package markdown

import (
	"bytes"
	"io"

	"github.com/gomarkdown/markdown/ast"
	"github.com/gomarkdown/markdown/html"
	"github.com/gomarkdown/markdown/parser"
)

// Renderer is an interface for implementing custom renderers.
type Renderer interface {
	// RenderNode renders markdown node to w.
	// It's called once for a leaf node.
	// It's called twice for non-leaf nodes:
	// * first with entering=true
	// * then with entering=false
	//
	// Return value is a way to tell the calling walker to adjust its walk
	// pattern: e.g. it can terminate the traversal by returning Terminate. Or it
	// can ask the walker to skip a subtree of this node by returning SkipChildren.
	// The typical behavior is to return GoToNext, which asks for the usual
	// traversal to the next node.
	RenderNode(w io.Writer, node ast.Node, entering bool) ast.WalkStatus

	// RenderHeader is a method that allows the renderer to produce some
	// content preceding the main body of the output document. The header is
	// understood in the broad sense here. For example, the default HTML
	// renderer will write not only the HTML document preamble, but also the
	// table of contents if it was requested.
	//
	// The method will be passed an entire document tree, in case a particular
	// implementation needs to inspect it to produce output.
	//
	// The output should be written to the supplied writer w. If your
	// implementation has no header to write, supply an empty implementation.
	RenderHeader(w io.Writer, ast ast.Node)

	// RenderFooter is a symmetric counterpart of RenderHeader.
	RenderFooter(w io.Writer, ast ast.Node)
}

// Parse parsers a markdown document using provided parser. If parser is nil,
// we use parser configured with parser.CommonExtensions.
//
// It returns AST (abstract syntax tree) that can be converted to another
// format using Render function.
func Parse(markdown []byte, p *parser.Parser) ast.Node {
	if p == nil {
		p = parser.New()
	}
	return p.Parse(markdown)
}

// Render uses renderer to convert parsed markdown document into a different format.
//
// To convert to HTML, pass html.Renderer
func Render(doc ast.Node, renderer Renderer) []byte {
	var buf bytes.Buffer
	renderer.RenderHeader(&buf, doc)
	ast.WalkFunc(doc, func(node ast.Node, entering bool) ast.WalkStatus {
		return renderer.RenderNode(&buf, node, entering)
	})
	renderer.RenderFooter(&buf, doc)
	return buf.Bytes()
}

// ToHTML converts markdownDoc to HTML.
//
// You can optionally pass a parser and renderer. This allows to customize
// a parser, use a customized html render or use completely custom renderer.
//
// If you pass nil for both, we use parser configured with parser.CommonExtensions
// and html.Renderer configured with html.CommonFlags.
func ToHTML(markdown []byte, p *parser.Parser, renderer Renderer) []byte {
	doc := Parse(markdown, p)
	if renderer == nil {
		opts := html.RendererOptions{
			Flags: html.CommonFlags,
		}
		renderer = html.NewRenderer(opts)
	}
	return Render(doc, renderer)
}