Routines for %-encoding and %-decoding.

These make a good storage format for mostly-textual data in
configuration, if it can't afford to reserve any character as a
delimiter. Assuming very few characters need to be escaped, the space
cost is lower than base64, and also you can read it by eye.

misc.h

@@ -116,6 +116,13 @@ void base64_encode_fp(FILE *fp, ptrlen data, int cpl);
 strbuf *base64_encode_sb(ptrlen data, int cpl);
 bool base64_valid(ptrlen data);
 
+void percent_encode_bs(BinarySink *bs, ptrlen data, const char *badchars);
+void percent_encode_fp(FILE *fp, ptrlen data, const char *badchars);
+strbuf *percent_encode_sb(ptrlen data, const char *badchars);
+void percent_decode_bs(BinarySink *bs, ptrlen data);
+void percent_decode_fp(FILE *fp, ptrlen data);
+strbuf *percent_decode_sb(ptrlen data);
+
 struct bufchain_granule;
 struct bufchain_tag {
     struct bufchain_granule *head, *tail;

utils/CMakeLists.txt

@@ -46,6 +46,8 @@ add_sources_from_current_dir(utils
   nullstrcmp.c
   out_of_memory.c
   parse_blocksize.c
+  percent_decode.c
+  percent_encode.c
   prompts.c
   ptrlen.c
   read_file_into.c

utils/percent_decode.c

@@ -0,0 +1,41 @@
+/*
+ * Decode %-encoding in URL style.
+ */
+
+#include <ctype.h>
+
+#include "misc.h"
+
+void percent_decode_bs(BinarySink *bs, ptrlen data)
+{
+    for (const char *p = data.ptr, *e = ptrlen_end(data); p < e; p++) {
+        char c = *p;
+        if (c == '%' && e-p >= 3 &&
+            isxdigit((unsigned char)p[1]) &&
+            isxdigit((unsigned char)p[2])) {
+            char hex[3];
+            hex[0] = p[1];
+            hex[1] = p[2];
+            hex[2] = '\0';
+            put_byte(bs, strtoul(hex, NULL, 16));
+            p += 2;
+        } else {
+            put_byte(bs, c);
+        }
+    }
+
+}
+
+void percent_decode_fp(FILE *fp, ptrlen data)
+{
+    stdio_sink ss;
+    stdio_sink_init(&ss, fp);
+    percent_decode_bs(BinarySink_UPCAST(&ss), data);
+}
+
+strbuf *percent_decode_sb(ptrlen data)
+{
+    strbuf *sb = strbuf_new();
+    percent_decode_bs(BinarySink_UPCAST(sb), data);
+    return sb;
+}

utils/percent_encode.c

@@ -0,0 +1,34 @@
+/*
+ * %-encoding in URL style.
+ *
+ * Defaults to escaping % itself (necessary for decoding to even
+ * work), and any C0 escape character. Further bad characters can be
+ * provided in 'badchars'.
+ */
+
+#include "misc.h"
+
+void percent_encode_bs(BinarySink *bs, ptrlen data, const char *badchars)
+{
+    for (const char *p = data.ptr, *e = ptrlen_end(data); p < e; p++) {
+        char c = *p;
+        if (c == '%' || c < ' ' || (badchars && strchr(badchars, c)))
+            put_fmt(bs, "%%%02X", (unsigned char)c);
+        else
+            put_byte(bs, c);
+    }
+}
+
+void percent_encode_fp(FILE *fp, ptrlen data, const char *badchars)
+{
+    stdio_sink ss;
+    stdio_sink_init(&ss, fp);
+    percent_encode_bs(BinarySink_UPCAST(&ss), data, badchars);
+}
+
+strbuf *percent_encode_sb(ptrlen data, const char *badchars)
+{
+    strbuf *sb = strbuf_new();
+    percent_encode_bs(BinarySink_UPCAST(sb), data, badchars);
+    return sb;
+}