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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 option.none;
import option.some;
type ebml_tag = rec(uint id, uint size);
type ebml_state = rec(ebml_tag ebml_tag, uint tag_pos, uint data_pos);
// TODO: When we have module renaming, make "reader" and "writer" separate
// modules within this file.
// EBML reading
type reader = rec(
io.reader reader,
mutable vec[ebml_state] states,
uint size
);
// TODO: eventually use u64 or big here
impure fn read_vint(&io.reader reader) -> uint {
auto a = reader.read_byte() as u8;
if (a & 0x80u8 != 0u8) { ret (a & 0x7fu8) as uint; }
auto b = reader.read_byte() as u8;
if (a & 0x40u8 != 0u8) {
ret (((a & 0x3fu8) as uint) << 8u) | (b as uint);
}
auto c = reader.read_byte() as u8;
if (a & 0x20u8 != 0u8) {
ret (((a & 0x1fu8) as uint) << 16u) | ((b as uint) << 8u) |
(c as uint);
}
auto d = reader.read_byte() as u8;
if (a & 0x10u8 != 0u8) {
ret (((a & 0x0fu8) as uint) << 24u) | ((b as uint) << 16u) |
((c as uint) << 8u) | (d as uint);
}
log "vint too big"; fail;
}
impure fn create_reader(&io.reader r) -> reader {
let vec[ebml_state] states = vec();
// Calculate the size of the stream.
auto pos = r.tell();
r.seek(0, io.seek_end);
auto size = r.tell() - pos;
r.seek(pos as int, io.seek_set);
ret rec(reader=r, mutable states=states, size=size);
}
impure fn bytes_left(&reader r) -> uint {
auto pos = r.reader.tell();
alt (_vec.last[ebml_state](r.states)) {
case (none[ebml_state]) { ret r.size - pos; }
case (some[ebml_state](?st)) {
ret st.data_pos + st.ebml_tag.size - pos;
}
}
}
impure fn read_tag(&reader r) -> ebml_tag {
auto id = read_vint(r.reader);
auto size = read_vint(r.reader);
ret rec(id=id, size=size);
}
// Reads a tag and moves the cursor to its first child or data segment.
impure fn move_to_first_child(&reader r) {
auto tag_pos = r.reader.tell();
auto t = read_tag(r);
auto data_pos = r.reader.tell();
r.states += vec(rec(ebml_tag=t, tag_pos=tag_pos, data_pos=data_pos));
}
// Reads a tag and skips over its contents, moving to its next sibling.
impure fn move_to_next_sibling(&reader r) {
auto t = read_tag(r);
r.reader.seek(t.size as int, io.seek_cur);
}
// Moves to the parent of this tag.
impure fn move_to_parent(&reader r) {
check (_vec.len[ebml_state](r.states) > 0u);
auto st = _vec.pop[ebml_state](r.states);
r.reader.seek(st.tag_pos as int, io.seek_set);
}
// Moves to the sibling of the current item with the given tag ID.
impure fn move_to_sibling_with_id(&reader r, uint tag_id) {
while (peek(r).id != tag_id) {
move_to_next_sibling(r);
}
}
// Moves to the first child of the current item with the given tag ID.
impure fn move_to_child_with_id(&reader r, uint tag_id) {
move_to_first_child(r);
move_to_sibling_with_id(r, tag_id);
}
// Reads the data segment of a tag.
impure fn read_data(&reader r) -> vec[u8] {
ret r.reader.read_bytes(bytes_left(r));
}
// Blows away the tag stack and moves the reader to the given byte position.
impure fn reset_reader(&reader r, uint pos) {
// FIXME: rustc "ty_var in trans.type_of" bug
let vec[ebml_state] states = vec();
r.states = states;
r.reader.seek(pos as int, io.seek_set);
}
impure fn peek(&reader r) -> ebml_tag {
check (bytes_left(r) > 0u);
auto pos = r.reader.tell();
auto t = read_tag(r);
r.reader.seek(pos as int, io.seek_set);
ret t;
}
// EBML writing
type writer = rec(io.buf_writer writer, mutable vec[uint] size_positions);
fn write_sized_vint(&io.buf_writer w, uint n, uint size) {
let vec[u8] buf;
alt (size) {
case (1u) {
buf = vec(0x80u8 | (n as u8));
}
case (2u) {
buf = vec(0x40u8 | ((n >> 8u) as u8),
(n & 0xffu) as u8);
}
case (3u) {
buf = vec(0x20u8 | ((n >> 16u) as u8),
((n >> 8u) & 0xffu) as u8,
(n & 0xffu) as u8);
}
case (4u) {
buf = vec(0x10u8 | ((n >> 24u) as u8),
((n >> 16u) & 0xffu) as u8,
((n >> 8u) & 0xffu) as u8,
(n & 0xffu) as u8);
}
case (_) {
log "vint to write too big";
fail;
}
}
w.write(buf);
}
fn write_vint(&io.buf_writer w, uint n) {
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; }
log "vint to write too big";
fail;
}
fn create_writer(&io.buf_writer w) -> writer {
let vec[uint] size_positions = vec();
ret rec(writer=w, mutable size_positions=size_positions);
}
// TODO: Provide a function to write the standard EBML header.
fn start_tag(&writer w, uint tag_id) {
// Write the tag ID.
write_vint(w.writer, tag_id);
// Write a placeholder four-byte size.
w.size_positions += vec(w.writer.tell());
let vec[u8] zeroes = vec(0u8, 0u8, 0u8, 0u8);
w.writer.write(zeroes);
}
fn end_tag(&writer w) {
auto last_size_pos = _vec.pop[uint](w.size_positions);
auto 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
|