From f2401ef479c0457c504e39bc2bf0655eae7b5d87 Mon Sep 17 00:00:00 2001
From: David Makray <dmakray@gmail.com>
Date: Sun, 31 Oct 2021 22:37:00 -0400
Subject: [PATCH] Factorial Calculation Algorithm

---
 algorithms/math/factorial.asm | 65 +++++++++++++++++++++++++++++++++++
 1 file changed, 65 insertions(+)
 create mode 100644 algorithms/math/factorial.asm

diff --git a/algorithms/math/factorial.asm b/algorithms/math/factorial.asm
new file mode 100644
index 0000000..3461912
--- /dev/null
+++ b/algorithms/math/factorial.asm
@@ -0,0 +1,65 @@
+section .data
+	hexoutdata	times 8 db 0, 10, 0
+	hexoutdatalength	equ $-hexoutdata
+
+section .text
+global _start
+_start:
+	mov eax, 4
+
+	call Factorial
+	bswap eax		;Fix little endian for proper hexidecimal readout
+	call HexConvert
+
+	mov eax, 4		;write to file
+	mov ebx, 1		;STDOUT handle
+	mov ecx, hexoutdata		;Message pointer
+	mov edx, hexoutdatalength		;Size of the message
+	int 80h			;System call
+	
+	xor ebx, ebx 		;Exit code is 0
+	mov eax, 1		;Terminate process code
+	int 80h		;System call
+
+
+HexConvert:
+	bswap eax
+	mov ecx, 8
+
+	hexloop:
+		mov ebx, eax
+		and ebx, 1111b
+
+		cmp ebx, 10
+		jb hexlessthanten
+		add ebx, 'a' - 10
+		jmp hexdone
+
+	hexlessthanten:
+		add ebx, '0'
+	hexdone:
+		mov edx, hexoutdata	;Base pointer
+		add edx, ecx	;Pointer offset
+		dec edx			;Correct to zero-based offset
+		mov esi, edx
+		mov [esi], bl
+		shr eax, 4
+		loop hexloop
+
+		mov eax, hexoutdata
+		add eax, 8
+		mov esi, eax
+		mov al, 0
+		mov [esi], al
+	ret
+
+Factorial:
+	mov ebx, 1
+fact_top:
+	imul ebx, eax
+	cmp eax, 2
+	dec eax
+	jne fact_top
+	mov eax, ebx
+	ret
+