Source File
	textview.go

Belonging Package
	github.com/pancsta/cview

package cview

import (
	"bytes"
	"regexp"
	"sync"
	"unicode"
	"unicode/utf8"

	"github.com/gdamore/tcell/v2"
	"github.com/lucasb-eyer/go-colorful"
	"github.com/mattn/go-runewidth"
	"github.com/rivo/uniseg"
)

var (
	// TabSize is the number of spaces with which a tab character will be replaced.
	TabSize = 4
)

var (
	openColorRegex  = regexp.MustCompile(`\[([a-zA-Z]*|#[0-9a-zA-Z]*)$`)
	openRegionRegex = regexp.MustCompile(`\["[a-zA-Z0-9_,;: \-\.]*"?$`)
)

// textViewIndex contains information about each line displayed in the text
// view.
type textViewIndex struct {
	Line            int    // The index into the "buffer" variable.
	Pos             int    // The index into the "buffer" line ([]byte position).
	NextPos         int    // The (byte) index of the next character in this buffer line.
	Width           int    // The screen width of this line.
	ForegroundColor string // The starting foreground color ("" = don't change, "-" = reset).
	BackgroundColor string // The starting background color ("" = don't change, "-" = reset).
	Attributes      string // The starting attributes ("" = don't change, "-" = reset).
	Region          []byte // The starting region ID.
}

// textViewRegion contains information about a region.
type textViewRegion struct {
	// The region ID.
	ID []byte

	// The starting and end screen position of the region as determined the last
	// time Draw() was called. A negative value indicates out-of-rect positions.
	FromX, FromY, ToX, ToY int
}

// TextView is a box which displays text. It implements the io.Writer interface
// so you can stream text to it. This does not trigger a redraw automatically
// but if a handler is installed via SetChangedFunc(), you can cause it to be
// redrawn. (See SetChangedFunc() for more details.)
//
// # Navigation
//
// If the text view is scrollable (the default), text is kept in a buffer which
// may be larger than the screen and can be navigated similarly to Vim:
//
//   - h, left arrow: Move left.
//   - l, right arrow: Move right.
//   - j, down arrow: Move down.
//   - k, up arrow: Move up.
//   - g, home: Move to the top.
//   - G, end: Move to the bottom.
//   - Ctrl-F, page down: Move down by one page.
//   - Ctrl-B, page up: Move up by one page.
//
// If the text is not scrollable, any text above the top visible line is
// discarded.
//
// Use SetInputCapture() to override or modify keyboard input.
//
// # Colors
//
// If dynamic colors are enabled via SetDynamicColors(), text color can be
// changed dynamically by embedding color strings in square brackets. This works
// the same way as anywhere else. Please see the package documentation for more
// information.
//
// # Regions and Highlights
//
// If regions are enabled via SetRegions(), you can define text regions within
// the text and assign region IDs to them. Text regions start with region tags.
// Region tags are square brackets that contain a region ID in double quotes,
// for example:
//
//	We define a ["rg"]region[""] here.
//
// A text region ends with the next region tag. Tags with no region ID ([""])
// don't start new regions. They can therefore be used to mark the end of a
// region. Region IDs must satisfy the following regular expression:
//
//	[a-zA-Z0-9_,;: \-\.]+
//
// Regions can be highlighted by calling the Highlight() function with one or
// more region IDs. This can be used to display search results, for example.
//
// The ScrollToHighlight() function can be used to jump to the currently
// highlighted region once when the text view is drawn the next time.
type TextView struct {
	*Box

	// The text buffer.
	buffer [][]byte

	// The last bytes that have been received but are not part of the buffer yet.
	recentBytes []byte

	// The last width and height of the text view.
	lastWidth, lastHeight int

	// The processed line index. This is nil if the buffer has changed and needs
	// to be re-indexed.
	index []*textViewIndex

	// The width of the text view buffer index.
	indexWidth int

	// If set to true, the buffer will be reindexed each time it is modified.
	reindex bool

	// The horizontal text alignment, one of AlignLeft, AlignCenter, or AlignRight.
	align int

	// The vertical text alignment, one of AlignTop, AlignMiddle, or AlignBottom.
	valign VerticalAlignment

	// Information about visible regions as of the last call to Draw().
	regionInfos []*textViewRegion

	// Indices into the "index" slice which correspond to the first line of the
	// first highlight and the last line of the last highlight. This is calculated
	// during re-indexing. Set to -1 if there is no current highlight.
	fromHighlight, toHighlight int

	// The screen space column of the highlight in its first line. Set to -1 if
	// there is no current highlight.
	posHighlight int

	// A set of region IDs that are currently highlighted.
	highlights map[string]struct{}

	// The screen width of the longest line in the index (not the buffer).
	longestLine int

	// The index of the first line shown in the text view.
	lineOffset int

	// The maximum number of newlines the text view will hold (0 = unlimited).
	maxLines int

	// If set to true, the text view will always remain at the end of the content.
	trackEnd bool

	// The number of characters to be skipped on each line (not in wrap mode).
	columnOffset int

	// The height of the content the last time the text view was drawn.
	pageSize int

	// If set to true, the text view will keep a buffer of text which can be
	// navigated when the text is longer than what fits into the box.
	scrollable bool

	// Visibility of the scroll bar.
	scrollBarVisibility ScrollBarVisibility

	// The scroll bar color.
	scrollBarColor tcell.Color

	// If set to true, lines that are longer than the available width are wrapped
	// onto the next line. If set to false, any characters beyond the available
	// width are discarded.
	wrap bool

	// The maximum line width when wrapping (0 = use TextView width).
	wrapWidth int

	// If set to true and if wrap is also true, lines are split at spaces or
	// after punctuation characters.
	wordWrap bool

	// The (starting) color of the text.
	textColor tcell.Color

	// The foreground color of highlighted text.
	highlightForeground tcell.Color

	// The background color of highlighted text.
	highlightBackground tcell.Color

	// If set to true, the text color can be changed dynamically by piping color
	// strings in square brackets to the text view.
	dynamicColors bool

	// If set to true, region tags can be used to define regions.
	regions bool

	// A temporary flag which, when true, will automatically bring the current
	// highlight(s) into the visible screen.
	scrollToHighlights bool

	// If true, setting new highlights will be a XOR instead of an overwrite
	// operation.
	toggleHighlights bool

	// An optional function which is called when the content of the text view has
	// changed.
	changed func()

	// An optional function which is called when the user presses one of the
	// following keys: Escape, Enter, Tab, Backtab.
	done func(tcell.Key)

	// An optional function which is called when one or more regions were
	// highlighted.
	highlighted func(added, removed, remaining []string)

	sync.RWMutex
	clicked func(regionId string)
}

// NewTextView returns a new text view.
func NewTextView() *TextView {
	return &TextView{
		Box:                 NewBox(),
		highlights:          make(map[string]struct{}),
		lineOffset:          -1,
		reindex:             true,
		scrollable:          true,
		scrollBarVisibility: ScrollBarAuto,
		scrollBarColor:      Styles.ScrollBarColor,
		align:               AlignLeft,
		valign:              AlignTop,
		wrap:                true,
		textColor:           Styles.PrimaryTextColor,
		highlightForeground: Styles.PrimitiveBackgroundColor,
		highlightBackground: Styles.PrimaryTextColor,
	}
}

// SetScrollable sets the flag that decides whether or not the text view is
// scrollable. If true, text is kept in a buffer and can be navigated. If false,
// the last line will always be visible.
func (t *TextView) SetScrollable(scrollable bool) {
	t.Lock()
	defer t.Unlock()

	t.scrollable = scrollable
	if !scrollable {
		t.trackEnd = true
	}
}

// SetScrollBarVisibility specifies the display of the scroll bar.
func (t *TextView) SetScrollBarVisibility(visibility ScrollBarVisibility) {
	t.Lock()
	defer t.Unlock()

	t.scrollBarVisibility = visibility
}

// SetScrollBarColor sets the color of the scroll bar.
func (t *TextView) SetScrollBarColor(color tcell.Color) {
	t.Lock()
	defer t.Unlock()

	t.scrollBarColor = color
}

// SetWrap sets the flag that, if true, leads to lines that are longer than the
// available width being wrapped onto the next line. If false, any characters
// beyond the available width are not displayed.
func (t *TextView) SetWrap(wrap bool) {
	t.Lock()
	defer t.Unlock()

	if t.wrap != wrap {
		t.index = nil
	}
	t.wrap = wrap
}

// SetWordWrap sets the flag that, if true and if the "wrap" flag is also true
// (see SetWrap()), wraps the line at spaces or after punctuation marks. Note
// that trailing spaces will not be printed.
//
// This flag is ignored if the "wrap" flag is false.
func (t *TextView) SetWordWrap(wrapOnWords bool) {
	t.Lock()
	defer t.Unlock()

	if t.wordWrap != wrapOnWords {
		t.index = nil
	}
	t.wordWrap = wrapOnWords
}

// SetTextAlign sets the horizontal alignment of the text. This must be either
// AlignLeft, AlignCenter, or AlignRight.
func (t *TextView) SetTextAlign(align int) {
	t.Lock()
	defer t.Unlock()

	if t.align != align {
		t.index = nil
	}
	t.align = align
}

// SetVerticalAlign sets the vertical alignment of the text. This must be
// either AlignTop, AlignMiddle, or AlignBottom.
func (t *TextView) SetVerticalAlign(valign VerticalAlignment) {
	t.Lock()
	defer t.Unlock()

	if t.valign != valign {
		t.index = nil
	}
	t.valign = valign
}

// SetTextColor sets the initial color of the text (which can be changed
// dynamically by sending color strings in square brackets to the text view if
// dynamic colors are enabled).
func (t *TextView) SetTextColor(color tcell.Color) {
	t.Lock()
	defer t.Unlock()

	t.textColor = color
}

// SetHighlightForegroundColor sets the foreground color of highlighted text.
func (t *TextView) SetHighlightForegroundColor(color tcell.Color) {
	t.Lock()
	defer t.Unlock()

	t.highlightForeground = color
}

// SetHighlightBackgroundColor sets the foreground color of highlighted text.
func (t *TextView) SetHighlightBackgroundColor(color tcell.Color) {
	t.Lock()
	defer t.Unlock()

	t.highlightBackground = color
}

// SetBytes sets the text of this text view to the provided byte slice.
// Previously contained text will be removed.
func (t *TextView) SetBytes(text []byte) {
	t.Lock()
	defer t.Unlock()

	t.clear()
	t.write(text)
}

// SetText sets the text of this text view to the provided string. Previously
// contained text will be removed.
func (t *TextView) SetText(text string) {
	t.SetBytes([]byte(text))
}

// GetBytes returns the current text of this text view. If "stripTags" is set
// to true, any region/color tags are stripped from the text.
func (t *TextView) GetBytes(stripTags bool) []byte {
	t.RLock()
	defer t.RUnlock()

	if !stripTags {
		if len(t.recentBytes) > 0 {
			return bytes.Join(append(t.buffer, t.recentBytes), []byte("\n"))
		}
		return bytes.Join(t.buffer, []byte("\n"))
	}

	buffer := bytes.Join(t.buffer, []byte("\n"))
	return StripTags(buffer, t.dynamicColors, t.regions)
}

// GetText returns the current text of this text view. If "stripTags" is set
// to true, any region/color tags are stripped from the text.
func (t *TextView) GetText(stripTags bool) string {
	return string(t.GetBytes(stripTags))
}

// GetBufferSize returns the number of lines and the length of the longest line
// in the text buffer. The screen size of the widget is available via GetRect.
func (t *TextView) GetBufferSize() (rows int, maxLen int) {
	t.RLock()
	defer t.RUnlock()

	return len(t.buffer), t.longestLine
}

// SetDynamicColors sets the flag that allows the text color to be changed
// dynamically. See class description for details.
func (t *TextView) SetDynamicColors(dynamic bool) {
	t.Lock()
	defer t.Unlock()

	if t.dynamicColors != dynamic {
		t.index = nil
	}
	t.dynamicColors = dynamic
}

// SetRegions sets the flag that allows to define regions in the text. See class
// description for details.
func (t *TextView) SetRegions(regions bool) {
	t.Lock()
	defer t.Unlock()

	if t.regions != regions {
		t.index = nil
	}
	t.regions = regions
}

// SetChangedFunc sets a handler function which is called when the text of the
// text view has changed. This is useful when text is written to this io.Writer
// in a separate goroutine. Doing so does not automatically cause the screen to
// be refreshed so you may want to use the "changed" handler to redraw the
// screen.
//
// Note that to avoid race conditions or deadlocks, there are a few rules you
// should follow:
//
//   - You can call Application.Draw() from this handler.
//   - You can call TextView.HasFocus() from this handler.
//   - During the execution of this handler, access to any other variables from
//     this primitive or any other primitive should be queued using
//     Application.QueueUpdate().
//
// See package description for details on dealing with concurrency.
func (t *TextView) SetChangedFunc(handler func()) {
	t.Lock()
	defer t.Unlock()

	t.changed = handler
}

// SetDoneFunc sets a handler which is called when the user presses on the
// following keys: Escape, Enter, Tab, Backtab. The key is passed to the
// handler.
func (t *TextView) SetDoneFunc(handler func(key tcell.Key)) {
	t.Lock()
	defer t.Unlock()

	t.done = handler
}

// SetHighlightedFunc sets a handler which is called when the list of currently
// highlighted regions change. It receives a list of region IDs which were newly
// highlighted, those that are not highlighted anymore, and those that remain
// highlighted.
//
// Note that because regions are only determined during drawing, this function
// can only fire for regions that have existed during the last call to Draw().
func (t *TextView) SetHighlightedFunc(handler func(added, removed, remaining []string)) {
	t.highlighted = handler
}

// SetClickedFunc Handler to run when a region is clicked.
func (t *TextView) SetClickedFunc(handler func(regionId string)) {
	t.clicked = handler
}

func (t *TextView) clipBuffer() {
	if t.maxLines <= 0 {
		return
	}

	lenbuf := len(t.buffer)
	if lenbuf > t.maxLines {
		t.buffer = t.buffer[lenbuf-t.maxLines:]
	}
}

// SetMaxLines sets the maximum number of newlines the text view will hold
// before discarding older data from the buffer.
func (t *TextView) SetMaxLines(maxLines int) {
	t.maxLines = maxLines
	t.clipBuffer()
}

// ScrollTo scrolls to the specified row and column (both starting with 0).
func (t *TextView) ScrollTo(row, column int) {
	t.Lock()
	defer t.Unlock()

	if !t.scrollable {
		return
	}
	t.lineOffset = row
	t.columnOffset = column
	t.trackEnd = false
}

// ScrollToBeginning scrolls to the top left corner of the text if the text view
// is scrollable.
func (t *TextView) ScrollToBeginning() {
	t.Lock()
	defer t.Unlock()

	if !t.scrollable {
		return
	}
	t.trackEnd = false
	t.lineOffset = 0
	t.columnOffset = 0
}

// ScrollToEnd scrolls to the bottom left corner of the text if the text view
// is scrollable. Adding new rows to the end of the text view will cause it to
// scroll with the new data.
func (t *TextView) ScrollToEnd() {
	t.Lock()
	defer t.Unlock()

	if !t.scrollable {
		return
	}
	t.trackEnd = true
	t.columnOffset = 0
}

// GetScrollOffset returns the number of rows and columns that are skipped at
// the top left corner when the text view has been scrolled.
func (t *TextView) GetScrollOffset() (row, column int) {
	t.RLock()
	defer t.RUnlock()

	return t.lineOffset, t.columnOffset
}

// Clear removes all text from the buffer.
func (t *TextView) Clear() {
	t.Lock()
	defer t.Unlock()

	t.clear()
}

func (t *TextView) clear() {
	t.buffer = nil
	t.recentBytes = nil
	if t.reindex {
		t.index = nil
	}
}

// Highlight specifies which regions should be highlighted. If highlight
// toggling is set to true (see SetToggleHighlights()), the highlight of the
// provided regions is toggled (highlighted regions are un-highlighted and vice
// versa). If toggling is set to false, the provided regions are highlighted and
// all other regions will not be highlighted (you may also provide nil to turn
// off all highlights).
//
// For more information on regions, see class description. Empty region strings
// are ignored.
//
// Text in highlighted regions will be drawn inverted, i.e. with their
// background and foreground colors swapped.
func (t *TextView) Highlight(regionIDs ...string) {
	t.Lock()

	// Toggle highlights.
	if t.toggleHighlights {
		var newIDs []string
	HighlightLoop:
		for regionID := range t.highlights {
			for _, id := range regionIDs {
				if regionID == id {
					continue HighlightLoop
				}
			}
			newIDs = append(newIDs, regionID)
		}
		for _, regionID := range regionIDs {
			if _, ok := t.highlights[regionID]; !ok {
				newIDs = append(newIDs, regionID)
			}
		}
		regionIDs = newIDs
	} // Now we have a list of region IDs that end up being highlighted.

	// Determine added and removed regions.
	var added, removed, remaining []string
	if t.highlighted != nil {
		for _, regionID := range regionIDs {
			if _, ok := t.highlights[regionID]; ok {
				remaining = append(remaining, regionID)
				delete(t.highlights, regionID)
			} else {
				added = append(added, regionID)
			}
		}
		for regionID := range t.highlights {
			removed = append(removed, regionID)
		}
	}

	// Make new selection.
	t.highlights = make(map[string]struct{})
	for _, id := range regionIDs {
		if id == "" {
			continue
		}
		t.highlights[id] = struct{}{}
	}
	t.index = nil

	// Notify.
	if t.highlighted != nil && (len(added) > 0 || len(removed) > 0) {
		t.Unlock()
		t.highlighted(added, removed, remaining)
	} else {
		t.Unlock()
	}
}

// GetHighlights returns the IDs of all currently highlighted regions.
func (t *TextView) GetHighlights() (regionIDs []string) {
	t.RLock()
	defer t.RUnlock()

	for id := range t.highlights {
		regionIDs = append(regionIDs, id)
	}
	return
}

// SetToggleHighlights sets a flag to determine how regions are highlighted.
// When set to true, the Highlight() function  will toggle the
// provided/selected regions. When set to false, Highlight()
// will simply highlight the provided regions.
func (t *TextView) SetToggleHighlights(toggle bool) {
	t.toggleHighlights = toggle
}

// ScrollToHighlight will cause the visible area to be scrolled so that the
// highlighted regions appear in the visible area of the text view. This
// repositioning happens the next time the text view is drawn. It happens only
// once so you will need to call this function repeatedly to always keep
// highlighted regions in view.
//
// Nothing happens if there are no highlighted regions or if the text view is
// not scrollable.
func (t *TextView) ScrollToHighlight() {
	t.Lock()
	defer t.Unlock()

	if len(t.highlights) == 0 || !t.scrollable || !t.regions {
		return
	}
	t.index = nil
	t.scrollToHighlights = true
	t.trackEnd = false
}

// GetRegionText returns the text of the region with the given ID. If dynamic
// colors are enabled, color tags are stripped from the text. Newlines are
// always returned as '\n' runes.
//
// If the region does not exist or if regions are turned off, an empty string
// is returned.
func (t *TextView) GetRegionText(regionID string) string {
	t.RLock()
	defer t.RUnlock()

	if !t.regions || len(regionID) == 0 {
		return ""
	}

	var (
		buffer          bytes.Buffer
		currentRegionID string
	)

	for _, str := range t.buffer {
		// Find all color tags in this line.
		var colorTagIndices [][]int
		if t.dynamicColors {
			colorTagIndices = colorPattern.FindAllIndex(str, -1)
		}

		// Find all regions in this line.
		var (
			regionIndices [][]int
			regions       [][][]byte
		)
		if t.regions {
			regionIndices = regionPattern.FindAllIndex(str, -1)
			regions = regionPattern.FindAllSubmatch(str, -1)
		}

		// Analyze this line.
		var currentTag, currentRegion int
		for pos, ch := range str {
			// Skip any color tags.
			if currentTag < len(colorTagIndices) && pos >= colorTagIndices[currentTag][0] && pos < colorTagIndices[currentTag][1] {
				if pos == colorTagIndices[currentTag][1]-1 {
					currentTag++
					if currentTag == len(colorTagIndices) {
						continue
					}
				}
				if colorTagIndices[currentTag][1]-colorTagIndices[currentTag][0] > 2 {
					continue
				}
			}

			// Skip any regions.
			if currentRegion < len(regionIndices) && pos >= regionIndices[currentRegion][0] && pos < regionIndices[currentRegion][1] {
				if pos == regionIndices[currentRegion][1]-1 {
					if currentRegionID == regionID {
						// This is the end of the requested region. We're done.
						return buffer.String()
					}
					currentRegionID = string(regions[currentRegion][1])
					currentRegion++
				}
				continue
			}

			// Add this rune.
			if currentRegionID == regionID {
				buffer.WriteByte(ch)
			}
		}

		// Add newline.
		if currentRegionID == regionID {
			buffer.WriteRune('\n')
		}
	}

	return escapePattern.ReplaceAllString(buffer.String(), `[$1$2]`)
}

// Focus is called when this primitive receives focus.
func (t *TextView) Focus(delegate func(p Primitive)) {
	t.Lock()
	defer t.Unlock()

	// Implemented here with locking because this is used by layout primitives.
	t.hasFocus = true
}

// HasFocus returns whether or not this primitive has focus.
func (t *TextView) HasFocus() bool {
	t.RLock()
	defer t.RUnlock()

	// Implemented here with locking because this may be used in the "changed"
	// callback.
	return t.hasFocus
}

// Write lets us implement the io.Writer interface. Tab characters will be
// replaced with TabSize space characters. A "\n" or "\r\n" will be interpreted
// as a new line.
func (t *TextView) Write(p []byte) (n int, err error) {
	t.Lock()
	changed := t.changed
	if changed != nil {
		// Notify at the end.
		defer changed()
	}
	defer t.Unlock()

	return t.write(p)
}

func (t *TextView) write(p []byte) (n int, err error) {
	// Copy data over.
	newBytes := append(t.recentBytes, p...)
	t.recentBytes = nil

	// If we have a trailing invalid UTF-8 byte, we'll wait.
	if r, _ := utf8.DecodeLastRune(p); r == utf8.RuneError {
		t.recentBytes = newBytes
		return len(p), nil
	}

	// If we have a trailing open dynamic color, exclude it.
	if t.dynamicColors {
		location := openColorRegex.FindIndex(newBytes)
		if location != nil {
			t.recentBytes = newBytes[location[0]:]
			newBytes = newBytes[:location[0]]
		}
	}

	// If we have a trailing open region, exclude it.
	if t.regions {
		location := openRegionRegex.FindIndex(newBytes)
		if location != nil {
			t.recentBytes = newBytes[location[0]:]
			newBytes = newBytes[:location[0]]
		}
	}

	// Transform the new bytes into strings.
	newBytes = bytes.Replace(newBytes, []byte{'\t'}, bytes.Repeat([]byte{' '}, TabSize), -1)
	for index, line := range bytes.Split(newBytes, []byte("\n")) {
		if index == 0 {
			if len(t.buffer) == 0 {
				t.buffer = [][]byte{line}
			} else {
				t.buffer[len(t.buffer)-1] = append(t.buffer[len(t.buffer)-1], line...)
			}
		} else {
			t.buffer = append(t.buffer, line)
		}
	}

	t.clipBuffer()

	// Reset the index.
	if t.reindex {
		t.index = nil
	}

	return len(p), nil
}

// SetWrapWidth set the maximum width of lines when wrapping is enabled.
// When set to 0 the width of the TextView is used.
func (t *TextView) SetWrapWidth(width int) {
	t.Lock()
	defer t.Unlock()

	t.wrapWidth = width
}

// SetReindexBuffer set a flag controlling whether the buffer is reindexed when
// it is modified. This improves the performance of TextViews whose contents
// always have line-breaks in the same location. This must be called after the
// buffer has been indexed.
func (t *TextView) SetReindexBuffer(reindex bool) {
	t.Lock()
	defer t.Unlock()

	t.reindex = reindex

	if reindex {
		t.index = nil
	}
}

// reindexBuffer re-indexes the buffer such that we can use it to easily draw
// the buffer onto the screen. Each line in the index will contain a pointer
// into the buffer from which on we will print text. It will also contain the
// color with which the line starts.
func (t *TextView) reindexBuffer(width int) {
	if t.index != nil && (!t.wrap || width == t.indexWidth) {
		return // Nothing has changed. We can still use the current index.
	}
	t.index = nil
	t.indexWidth = width
	t.fromHighlight, t.toHighlight, t.posHighlight = -1, -1, -1

	// If there's no space, there's no index.
	if width < 1 {
		return
	}

	if t.wrapWidth > 0 && t.wrapWidth < width {
		width = t.wrapWidth
	}

	// Initial states.
	var regionID []byte
	var (
		highlighted                                  bool
		foregroundColor, backgroundColor, attributes string
	)

	// Go through each line in the buffer.
	for bufferIndex, buf := range t.buffer {
		colorTagIndices, colorTags, regionIndices, regions, escapeIndices, strippedStr, _ := decomposeText(buf, t.dynamicColors, t.regions)

		// Split the line if required.
		var splitLines []string
		str := string(strippedStr)
		if t.wrap && len(str) > 0 {
			for len(str) > 0 {
				extract := runewidth.Truncate(str, width, "")
				if len(extract) == 0 {
					// We'll extract at least one grapheme cluster.
					gr := uniseg.NewGraphemes(str)
					gr.Next()
					_, to := gr.Positions()
					extract = str[:to]
				}
				if t.wordWrap && len(extract) < len(str) {
					// Add any spaces from the next line.
					if spaces := spacePattern.FindStringIndex(str[len(extract):]); spaces != nil && spaces[0] == 0 {
						extract = str[:len(extract)+spaces[1]]
					}

					// Can we split before the mandatory end?
					matches := boundaryPattern.FindAllStringIndex(extract, -1)
					if len(matches) > 0 {
						// Yes. Let's split there.
						extract = extract[:matches[len(matches)-1][1]]
					}
				}
				splitLines = append(splitLines, extract)
				str = str[len(extract):]
			}
		} else {
			// No need to split the line.
			splitLines = []string{str}
		}

		// Create index from split lines.
		var originalPos, colorPos, regionPos, escapePos int
		for _, splitLine := range splitLines {
			line := &textViewIndex{
				Line:            bufferIndex,
				Pos:             originalPos,
				ForegroundColor: foregroundColor,
				BackgroundColor: backgroundColor,
				Attributes:      attributes,
				Region:          regionID,
			}

			// Shift original position with tags.
			lineLength := len(splitLine)
			remainingLength := lineLength
			tagEnd := originalPos
			totalTagLength := 0
			for {
				// Which tag comes next?
				nextTag := make([][3]int, 0, 3)
				if colorPos < len(colorTagIndices) {
					nextTag = append(nextTag, [3]int{colorTagIndices[colorPos][0], colorTagIndices[colorPos][1], 0}) // 0 = color tag.
				}
				if regionPos < len(regionIndices) {
					nextTag = append(nextTag, [3]int{regionIndices[regionPos][0], regionIndices[regionPos][1], 1}) // 1 = region tag.
				}
				if escapePos < len(escapeIndices) {
					nextTag = append(nextTag, [3]int{escapeIndices[escapePos][0], escapeIndices[escapePos][1], 2}) // 2 = escape tag.
				}
				minPos := -1
				tagIndex := -1
				for index, pair := range nextTag {
					if minPos < 0 || pair[0] < minPos {
						minPos = pair[0]
						tagIndex = index
					}
				}

				// Is the next tag in range?
				if tagIndex < 0 || minPos > tagEnd+remainingLength {
					break // No. We're done with this line.
				}

				// Advance.
				strippedTagStart := nextTag[tagIndex][0] - originalPos - totalTagLength
				tagEnd = nextTag[tagIndex][1]
				tagLength := tagEnd - nextTag[tagIndex][0]
				if nextTag[tagIndex][2] == 2 {
					tagLength = 1
				}
				totalTagLength += tagLength
				remainingLength = lineLength - (tagEnd - originalPos - totalTagLength)

				// Process the tag.
				switch nextTag[tagIndex][2] {
				case 0:
					// Process color tags.
					foregroundColor, backgroundColor, attributes = styleFromTag(foregroundColor, backgroundColor, attributes, colorTags[colorPos])
					colorPos++
				case 1:
					// Process region tags.
					regionID = regions[regionPos][1]
					_, highlighted = t.highlights[string(regionID)]

					// Update highlight range.
					if highlighted {
						line := len(t.index)
						if t.fromHighlight < 0 {
							t.fromHighlight, t.toHighlight = line, line
							t.posHighlight = runewidth.StringWidth(splitLine[:strippedTagStart])
						} else if line > t.toHighlight {
							t.toHighlight = line
						}
					}

					regionPos++
				case 2:
					// Process escape tags.
					escapePos++
				}
			}

			// Advance to next line.
			originalPos += lineLength + totalTagLength

			// Append this line.
			line.NextPos = originalPos
			line.Width = runewidth.StringWidth(splitLine)
			t.index = append(t.index, line)
		}

		// Word-wrapped lines may have trailing whitespace. Remove it.
		if t.wrap && t.wordWrap {
			for _, line := range t.index {
				str := t.buffer[line.Line][line.Pos:line.NextPos]
				trimmed := bytes.TrimRightFunc(str, unicode.IsSpace)
				if len(trimmed) != len(str) {
					oldNextPos := line.NextPos
					line.NextPos -= len(str) - len(trimmed)
					line.Width -= runewidth.StringWidth(string(t.buffer[line.Line][line.NextPos:oldNextPos]))
				}
			}
		}
	}

	// Calculate longest line.
	t.longestLine = 0
	for _, line := range t.index {
		if line.Width > t.longestLine {
			t.longestLine = line.Width
		}
	}
}

// Draw draws this primitive onto the screen.
func (t *TextView) Draw(screen tcell.Screen) {
	if !t.GetVisible() {
		return
	}

	t.Box.Draw(screen)

	t.Lock()
	defer t.Unlock()

	// Get the available size.
	x, y, width, height := t.GetInnerRect()
	if height == 0 {
		return
	}
	t.pageSize = height

	if t.index == nil || width != t.lastWidth || height != t.lastHeight {
		t.reindexBuffer(width)
	}
	t.lastWidth, t.lastHeight = width, height

	showVerticalScrollBar := t.scrollBarVisibility == ScrollBarAlways || (t.scrollBarVisibility == ScrollBarAuto && len(t.index) > height)
	if showVerticalScrollBar {
		width-- // Subtract space for scroll bar.
	}

	t.reindexBuffer(width)
	if t.regions {
		t.regionInfos = nil
	}

	// Draw scroll bar last.
	defer func() {
		if !showVerticalScrollBar {
			return
		}

		items := len(t.index)
		cursor := int(float64(len(t.index)) * (float64(t.lineOffset) / float64(len(t.index)-height)))

		// Render cursor at the bottom when tracking end
		if t.trackEnd && items <= height {
			items = height + 1
			cursor = height
		}

		for printed := 0; printed < height; printed++ {
			RenderScrollBar(screen, t.scrollBarVisibility, x+width, y+printed, height, items, cursor, printed, t.hasFocus, t.scrollBarColor)
		}
	}()

	// If we don't have an index, there's nothing to draw.
	if t.index == nil {
		return
	}

	// Move to highlighted regions.
	if t.regions && t.scrollToHighlights && t.fromHighlight >= 0 {
		// Do we fit the entire height?
		if t.toHighlight-t.fromHighlight+1 < height {
			// Yes, let's center the highlights.
			t.lineOffset = (t.fromHighlight + t.toHighlight - height) / 2
		} else {
			// No, let's move to the start of the highlights.
			t.lineOffset = t.fromHighlight
		}

		// If the highlight is too far to the right, move it to the middle.
		if t.posHighlight-t.columnOffset > 3*width/4 {
			t.columnOffset = t.posHighlight - width/2
		}

		// If the highlight is offscreen on the left, move it onscreen.
		if t.posHighlight-t.columnOffset < 0 {
			t.columnOffset = t.posHighlight - width/4
		}
	}
	t.scrollToHighlights = false

	// Adjust line offset.
	if t.lineOffset+height > len(t.index) {
		t.trackEnd = true
	}
	if t.trackEnd {
		t.lineOffset = len(t.index) - height
	}
	if t.lineOffset < 0 {
		t.lineOffset = 0
	}

	// Adjust column offset.
	if t.align == AlignLeft {
		if t.columnOffset+width > t.longestLine {
			t.columnOffset = t.longestLine - width
		}
		if t.columnOffset < 0 {
			t.columnOffset = 0
		}
	} else if t.align == AlignRight {
		if t.columnOffset-width < -t.longestLine {
			t.columnOffset = width - t.longestLine
		}
		if t.columnOffset > 0 {
			t.columnOffset = 0
		}
	} else { // AlignCenter.
		half := (t.longestLine - width) / 2
		if half > 0 {
			if t.columnOffset > half {
				t.columnOffset = half
			}
			if t.columnOffset < -half {
				t.columnOffset = -half
			}
		} else {
			t.columnOffset = 0
		}
	}

	// Calculate offset to apply vertical alignment
	verticalOffset := 0
	if len(t.index) < height {
		if t.valign == AlignMiddle {
			verticalOffset = (height - len(t.index)) / 2
		} else if t.valign == AlignBottom {
			verticalOffset = height - len(t.index)
		}
	}

	// Draw the buffer.
	defaultStyle := tcell.StyleDefault.Foreground(t.textColor).Background(t.backgroundColor)
	for line := t.lineOffset; line < len(t.index); line++ {
		// Are we done?
		if line-t.lineOffset >= height {
			break
		}

		// Get the text for this line.
		index := t.index[line]
		text := t.buffer[index.Line][index.Pos:index.NextPos]
		foregroundColor := index.ForegroundColor
		backgroundColor := index.BackgroundColor
		attributes := index.Attributes
		regionID := index.Region
		if t.regions {
			if len(t.regionInfos) > 0 && !bytes.Equal(t.regionInfos[len(t.regionInfos)-1].ID, regionID) {
				// End last region.
				t.regionInfos[len(t.regionInfos)-1].ToX = x
				t.regionInfos[len(t.regionInfos)-1].ToY = y + line - t.lineOffset
			}
			if len(regionID) > 0 && (len(t.regionInfos) == 0 || !bytes.Equal(t.regionInfos[len(t.regionInfos)-1].ID, regionID)) {
				// Start a new region.
				t.regionInfos = append(t.regionInfos, &textViewRegion{
					ID:    regionID,
					FromX: x,
					FromY: y + line - t.lineOffset,
					ToX:   -1,
					ToY:   -1,
				})
			}
		}

		// Process tags.
		colorTagIndices, colorTags, regionIndices, regions, escapeIndices, strippedText, _ := decomposeText(text, t.dynamicColors, t.regions)

		// Calculate the position of the line.
		var skip, posX int
		if t.align == AlignLeft {
			posX = -t.columnOffset
		} else if t.align == AlignRight {
			posX = width - index.Width - t.columnOffset
		} else { // AlignCenter.
			posX = (width-index.Width)/2 - t.columnOffset
		}
		if posX < 0 {
			skip = -posX
			posX = 0
		}

		drawAtY := y + line - t.lineOffset + verticalOffset

		// Print the line.
		if drawAtY >= 0 {
			var colorPos, regionPos, escapePos, tagOffset, skipped int
			iterateString(string(strippedText), func(main rune, comb []rune, textPos, textWidth, screenPos, screenWidth int) bool {
				// Process tags.
				for {
					if colorPos < len(colorTags) && textPos+tagOffset >= colorTagIndices[colorPos][0] && textPos+tagOffset < colorTagIndices[colorPos][1] {
						// Get the color.
						foregroundColor, backgroundColor, attributes = styleFromTag(foregroundColor, backgroundColor, attributes, colorTags[colorPos])
						tagOffset += colorTagIndices[colorPos][1] - colorTagIndices[colorPos][0]
						colorPos++
					} else if regionPos < len(regionIndices) && textPos+tagOffset >= regionIndices[regionPos][0] && textPos+tagOffset < regionIndices[regionPos][1] {
						// Get the region.
						if len(regionID) > 0 && len(t.regionInfos) > 0 && bytes.Equal(t.regionInfos[len(t.regionInfos)-1].ID, regionID) {
							// End last region.
							t.regionInfos[len(t.regionInfos)-1].ToX = x + posX
							t.regionInfos[len(t.regionInfos)-1].ToY = y + line - t.lineOffset
						}
						regionID = regions[regionPos][1]
						if len(regionID) > 0 {
							// Start new region.
							t.regionInfos = append(t.regionInfos, &textViewRegion{
								ID:    regionID,
								FromX: x + posX,
								FromY: y + line - t.lineOffset,
								ToX:   -1,
								ToY:   -1,
							})
						}
						tagOffset += regionIndices[regionPos][1] - regionIndices[regionPos][0]
						regionPos++
					} else {
						break
					}
				}

				// Skip the second-to-last character of an escape tag.
				if escapePos < len(escapeIndices) && textPos+tagOffset == escapeIndices[escapePos][1]-2 {
					tagOffset++
					escapePos++
				}

				// Mix the existing style with the new style.
				_, _, existingStyle, _ := screen.GetContent(x+posX, drawAtY)
				_, background, _ := existingStyle.Decompose()
				style := overlayStyle(background, defaultStyle, foregroundColor, backgroundColor, attributes)

				// Do we highlight this character?
				var highlighted bool
				if len(regionID) > 0 {
					if _, ok := t.highlights[string(regionID)]; ok {
						highlighted = true
					}
				}
				if highlighted {
					fg := t.highlightForeground
					bg := t.highlightBackground
					if fg == tcell.ColorDefault {
						fg = Styles.PrimaryTextColor
						if fg == tcell.ColorDefault {
							fg = tcell.ColorWhite.TrueColor()
						}
					}
					if bg == tcell.ColorDefault {
						r, g, b := fg.RGB()
						c := colorful.Color{R: float64(r) / 255, G: float64(g) / 255, B: float64(b) / 255}
						_, _, li := c.Hcl()
						if li < .5 {
							bg = tcell.ColorWhite.TrueColor()
						} else {
							bg = tcell.ColorBlack.TrueColor()
						}
					}
					style = style.Foreground(fg).Background(bg)
				}

				// Skip to the right.
				if !t.wrap && skipped < skip {
					skipped += screenWidth
					return false
				}

				// Stop at the right border.
				if posX+screenWidth > width {
					return true
				}

				// Draw the character.
				for offset := screenWidth - 1; offset >= 0; offset-- {
					if offset == 0 {
						screen.SetContent(x+posX+offset, drawAtY, main, comb, style)
					} else {
						screen.SetContent(x+posX+offset, drawAtY, ' ', nil, style)
					}
				}

				// Advance.
				posX += screenWidth
				return false
			})
		}
	}

	// If this view is not scrollable, we'll purge the buffer of lines that have
	// scrolled out of view.
	if !t.scrollable && t.lineOffset > 0 {
		if t.lineOffset >= len(t.index) {
			t.buffer = nil
		} else {
			t.buffer = t.buffer[t.index[t.lineOffset].Line:]
		}
		t.index = nil
		t.lineOffset = 0
	}
}

// InputHandler returns the handler for this primitive.
func (t *TextView) InputHandler() func(event *tcell.EventKey, setFocus func(p Primitive)) {
	return t.WrapInputHandler(func(event *tcell.EventKey, setFocus func(p Primitive)) {
		key := event.Key()

		if HitShortcut(event, Keys.Cancel, Keys.Select, Keys.Select2, Keys.MovePreviousField, Keys.MoveNextField) {
			if t.done != nil {
				t.done(key)
			}
			return
		}

		t.Lock()
		defer t.Unlock()

		if !t.scrollable {
			return
		}

		if HitShortcut(event, Keys.MoveFirst, Keys.MoveFirst2) {
			t.trackEnd = false
			t.lineOffset = 0
			t.columnOffset = 0
		} else if HitShortcut(event, Keys.MoveLast, Keys.MoveLast2) {
			t.trackEnd = true
			t.columnOffset = 0
		} else if HitShortcut(event, Keys.MoveUp, Keys.MoveUp2) {
			t.trackEnd = false
			t.lineOffset--
		} else if HitShortcut(event, Keys.MoveDown, Keys.MoveDown2) {
			t.lineOffset++
		} else if HitShortcut(event, Keys.MoveLeft, Keys.MoveLeft2) {
			t.columnOffset--
		} else if HitShortcut(event, Keys.MoveRight, Keys.MoveRight2) {
			t.columnOffset++
		} else if HitShortcut(event, Keys.MovePreviousPage) {
			t.trackEnd = false
			t.lineOffset -= t.pageSize
		} else if HitShortcut(event, Keys.MoveNextPage) {
			t.lineOffset += t.pageSize
		}
	})
}

// MouseHandler returns the mouse handler for this primitive.
func (t *TextView) MouseHandler() func(action MouseAction, event *tcell.EventMouse, setFocus func(p Primitive)) (consumed bool, capture Primitive) {
	return t.WrapMouseHandler(func(action MouseAction, event *tcell.EventMouse, setFocus func(p Primitive)) (consumed bool, capture Primitive) {
		x, y := event.Position()
		if !t.InRect(x, y) {
			return false, nil
		}

		switch action {
		case MouseLeftClick:
			if t.regions {
				// Find a region to highlight.
				for _, region := range t.regionInfos {
					if y == region.FromY && x < region.FromX ||
						y == region.ToY && x >= region.ToX ||
						region.FromY >= 0 && y < region.FromY ||
						region.ToY >= 0 && y > region.ToY {
						continue
					}
					if t.clicked != nil {
						t.clicked(string(region.ID))
					}
					break
				}
			}
			consumed = true
			setFocus(t)
		case MouseScrollUp:
			if t.scrollable {
				t.trackEnd = false
				t.lineOffset--
				consumed = true
			}
		case MouseScrollDown:
			if t.scrollable {
				t.lineOffset++
				consumed = true
			}
		}

		return
	})
}

The pages are generated with Golds v0.8.2. (GOOS=linux GOARCH=amd64)
Golds is a Go 101 project developed by Tapir Liu.
PR and bug reports are welcome and can be submitted to the issue list.
Please follow @zigo_101 (reachable from the left QR code) to get the latest news of Golds.