namespace XFESystem.Kernel;

/// <summary>Validated SFNT metrics convert a CSS em size to Cosmos' hhea pixel height.</summary>
public sealed class NativeWebFontMetrics
{
    public double LineToEm { get; }
    public double AscentToEm { get; }
    public double NaturalLineToEm { get; }
    public NativeWebFontMetrics(byte[] data)
    {
        ArgumentNullException.ThrowIfNull(data);
        int U16(int p) { if(p<0||p>data.Length-2)throw new InvalidOperationException("Font table is truncated.");return data[p]<<8|data[p+1]; }
        int S16(int p)=>unchecked((short)U16(p));
        int U32(int p) { if(p<0||p>data.Length-4)throw new InvalidOperationException("Font table is truncated.");uint value=(uint)(data[p]<<24|data[p+1]<<16|data[p+2]<<8|data[p+3]);if(value>int.MaxValue)throw new InvalidOperationException("Font table offset is excessive.");return (int)value; }
        int count=U16(4),head=-1,hhea=-1;
        if(count>256||12L+count*16>data.Length)throw new InvalidOperationException("Font directory is invalid.");
        for(int i=0;i<count;i++)
        {
            int p=12+i*16,offset=U32(p+8),length=U32(p+12);
            if(offset>data.Length-length)throw new InvalidOperationException("Font table is outside the font.");
            if(data[p]=='h'&&data[p+1]=='e'&&data[p+2]=='a'&&data[p+3]=='d'&&length>=20)head=offset;
            if(data[p]=='h'&&data[p+1]=='h'&&data[p+2]=='e'&&data[p+3]=='a'&&length>=10)hhea=offset;
        }
        if(head<0||hhea<0)throw new InvalidOperationException("Font head/hhea table is missing.");
        int em=U16(head+18),ascent=S16(hhea+4),descent=S16(hhea+6),gap=S16(hhea+8);
        if(em is <16 or >16384||ascent<=descent)throw new InvalidOperationException("Font line metrics are invalid.");
        LineToEm=(ascent-descent)/(double)em; AscentToEm=ascent/(double)em; NaturalLineToEm=(ascent-descent+gap)/(double)em;
    }
}