1 files changed, 193 insertions, 0 deletions
diff --git a/mcproto/decode.ha b/mcproto/decode.ha
new file mode 100644
index 0000000..bd020c4
--- /dev/null
+++ b/mcproto/decode.ha
@@ -0,0 +1,193 @@
+use encoding::utf8;
+use endian;
+use fmt;
+use io;
+use strings;
+use trace;
+use uuid;
+
+export type Decoder = struct {
+	input: []u8,
+	pos: size,
+	tracer: *trace::tracer,
+};
+
+export type Context = struct {
+	dec: *Decoder,
+	pos: size,
+	fmt: str,
+	fields: []fmt::field,
+	up: nullable *Context,
+};
+
+export fn log(
+	ctx: *Context,
+	lvl: trace::level,
+	fmt: str,
+	fields: fmt::field...
+) void = {
+	log_(ctx, null, lvl, fmt, fields...);
+};
+
+fn log_(
+	ctx: *Context,
+	trace_ctx: nullable *trace::context,
+	lvl: trace::level,
+	fmt: str,
+	fields: fmt::field...
+) void = {
+	let s = "";
+	defer free(s);
+	if (len(ctx.fmt) != 0) {
+		s = fmt::asprintf(ctx.fmt, ctx.fields...);
+		// TODO: is this legal? works at the moment due to qbe
+		// semantics, but who knows...
+		trace_ctx = &trace::context {
+			fmt = "{} (offset {})",
+			fields = [s, ctx.pos],
+			next = trace_ctx,
+		};
+	};
+	match (ctx.up) {
+	case let ctx_: *Context =>
+		log_(ctx_, trace_ctx, lvl, fmt, fields...);
+	case null =>
+		trace::log(ctx.dec.tracer, trace_ctx, lvl, fmt, fields...);
+	};
+};
+
+export fn error(ctx: *Context, fmt: str, fields: fmt::field...) trace::failed = {
+	log(ctx, trace::level::ERROR, fmt, fields...);
+	return trace::failed;
+};
+
+export fn root(dec: *Decoder) Context = {
+	return Context {
+		dec = dec,
+		pos = dec.pos,
+		...
+	};
+};
+
+export fn context(ctx: *Context, fmt: str, fields: fmt::field...) Context = {
+	return Context {
+		dec = ctx.dec,
+		pos = ctx.dec.pos,
+		fmt = fmt,
+		fields = fields,
+		up = ctx,
+	};
+};
+
+export fn decode_nbytes(ctx: *Context, length: size)
+		([]u8 | trace::failed) = {
+	const dec = ctx.dec;
+	if (len(dec.input) - dec.pos < length) {
+		return error(ctx, "Expected {} bytes, found only {}",
+			length, len(dec.input) - dec.pos);
+	};
+	const res = dec.input[dec.pos..dec.pos + length];
+	dec.pos += length;
+	return res;
+};
+
+export fn decode_byte(ctx: *Context) (u8 | trace::failed) = {
+	const b = decode_nbytes(ctx, 1)?;
+	return b[0];
+};
+export fn decode_short(ctx: *Context) (u16 | trace::failed) = {
+	const b = decode_nbytes(ctx, 2)?;
+	return endian::begetu16(b);
+};
+export fn decode_int(ctx: *Context) (u32 | trace::failed) = {
+	const b = decode_nbytes(ctx, 4)?;
+	return endian::begetu32(b);
+};
+export fn decode_long(ctx: *Context) (u64 | trace::failed) = {
+	const b = decode_nbytes(ctx, 8)?;
+	return endian::begetu64(b);
+};
+
+export fn decode_bool(ctx: *Context) (bool | trace::failed) = {
+	const b = decode_byte(ctx)?;
+	if (b >= 2) {
+		return error(ctx, "Invalid boolean");
+	};
+	return b != 0;
+};
+
+export fn decode_float(ctx: *Context) (f32 | trace::failed) = {
+	const v = decode_int(ctx)?;
+	return *(&v: *f32);
+};
+export fn decode_double(ctx: *Context) (f64 | trace::failed) = {
+	const v = decode_long(ctx)?;
+	return *(&v: *f64);
+};
+
+export fn try_decode_varint(ctx: *Context) (i32 | !(trace::failed | TooShort)) = {
+	let res = 0u32;
+	const dec = ctx.dec;
+
+	for (let i = 0u32; dec.pos + i < len(dec.input); i += 1) {
+		const b = dec.input[dec.pos + i];
+
+		if (i == 4 && b & 0xf0 != 0) {
+			return error(ctx, "VarInt too long");
+		};
+
+		res |= (b & 0x7f): u32 << (7 * i);
+
+		if (b & 0x80 == 0) {
+			dec.pos += i + 1;
+			return res: i32;
+		};
+	};
+
+	return TooShort;
+};
+
+export fn decode_varint(ctx: *Context) (i32 | trace::failed) = {
+	match (try_decode_varint(ctx)) {
+	case let res: i32 =>
+		return res;
+	case =>
+		return error(ctx, "VarInt too short");
+	};
+};
+
+export fn decode_string(ctx: *Context, maxlen: size) (str | trace::failed) = {
+	const length = decode_varint(&context(ctx, "string length"))?;
+	const length = length: size;
+
+	if (length >= maxlen * 4) {
+		return error(ctx,
+			"String length {} exceeds limit of {} bytes",
+			length, maxlen * 4);
+	};
+
+	const ctx_ = context(ctx, "string data ({} bytes)", length);
+	const bytes = decode_nbytes(&ctx_, length)?;
+	match (strings::fromutf8(bytes)) {
+	case let string: str =>
+		// don't bother checking length in code points. doesn't seem
+		// very useful.
+		return string;
+	case utf8::invalid =>
+		return error(&ctx_, "Invalid UTF-8");
+	};
+};
+
+export fn decode_uuid(ctx: *Context) (uuid::uuid | trace::failed) = {
+	let uuid: [16]u8 = [0...];
+	uuid[..] = decode_nbytes(ctx, 16)?;
+	return uuid;
+};
+
+export fn expect_end(ctx: *Context) (void | trace::failed) = {
+	if (ctx.dec.pos != len(ctx.dec.input)) {
+		return error(ctx,
+			"Expected end of input, but found {} extra bytes starting at {}",
+			len(ctx.dec.input) - ctx.dec.pos, ctx.dec.pos);
+	};
+};