let there be midi

js/lib/jasmid/stream.js

@@ -0,0 +1,68 @@
+/* Wrapper for accessing strings through sequential reads */
+function Stream(str) {
+	var position = 0;
+	
+	function read(length) {
+		var result = str.substr(position, length);
+		position += length;
+		return result;
+	}
+	
+	/* read a big-endian 32-bit integer */
+	function readInt32() {
+		var result = (
+			(str.charCodeAt(position) << 24)
+			+ (str.charCodeAt(position + 1) << 16)
+			+ (str.charCodeAt(position + 2) << 8)
+			+ str.charCodeAt(position + 3));
+		position += 4;
+		return result;
+	}
+
+	/* read a big-endian 16-bit integer */
+	function readInt16() {
+		var result = (
+			(str.charCodeAt(position) << 8)
+			+ str.charCodeAt(position + 1));
+		position += 2;
+		return result;
+	}
+	
+	/* read an 8-bit integer */
+	function readInt8() {
+		var result = str.charCodeAt(position);
+		position += 1;
+		return result;
+	}
+	
+	function eof() {
+		return position >= str.length;
+	}
+	
+	/* read a MIDI-style variable-length integer
+		(big-endian value in groups of 7 bits,
+		with top bit set to signify that another byte follows)
+	*/
+	function readVarInt() {
+		var result = 0;
+		while (true) {
+			var b = readInt8();
+			if (b & 0x80) {
+				result += (b & 0x7f);
+				result <<= 7;
+			} else {
+				/* b is the last byte */
+				return result + b;
+			}
+		}
+	}
+	
+	return {
+		'eof': eof,
+		'read': read,
+		'readInt32': readInt32,
+		'readInt16': readInt16,
+		'readInt8': readInt8,
+		'readVarInt': readVarInt
+	}
+}