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|
// Simple Extensible Binary Markup Language (ebml) reader and writer on a
// cursor model. See the specification here:
// http://www.matroska.org/technical/specs/rfc/index.html
import core::option;
import option::{some, none};
type ebml_tag = {id: uint, size: uint};
type ebml_state = {ebml_tag: ebml_tag, tag_pos: uint, data_pos: uint};
// TODO: When we have module renaming, make "reader" and "writer" separate
// modules within this file.
// ebml reading
type doc = {data: @[u8], start: uint, end: uint};
fn vint_at(data: [u8], start: uint) -> {val: uint, next: uint} {
let a = data[start];
if a & 0x80u8 != 0u8 { ret {val: a & 0x7fu8 as uint, next: start + 1u}; }
if a & 0x40u8 != 0u8 {
ret {val: (a & 0x3fu8 as uint) << 8u | (data[start + 1u] as uint),
next: start + 2u};
} else if a & 0x20u8 != 0u8 {
ret {val:
(a & 0x1fu8 as uint) << 16u |
(data[start + 1u] as uint) << 8u |
(data[start + 2u] as uint),
next: start + 3u};
} else if a & 0x10u8 != 0u8 {
ret {val:
(a & 0x0fu8 as uint) << 24u |
(data[start + 1u] as uint) << 16u |
(data[start + 2u] as uint) << 8u |
(data[start + 3u] as uint),
next: start + 4u};
} else { #error("vint too big"); fail; }
}
fn new_doc(data: @[u8]) -> doc {
ret {data: data, start: 0u, end: vec::len::<u8>(*data)};
}
fn doc_at(data: @[u8], start: uint) -> doc {
let elt_tag = vint_at(*data, start);
let elt_size = vint_at(*data, elt_tag.next);
let end = elt_size.next + elt_size.val;
ret {data: data, start: elt_size.next, end: end};
}
fn maybe_get_doc(d: doc, tg: uint) -> option::t<doc> {
let pos = d.start;
while pos < d.end {
let elt_tag = vint_at(*d.data, pos);
let elt_size = vint_at(*d.data, elt_tag.next);
pos = elt_size.next + elt_size.val;
if elt_tag.val == tg {
ret some::<doc>({data: d.data, start: elt_size.next, end: pos});
}
}
ret none::<doc>;
}
fn get_doc(d: doc, tg: uint) -> doc {
alt maybe_get_doc(d, tg) {
some(d) { ret d; }
none {
#error("failed to find block with tag %u", tg);
fail;
}
}
}
fn docs(d: doc, it: block(uint, doc)) {
let pos = d.start;
while pos < d.end {
let elt_tag = vint_at(*d.data, pos);
let elt_size = vint_at(*d.data, elt_tag.next);
pos = elt_size.next + elt_size.val;
it(elt_tag.val, {data: d.data, start: elt_size.next, end: pos});
}
}
fn tagged_docs(d: doc, tg: uint, it: block(doc)) {
let pos = d.start;
while pos < d.end {
let elt_tag = vint_at(*d.data, pos);
let elt_size = vint_at(*d.data, elt_tag.next);
pos = elt_size.next + elt_size.val;
if elt_tag.val == tg {
it({data: d.data, start: elt_size.next, end: pos});
}
}
}
fn doc_data(d: doc) -> [u8] { ret vec::slice::<u8>(*d.data, d.start, d.end); }
fn be_uint_from_bytes(data: @[u8], start: uint, size: uint) -> uint {
let sz = size;
assert (sz <= 4u);
let val = 0u;
let pos = start;
while sz > 0u {
sz -= 1u;
val += (data[pos] as uint) << sz * 8u;
pos += 1u;
}
ret val;
}
fn doc_as_uint(d: doc) -> uint {
ret be_uint_from_bytes(d.data, d.start, d.end - d.start);
}
// ebml writing
type writer = {writer: io::writer, mutable size_positions: [uint]};
fn write_sized_vint(w: io::writer, n: uint, size: uint) {
let buf: [u8];
alt size {
1u { buf = [0x80u8 | (n as u8)]; }
2u { buf = [0x40u8 | (n >> 8u as u8), n & 0xffu as u8]; }
3u {
buf =
[0x20u8 | (n >> 16u as u8), n >> 8u & 0xffu as u8,
n & 0xffu as u8];
}
4u {
buf =
[0x10u8 | (n >> 24u as u8), n >> 16u & 0xffu as u8,
n >> 8u & 0xffu as u8, n & 0xffu as u8];
}
_ { #error("vint to write too big"); fail; }
}
w.write(buf);
}
fn write_vint(w: io::writer, n: uint) {
if n < 0x7fu { write_sized_vint(w, n, 1u); ret; }
if n < 0x4000u { write_sized_vint(w, n, 2u); ret; }
if n < 0x200000u { write_sized_vint(w, n, 3u); ret; }
if n < 0x10000000u { write_sized_vint(w, n, 4u); ret; }
#error("vint to write too big");
fail;
}
fn create_writer(w: io::writer) -> writer {
let size_positions: [uint] = [];
ret {writer: w, mutable size_positions: size_positions};
}
// TODO: Provide a function to write the standard ebml header.
fn start_tag(w: writer, tag_id: uint) {
// Write the tag ID:
write_vint(w.writer, tag_id);
// Write a placeholder four-byte size.
w.size_positions += [w.writer.tell()];
let zeroes: [u8] = [0u8, 0u8, 0u8, 0u8];
w.writer.write(zeroes);
}
fn end_tag(w: writer) {
let last_size_pos = vec::pop::<uint>(w.size_positions);
let cur_pos = w.writer.tell();
w.writer.seek(last_size_pos as int, io::seek_set);
write_sized_vint(w.writer, cur_pos - last_size_pos - 4u, 4u);
w.writer.seek(cur_pos as int, io::seek_set);
}
// TODO: optionally perform "relaxations" on end_tag to more efficiently
// encode sizes; this is a fixed point iteration
|
