extremely basic functions for the jpg file format

jpg.d

@@ -0,0 +1,88 @@
+module arsd.jpg;
+
+import std.typecons;
+import std.stdio;
+import std.conv;
+
+struct JpegSection {
+	ushort length;
+	ubyte identifier;
+	ubyte[] data;
+}
+
+// gives as a range of file sections
+struct LazyJpegFile {
+	File f;
+	JpegSection _front;
+	bool _frontIsValid;
+	this(File f) {
+		this.f = f;
+
+		ubyte[2] headerBuffer;
+		auto data = f.rawRead(headerBuffer);
+		if(data != [0xff, 0xd8])
+			throw new Exception("no jpeg header");
+		popFront(); // prime
+	}
+
+	void popFront() {
+		ubyte[4] startingBuffer;
+		auto read = f.rawRead(startingBuffer);
+		if(read.length != 4) {
+			_frontIsValid = false;
+			return; // end of file
+		}
+
+		if(startingBuffer[0] != 0xff)
+			throw new Exception("not lined up in file");
+
+		_front.identifier = startingBuffer[1];
+		_front.length = cast(ushort) (startingBuffer[2]) * 256 + startingBuffer[3];
+
+		if(_front.length < 2)
+			throw new Exception("wtf");
+		_front.length -= 2; // the length in the file includes the block header, but we just want the data here
+
+		_front.data = new ubyte[](_front.length);
+		read = f.rawRead(_front.data);
+		if(read.length != _front.length)
+			throw new Exception("didn't read the file right, got " ~ to!string(read.length) ~ " instead of " ~ to!string(_front.length));
+
+		_frontIsValid = true;
+	}
+
+	JpegSection front() {
+		return _front;
+	}
+
+	bool empty() {
+		return !_frontIsValid;
+	}
+}
+
+// http://www.obrador.com/essentialjpeg/headerinfo.htm
+Tuple!(int, int) getSizeFromFile(string filename) {
+	import std.stdio;
+
+	auto file = File(filename, "rb");
+
+	auto jpeg = LazyJpegFile(file);
+
+	auto firstSection = jpeg.front();
+	jpeg.popFront();
+
+	//if(firstSection.identifier != 0xe0)
+		//throw new Exception("bad header");
+
+	for(; !jpeg.empty(); jpeg.popFront()) {
+		if(jpeg.front.identifier != 0xc0)
+			continue;
+		auto data = jpeg.front.data[1..$]; // skip the precision byte
+
+		ushort height = data[0] * 256 + data[1];
+		ushort width  = data[2] * 256 + data[3];
+		return tuple(cast(int) width, cast(int) height);
+	}
+
+	throw new Exception("idk about the length");
+}