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|
(* Variable-byte encoding of 8-byte integers (starting from 0). *)
open Printf
open Bi_outbuf
open Bi_inbuf
type uint = int
(* Word size in bytes *)
let word_size =
if 0x7fffffff = -1 then 4
else 8
(* Maximum int size in bits *)
let max_int_bits =
8 * word_size - 1
(* Maximum length of a vint decodable into an OCaml int,
maximum value of the highest byte of the largest vint supported *)
let max_vint_bytes, max_highest_byte =
if max_int_bits mod 7 = 0 then
let m = max_int_bits / 7 in
let h = 1 lsl 7 - 1 in
m, h
else
let m = max_int_bits / 7 + 1 in
let h = 1 lsl (max_int_bits mod 7) - 1 in
m, h
let check_highest_byte x =
if x > max_highest_byte then
Bi_util.error "Vint exceeding range of OCaml ints"
let unsigned_of_signed i =
if i >= 0 then
(*
0 -> 0
1 -> 2
2 -> 4
3 -> 6
*)
i lsl 1
else
(*
-1 -> 1
-2 -> 3
-3 -> 5
*)
((-1-i) lsl 1) lor 1
let signed_of_unsigned i =
if i land 1 = 0 then i lsr 1
else -1 - (i lsr 1)
let write_uvint buf i =
Bi_outbuf.extend buf max_vint_bytes;
let x = ref i in
while !x lsr 7 <> 0 do
let byte = 0x80 lor (!x land 0x7f) in
Bi_outbuf.unsafe_add_char buf (Char.chr byte);
x := !x lsr 7;
done;
Bi_outbuf.unsafe_add_char buf (Char.chr !x)
let write_svint buf i =
write_uvint buf (unsigned_of_signed i)
(* convenience *)
let uvint_of_uint ?buf i =
let buffer =
match buf with
| None -> Bi_outbuf.create 10
| Some b -> b
in
Bi_outbuf.clear buffer;
write_uvint buffer i;
Bi_outbuf.contents buffer
let svint_of_int ?buf i =
uvint_of_uint ?buf (unsigned_of_signed i)
let read_uvint ib =
let avail = Bi_inbuf.try_preread ib max_vint_bytes in
let s = ib.i_s in
let pos = ib.i_pos in
let x = ref 0 in
(try
for i = 0 to avail - 1 do
let b = Char.code s.[pos+i] in
x := ((b land 0x7f) lsl (7*i)) lor !x;
if b < 0x80 then (
ib.i_pos <- pos + i + 1;
if i + 1 = max_vint_bytes then
check_highest_byte b;
raise Exit
)
done;
Bi_util.error "Unterminated vint or vint exceeding range of OCaml ints"
with Exit -> ()
);
!x
let read_svint ib =
signed_of_unsigned (read_uvint ib)
(* convenience *)
let check_end_of_input ib =
if Bi_inbuf.try_preread ib 1 > 0 then
Bi_util.error "Junk input after end of vint"
let uint_of_uvint s =
let ib = Bi_inbuf.from_string s in
let x = read_uvint ib in
check_end_of_input ib;
x
let int_of_svint s =
let ib = Bi_inbuf.from_string s in
let x = read_svint ib in
check_end_of_input ib;
x
(*
Testing
*)
let string_of_list l =
let ob = Bi_outbuf.create 100 in
List.iter (write_uvint ob) l;
Bi_outbuf.contents ob
let rec read_list ib =
if ib.i_pos < ib.i_len then
let x = read_uvint ib in
x :: read_list ib
else
[]
let list_of_string s =
read_list (Bi_inbuf.from_string s)
let print_list l =
List.iter (
fun i ->
printf "dec %i\nhex %x\nbin %s\n" i i
(Bi_util.print_bits (Bi_util.string8_of_int i))
) l
let test () =
let l = [
0;
0xfffffff;
(0x01020304 lsl 32) lor 0x05060708;
max_int;
min_int
] in
printf "Input:\n";
print_list l;
let l' = list_of_string (string_of_list l) in
printf "Output:\n";
print_list l';
if l = l' then
print_endline "SUCCESS"
else
print_endline "FAILURE"
|
