; Intel x86 asm implementation of rot13 using a table
; by Tim Newsome <nuisance@cmu.edu>
; Compile with: nasm rot13.asm;cc -o rot13 rot13.o
; This could probably be tons faster, it's my first intel asm. (Any
; optimizations I made were for Pentium

	GLOBAL	main

	SEGMENT	.text

; Do fu so it can be turned into a binary or something
main:
.loop:
	; function calls are slow ;-)
	mov	eax, 3		; magic number for read()
	xor	ebx, ebx	; file handle
	mov	ecx, buffer	; buffer to fill
	mov	edx, 4096	; how many bytes to read
	int	0x80

	; if the program doesn't work, replace ebx with a 0
	cmp	eax, ebx	; ebx is 0 from the filehandle (we hope)
	jle	.done
; now eax contains the # of bytes read.

.rot_buffer:
	mov	edx, buffer;
	mov	ecx, table

.rotloop:
	mov	ebx, [edx]	; read four bytes

	mov	byte cl, bl	; offset into table
	mov	byte bl, [ecx]

	mov	byte cl, bh	; offset into table
	mov	byte bh, [ecx]

	rol	ebx, 16		; swap bottom and top 2 bytes

	mov	byte cl, bl	; offset into table
	mov	byte bl, [ecx]

	mov	byte cl, bh	; offset into table
	mov	byte bh, [ecx]

	rol	ebx, 16		; swap bottom and top 2 bytes

	mov	[edx], ebx

.next:
	add	edx, 4
	sub	eax, 4
	;inc	edx
	;dec	eax
	cmp	eax, 0
	jg	.rotloop

.write:
	mov	eax, 4		; magic number for write()
	mov	ebx, 1		; file handle (stdout)
	mov	ecx, buffer	; pointer to data
	sub	edx, buffer	; number of bytes to write
	int	0x80

	jmp	.loop

.done:
	mov	eax, 1		; magic number for exit
	xor	ebx, ebx	; exit code (0)
	int	0x80

; this alignment allows for faster addressing
	SECTION .data align=256
table:
	db	0
	db	1
	db	2
	db	3
	db	4
	db	5
	db	6
	db	7
	db	8
	db	9
	db	10
	db	11
	db	12
	db	13
	db	14
	db	15
	db	16
	db	17
	db	18
	db	19
	db	20
	db	21
	db	22
	db	23
	db	24
	db	25
	db	26
	db	27
	db	28
	db	29
	db	30
	db	31
	db	32
	db	33
	db	34
	db	35
	db	36
	db	37
	db	38
	db	39
	db	40
	db	41
	db	42
	db	43
	db	44
	db	45
	db	46
	db	47
	db	48
	db	49
	db	50
	db	51
	db	52
	db	53
	db	54
	db	55
	db	56
	db	57
	db	58
	db	59
	db	60
	db	61
	db	62
	db	63
	db	64
	db	'N'
	db	79
	db	80
	db	81
	db	82
	db	83
	db	84
	db	85
	db	86
	db	87
	db	88
	db	89
	db	'Z'
	db	'A'
	db	66
	db	67
	db	68
	db	69
	db	70
	db	71
	db	72
	db	73
	db	74
	db	75
	db	76
	db	77
	db	91
	db	92
	db	93
	db	94
	db	95
	db	96
	db	'n'
	db	111
	db	112
	db	113
	db	114
	db	115
	db	116
	db	117
	db	118
	db	119
	db	120
	db	121
	db	'z'
	db	'a'
	db	98
	db	99
	db	100
	db	101
	db	102
	db	103
	db	104
	db	105
	db	106
	db	107
	db	108
	db	109
	db	123
	db	124
	db	125
	db	126
	db	127
	db	128
	db	129
	db	130
	db	131
	db	132
	db	133
	db	134
	db	135
	db	136
	db	137
	db	138
	db	139
	db	140
	db	141
	db	142
	db	143
	db	144
	db	145
	db	146
	db	147
	db	148
	db	149
	db	150
	db	151
	db	152
	db	153
	db	154
	db	155
	db	156
	db	157
	db	158
	db	159
	db	160
	db	161
	db	162
	db	163
	db	164
	db	165
	db	166
	db	167
	db	168
	db	169
	db	170
	db	171
	db	172
	db	173
	db	174
	db	175
	db	176
	db	177
	db	178
	db	179
	db	180
	db	181
	db	182
	db	183
	db	184
	db	185
	db	186
	db	187
	db	188
	db	189
	db	190
	db	191
	db	192
	db	193
	db	194
	db	195
	db	196
	db	197
	db	198
	db	199
	db	200
	db	201
	db	202
	db	203
	db	204
	db	205
	db	206
	db	207
	db	208
	db	209
	db	210
	db	211
	db	212
	db	213
	db	214
	db	215
	db	216
	db	217
	db	218
	db	219
	db	220
	db	221
	db	222
	db	223
	db	224
	db	225
	db	226
	db	227
	db	228
	db	229
	db	230
	db	231
	db	232
	db	233
	db	234
	db	235
	db	236
	db	237
	db	238
	db	239
	db	240
	db	241
	db	242
	db	243
	db	244
	db	245
	db	246
	db	247
	db	248
	db	249
	db	250
	db	251
	db	252
	db	253
	db	254
	db	255

	SECTION	.bss
buffer:
	resb	4096		; 4kb buffer