package svg

import "regexp"
import "fmt"
import "strconv"
import "encoding/xml"

type Unit string

// Relative units are:
// em: the 'font-size' of the relevant font
// ex: the 'x-height' of the relevant font
// px: pixels, relative to the viewing device
// ebisvg uses exactly one ebiten pixel for px unit.
// const UnitEx = Unit(ex)
// As the CSS2 standard states "1px thus corresponds to about 0.28 mm"
// This then means that there are 3.571428571428571 pixels per mm.
// ebisvg uses this scale factor for the absolute SVG units.
//
const MM2PX = 3.571428571428571
const IN2MM = 254
const PT2IN = 1 / 72
const PC2IN = 1 / 6

const UnitNo = Unit("")
const UnitPx = Unit("px")
const UnitEm = Unit("em")
const UnitEx = Unit("ex")
const UnitIn = Unit("in")
const UnitCm = Unit("cm")
const UnitMm = Unit("mm")
const UnitPt = Unit("pt")
const UnitPc = Unit("pc")
const UnitPe = Unit("%")

type Length struct {
	Magnitude float32
	Unit
}

func (l Length) Pixels(parent ...float32) float32 {

	switch l.Unit {
	case UnitNo, UnitPx:
		return l.Magnitude
	case UnitEx, UnitEm, UnitPe:
		if len(parent) > 0 {
			return l.Magnitude * parent[0]
		} else {
			return l.Magnitude
		}

	case UnitPc:
		return l.Magnitude * PC2IN * IN2MM * MM2PX
	case UnitPt:
		return l.Magnitude * PT2IN * IN2MM * MM2PX
	case UnitMm:
		return l.Magnitude * MM2PX
	case UnitCm:
		return l.Magnitude * MM2PX * 100
	case UnitIn:
		return l.Magnitude * IN2MM * MM2PX
	default:
		return l.Magnitude
	}
}

var ParseLengthRe = regexp.MustCompile(`([0-9]*\.?[0-9]*)(em|ex|px|in|cm|mm|pt|pc|%)?`)

func ParseLength(s string) (Length, error) {
	parts := ParseLengthRe.FindStringSubmatch(s)
	res := Length{}
	if len(parts) < 2 {
		return res, fmt.Errorf("empty length")
	}
	magnitude, err := strconv.ParseFloat(parts[1], 32)
	if err != nil {
		return res, err
	}
	res.Magnitude = float32(magnitude)
	if len(parts) > 2 {
		res.Unit = Unit(parts[2])
	}
	return res, nil
}

func (l *Length) UnmarshalXMLAttr(attr xml.Attr) error {
	length, err := ParseLength(attr.Value)
	if err != nil {
		return err
	}
	*l = length
	return nil
}

var _ xml.UnmarshalerAttr = &Length{}