2 files changed, 154 insertions, 0 deletions
diff --git a/src/gmi/gemtext/gemtext.go b/src/gmi/gemtext/gemtext.go
new file mode 100644
index 0000000..5c8f594
--- /dev/null
+++ b/src/gmi/gemtext/gemtext.go
@@ -0,0 +1,88 @@
+// Package gemtext contains code related to processing and producing gemtext
+// documents.
+package gemtext
+
+import (
+	"bufio"
+	"errors"
+	"fmt"
+	"io"
+	"net/url"
+	"path"
+	"regexp"
+	"strings"
+)
+
+func hasImgExt(p string) bool {
+	switch path.Ext(strings.ToLower(p)) {
+	case ".jpg", ".jpeg", ".png", ".gif", ".svg":
+		return true
+	default:
+		return false
+	}
+}
+
+// matches `=> dstURL [optional description]`
+var linkRegexp = regexp.MustCompile(`^=>\s+(\S+)\s*(.*?)\s*$`)
+
+// ToMarkdown reads a gemtext formatted body from the Reader and writes
+// the markdown version of that body to the Writer.
+//
+// gmiGateway, if given, is used for all `gemini://` links. The `gemini://`
+// prefix will be stripped, and replaced with the given URL.
+func ToMarkdown(dst io.Writer, src io.Reader, gmiGateway *url.URL) error {
+
+	bufSrc := bufio.NewReader(src)
+
+	for {
+
+		line, err := bufSrc.ReadString('\n')
+		if err != nil && !errors.Is(err, io.EOF) {
+			return fmt.Errorf("reading: %w", err)
+		}
+
+		last := err == io.EOF
+
+		if match := linkRegexp.FindStringSubmatch(line); len(match) > 0 {
+
+			u, err := url.Parse(match[1])
+			if err != nil {
+				return fmt.Errorf("link to invalid url %q: %w", match[1], err)
+			}
+
+			if u.Scheme == "gemini" && gmiGateway != nil {
+				newUStr := gmiGateway.String() + u.Host + u.Path
+				if u, err = url.Parse(newUStr); err != nil {
+					return fmt.Errorf("parsing proxied URL %q: %w", newUStr, err)
+				}
+			}
+
+			isImg := hasImgExt(u.Path)
+
+			descr := match[2]
+
+			if descr != "" {
+				// ok
+			} else if isImg {
+				descr = "Image"
+			} else {
+				descr = "Link"
+			}
+
+			line = fmt.Sprintf("[%s](%s)\n", descr, u.String())
+
+			if isImg {
+				line = "!" + line
+			}
+		}
+
+		if _, err := dst.Write([]byte(line)); err != nil {
+			return fmt.Errorf("writing: %w", err)
+		}
+
+		if last {
+			return nil
+		}
+	}
+
+}
diff --git a/src/gmi/gemtext/gemtext_test.go b/src/gmi/gemtext/gemtext_test.go
new file mode 100644
index 0000000..fe58a64
--- /dev/null
+++ b/src/gmi/gemtext/gemtext_test.go
@@ -0,0 +1,66 @@
+package gemtext
+
+import (
+	"bytes"
+	"net/url"
+	"strconv"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestToMarkdown(t *testing.T) {
+
+	gmiGateway, _ := url.Parse("https://gateway.com/x/")
+
+	tests := []struct {
+		in, exp string
+	}{
+		{
+			in:  "",
+			exp: "",
+		},
+		{
+			in:  "=> foo",
+			exp: "[Link](foo)\n",
+		},
+		{
+			in:  "what\n=> foo\n=> bar",
+			exp: "what\n[Link](foo)\n[Link](bar)\n",
+		},
+		{
+			in:  "=> foo description is here   ",
+			exp: "[description is here](foo)\n",
+		},
+		{
+			in:  "=> img.png",
+			exp: "![Image](img.png)\n",
+		},
+		{
+			in:  "=> img.png description is here   ",
+			exp: "![description is here](img.png)\n",
+		},
+		{
+			in:  "=> gemini://somewhere.com/foo Somewhere",
+			exp: "[Somewhere](https://gateway.com/x/somewhere.com/foo)\n",
+		},
+		{
+			in:  "=> gemini://somewhere.com:420/foo Somewhere",
+			exp: "[Somewhere](https://gateway.com/x/somewhere.com:420/foo)\n",
+		},
+		{
+			in:  "=> gemini://somewhere.com:420/foo?bar=baz Somewhere",
+			exp: "[Somewhere](https://gateway.com/x/somewhere.com:420/foo?bar=baz)\n",
+		},
+	}
+
+	for i, test := range tests {
+		t.Run(strconv.Itoa(i), func(t *testing.T) {
+
+			got := new(bytes.Buffer)
+			err := ToMarkdown(got, bytes.NewBufferString(test.in), gmiGateway)
+			assert.NoError(t, err)
+			assert.Equal(t, test.exp, got.String())
+		})
+	}
+}