[readline] Add history support

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/readline/readline.h

@@ -9,6 +9,49 @@
 
 FILE_LICENCE ( GPL2_OR_LATER );
 
+/** A readline history entry */
+struct readline_history_entry {
+	/** Persistent copy of string */
+	char *string;
+	/** Temporary copy of string
+	 *
+	 * The temporary copy exists only during the call to
+	 * readline().
+	 */
+	char *temp;
+};
+
+/** Maximum depth of a readline history buffer
+ *
+ * Must be one less than a power of two.
+ */
+#define READLINE_HISTORY_MAX_DEPTH ( ( 1 << 3 ) - 1 )
+
+/** A readline history buffer */
+struct readline_history {
+	/** History entries
+	 *
+	 * This is a circular buffer, with entries in chronological
+	 * order.  The "next" entry is always empty except during a
+	 * call to readline().
+	 */
+	struct readline_history_entry entries[READLINE_HISTORY_MAX_DEPTH + 1];
+	/** Position of next entry within buffer
+	 *
+	 * This is incremented monotonically each time an entry is
+	 * added to the buffer.
+	 */
+	unsigned int next;
+	/** Current depth within history buffer
+	 *
+	 * This is valid only during the call to readline()
+	 */
+	unsigned int depth;
+};
+
+extern void history_free ( struct readline_history *history );
+extern char * __malloc readline_history ( const char *prompt,
+					  struct readline_history *history );
 extern char * __malloc readline ( const char *prompt );
 
 #endif /* _READLINE_H */