Add YAML front matter parsing support

Fixes #1. Only a subset of Hugo front matter props are supported, namely
title/date.

cmd/md2gmn/main.go

@@ -13,7 +13,8 @@
 // You should have received a copy of the GNU General Public License
 // along with gmnhg. If not, see <https://www.gnu.org/licenses/>.
 
-// md2gmn converts Markdown text files to text/gemini.
+// md2gmn converts Markdown text files to text/gemini. It panics on
+// invalid input.
 package main
 
 import (
@@ -46,5 +47,10 @@ func main() {
 		panic(err)
 	}
 
-	os.Stdout.Write(gemini.RenderMarkdown(text))
+	geminiContent, err := gemini.RenderMarkdown(text)
+	if err != nil {
+		panic(err)
+	}
+
+	os.Stdout.Write(geminiContent)
 }

go.mod

@@ -2,4 +2,7 @@ module git.tdem.in/tdemin/gmnhg
 
 go 1.15
 
-require github.com/gomarkdown/markdown v0.0.0-20201024011455-45c732cc8a6b
+require (
+	github.com/gomarkdown/markdown v0.0.0-20201024011455-45c732cc8a6b
+	gopkg.in/yaml.v2 v2.3.0
+)

go.sum

@@ -1,3 +1,6 @@
 github.com/gomarkdown/markdown v0.0.0-20201024011455-45c732cc8a6b h1:Om9FdD4lzIJELyJxwr9EWSjaG6GMUNS3iebnhrGevhI=
 github.com/gomarkdown/markdown v0.0.0-20201024011455-45c732cc8a6b/go.mod h1:aii0r/K0ZnHv7G0KF7xy1v0A7s2Ljrb5byB7MO5p6TU=
 golang.org/dl v0.0.0-20190829154251-82a15e2f2ead/go.mod h1:IUMfjQLJQd4UTqG1Z90tenwKoCX93Gn3MAQJMOSBsDQ=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v2 v2.3.0 h1:clyUAQHOM3G0M3f5vQj7LuJrETvjVot3Z5el9nffUtU=
+gopkg.in/yaml.v2 v2.3.0/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=

internal/gemini/renderer.go

@@ -22,6 +22,7 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"time"
 
 	"github.com/gomarkdown/markdown/ast"
 )
@@ -35,14 +36,30 @@ var (
 	itemIndent  = []byte{'\t'}
 )
 
+const timestampFormat = "2006-01-02 15:04"
+
+// Metadata provides data necessary for proper post rendering.
+type Metadata interface {
+	Title() string
+	Date() time.Time
+}
+
 // Renderer implements markdown.Renderer.
-type Renderer struct{}
+type Renderer struct {
+	Metadata Metadata
+}
 
 // NewRenderer returns a new Renderer.
 func NewRenderer() Renderer {
 	return Renderer{}
 }
 
+// NewRendererWithMetadata returns a new Renderer initialized with post
+// metadata.
+func NewRendererWithMetadata(m Metadata) Renderer {
+	return Renderer{Metadata: m}
+}
+
 func (r Renderer) link(w io.Writer, node *ast.Link, entering bool) {
 	if entering {
 		w.Write(linkPrefix)
@@ -290,12 +307,15 @@ func (r Renderer) RenderNode(w io.Writer, node ast.Node, entering bool) ast.Walk
 	return ast.GoToNext
 }
 
-// RenderHeader implements Renderer.RenderHeader().
+// RenderHeader implements Renderer.RenderHeader(). It renders metadata
+// at the top of the post if any has been provided.
 func (r Renderer) RenderHeader(w io.Writer, node ast.Node) {
-	// likely doesn't need any code
+	if r.Metadata != nil {
+		// TODO: Renderer.RenderHeader: check whether date is mandatory
+		// in Hugo
+		w.Write([]byte(fmt.Sprintf("# %s\n\n%s\n\n", r.Metadata.Title(), r.Metadata.Date().Format(timestampFormat))))
+	}
 }
 
 // RenderFooter implements Renderer.RenderFooter().
-func (r Renderer) RenderFooter(w io.Writer, node ast.Node) {
-	// likely doesn't need any code either
-}
+func (r Renderer) RenderFooter(w io.Writer, node ast.Node) {}

render.go

@@ -14,20 +14,64 @@
 // along with gmnhg. If not, see <https://www.gnu.org/licenses/>.
 
 // Package gemini provides functions to convert Markdown files to
-// Gemtext.
+// Gemtext. It supports the use of YAML front matter in Markdown.
 package gemini
 
 import (
+	"bytes"
+	"fmt"
+	"time"
+
 	"git.tdem.in/tdemin/gmnhg/internal/gemini"
 	"github.com/gomarkdown/markdown"
 	"github.com/gomarkdown/markdown/parser"
+	"gopkg.in/yaml.v2"
 )
 
-// RenderMarkdown converts Markdown text to text/gemini using gomarkdown.
-//
-// gomarkdown doesn't return any errors, nor does this function.
-func RenderMarkdown(md []byte) (geminiText []byte) {
+// hugoMetadata implements gemini.Metadata, providing the bare minimum
+// of possible post props.
+type hugoMetadata struct {
+	PostTitle string    `yaml:"title"`
+	PostDate  time.Time `yaml:"date"`
+}
+
+func (h hugoMetadata) Title() string {
+	return h.PostTitle
+}
+
+func (h hugoMetadata) Date() time.Time {
+	return h.PostDate
+}
+
+var yamlDelimiter = []byte("---\n")
+
+// RenderMarkdown converts Markdown text to text/gemini using
+// gomarkdown, appending Hugo YAML front matter data if any is present
+// to the post header.
+func RenderMarkdown(md []byte) (geminiText []byte, err error) {
+	var metadata hugoMetadata
+	if len(md) > len(yamlDelimiter)*2 {
+		// only allow front matter at file start
+		if bytes.Index(md, yamlDelimiter) != 0 {
+			goto parse
+		}
+		blockEnd := bytes.Index(md[len(yamlDelimiter):], yamlDelimiter)
+		if blockEnd == -1 {
+			goto parse
+		}
+		yamlContent := md[len(yamlDelimiter) : blockEnd+len(yamlDelimiter)]
+		if err := yaml.Unmarshal(yamlContent, &metadata); err != nil {
+			return nil, fmt.Errorf("invalid front matter: %w", err)
+		}
+		md = md[blockEnd+len(yamlDelimiter)*2:]
+	}
+parse:
 	ast := markdown.Parse(md, parser.NewWithExtensions(parser.CommonExtensions))
-	geminiContent := markdown.Render(ast, gemini.NewRenderer())
-	return geminiContent
+	var geminiContent []byte
+	if metadata.PostTitle != "" {
+		geminiContent = markdown.Render(ast, gemini.NewRendererWithMetadata(metadata))
+	} else {
+		geminiContent = markdown.Render(ast, gemini.NewRenderer())
+	}
+	return geminiContent, nil
 }