first bits working

src/tests/test01.vtc

+# looks like -*- perl -*-
+
 varnishtest "Test convert vmod"
 
 server s1 {
@@ -9,14 +11,17 @@ varnish v1 -vcl+backend {
 	import convert from "${vmod_topbuild}/src/.libs/libvmod_convert.so";
 
 	sub vcl_deliver {
-		set resp.http.hello = convert.hello("World");
+	    set resp.http.hex =
+		convert.transcode(IDENTITY, HEXLC, req.url +
+				  "Hel" + "lo " + "" + "world" +
+				  req.url);
 	}
 } -start
 
 client c1 {
 	txreq -url "/"
 	rxresp
-	expect resp.http.hello == "Hello, World"
+	expect resp.http.hex == "2f48656c6c6f20776f726c642f"
 }
 
 client c1 -run

src/vmod_convert.c

@@ -23,6 +23,101 @@ static const struct vmod_priv const null_blob[1] =
 	}
 };
 
+static inline char
+hexnibblelc(char c)
+{
+	return ((c > 9) ? ('a' + (c - 0xa)) : ('0' + c));
+}
+
+
+/*
+ * en/decoders need to work for the string_list vargs case (implicit length) as
+ * well as for the blob case (explicit length). Fortunately, the blob case is
+ * not varadic, so we can assume just one length argument _or_ varags
+ *
+ * rblob: alloc additional whitespace so any result can also be (safely)
+ *	  used as a string.
+ */
+
+static VCL_BLOB
+hex_encode_va(struct ws *ws, struct vmod_priv *rblob, const ssize_t len,
+    const char *in, va_list va)
+{
+	char *p = ws->f;
+	char *out = p;	// should be const, but (struct vmod_priv).priv isnt
+	const char *e = out + WS_Reserve(ws, 0);
+	ssize_t insz;
+	int i;
+
+	AN(rblob);
+
+	if (in == vrt_magic_string_end)
+		goto nodata;
+
+	if (len != -1)
+		insz = len;
+	else if (in == NULL)
+		insz = 0;
+	else
+		insz = strlen(in);
+
+	fprintf(stderr, "magic_string_end %p\n", vrt_magic_string_end);
+
+	do {
+		if (insz == 0)
+			goto next;
+		fprintf(stderr, "debug in %p %s\n", in, in);
+		// p + 2 includes terminating null byte
+		for (i = 0; i < insz && p + 2 < e; i++) {
+			*p++ = hexnibblelc((in[i] & 0xf0) >> 4);
+			*p++ = hexnibblelc( in[i] & 0x0f);
+		}
+		if (i != insz)
+			goto err;
+
+	  next:
+		in = va_arg(va, const char *);
+		if (in == vrt_magic_string_end)
+			break;
+		insz = (in != NULL) ? strlen(in) : 0;
+	} while(1);
+
+	if (p == out)
+		goto nodata;
+
+	*p++ = '\0';
+	assert(p < e);
+
+	rblob->priv = out;
+	rblob->len = p - out - 1;
+	rblob->free = NULL;
+
+	WS_ReleaseP(ws, p);
+
+	return rblob;
+
+  nodata:
+	WS_Release(ws, 0);
+	return null_blob;
+
+  err:
+	WS_Release(ws, 0);
+	return NULL;
+}
+
+static VCL_BLOB
+hex_encode(struct ws *ws, struct vmod_priv *rblob, const ssize_t len,
+    const char *in, ...)
+{
+	va_list ap;
+	VCL_BLOB r;
+	va_start(ap, in);
+	r = hex_encode_va(ws, rblob, len, in, ap);
+	va_end(ap);
+	return r;
+}
+
+
 /* Functions */
 
 VCL_BLOB
@@ -36,18 +131,41 @@ vmod_decode(VRT_CTX, VCL_ENUM decs, const char *p, ...) {
 VCL_STRING
 vmod_encode(VRT_CTX, VCL_ENUM encs, VCL_BLOB b) {
 	enum encoding enc = parse_encoding(encs);
+	struct vmod_priv tmp[1];
+	const struct vmod_priv *r;
+
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	assert(enc != _INVALID);
-	return "XXX";
+
+	assert(enc == HEXLC);	// XXX
+	r = hex_encode(ctx->ws, tmp, b->len, b->priv, vrt_magic_string_end);
+	if (r == NULL)
+		return NULL;
+	return r->priv;
 }
-VCL_STRING
+
+VCL_STRING __match_proto__(td_convert_transcode)
 vmod_transcode(VRT_CTX, VCL_ENUM decs, VCL_ENUM encs, const char *p, ...) {
 	enum encoding dec = parse_encoding(decs);
 	enum encoding enc = parse_encoding(encs);
+	struct vmod_priv tmp[1];
+
+	va_list ap;
+	VCL_BLOB r;
+
 	CHECK_OBJ_NOTNULL(ctx, VRT_CTX_MAGIC);
 	assert(dec != _INVALID);
 	assert(enc != _INVALID);
-	return p;
+
+	// XXX DO THE DECODE BIT
+
+	assert(enc == HEXLC);	// XXX
+	va_start(ap, p);
+	r = hex_encode_va(ctx->ws, tmp, -1, p, ap);
+	va_end(ap);
+	if (r == NULL)
+		return NULL;
+	return r->priv;
 }
 VCL_BLOB
 vmod_transcode_blob(VRT_CTX, VCL_ENUM decs, VCL_ENUM encs, VCL_BLOB b) {