feat(creole): Support [[https://example.com|links]]

This patch adds support for freelinks.

This patch also introduced hprint() – a function to print text while
escaping HTML characters. This isn't very DRY since that's also
implemented in do_replacements(), but the abstraction necessary to reuse
this functionality introduced disproportional amount of mental overhead.

2 files changed, 100 insertions, 12 deletions

diff --git a/src/creole-test.c b/src/creole-test.c
index 50ccd8b..c5b6590 100644
--- a/src/creole-test.c
+++ b/src/creole-test.c
@@ -55,6 +55,41 @@ struct {
 		.input   =  "======= Header =",
 		.output  =  "<p>======= Header =</p>"
 	},
+	{
+		.name    =  "Unnamed link",
+		.input   =  "[[MyPage]]",
+		.output  =  "<p><a href=\"MyPage\">MyPage</a></p>"
+	},
+	{
+		.name    =  "Named link",
+		.input   =  "[[MyPage|My page]]",
+		.output  =  "<p><a href=\"MyPage\">My page</a></p>"
+	},
+	{
+		.name    =  "Escaped link",
+		.input   =  "A paragraph with an ~[[escaped link]].",
+		.output  =  "<p>A paragraph with an [[escaped link]].</p>"
+	},
+	{
+		.name    =  "Link with an escaped end",
+		.input   =  "[[https://example.com|A link with an escaped ~]] end]]",
+		.output  =  "<p><a href=\"https://example.com\">A link with an escaped ]] end</a></p>"
+	},
+	{
+		.name    =  "Link with empty text",
+		.input   =  "[[https://example.com|]]",
+		.output  =  "<p><a href=\"https://example.com\"></a></p>"
+	},
+	{
+		.name    =  "Link with empty address",
+		.input   =  "[[|Hello]]",
+		.output  =  "<p><a href=\"\">Hello</a></p>"
+	},
+	{
+		.name    =  "Empty link",
+		.input   =  "[[]]",
+		.output  =  "<p><a href=\"\"></a></p>"
+	},
 #if 0
 	{
 		.name    =  "Simple unordered list",
@@ -132,16 +167,6 @@ struct {
 		            "<a href=\"http //example.com/examplepage\">Example Page</a></p>"
 	},
 	{
-		.name    =  "Unnamed link",
-		.input   =  "[[MyPage]]",
-		.output  =  "<p><a href=\"MyPage\">MyPage</a></p>"
-	},
-	{
-		.name    =  "Named link",
-		.input   =  "[[MyPage|My page]]",
-		.output  =  "<p><a href=\"MyPage\">My page</a></p>"
-	},
-	{
 		.name    =  "Image",
 		.input   =  "{{image.gif|my image}}",
 		.output  =  "<p><img src=\"image.gif\" alt=\"my image\"/></p>"
diff --git a/src/creole.c b/src/creole.c
index 7472a3b..79a767d 100644
--- a/src/creole.c
+++ b/src/creole.c
@@ -16,6 +16,24 @@ void process(const char *begin, const char *end, bool new_block, FILE *out);
 int do_headers(const char *begin, const char *end, bool new_block, FILE *out);
 int do_paragraph(const char *begin, const char *end, bool new_block, FILE *out);
 int do_replacements(const char *begin, const char *end, bool new_block, FILE *out);
+int do_link(const char *begin, const char *end, bool new_block, FILE *out);
+
+// Prints string escaped.
+void hprint(FILE *out, const char *begin, const char *end) {
+	for (const char *p = begin; p != end; p++) {
+		if (*p == '&') {
+			fputs("&amp;", out);
+		} else if (*p == '"') {
+			fputs("&quot;", out);
+		} else if (*p == '>') {
+			fputs("&gt;", out);
+		} else if (*p == '<') {
+			fputs("&lt;", out);
+		} else {
+			fputc(*p, out);
+		}
+	}
+}
 
 // A parser takes a (sub)string and returns the number of characters consumed, if any.
 //
@@ -23,7 +41,7 @@ int do_replacements(const char *begin, const char *end, bool new_block, FILE *ou
 // The sign of the return value determines whether a new block should begin, after the consumed text.
 typedef int (* parser_t)(const char *begin, const char *end, bool new_block, FILE *out);
 
-static parser_t parsers[] = { do_headers, do_paragraph, do_replacements };
+static parser_t parsers[] = { do_headers, do_paragraph, do_link, do_replacements };
 
 int do_headers(const char *begin, const char *end, bool new_block, FILE *out) {
 	if (!new_block) { // Headers are block-level elements.
@@ -93,8 +111,10 @@ static struct {
 	const char *from, *to;
 } replacements[] = {
 	// Escaped special characters
-	// TODO (e.g. "\\{" should become "{" so users can escape and void it being parsed as markup
+	{"~[[", "[["},
+	{"~]]", "]]"}, // NOTE: This pattern is duplicated in do_link().
 	// Characters that have special meaning in HTML
+	// NOTE: These rules are duplicated in hprint().
 	{"<", "&lt;"},
 	{">", "&gt;"},
 	{"\"", "&quot;"},
@@ -117,6 +137,49 @@ int do_replacements(const char *begin, const char *end, bool new_block, FILE *ou
 	return 0;
 }
 
+int do_link(const char *begin, const char *end, bool new_block, FILE *out)
+{
+	// Links start with "[[".
+	if (begin + 2 >= end || begin[0] != '[' || begin[1] != '[') {
+		return 0;
+	}
+	const char *start = begin + 2;
+
+	// Find the matching, unescaped "]]".
+	// Poor man's for...else
+	const char *stop = start - 1;
+	do {
+		stop = strnstr(stop + 1, "]]", end - (stop + 1));
+	} while (stop != NULL && stop[-1] == '~');
+	if (stop == NULL) {
+		return 0;
+	}
+
+	// FIXME: How do we handle WikiWord style links? Should we just append ".html" if is_wikiword()?
+
+	const char *pipe = strnstr(start, "|", stop - start);
+	if (pipe != NULL) {
+		const char *link_address_start = start;
+		const char *link_address_stop = pipe;
+		fprintf(out, "<a href=\"");
+		hprint(out, link_address_start, link_address_stop);
+		fprintf(out, "\">");
+
+		const char *link_text_start = pipe + 1;
+		const char *link_text_stop = stop;
+		process(link_text_start, link_text_stop, false, out);
+		fprintf(out, "</a>");
+	} else {
+		fprintf(out, "<a href=\"");
+		hprint(out, start, stop);
+		fprintf(out, "\">");
+		hprint(out, start, stop); // Don't parse markup when we know it's a link.
+		fprintf(out, "</a>");
+	}
+
+	return stop - start + 4 /* [[]] */;
+}
+
 void process(const char *begin, const char *end, bool new_block, FILE *out) {
 	const char *p = begin;
 	while (p < end) {