Add rot13

rot13/.gitignore

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+rot13*
+.gdb_history

rot13/README.md

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+Rot13
+=====
+## A rot13 program in amd64 linux asm.
+
+Be warned: it does not appear to be possible to enter more than 4096 bytes,
+because if you do, it gets truncated.

rot13/main.asm

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+global _start
+
+; bss is the section for data like buffers that is zeroed out
+section .bss
+	; Reserve 1KiB for the buffer size. Do not reserve more than `(2^65)-1` bytes,
+	; as that is the maximum size allowed (the counter register and/or the bytes
+	; read will overflow and cause bugs)
+	buf: resb 8192
+	; This should be equal to the buffer size, if it is smaller it's a waste, if
+	; it's bigger it can segfault.
+	BUF_S: equ 8192
+
+section .text
+_start:
+	; Read into buffer
+	xor rax, rax
+	mov rsi, buf
+	mov rdx,BUF_S
+	syscall
+	; Clear rcx (rcx to be the counter)
+	xor rcx,rcx
+
+rot_loop:
+	; If counter is equal to max bytes,
+	cmp rcx, rax
+	; Done, jump to finish.
+	je done
+	; Set lowest byte of rdx to buf[rcx].
+	mov dl, byte [buf+rcx]
+	; If dl is greater than or equal to 'a',
+	cmp dl, 97
+	; Go to lowercase.
+	jge lower
+	; If dl is greater than or equal to 'A',
+	cmp dl, 65
+	; Go to uppercase.
+	jge upper
+
+	; Else, it's not a letter, inc rcx and go back to start of loop.
+	inc rcx
+	jmp rot_loop
+
+lower:
+	; If dl is greater than 'z',
+	cmp dl, 122
+	; It's not a letter, so go to special char.
+	jg special
+	; If dl is less than or equal to 'm',
+	cmp dl, 109
+	; Add 13 letters.
+	jle up13
+	; Else, subtract 13.
+	jmp down13
+
+; This loop does the exact thing as `lower` but uses uppercase letters instead.
+upper:
+	cmp dl, 90
+	jg special
+	cmp dl, 77
+	jle up13
+	jmp down13
+
+; This handles special chars, it just increments rcx and goes to the start of
+; the loop.
+special:
+	inc rcx
+	jmp rot_loop
+
+; Adds 13 to dl, increments rcx, and goes to loop start.
+up13:
+	add dl, 13
+	mov byte [buf+rcx], dl
+	inc rcx
+	jmp rot_loop
+
+; Same as up13 but subtracts instead.
+down13:
+	sub dl, 13
+	mov byte [buf+rcx], dl
+	inc rcx
+	jmp rot_loop
+
+; If done, print buffer (it has been rot13ed) and exit(0)
+done:
+	mov rdx, rax
+	mov rax, 1
+	mov rdi, 1
+	mov rsi, buf
+	syscall
+
+	cmp byte [buf+rax-1], 0xA
+	jne _start
+	
+	mov rax, 60
+	xor rdi,rdi
+	syscall
+
+; vim: set ft=nasm:
+; vim: set ai:

rot13/make.sh

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+#!/usr/bin/env bash
+if [ "$1" == "rot13.dbg" ]; then
+	nasm -felf64 -g main.asm
+	ld main.o -o rot13.dbg
+	exit
+fi
+nasm -felf64 main.asm
+ld main.o -s -o rot13