.globl _start
.option rvc # enable compressed instructions

.data
writebuf: .ascii "00000000: 00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00 | ................\n"
.equ hexstart, 10
.equ hexend, hexstart+3*16
.equ charstart, hexend+3
.equ linelen, (. - writebuf)
.equ readlen, 16
readbuf: .zero readlen

.text
_start:
    # set up the gp register so that relaxed loads work
    .option push
    .option norelax
    la gp, __global_pointer$
    .option pop

    # fd in s6
    # data offset in s7
    li s6, 0
    li s7, 0 #0xabad1dea


loop:
    li a7, 63      # sys_read
    mv a0, s6      # fd
    la a1, readbuf # pointer to buffer
    li a2, readlen  # 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
    
    li t5, 10

    la s0, writebuf

    .macro hexnibble src, shift, dest
    slli t1, \src, (64-\shift-4) # put high nibble of s7 in t1
    srli t1, t1, 64-4
    addi t2, t1, 0x30 # convert to decimal digit
    bgt t5, t1, 0f # if digit >= 10, use switch to alpha
    addi t2, t1, 0x61-10
0:
    # copy digit to writebuf
    sb t2, \dest
    .endm

    .macro hexbyte src, shift, dest
    hexnibble \src, (\shift+4), \dest
    hexnibble \src, \shift, 1+\dest
    .endm

    hexbyte s7, 24, 0(s0)
    hexbyte s7, 16, 2(s0)
    hexbyte s7, 8, 4(s0)
    hexbyte s7, 0, 6(s0)

    la s1, readbuf

    # it's perfectly fine to process more
    # bytes than we read here, since the buffer
    # sizes are both fixed
    .macro inpbyte srcindex, dstindex
    lbu t0, \srcindex(s1)
    hexbyte t0, 0, \dstindex(s0)
    .endm

    # TODO: load 8 bytes at a time
    #  (need a way to check endianness)
    inpbyte 0, 10
    inpbyte 1, 13
    inpbyte 2, 16
    inpbyte 3, 19
    inpbyte 4, 22
    inpbyte 5, 25
    inpbyte 6, 28
    inpbyte 7, 31

    inpbyte 8, 35
    inpbyte 9, 38
    inpbyte 10, 41
    inpbyte 11, 44
    inpbyte 12, 47
    inpbyte 13, 50
    inpbyte 14, 53
    inpbyte 15, 56


    li t0, 0
    li t4, 0x7e-0x20
    # the ascii printable range is 0x20-0x7e
    #
    #      0123456789ABCDEF
    #   20  !"#$%&'()*+,-./
    #   30 0123456789:;<=>?
    #   40 @ABCDEFGHIJKLMNO
    #   50 PQRSTUVWXYZ[\]^_
    #   60 `abcdefghijklmno
    #   70 pqrstuvwxyz{|}~
printable_loop:
    add t1, s1, t0
    lbu t2, 0(t1)
    addi t3, t2, -0x20
    bleu t3, t4, 0f
    li t2, 0x2e # '.' 
    0:
    add t1, s0, t0
    sb t2, 61(t1)
    addi t0, t0, 1
    blt t0, a0, printable_loop

    li t0, readlen
    blt a0, t0, fixup

    mv s0, a0 # save length read
    add s7, s7, s0 # file offset += s0

    li a7, 64      # sys_write
    li a0, 1       # fd = stdout
    la a1, writebuf     # pointer to string
    li a2, linelen      # size of data
    ecall

    bltz a0, error
    j loop

fixup:
    # we read <16 bytes, so we need to 
    # 1) replace the hex bytes >a0 with spaces
    # 2) adjust the line length
    # 3) print it
    
    add t0, a0, a0 # t0 = len*3
    add t0, t0, a0
    addi t0, t0, hexstart
    add t0, s0, t0
    addi t1, s0, hexend
    li t2, 0x20
1:
    sb t2, 0(t0)
    addi t0, t0, 1
    blt t0, t1, 1b

    addi t0, a0, charstart
    add t1, t0, s0
    li t2, 0xa # \n
    sb t2, 0(t1)
    li a7, 64      # sys_write
    li a0, 1       # fd = stdout
    la a1, writebuf     # pointer to string
    addi a2, t0, 1      # size of data
    ecall
    bltz a0, error
    j done

error:
    li a7, 93 # exit
    li a0, 1
    ecall
    
done:
    li a7, 93 # exit
    li a0, 0
    ecall