package main

import "fmt"
import "io"
import "os"
import "strings"

// Html is either Element or Text
type Html interface {
  io.WriterTo
  // isHtml is a non-functional marker to differentiate Html from interface{}
  isHtml()
}

func Reader() io.Reader {
  // TODO in-order traversal of HTML tree to spit out bytes
  return nil
}

func String() string {
  panic("this could be trivially implemented with a Reader and a bytes.Buffer")
}

type Attrib struct {
  Key string
  Value string
}

type element struct {
  tagName string
  attribs []Attrib
  empty bool
  children []Html
}

// writeTo attempts to write the strings in ss to w, incrementing n by the
// number of bytes written.
func writeTo(w io.Writer, n *int64, ss ...string) error {
  for _, s := range ss {
    nCurr, err := io.WriteString(w, s)
    *n += int64(nCurr)
    if err != nil {
      return err
    }
  }
  return nil
}

func (a Attrib) WriteTo(w io.Writer) (n int64, err error) {
  err = writeTo(w, &n, a.Key)
  if err != nil {
    return
  }
  if a.Value != "" {
    err = writeTo(w, &n, "='", a.Value, "'")
  }
  return
}

func (e *element) WriteTo(w io.Writer) (n int64, err error) {
  err = writeTo(w, &n, "<", e.tagName)
  if err != nil {
    return
  }

  for _, attrib := range e.attribs {
    err = writeTo(w, &n, " ")
    if err != nil {
      return
    }

    var nCurr int64
    nCurr, err = attrib.WriteTo(w)
    n += int64(nCurr)
    if err != nil {
      return
    }
  }

  err = writeTo(w, &n, ">")
  if err != nil {
    return
  }

  if !e.empty {
    for _, child := range e.children {
      var nCurr int64
      nCurr, err = child.WriteTo(w)
      n += int64(nCurr)
      if err != nil {
        return
      }
    }

    err = writeTo(w, &n, "</", e.tagName, ">")
  }
  return
}

func (*element) isHtml() {}

func emptyElement(tagName string, attribs []Attrib) Html {
  return &element{
    tagName: tagName,
    attribs: attribs,
    empty: true,
  }
}

func elementFunc(tagName string, attribs []Attrib) func(...Html) Html {
  return func(children ...Html) Html {
    return &element{
      tagName: tagName,
      attribs: attribs,
      children: children,
      empty: false,
    }
  }
}

type text string

func Text(s string) Html {
  return text(s)
}

func (text) isHtml() {}

func (t text) WriteTo(w io.Writer) (n int64, err error) {
  err = writeTo(w, &n, string(t))
  return
}

type rText struct {
  io.Reader
}

func RText(r io.Reader) Html {
  return rText{Reader: r}
}

func (rText) isHtml() {}

func (rt rText) WriteTo(w io.Writer) (n int64, err error) {
  return io.Copy(w, rt.Reader)
}

// Html5 functions

func Div(attribs ...Attrib) func(children ...Html) Html {
  return elementFunc("div", attribs)
}

func Img(attribs ...Attrib) Html {
  return emptyElement("img", attribs)
}

func Id(id string) Attrib {
  return Attrib{
    Key: "id",
    Value: id,
  }
}

func Disabled() Attrib {
  return Attrib {
    Key: "disabled",
  }
}

func main() {
  var page = Div(Disabled(), Id("container"))(
    Img(Id("profile-photo")),
    Text("hello"),
    Img(Disabled()),
    RText(strings.NewReader("goodbye!")),
  )

  n, err := page.WriteTo(os.Stdout)
  fmt.Println()
  fmt.Println(n)
  fmt.Println(err)
}