From c9d28a108d6df59a895dd10f47e834ceba20f98a Mon Sep 17 00:00:00 2001
From: Brian Picciano <me@mediocregopher.com>
Date: Thu, 4 Jul 2024 18:25:17 +0200
Subject: Add gateway_url parameter in gemtext template

---
 README.md                                    | 43 +++++++++++++++++++++-----
 example/Caddyfile                            |  4 ++-
 http/handlers/templates/functions/gemtext.go | 46 ++++++++++++++++++++++++++--
 3 files changed, 81 insertions(+), 12 deletions(-)

diff --git a/README.md b/README.md
index a914f51..30a3285 100644
--- a/README.md
+++ b/README.md
@@ -20,11 +20,45 @@ function][mdfunc] in its usage. It can be enabled by being included in the
 ```text
 templates {
     extensions {
-        gemtext
+        gemtext {
+            # All parameters are optional
+            gateway_url "https://some.gateway/x/"
+        }
     }
 }
 ```
 
+See the `template.localhost` virtual host in `example/Caddyfile`, and the
+associated `example/tpl/render_gemtext.html` template file, for an example of
+how to use this directive.
+
+[gemtext]: https://geminiprotocol.net/docs/gemtext.gmi
+[mdfunc]: https://caddyserver.com/docs/modules/http.handlers.templates#markdown
+
+#### Parameters
+
+Optional parameters to the gemtext extension include:
+
+**gateway_url**
+
+If given then any `gemini://` URLs encountered as links within
+the document will be appended to this URL, having their `gemini://` scheme
+stripped off first.
+
+e.g. if `gateway_url` is `https://some.gateway/x/` then the following line:
+
+```text
+=> gemini://geminiprotocol.net Check it out!
+```
+
+becomes
+
+```html
+<a href="https://some.gateway/x/geminiprotocol.net">Check it out!</a>
+```
+
+#### Template function
+
 Within a template being rendered the `gemtext` function will be available and
 can be passed any string. The function will return a struct with the following
 fields:
@@ -36,13 +70,6 @@ fields:
 * `Title`: A suggested title, based on the first `# Header` line found in the
   gemtext input.
 
-See the `template.localhost` virtual host in `example/Caddyfile`, and the
-associated `example/tpl/render_gemtext.html` template file, for an example of
-how to use the template function.
-
-[gemtext]: https://geminiprotocol.net/docs/gemtext.gmi
-[mdfunc]: https://caddyserver.com/docs/modules/http.handlers.templates#markdown
-
 ## Development
 
 A nix-based development environment is provided with the correct versions of all
diff --git a/example/Caddyfile b/example/Caddyfile
index 8d9dd50..05ad0ba 100644
--- a/example/Caddyfile
+++ b/example/Caddyfile
@@ -26,7 +26,9 @@ http://template.localhost {
 			# Include the gemtext extention to make the gemtext function
 			# available within the template.
 			extensions {
-				gemtext
+				gemtext {
+					gateway_url "https://gemini.tildeverse.org/?gemini://"
+				}
 			}
 		}
 
diff --git a/http/handlers/templates/functions/gemtext.go b/http/handlers/templates/functions/gemtext.go
index 57ea751..179d558 100644
--- a/http/handlers/templates/functions/gemtext.go
+++ b/http/handlers/templates/functions/gemtext.go
@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"html"
 	"io"
+	"net/url"
 	"strings"
 	"text/template"
 
@@ -30,7 +31,22 @@ func init() {
 	)
 }
 
-type Gemtext struct{}
+type Gemtext struct {
+
+	// If given then any `gemini://` URLs encountered as links within the
+	// document will be appended to this URL, having their `gemini://` scheme
+	// stripped off first.
+	//
+	// e.g. if `gateway_url` is `https://some.gateway/x/` then the following
+	// line:
+	//
+	//	=> gemini://geminiprotocol.net Check it out!
+	//
+	// becomes
+	//
+	//	<a href="https://some.gateway/x/geminiprotocol.net">Check it out!</a>
+	GatewayURL string `json:"gateway_url,omitempty"`
+}
 
 var _ templates.CustomFunctions = (*Gemtext)(nil)
 
@@ -56,7 +72,7 @@ type gemtextResult struct {
 	Body  string
 }
 
-func (*Gemtext) funcGemtext(input any) (gemtextResult, error) {
+func (g *Gemtext) funcGemtext(input any) (gemtextResult, error) {
 	var (
 		r         = bufio.NewReader(strings.NewReader(caddy.ToString(input)))
 		w         = new(bytes.Buffer)
@@ -126,9 +142,17 @@ loop:
 				urlStr = line
 				label  = urlStr
 			)
+
 			if i := strings.IndexAny(urlStr, " \t"); i > -1 {
 				urlStr, label = urlStr[:i], sanitizeText(urlStr[i:])
 			}
+
+			if g.GatewayURL != "" {
+				if u, err := url.Parse(urlStr); err == nil && u.Scheme == "gemini" {
+					urlStr = g.GatewayURL + u.Host + u.Path
+				}
+			}
+
 			write("<p><a href=\"%s\">%s</a></p>\n", urlStr, label)
 
 		case strings.HasPrefix(line, "###"):
@@ -160,7 +184,23 @@ loop:
 }
 
 // UnmarshalCaddyfile implements caddyfile.Unmarshaler.
-func (*Gemtext) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
+func (g *Gemtext) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 	d.Next() // consume directive name
+
+	for nesting := d.Nesting(); d.NextBlock(nesting); {
+		v := d.Val()
+		switch v {
+		case "gateway_url":
+			if !d.Args(&g.GatewayURL) {
+				return d.ArgErr()
+			} else if _, err := url.Parse(g.GatewayURL); err != nil {
+				return fmt.Errorf("invalid gateway url: %w", err)
+			}
+
+		default:
+			return fmt.Errorf("unknown directive %q", v)
+		}
+	}
+
 	return nil
 }
-- 
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