1 files changed, 145 insertions, 0 deletions
diff --git a/internal/gemtext/gemtext.go b/internal/gemtext/gemtext.go
new file mode 100644
index 0000000..83a1be3
--- /dev/null
+++ b/internal/gemtext/gemtext.go
@@ -0,0 +1,145 @@
+// Package gemtext implements shared logic related to gemtext files.
+package gemtext
+
+import (
+	"bufio"
+	"bytes"
+	"errors"
+	"fmt"
+	"html"
+	"io"
+	"strings"
+)
+
+// HTMLTranslator is used to translate a gemtext file into equivalent HTML DOM
+// elements.
+type HTMLTranslator struct {
+	// RenderLink, if given, can be used to override how links are rendered.
+	RenderLink func(w io.Writer, url, label string) error
+}
+
+// HTML contains the result of a translation from gemtext. The Body will be the
+// translated body itself, and Title will correspond to the first primary header
+// of the gemtext file, if there was one.
+type HTML struct {
+	Title string
+	Body  string
+}
+
+// Translate will read a gemtext file from the Reader and return it as an HTML
+// document.
+func (t HTMLTranslator) Translate(src io.Reader) (HTML, error) {
+	var (
+		r         = bufio.NewReader(src)
+		w         = new(bytes.Buffer)
+		title     string
+		pft, list bool
+		writeErr  error
+	)
+
+	sanitizeText := func(str string) string {
+		return html.EscapeString(strings.TrimSpace(str))
+	}
+
+	write := func(fmtStr string, args ...any) {
+		if writeErr != nil {
+			return
+		}
+		fmt.Fprintf(w, fmtStr, args...)
+	}
+
+loop:
+	for {
+		if writeErr != nil {
+			return HTML{}, fmt.Errorf("writing line: %w", writeErr)
+		}
+
+		line, err := r.ReadString('\n')
+
+		switch {
+		case errors.Is(err, io.EOF):
+			break loop
+
+		case err != nil:
+			return HTML{}, fmt.Errorf("reading next line: %w", err)
+
+		case strings.HasPrefix(line, "```"):
+			if !pft {
+				write("<pre>\n")
+				pft = true
+			} else {
+				write("</pre>\n")
+				pft = false
+			}
+			continue
+
+		case pft:
+			write(line)
+			continue
+
+		case len(strings.TrimSpace(line)) == 0:
+			continue
+		}
+
+		// list case is special, because it requires a prefix and suffix tag
+		if strings.HasPrefix(line, "*") {
+			if !list {
+				write("<ul>\n")
+			}
+			write("<li>%s</li>\n", sanitizeText(line[1:]))
+			list = true
+			continue
+		} else if list {
+			write("</ul>\n")
+			list = false
+		}
+
+		switch {
+		case strings.HasPrefix(line, "=>"):
+			var (
+				line   = strings.TrimSpace(line[2:])
+				urlStr = line
+				label  = urlStr
+			)
+
+			if i := strings.IndexAny(urlStr, " \t"); i > -1 {
+				urlStr, label = urlStr[:i], sanitizeText(urlStr[i:])
+			}
+
+			if t.RenderLink == nil {
+				write("<p><a href=\"%s\">%s</a></p>\n", urlStr, label)
+			} else {
+				if err := t.RenderLink(w, urlStr, label); err != nil {
+					return HTML{}, fmt.Errorf(
+						"rendering link %q (label:%q): %w", urlStr, label, err,
+					)
+				}
+			}
+
+		case strings.HasPrefix(line, "###"):
+			write("<h3>%s</h3>\n", sanitizeText(line[3:]))
+
+		case strings.HasPrefix(line, "##"):
+			write("<h2>%s</h2>\n", sanitizeText(line[2:]))
+
+		case strings.HasPrefix(line, "#"):
+			line = sanitizeText(line[1:])
+			if title == "" {
+				title = line
+			}
+			write("<h1>%s</h1>\n", line)
+
+		case strings.HasPrefix(line, ">"):
+			write("<blockquote>%s</blockquote>\n", sanitizeText(line[1:]))
+
+		default:
+			line = strings.TrimSpace(line)
+			write("<p>%s</p>\n", line)
+		}
+	}
+
+	return HTML{
+		Title: title,
+		Body:  w.String(),
+	}, nil
+}