color.h: document and modernize header

Add documentation explaining the functions in color.h.
While at it, migrate the function `color_set` into grep.c,
where the only callers are.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

color.c

@@ -161,11 +161,6 @@ int color_parse(const char *value, char *dst)
 	return color_parse_mem(value, strlen(value), dst);
 }
 
-void color_set(char *dst, const char *color_bytes)
-{
-	xsnprintf(dst, COLOR_MAXLEN, "%s", color_bytes);
-}
-
 /*
  * Write the ANSI color codes for "c" to "out"; the string should
  * already have the ANSI escape code in it. "out" should have enough
@@ -399,8 +394,6 @@ static int color_vfprintf(FILE *fp, const char *color, const char *fmt,
 	return r;
 }
 
-
-
 int color_fprintf(FILE *fp, const char *color, const char *fmt, ...)
 {
 	va_list args;

color.h

@@ -76,22 +76,46 @@ int git_color_config(const char *var, const char *value, void *cb);
 int git_color_default_config(const char *var, const char *value, void *cb);
 
 /*
- * Set the color buffer (which must be COLOR_MAXLEN bytes)
- * to the raw color bytes; this is useful for initializing
- * default color variables.
+ * Parse a config option, which can be a boolean or one of
+ * "never", "auto", "always". Return a constant of
+ * GIT_COLOR_NEVER for "never" or negative boolean,
+ * GIT_COLOR_ALWAYS for "always" or a positive boolean,
+ * and GIT_COLOR_AUTO for "auto".
  */
-void color_set(char *dst, const char *color_bytes);
-
 int git_config_colorbool(const char *var, const char *value);
+
+/*
+ * Return a boolean whether to use color, where the argument 'var' is
+ * one of GIT_COLOR_UNKNOWN, GIT_COLOR_NEVER, GIT_COLOR_ALWAYS, GIT_COLOR_AUTO.
+ */
 int want_color(int var);
+
+/*
+ * Translate a Git color from 'value' into a string that the terminal can
+ * interpret and store it into 'dst'. The Git color values are of the form
+ * "foreground [background] [attr]" where fore- and background can be a color
+ * name ("red"), a RGB code (#0xFF0000) or a 256-color-mode from the terminal.
+ */
 int color_parse(const char *value, char *dst);
 int color_parse_mem(const char *value, int len, char *dst);
+
+/*
+ * Output the formatted string in the specified color (and then reset to normal
+ * color so subsequent output is uncolored). Omits the color encapsulation if
+ * `color` is NULL. The `color_fprintf_ln` prints a new line after resetting
+ * the color.  The `color_print_strbuf` prints the contents of the given
+ * strbuf (BUG: but only up to its first NUL character).
+ */
 __attribute__((format (printf, 3, 4)))
 int color_fprintf(FILE *fp, const char *color, const char *fmt, ...);
 __attribute__((format (printf, 3, 4)))
 int color_fprintf_ln(FILE *fp, const char *color, const char *fmt, ...);
 void color_print_strbuf(FILE *fp, const char *color, const struct strbuf *sb);
 
+/*
+ * Check if the given color is GIT_COLOR_NIL that means "no color selected".
+ * The caller needs to replace the color with the actual desired color.
+ */
 int color_is_nil(const char *color);
 
 #endif /* COLOR_H */

grep.c

@@ -18,6 +18,11 @@ static void std_output(struct grep_opt *opt, const void *buf, size_t size)
 	fwrite(buf, size, 1, stdout);
 }
 
+static void color_set(char *dst, const char *color_bytes)
+{
+	xsnprintf(dst, COLOR_MAXLEN, "%s", color_bytes);
+}
+
 /*
  * Initialize the grep_defaults template with hardcoded defaults.
  * We could let the compiler do this, but without C99 initializers