lib: Add an EBML reader module

2 files changed, 110 insertions, 0 deletions

diff --git a/src/lib/ebml.rs b/src/lib/ebml.rs
new file mode 100644
index 00000000..7b413c63
--- /dev/null
+++ b/src/lib/ebml.rs
@@ -0,0 +1,109 @@
+// 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 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();
+    if (a & 0x80u8 != 0u8) { ret (a & 0x7fu8) as uint; }
+    auto b = reader.read_byte();
+    if (a & 0x40u8 != 0u8) {
+        ret (((a & 0x3fu8) as uint) << 8u) | (b as uint);
+    }
+    auto c = reader.read_byte();
+    if (a & 0x20u8 != 0u8) {
+        ret (((a & 0x1fu8) as uint) << 16u) | ((b as uint) << 8u) |
+            (c as uint);
+    }
+    auto d = reader.read_byte();
+    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.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 pos = r.reader.tell();
+    auto t = read_tag(r);
+    r.states += vec(rec(ebml_tag=t, pos=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.pos as int, io.seek_set);
+}
+
+// Reads the data segment of a tag.
+impure fn read_data(&reader r) -> vec[u8] {
+    ret r.reader.read_bytes(bytes_left(r));
+}
+
+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.writer writer, mutable vec[ebml_state] states);
+
+// TODO
+
diff --git a/src/lib/std.rc b/src/lib/std.rc
index 7a3cdbb2..e434769a 100644
--- a/src/lib/std.rc
+++ b/src/lib/std.rc
@@ -77,6 +77,7 @@ mod dbg;
 mod bitv;
 mod sort;
 mod sha1;
+mod ebml;
 
 // Local Variables:
 // mode: rust;