ; rdi + rdi
+	add rdi, rax ; rdi + rdi + rdi = 3 * rdi
+	inc rdi
+	jmp .loop_start
+
+.loop_exit:
+	mov rax, r9
+	ret
\ No newline at end of file