tag: add a ‘%’ formatter

Range tags can now be rendered as a percentage value, by using a ‘%’
formatter:

  {tag_name:%}

CHANGELOG.md

@@ -33,6 +33,7 @@
 * network: `ssid`, `signal`, `rx-bitrate` and `rx-bitrate` tags.
 * network: `poll-interval` option (for the new `signal` and
   `*-bitrate` tags).
+* tags: percentage formatter, for range tags: `{tag_name:%}`.
 
 
 ### Changed

doc/yambar-tags.5.scd

@@ -38,9 +38,50 @@ The available tag *types* are:
 
 # FORMATTING
 
-As mentioned above, each tag type has a default representation that is
+A tag may be followed by one or more formatters that alter the tags
-used when the tag is rendered by a string particle.
+rendition.
+
+Formatters are added by appending a ':' separated list of formatter
+names:
+
+	"{tag_name:max:hex}"
+
+In the table below, "kind" describes the type of action performed by
+the formatter:
+
+- *format*: changes the representation of the tag's value
+- *selector*: changes what to render
+
+In general, formatters of the same kind cannot be combined; if
+multiple formatters of the same kind are specified, the last one will
+be used.
+
+[[ *Formatter*
+:[ *Kind*
+:[ *Description*
+:[ *Applies to*]
+|  hex
+:  format
+:  Renders a tag's value in hex
+:  All tag types
+|  oct
+:  format
+:  Renders a tag's value in octal
+:  All tag types
+|  %
+:  format
+:  Renders a range tag's value as a percentage value
+:  Range tags
+|  min
+:  selector
+:  Renders a range tag's mininum value
+:  Range tags
+|  max
+:  selector
+:  Renders a range tag's maximum value
+:  Range tags
+|  unit
+:  selector
+:  Renders a realtime tag's unit (e.g. "s", or "ms")
+:  Realtime tags
 
-All integer, floating point and boolean tag types can be modified to
-instead be rendered in hexadecimal or octal form, by appending either
-the *:hex* or *:oct* suffixes. For example, _\"{tag_name:hex}\"_.

tag.c

@@ -459,7 +459,7 @@ tags_expand_template(const char *template, const struct tag_set *tags)
         sbuf_append_at_most(&formatted, template, begin - template);
 
         /* Parse arguments */
-        enum { FMT_DEFAULT, FMT_HEX, FMT_OCT } format = FMT_DEFAULT;
+        enum { FMT_DEFAULT, FMT_HEX, FMT_OCT, FMT_PERCENT } format = FMT_DEFAULT;
         enum { VALUE_VALUE, VALUE_MIN, VALUE_MAX, VALUE_UNIT } kind = VALUE_VALUE;
 
         for (size_t i = 0; i < MAX_TAG_ARGS; i++) {
@@ -469,12 +469,16 @@ tags_expand_template(const char *template, const struct tag_set *tags)
                 format = FMT_HEX;
             else if (strcmp(tag_args[i], "oct") == 0)
                 format = FMT_OCT;
+            else if (strcmp(tag_args[i], "%") == 0)
+                format = FMT_PERCENT;
             else if (strcmp(tag_args[i], "min") == 0)
                 kind = VALUE_MIN;
             else if (strcmp(tag_args[i], "max") == 0)
                 kind = VALUE_MAX;
             else if (strcmp(tag_args[i], "unit") == 0)
                 kind = VALUE_UNIT;
+            else
+                LOG_WARN("invalid tag formatter: %s", tag_args[i]);
         }
 
         /* Copy tag value */
@@ -493,20 +497,40 @@ tags_expand_template(const char *template, const struct tag_set *tags)
                 sbuf_append(&formatted, str);
                 break;
             }
+
+            case FMT_PERCENT: {
+                const long min = tag->min(tag);
+                const long max = tag->max(tag);
+                const long cur = tag->as_int(tag);
+
+                char str[4];
+                snprintf(str, sizeof(str), "%lu", (cur - min) * 100 / (max - min));
+                sbuf_append(&formatted, str);
+                break;
+            }
             }
             break;
 
         case VALUE_MIN:
         case VALUE_MAX: {
-            const long value = kind == VALUE_MIN ? tag->min(tag) : tag->max(tag);
+            const long min = tag->min(tag);
+            const long max = tag->max(tag);
+            long value = kind == VALUE_MIN ? min : max;
 
             const char *fmt;
             switch (format) {
             case FMT_DEFAULT: fmt = "%ld"; break;
             case FMT_HEX:     fmt = "%lx"; break;
             case FMT_OCT:     fmt = "%lo"; break;
+
+            case FMT_PERCENT:
+                value = (value - min) * 100 / (max - min);
+                fmt = "%lu";
+                break;
             }
 
+            
+
             char str[24];
             snprintf(str, sizeof(str), fmt, value);
             sbuf_append(&formatted, str);