Writing TTF tables using abstract

HmtxTable.hx

package format.ttf.tables;
// HMTX
typedef Metric = {
	advanceWidth:Int,
	leftSideBearing:Int
}
typedef THmtxTable = {
    metrics: Array<Metric>,
    numberOfHMetrics: Int,
    numGlyphs: Int
}
@:forward
abstract HmtxTable( THmtxTable ) to THmtxTable { 
    public
    function new( tHmtxTable: THmtxTable ){
        this = tHmtxTable;
    }
    // hmtx (horizontal metrics) table 
    public static inline
    function read( bytes, maxp, hhea ):Array<Metric> {
        if (bytes == null)
            throw 'no hmtx table found';
        var input = new BytesInput(bytes);
        input.bigEndian = true;
        var metrics = new ArrayMetric();//:Array<Metric> = new Array();
        for (i in 0...hhea.numberOfHMetrics) {
            metrics.push({
                advanceWidth: input.readUInt16(),
                leftSideBearing: input.readInt16() // FWord
            });
        }
        var len = maxp.numGlyphs - hhea.numberOfHMetrics;
        var lastAdvanceWidth = metrics[metrics.length - 1].advanceWidth;
        for (i in 0...len) {
            metrics.push({advanceWidth: lastAdvanceWidth, leftSideBearing: input.readInt16()});
        }
                return new HmtxTable( { metrics:          metrics
                                      , numberOfHMetrics: hhea.numberOfHMetrics
                                      , numGlyhs:         maxp.numGlyhs } );
    }
    public inline
    function write( o: haxe.io.Output ): haxe.io.Output {
        var j = 0;
        var m: Metric;
        for( i in 0...this.numberOfHMetrics ){
            m = this.metrics[ j ];
            o.writeUInt16( m.advanceWidth );
            o.writeInt16(  m.advanceWidth );
            j++;
        }
        var len = this.numGlyphs - this.numberOfHMetrics;
        for( i in 0...len ){
            m = this.metrics[ j ];
            o.writeInt16( m.leftSideBearing );
            j++;
        }
        return o;
    }
}