.globl _start
.option rvc # enable compressed instructions
.option norelax

.data
sigaction_ignore:
    .dword 1 # sa_handler = SIG_IGN
    .dword 0 # sa_flags
    .dword 0  # sa_mask

.bss
.equ bufsize, 4<<10
buf: .zero bufsize

.bss
crc_table: .zero 256*4

.text
_start:
    # ignore SIGPIPE
    li a7, 134  # sys_rt_sigaction
    li a0, 13   # SIGPIPE
    la a1, sigaction_ignore # action
    li a2, 0    # oldact = NULL
                # if this is a non-null pointer the kernel will
                # return a copy of the old action to us, but we don't care
    li a3, 8    # signal set size (bytes)
    ecall

# TODO: error messages

    call init_table

    li s1, 0 # CRC

loop:
    li a7, 63      # sys_read
    li a0, 0       # fd = stdin
    la a1, buf     # pointer to buffer
    li a2, bufsize  # size of buffer
    ecall
    #li a0, 0

    # return in a0
    #   negative => error code
    #   positive => number of byte read
    #   zero     => end of input
    bltz a0, error
    beqz a0, done

    mv s0, a0

    # loop over buffer and update crc
    mv a0, s1
    la a1, buf
    mv a2, s0 # buf size
    call crc_update
    mv s1, a0

    bnez s0, loop # loop if not end of input

error:
    li a7, 93 # sys_exit
    li a0, 1  # status code = failure
    ecall

done:
    #la a0, crc_table
    #addi a0, a0, 4*0x80
    #lwu a0, (a0)
    #mv s1, a0

    # encode crc
    la a0, buf
    mv a1, s1
    call u32hex

    # add newline
    la a1, buf
    li t0, 10 # '\n'
    sb t0, 8(a1)

    # write crc
    li a7, 64      # sys_write
    li a0, 1       # fd = stdout
    li a2, 9       # size of data
    ecall
    #mv s0, a0

    # check for EPIPE
    li t0, -32
    beq a0, t0, ok # EPIPE
    bltz a0, error

ok:
    li a7, 93 # sys_exit
    li a0, 0  # status code = success
    ecall

#............................................

.equ poly, 0xEDB88320

.text
init_table:
    la   s0, crc_table
    li   t6, 256
    li   a0, poly
iloop:
    addi t6, t6, -1
    mv   t0, t6
    li   t2, 8
iloop2:
    andi t1, t0, 1
    srli t0, t0, 1
    beqz t1, 0f
    xor  t0, t0, a0
0:
    addi t2, t2, -1
    bnez t2, iloop2

    slli t1, t6, 2
    add  t1, t1, s0
    sw   t0, (t1)
    bnez t6, iloop
    ret

#............................................

.macro notw rd, rs
    not    \rd, \rs
    #zext.w \rd, \rd
    #add.uw  \rd, \rs, 0
    slli   \rd, \rd, 32
    srli   \rd, \rd, 32
.endm

# inputs:
# a0: crc
# a1: buf
# a2: buf size
#
# returns:
# a0 - new crc
#
# clobbers: t0, t1, t2, t3
.text
crc_update:
    mv t1, s0
    mv t2, s1
    mv t3, s2
    # invert crc
    notw a0, a0

    mv s0, a0 # crc
    mv s1, a1 # buffer
    la s2, crc_table

    beqz a2, udone

uloop:
    lbu  a1, (s1)    # load byte
    andi t0, a0, 0xff # a1 = crc&0xff ^ byte
    xor  a1, a1, t0
    srli a0, a0, 8    # crc >>= 8
    slli a1, a1, 2    # convert a1 to table address
    add  a1, a1, s2
    lwu  a1, (a1)    # load crc table entry
    xor  a0, a0, a1   # crc ^= table entry

    addi s1, s1, 1
    addi a2, a2, -1
    bnez a2, uloop

udone:
    # restore saved registers
    mv s0, t1
    mv s1, t2
    mv s2, t3
    # invert crc
    #li   t0, -1
    #srli t0, t0, 32
    notw a0, a0
    ret