package mdconv

import (
	"io"
	"text/template"

	"tildegit.org/tjp/gus/gemtext"
	"tildegit.org/tjp/gus/gemtext/internal"
)

// Convert writes markdown to a writer from the provided gemtext document.
//
// Templates can be provided to override the output for different line types.
// The templates supported are:
//   - "header" is called before any lines and is passed the full Document.
//   - "footer" is called after the lines and is passed the full Document.
//   - "textline" is called once per line of text and is passed a gemtext.TextLine.
//   - "linkline" is called once per link line and is passed a gemtext.LinkLine.
//   - "preformattedtextlines" is called once for a block of preformatted text and is
//     passed a slice of gemtext.PreformattedTextLines.
//   - "heading1line" is called once per h1 line and is passed a gemtext.Heading1Line.
//   - "heading2line" is called once per h2 line and is passed a gemtext.Heading2Line.
//   - "heading3line" is called once per h3 line and is passed a gemtext.Heading3Line.
//   - "listitemlines" is called once for a block of contiguous list item lines and
//     is passed a slice of gemtext.ListItemLines.
//   - "quoteline" is passed once per blockquote line and is passed a gemtext.QuoteLine.
//
// There exist default implementations of each of these templates, so the "overrides"
// argument can be nil.
func Convert(wr io.Writer, doc gemtext.Document, overrides *template.Template) error {
	if err := internal.ValidateLinks(doc); err != nil {
		return err
	}

	tmpl, err := baseTmpl.Clone()
	if err != nil {
		return err
	}

	tmpl, err = internal.AddAllTemplates(tmpl, overrides)
	if err != nil {
		return err
	}

	for _, item := range internal.RenderItems(doc) {
		if err := tmpl.ExecuteTemplate(wr, item.Template, item.Object); err != nil {
			return err
		}
	}

	return nil
}

var baseTmpl = template.Must(template.New("mdconv").Parse(`
{{ define "header" }}{{ end }}
{{ define "textline" }}{{ . }}{{ end }}
{{ define "linkline" -}}
	=> [{{ if eq .Label "" }}{{ .URL }}{{ else }}{{ .Label }}{{ end }}]({{ .URL }})
{{ end }}
{{ define "preformattedtextlines" }}` + "```\n" + `{{ range . }}{{ . }}{{ end }}` + "```\n" + `{{ end }}
{{ define "heading1line" }}# {{ .Body }}
{{ end }}
{{ define "heading2line" }}## {{ .Body }}
{{ end }}
{{ define "heading3line" }}### {{ .Body }}
{{ end }}
{{ define "listitemlines" }}{{ range . }}* {{ .Body }}
{{ end }}{{ end }}
{{ define "quoteline" }}> {{ .Body }}
{{ end }}
{{ define "footer" }}{{ end }}
`))