slash: optimize screen refresh

Erasing the line and rewriting everything on every change is simple, but
works poorly with slow serial speeds and can still create visible
refresh artifacts with faster speeds.

Rewrite the refresh routine to keep track of the screen cursor and line
length and try to refresh fewer bytes. We still need a boolean to force
a full reset after we print completion lists. A full buffer reset is
also used when browsing the history, which could probably be optimized
further.

Signed-off-by: Jeppe Ledet-Pedersen <jlp@satlab.com>

Author: jledet

include/slash/slash.h

@@ -267,10 +267,13 @@ struct slash_command {
  * @line_size: Size in bytes of the line buffer.
  * @prompt: Current prompt string.
  * @prompt_length: Length in bytes of the prompt string.
- * @prompt_print_length: Length in printable characters of the prompt string.
  * @buffer: Pointer to line buffer memory.
  * @cursor: Current index of cursor in line buffer.
+ * @cursor_screen: Current index of cursor in line buffer on screen.
  * @length: Current amount of used bytes in line buffer.
+ * @length_screen: Current amount of used bytes in line buffer on screen.
+ * @refresh_full: Force a full screen refresh including prompt.
+ * @refresh_buffer: Force a screen refresh of buffer.
  * @last_char: Last input character.
  * @history_size: Size in byte of the history buffer.
  * @history_depth: Number of history entries browsed back.
@@ -306,10 +309,13 @@ struct slash {
 	size_t line_size;
 	const char *prompt;
 	size_t prompt_length;
-	size_t prompt_print_length;
 	char *buffer;
 	size_t cursor;
+	size_t cursor_screen;
 	size_t length;
+	size_t length_screen;
+	bool refresh_full;
+	bool refresh_buffer;
 	char last_char;
 
 	/* History */

src/slash.c

@@ -113,26 +113,6 @@ int slash_getopt(struct slash *slash, const char *opts)
 }
 
 /* Terminal handling */
-static size_t slash_escaped_strlen(const char *s)
-{
-	int len = 0;
-	bool escaped = false;
-
-	while (*s) {
-		if (escaped) {
-			if (*s == 'm')
-				escaped = false;
-		} else if (*s == ESC) {
-			escaped = true;
-		} else {
-			len++;
-		}
-		s++;
-	}
-
-	return len;
-}
-
 static int slash_rawmode_enable(struct slash *slash)
 {
 #ifdef SLASH_HAVE_TERMIOS_H
@@ -697,6 +677,7 @@ static void slash_complete(struct slash *slash)
 		if (command) {
 			slash_printf(slash, "\n");
 			slash_command_usage(slash, command);
+			slash->refresh_full = true;
 		} else {
 			slash_bell(slash);
 		}
@@ -717,6 +698,7 @@ static void slash_complete(struct slash *slash)
 				slash_command_description(slash, cur);
 			}
 		}
+		slash->refresh_full = true;
 	}
 }
 
@@ -888,6 +870,7 @@ static bool slash_history_next(struct slash *slash)
 	slash->buffer[srclen] = '\0';
 	slash->history_cursor = src;
 	slash->cursor = slash->length = srclen;
+	slash->refresh_buffer = true;
 
 	/* Rewind if use to store buffer temporarily */
 	if (!slash->history_depth && slash->history_cursor != slash->history_tail)
@@ -917,33 +900,64 @@ static bool slash_history_previous(struct slash *slash)
 	slash->buffer[srclen] = '\0';
 	slash->history_cursor = src;
 	slash->cursor = slash->length = srclen;
+	slash->refresh_buffer = true;
 
 	return true;
 }
 
 /* Line editing */
 int slash_refresh(struct slash *slash)
 {
-	char esc[16];
+	const char *esc = ESCAPE("K");
+
+	if (slash->refresh_full) {
+		slash_putchar(slash, '\r');
+		if (slash_write(slash, slash->prompt, slash->prompt_length) < 0)
+			return -1;
+		if (slash_write(slash, esc, strlen(esc)) < 0)
+			return -1;
+		slash->cursor_screen = 0;
+		slash->length_screen = 0;
+		slash->refresh_full = false;
+	}
 
-	/* Ensure line is zero terminated */
-	slash->buffer[slash->length] = '\0';
+	if (slash->refresh_buffer) {
+		while (slash->cursor_screen > 0) {
+			slash_putchar(slash, '\b');
+			slash->cursor_screen--;
+		}
+		slash->refresh_buffer = false;
+	}
+
+	while (slash->cursor_screen != slash->cursor) {
+		if (slash->cursor_screen < slash->cursor) {
+			slash_putchar(slash, slash->buffer[slash->cursor_screen]);
+			slash->cursor_screen++;
+		} else if (slash->cursor_screen > slash->cursor) {
+			slash_putchar(slash, '\b');
+			slash->cursor_screen--;
+		}
+	}
 
-	/* Move cursor to left edge */
-	if (slash_putchar(slash, '\r') < 0)
-		return -1;
+	if (slash->length_screen != slash->length) {
+		if (slash_write(slash,
+				&slash->buffer[slash->cursor],
+				slash->length - slash->cursor) < 0)
+			return -1;
+		slash->cursor_screen = slash->length;
 
-	/* Write the prompt and the current buffer content */
-	if (slash_write(slash, slash->prompt, slash->prompt_length) < 0)
-		return -1;
-	if (slash_write(slash, slash->buffer, slash->length) < 0)
-		return -1;
+		if (slash->length_screen > slash->length) {
+			if (slash_write(slash, esc, strlen(esc)) < 0)
+				return -1;
+		}
 
-	/* Erase to the right and move cursor to original position. */
-	snprintf(esc, sizeof(esc), ESCAPE("K") "\r" ESCAPE("%uC"),
-		(unsigned int)(slash->cursor + slash->prompt_print_length));
-	if (slash_write(slash, esc, strlen(esc)) < 0)
-		return -1;
+		while (slash->cursor_screen > slash->cursor) {
+			slash_putchar(slash, '\b');
+			slash->cursor_screen--;
+		}
+
+		slash->length_screen = slash->length;
+	}
 
 	return 0;
 }
@@ -953,30 +967,21 @@ static void slash_insert(struct slash *slash, int c)
 	if (slash->length + 1 > slash->line_size)
 		return;
 
-	/* Fast path if we're adding a character to the end of the line */
-	if (slash->cursor == slash->length) {
-		slash->buffer[slash->cursor] = c;
-		slash->cursor++;
-		slash->length++;
-		slash->buffer[slash->length] = '\0';
-		slash_putchar(slash, slash->buffer[slash->cursor - 1]);
-	} else {
-		memmove(&slash->buffer[slash->cursor + 1],
-			&slash->buffer[slash->cursor],
-			slash->length - slash->cursor);
-		slash->buffer[slash->cursor] = c;
-		slash->cursor++;
-		slash->length++;
-		slash->buffer[slash->length] = '\0';
-		slash_refresh(slash);
-	}
+	memmove(&slash->buffer[slash->cursor + 1],
+		&slash->buffer[slash->cursor],
+		slash->length - slash->cursor);
+	slash->buffer[slash->cursor] = c;
+	slash->cursor++;
+	slash->length++;
+	slash->buffer[slash->length] = '\0';
 }
 
 void slash_reset(struct slash *slash)
 {
 	slash->buffer[0] = '\0';
 	slash->length = 0;
 	slash->cursor = 0;
+	slash->refresh_full = true;
 }
 
 static void slash_arrow_up(struct slash *slash)
@@ -1018,6 +1023,7 @@ void slash_clear_screen(struct slash *slash)
 {
 	const char *esc = ESCAPE("H") ESCAPE("2J");
 	slash_write(slash, esc, strlen(esc));
+	slash->refresh_full = true;
 }
 
 static void slash_backspace(struct slash *slash)
@@ -1065,7 +1071,6 @@ void slash_set_prompt(struct slash *slash, const char *prompt)
 {
 	slash->prompt = prompt;
 	slash->prompt_length = strlen(prompt);
-	slash->prompt_print_length = slash_escaped_strlen(prompt);
 }
 
 char *slash_readline(struct slash *slash)
@@ -1108,8 +1113,6 @@ char *slash_readline(struct slash *slash)
 					slash->cursor = slash->length;
 			}
 			escaped = false;
-
-			slash_refresh(slash);
 		} else if (iscntrl(c)) {
 			switch (c) {
 			case CONTROL('A'):
@@ -1141,6 +1144,7 @@ char *slash_readline(struct slash *slash)
 				break;
 			case CONTROL('K'):
 				slash->length = slash->cursor;
+				slash->buffer[slash->length] = '\0';
 				break;
 			case CONTROL('L'):
 				slash_clear_screen(slash);
@@ -1157,6 +1161,7 @@ char *slash_readline(struct slash *slash)
 			case CONTROL('U'):
 				slash->cursor = 0;
 				slash->length = 0;
+				slash->buffer[0] = '\0';
 				break;
 			case CONTROL('W'):
 				slash_delete_word(slash);
@@ -1179,13 +1184,13 @@ char *slash_readline(struct slash *slash)
 				/* Unknown control */
 				break;
 			}
-
-			slash_refresh(slash);
 		} else if (isprint(c)) {
 			/* Add to buffer */
 			slash_insert(slash, c);
 		}
 
+		slash_refresh(slash);
+
 		slash->last_char = c;
 	}