term: implement term.clear() for windows (#8650)

vlib/term/term.v

@@ -119,11 +119,3 @@ fn supports_escape_sequences(fd int) bool {
 		return is_atty(fd) > 0
 	}
 }
-
-// clear clears current terminal screen.
-pub fn clear() {
-	$if !windows {
-		print('\x1b[2J')
-		print('\x1b[H')
-	}
-}

vlib/term/term_nix.c.v

@@ -93,3 +93,9 @@ pub fn set_terminal_title(title string) bool {
 	print('\033]0;${title}\007')
 	return true
 }
+
+// clear clears current terminal screen.
+pub fn clear() {
+	print('\x1b[2J')
+	print('\x1b[H')
+}

vlib/term/term_windows.c.v

@@ -4,12 +4,14 @@ import os
 
 [typedef]
 struct C.COORD {
+mut:
 	X i16
 	Y i16
 }
 
 [typedef]
 struct C.SMALL_RECT {
+mut:
 	Left   u16
 	Top    u16
 	Right  u16
@@ -20,6 +22,7 @@ struct C.SMALL_RECT {
 // https://docs.microsoft.com/en-us/windows/console/console-screen-buffer-info-str
 [typedef]
 struct C.CONSOLE_SCREEN_BUFFER_INFO {
+mut:
 	dwSize              C.COORD
 	dwCursorPosition    C.COORD
 	wAttributes         u16
@@ -27,12 +30,31 @@ struct C.CONSOLE_SCREEN_BUFFER_INFO {
 	dwMaximumWindowSize C.COORD
 }
 
+union C.uChar {
+mut:
+	UnicodeChar rune
+	AsciiChar   byte
+}
+
+[typedef]
+struct C.CHAR_INFO {
+mut:
+	Char C.uChar
+	Attributes u16
+}
+
 // ref - https://docs.microsoft.com/en-us/windows/console/getconsolescreenbufferinfo
 fn C.GetConsoleScreenBufferInfo(handle os.HANDLE, info &C.CONSOLE_SCREEN_BUFFER_INFO) bool
 
 // ref - https://docs.microsoft.com/en-us/windows/console/setconsoletitle
 fn C.SetConsoleTitle(title &u16) bool
 
+// ref - https://docs.microsoft.com/en-us/windows/console/setconsolecursorposition
+fn C.SetConsoleCursorPosition(handle os.HANDLE, coord C.COORD) bool
+
+// ref - https://docs.microsoft.com/en-us/windows/console/scrollconsolescreenbuffer
+fn C.ScrollConsoleScreenBuffer(output os.HANDLE, scroll_rect &C.SMALL_RECT, clip_rect &C.SMALL_RECT, des C.COORD, fill C.CHAR_INFO) bool
+
 // get_terminal_size returns a number of colums and rows of terminal window.
 pub fn get_terminal_size() (int, int) {
 	if is_atty(1) > 0 && os.getenv('TERM') != 'dumb' {
@@ -64,3 +86,40 @@ pub fn set_terminal_title(title string) bool {
 	title_change := C.SetConsoleTitle(title.to_wide())
 	return title_change
 }
+
+// clear clears current terminal screen.
+// Implementation taken from https://docs.microsoft.com/en-us/windows/console/clearing-the-screen#example-2.
+pub fn clear() {
+	hconsole := C.GetStdHandle(C.STD_OUTPUT_HANDLE)
+	mut csbi := C.CONSOLE_SCREEN_BUFFER_INFO{}
+	mut scrollrect := C.SMALL_RECT{}
+	mut scrolltarget := C.COORD{}
+	mut fill := C.CHAR_INFO{}
+
+	// Get the number of character cells in the current buffer.
+	if !C.GetConsoleScreenBufferInfo(hconsole, &csbi) {
+		return
+	}
+	// Scroll the rectangle of the entire buffer.
+	scrollrect.Left = 0
+	scrollrect.Top = 0
+	scrollrect.Right = u16(csbi.dwSize.X)
+	scrollrect.Bottom = u16(csbi.dwSize.Y)
+
+	// Scroll it upwards off the top of the buffer with a magnitude of the entire height.
+	scrolltarget.X = 0
+	scrolltarget.Y = (0 - csbi.dwSize.Y)
+
+	// Fill with empty spaces with the buffer's default text attribute.
+	fill.Char.UnicodeChar = rune(` `)
+	fill.Attributes = csbi.wAttributes
+
+	// Do the scroll
+	C.ScrollConsoleScreenBuffer(hconsole, &scrollrect, C.NULL, scrolltarget, &fill)
+
+	// Move the cursor to the top left corner too.
+	csbi.dwCursorPosition.X = 0
+	csbi.dwCursorPosition.Y = 0
+
+	C.SetConsoleCursorPosition(hconsole, csbi.dwCursorPosition)
+}