function Gauge( canvas, options ) {
	this.canvas = canvas;
	options = options || {};
	this.settings = {
		value: options.value || 0,
		pointerValue: options.value || 0,
		label: options.label || '',
		min: options.min || 0,
		max: options.max || 100,
		majorTicks: options.majorTicks || 5,
		minorTicks: options.minorTicks || 2, // small ticks inside each major tick
		greenFrom: options.greenFrom || 0,
		greenTo: options.greenTo || 0,
		yellowFrom: options.yellowFrom || 0,
		yellowTo: options.yellowTo || 0,
		redFrom: options.redFrom || 0,
		redTo: options.redTo || 0
	};


	this.drawBackground = function( ) {
		var fill = [ '#111', '#ccc', '#ddd', '#eee' ];
		var rad = [ this.radius, this.radius - 1, this.radius * 0.98, 
			this.radius * 0.95 ];

		this.c2d.rotate( this.startDeg ); 
		for ( var i = 0; i < fill.length; i++ ) {
			this.c2d.fillStyle = fill[ i ];
			this.c2d.beginPath();
			this.c2d.arc( 0, 0, rad[ i ], 0, this.spanDeg, false );
			this.c2d.fill();
		}
	}

	this.drawRange = function( from, to, style, span ) {
		if ( to > from ) {
			var span = this.spanDeg * ( to - from ) / 100;

			this.c2d.rotate( this.startDeg ); 
			this.c2d.fillStyle = style;
			this.c2d.rotate( this.spanDeg * from / 100 );    
			this.c2d.beginPath();
			this.c2d.moveTo( this.innerRadius, 0 );
			this.c2d.lineTo( this.outerRadius, 0 );
			this.c2d.arc( 0, 0, this.outerRadius, 0, span, false );
			this.c2d.rotate( span );
			this.c2d.lineTo( this.innerRadius, 0 );
			this.c2d.arc( 0, 0, this.innerRadius, 0, - span, true ); 
			this.c2d.fill();
		}
	}

	this.drawTicks = function( majorTicks, minorTicks ) {
		// major ticks
		this.c2d.rotate( this.startDeg ); 
		this.c2d.lineWidth = this.radius * 0.025;
		var majorSpan = this.spanDeg / ( majorTicks - 1 ); 
		for ( var i = 0; i < majorTicks; i++ ) {
			this.c2d.beginPath();
			this.c2d.moveTo( this.innerRadius,0 );
			this.c2d.lineTo( this.outerRadius,0 );
			this.c2d.stroke();

			// minor ticks
			if ( i + 1 < majorTicks ) {
				this.c2d.save();
				this.c2d.lineWidth = this.radius * 0.01;
				var minorSpan = majorSpan / ( minorTicks + 1 );
				for ( var j = 0; j < minorTicks; j++ ) {
					this.c2d.rotate( minorSpan );
					this.c2d.beginPath();
					this.c2d.moveTo( this.innerRadius + ( this.outerRadius - this.innerRadius ) / 3, 0 );
					this.c2d.lineTo( this.outerRadius, 0 );
					this.c2d.stroke();
				}
				this.c2d.restore();
			}
			this.c2d.rotate( majorSpan );
		}
	}

	this.drawPointer = function( value ) {
		function pointer( ctx ) {
			ctx.c2d.beginPath();
			ctx.c2d.moveTo( - ctx.radius * 0.2, 0 );
			ctx.c2d.lineTo(	0, ctx.radius * 0.05 );
			ctx.c2d.lineTo( ctx.radius * 0.8, 0 );
			ctx.c2d.lineTo( 0, - ctx.radius * 0.05 );
			ctx.c2d.lineTo( - ctx.radius * 0.2, 0 );
		}
		this.c2d.rotate( this.startDeg ); 
		this.c2d.rotate( this.spanDeg * value / 100 );    
		this.c2d.lineWidth = this.radius * 0.015;
		this.c2d.fillStyle = 'rgba(255, 100, 0, 0.7)';
		pointer( this );
		this.c2d.fill();
		this.c2d.strokeStyle = 'rgba(255, 100, 100, 0.9)';
		pointer( this );
		this.c2d.stroke();
		// center circle
		this.c2d.fillStyle = 'rgba(0, 100, 255, 1)';
		this.c2d.beginPath();
		this.c2d.arc( 0, 0, this.radius * 0.1, 0, Math.PI * 2, true );
		this.c2d.fill();
		this.c2d.strokeStyle = 'rgba(0, 0, 255, 1)';
		this.c2d.beginPath();
		this.c2d.arc( 0, 0, this.radius * 0.1, 0, Math.PI * 2, true );
		this.c2d.stroke();
	}
	
	this.drawCaption = function( label ) {
		if ( label ) {
			var fontSize = this.radius / 5;
			this.c2d.font = fontSize.toFixed(0) + 'px sans-serif';
			var metrics = this.c2d.measureText( label );
			this.c2d.fillStyle = 'rgb(0, 0, 0)';
			var px = - metrics.width / 2;
			var py = - this.radius * 0.4 + fontSize / 2;
			this.c2d.fillText( label, px, py );
		}
	}

	this.drawValues = function( min, max, value, decimals ) {
		function formatNum( value, decimals ) {
			var ret = value.toFixed( decimals );
			while ( ( decimals > 0 ) && ret.match( /^\d+\.(\d+)?0$/ ) ) {
					decimals -= 1;
					ret = value.toFixed( decimals );
			}
			return ret;
		}
		// value text
		var fontSize = this.radius / 5;
		this.c2d.font = fontSize.toFixed(0) + 'px sans-serif';
		var metrics = this.c2d.measureText( formatNum( value, decimals ) );
		this.c2d.fillStyle = 'rgb(0, 0, 0)';
		this.c2d.fillText( formatNum( value, decimals ), - metrics.width / 2, this.radius * 0.72 );

		// min label
		this.save();
		var deg = Math.PI * 14.5/8;
		this.c2d.translate( this.radius * 0.65 * Math.sin( deg ), 
			this.radius * 0.65 * Math.cos( deg ) );
		var fontSize = this.radius / 8;
		this.c2d.font = fontSize.toFixed(0) + 'px sans-serif';
		var metrics = this.c2d.measureText( formatNum( min, decimals ) );
		this.c2d.fillStyle = 'rgb(0, 0, 0)';
		this.c2d.fillText( formatNum( min, decimals ), 0, 0 );
		this.restore();

		// max label
		this.save();
		var deg = Math.PI * 17.5/8;
		this.c2d.translate( this.radius * 0.65 * Math.sin( deg ), 
			this.radius * 0.65 * Math.cos( deg ) );
		var fontSize = this.radius / 8;
		this.c2d.font = fontSize.toFixed(0) + 'px sans-serif';
		var metrics = this.c2d.measureText( formatNum( max, decimals ) );
		this.c2d.fillStyle = 'rgb(0, 0, 0)';
		this.c2d.fillText( formatNum( max, decimals ), - metrics.width, 0 );
		this.restore();
	}
	this.draw();

	return this;
}

Gauge.prototype.setValue = function( value ) {
	var timer = null;
	var that = this;
	var increment = Math.abs( that.settings.value - value ) / 25;

	function adjustValue() {
		if ( that.settings.pointerValue < value ) {
			that.settings.pointerValue += increment;
			if ( that.settings.pointerValue + increment >= value ) {
				that.settings.pointerValue = value;
				clearInterval( timer );
			}
		} else {
			that.settings.pointerValue -= increment;
			if ( that.settings.pointerValue - increment <= value ) {
				that.settings.pointerValue = value; 
				clearInterval( timer );
			}
		}
		that.draw();
	}

	if ( this.settings.value != value ) {
		this.settings.value = value;
		timer = setInterval( adjustValue, 40 );
	}
}

Gauge.prototype.draw = function() {

	if ( ! this.canvas.getContext ) return; //-->

	// settings normalized to a [0, 100] interval
	function normalize( settings ) {
		var span = settings.max - settings.min;

		return {
			min: 0,
			max: 100,
			value: ( settings.value - settings.min ) / span * 100,
			pointerValue: ( settings.pointerValue - settings.min ) / span * 100,
			label: settings.label || '',
			greenFrom: ( settings.greenFrom - settings.min ) / span * 100,
			greenTo: ( settings.greenTo - settings.min ) / span * 100,
			yellowFrom: ( settings.yellowFrom - settings.min ) / span * 100,
			yellowTo: ( settings.yellowTo - settings.min ) / span * 100,
			redFrom: ( settings.redFrom - settings.min ) / span * 100,
			redTo: ( settings.redTo - settings.min ) / span * 100,
			// also fix some possible invalid settings
			majorTicks: Math.max( 2, settings.majorTicks ),
			minorTicks: Math.max( 0, settings.minorTicks ),
			decimals: Math.max( 0, 3 - ( settings.max - settings.min ).toFixed( 0 ).length )
		};
	}

	// draw context contains a set of values useful for
	// most drawing operations.
	var relSettings = normalize( this.settings );
	var drawCtx = {
		c2d: this.canvas.getContext( '2d' ),
		startDeg: Math.PI * 5.5 / 8,
		spanDeg: Math.PI * 13 / 8,
		save: function() {
			this.c2d.save();
		},
		restore: function() {
			this.c2d.restore();
		},
		call: function( fn ) {
			var args = Array.prototype.slice.call( arguments );
			this.save();
			this.translateCenter();
			fn.apply( this, args.slice( 1 ) );
			this.restore();
		},
		clear: function() {
			this.c2d.clearRect( 0, 0, this.width, this.height );
		},
		translateCenter: function() {
			this.c2d.translate( this.centerX, this.centerY );
		}
	}
	drawCtx.width = drawCtx.c2d.canvas.width;
	drawCtx.height = drawCtx.c2d.canvas.height;
	drawCtx.radius = Math.min( drawCtx.width / 2 - 4, drawCtx.height / 2 - 4 );
	drawCtx.innerRadius = drawCtx.radius * 0.7;
	drawCtx.outerRadius = drawCtx.radius * 0.9;
	drawCtx.centerX = drawCtx.radius + 4;
	drawCtx.centerY = drawCtx.radius + 4 + ( drawCtx.radius - 
		drawCtx.radius * Math.sin( drawCtx.startDeg ) ) / 2;

	// draw everything
	drawCtx.clear();
	drawCtx.call( this.drawBackground );
	drawCtx.call( this.drawRange, relSettings.redFrom, relSettings.redTo, 'rgba(255, 0, 0, 0.2)');
	drawCtx.call( this.drawRange, relSettings.greenFrom, relSettings.greenTo, 'rgba(0, 255, 0, 0.2)' );
	drawCtx.call( this.drawRange, relSettings.yellowFrom, relSettings.yellowTo, 'rgba(255, 215, 0, 0.2)' );
	drawCtx.call( this.drawTicks, relSettings.majorTicks, relSettings.minorTicks );
	drawCtx.call( this.drawPointer, relSettings.pointerValue );
	drawCtx.call( this.drawCaption, relSettings.label );
	drawCtx.call( this.drawValues, this.settings.min, this.settings.max, this.settings.value, relSettings.decimals ); 
}