# Null-terminated vs length-prefixed ascii strings.
#
# By default we create strings with a 4-byte length prefix rather than a null suffix.
# However we still need null-prefixed strings when interacting with the Linux
# kernel in a few places. This layer implements a function for comparing
# a null-terminated 'kernel string' with a length-prefixed 'SubX string'.
#
# To run (from the subx directory):
#   $ ./subx translate examples/ex11.subx -o examples/ex11
#   $ ./subx run examples/ex11  # runs a series of tests
#   ......  # all tests pass
#
# (We can't yet run the tests when given a "test" commandline argument,
# because checking for it would require the function being tested! Breakage
# would cause tests to not run, rather than to fail as we'd like.)

== code
#   instruction                     effective address                                                   register    displacement    immediate
# . op          subop               mod             rm32          base        index         scale       r32
# . 1-3 bytes   3 bits              2 bits          3 bits        3 bits      3 bits        2 bits      2 bits      0/1/2/4 bytes   0/1/2/4 bytes

Entry:  # run all tests
    e8/call  run-tests/disp32  # 'run-tests' is a function created automatically by SubX. It calls all functions that start with 'test-'.
    # syscall(exit, EAX)
    89/copy                         3/mod/direct    3/rm32/EBX    .           .             .           0/r32/EAX   .               .                 # copy EAX to EBX
    b8/copy-to-EAX  1/imm32/exit
    cd/syscall  0x80/imm8

# compare a null-terminated ascii string with a more idiomatic length-prefixed byte array
# reason for the name: the only place we should have null-terminated ascii strings is from commandline args
kernel-string-equal?:  # s : null-terminated ascii string, benchmark : length-prefixed ascii string -> EAX : boolean
    # pseudocode:
    #   n = benchmark->length
    #   s1 = s
    #   s2 = benchmark->data
    #   i = 0
    #   while i < n
    #     c1 = *s1
    #     c2 = *s2
    #     if (c1 == 0) return false
    #     if (c1 != c2) return false
    #     ++s1, ++s2, ++i
    #   return *s1 == 0
    #
    # registers:
    #   i: ECX
    #   n: EDX
    #   s1: EDI
    #   s2: ESI
    #   c1: EAX
    #   c2: EBX
    #
    # . prolog
    55/push-EBP
    89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    # . save registers
    51/push-ECX
    52/push-EDX
    53/push-EBX
    56/push-ESI
    57/push-EDI
    # s1/EDI = s
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           7/r32/EDI   8/disp8         .                 # copy *(EBP+8) to EDI
    # n/EDX = benchmark->length
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           2/r32/EDX   0xc/disp8       .                 # copy *(EBP+12) to EDX
    8b/copy                         0/mod/indirect  2/rm32/EDX    .           .             .           2/r32/EDX   .               .                 # copy *EDX to EDX
    # s2/ESI = benchmark->data
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           6/r32/ESI   0xc/disp8       .                 # copy *(EBP+12) to ESI
    81          0/subop/add         3/mod/direct    6/rm32/ESI    .           .             .           .           .               4/imm32           # add to ESI
    # i/ECX = c1/EAX = c2/EBX = 0
    b9/copy-to-ECX  0/imm32/exit
    b8/copy-to-EAX  0/imm32
    bb/copy-to-EBX  0/imm32
$kernel-string-equal?:loop:
    # if (i >= n) break
    39/compare                      3/mod/direct    1/rm32/ECX    .           .             .           2/r32/EDX   .               .                 # compare ECX with EDX
    7d/jump-if-greater-or-equal  $kernel-string-equal?:break/disp8
    # c1 = *s1
    8a/copy-byte                    0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/AL    .               .                 # copy byte at *EDI to AL
    # c2 = *s2
    8a/copy-byte                    0/mod/indirect  6/rm32/ESI    .           .             .           3/r32/BL    .               .                 # copy byte at *ESI to BL
    # if (c1 == 0) return false
    3d/compare-EAX-and  0/imm32
    74/jump-if-equal  $kernel-string-equal?:false/disp8
    # if (c1 != c2) return false
    39/compare                      3/mod/direct    0/rm32/EAX    .           .             .           3/r32/EBX   .               .                 # compare EAX and EBX
    75/jump-if-not-equal  $kernel-string-equal?:false/disp8
    # ++i
    41/increment-ECX
    # ++s1
    47/inc-EDI
    # ++s2
    46/inc-ESI
    eb/jump  $kernel-string-equal?:loop/disp8
$kernel-string-equal?:break:
    # return *s1 == 0
    8a/copy-byte                    0/mod/indirect  7/rm32/EDI    .           .             .           0/r32/AL    .               .                 # copy byte at *EDI to AL
    3d/compare-EAX-and  0/imm32
    75/jump-if-not-equal  $kernel-string-equal?:false/disp8
$kernel-string-equal?:true:
    b8/copy-to-EAX  1/imm32
    eb/jump  $kernel-string-equal?:end/disp8
$kernel-string-equal?:false:
    b8/copy-to-EAX  0/imm32
$kernel-string-equal?:end:
    # . restore registers
    5f/pop-to-EDI
    5e/pop-to-ESI
    5b/pop-to-EBX
    5a/pop-to-EDX
    59/pop-to-ECX
    # . epilog
    89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
    5d/pop-to-EBP
    c3/return

# - tests

test-compare-null-kernel-string-with-empty-array:
    # EAX = kernel-string-equal?(Null-kernel-string, "")
    # . . push args
    68/push  ""/imm32
    68/push  Null-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
    # check-ints-equal(EAX, 1, msg)
    # . . push args
    68/push  "F - test-compare-null-kernel-string-with-empty-array"/imm32
    68/push  1/imm32/true
    50/push-EAX
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
    c3/return

test-compare-null-kernel-string-with-non-empty-array:
    # EAX = kernel-string-equal?(Null-kernel-string, "Abc")
    # . . push args
    68/push  "Abc"/imm32
    68/push  Null-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
    # check-ints-equal(EAX, 0, msg)
    # . . push args
    68/push  "F - test-compare-null-kernel-string-with-non-empty-array"/imm32
    68/push  0/imm32/false
    50/push-EAX
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
    c3/return

test-compare-kernel-string-with-equal-array:
    # EAX = kernel-string-equal?(_test-Abc-kernel-string, "Abc")
    # . . push args
    68/push  "Abc"/imm32
    68/push  _test-Abc-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
    # check-ints-equal(EAX, 1, msg)
    # . . push args
    68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
    68/push  1/imm32/true
    50/push-EAX
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
    c3/return

test-compare-kernel-string-with-inequal-array:
    # EAX = kernel-string-equal?(_test-Abc-kernel-string, "Adc")
    # . . push args
    68/push  "Adc"/imm32
    68/push  _test-Abc-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
    # check-ints-equal(EAX, 0, msg)
    # . . push args
    68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
    68/push  0/imm32/false
    50/push-EAX
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
    c3/return

test-compare-kernel-string-with-empty-array:
    # EAX = kernel-string-equal?(_test-Abc-kernel-string, "")
    # . . push args
    68/push  ""/imm32
    68/push  _test-Abc-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
    # check-ints-equal(EAX, 0, msg)
    # . . push args
    68/push  "F - test-compare-kernel-string-with-equal-array"/imm32
    68/push  0/imm32/false
    50/push-EAX
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
    c3/return

test-compare-kernel-string-with-shorter-array:
    # EAX = kernel-string-equal?(_test-Abc-kernel-string, "Ab")
    # . . push args
    68/push  "Ab"/imm32
    68/push  _test-Abc-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
    # check-ints-equal(EAX, 0, msg)
    # . . push args
    68/push  "F - test-compare-kernel-string-with-shorter-array"/imm32
    68/push  0/imm32/false
    50/push-EAX
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
    c3/return

test-compare-kernel-string-with-longer-array:
    # EAX = kernel-string-equal?(_test-Abc-kernel-string, "Abcd")
    # . . push args
    68/push  "Abcd"/imm32
    68/push  _test-Abc-kernel-string/imm32
    # . . call
    e8/call  kernel-string-equal?/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               8/imm32           # add to ESP
    # check-ints-equal(EAX, 0, msg)
    # . . push args
    68/push  "F - test-compare-kernel-string-with-longer-array"/imm32
    68/push  0/imm32/false
    50/push-EAX
    # . . call
    e8/call  check-ints-equal/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               0xc/imm32         # add to ESP
    c3/return

# - helpers

# print msg to stderr if a != b, otherwise print "."
check-ints-equal:  # (a : int, b : int, msg : (address array byte)) -> boolean
    # . prolog
    55/push-EBP
    89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    # . save registers
    51/push-ECX
    53/push-EBX
    # load args into EAX, EBX and ECX
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           0/r32/EAX   8/disp8         .                 # copy *(EBP+8) to EAX
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           3/r32/EBX   0xc/disp8       .                 # copy *(EBP+12) to EBX
    # if (EAX == b/EBX) print('.') and return
    39/compare                      3/mod/direct    0/rm32/EAX    .           .             .           3/r32/EBX   .               .                 # compare EAX and EBX
    75/jump-if-unequal  $check-ints-equal:else/disp8
    # . write-stderr('.')
    # . . push args
    68/push  "."/imm32
    # . . call
    e8/call  write-stderr/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
    # . return
    eb/jump  $check-ints-equal:end/disp8
    # otherwise print(msg)
$check-ints-equal:else:
    # copy msg into ECX
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           1/r32/ECX   0x10/disp8       .                # copy *(EBP+16) to ECX
    # print(ECX)
    # . . push args
    51/push-ECX
    # . . call
    e8/call  write-stderr/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
    # print newline
    # . . push args
    68/push  Newline/imm32
    # . . call
    e8/call  write-stderr/disp32
    # . . discard args
    81          0/subop/add         3/mod/direct    4/rm32/ESP    .           .             .           .           .               4/imm32           # add to ESP
$check-ints-equal:end:
    # . restore registers
    5b/pop-to-EBX
    59/pop-to-ECX
    # end
    89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
    5d/pop-to-EBP
    c3/return

write-stderr:  # s : (address array byte) -> <void>
    # . prolog
    55/push-EBP
    89/copy                         3/mod/direct    5/rm32/EBP    .           .             .           4/r32/ESP   .               .                 # copy ESP to EBP
    # . save registers
    50/push-EAX
    51/push-ECX
    52/push-EDX
    53/push-EBX
    # syscall(write, 2/stderr, (data) s+4, (size) *s)
    # . . fd = 2 (stderr)
    bb/copy-to-EBX  2/imm32
    # . . x = s+4
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           1/r32/ECX   8/disp8         .                 # copy *(EBP+8) to ECX
    81          0/subop/add         3/mod/direct    1/rm32/ECX    .           .             .           .           .               4/imm32           # add to ECX
    # . . size = *s
    8b/copy                         1/mod/*+disp8   5/rm32/EBP    .           .             .           2/r32/EDX   8/disp8         .                 # copy *(EBP+8) to EDX
    8b/copy                         0/mod/indirect  2/rm32/EDX    .           .             .           2/r32/EDX   .               .                 # copy *EDX to EDX
    # . . syscall
    b8/copy-to-EAX  4/imm32/write
    cd/syscall  0x80/imm8
    # . restore registers
    5b/pop-to-EBX
    5a/pop-to-EDX
    59/pop-to-ECX
    58/pop-to-EAX
    # . end
    89/copy                         3/mod/direct    4/rm32/ESP    .           .             .           5/r32/EBP   .               .                 # copy EBP to ESP
    5d/pop-to-EBP
    c3/return

== data

Newline:
    # size
    1/imm32
    # data
    0a/newline

# for kernel-string-equal tests
Null-kernel-string:
    00/null

_test-Abc-kernel-string:
    41/A 62/b 63/c 00/null

# . . vim:nowrap:textwidth=0