Add Grapheme cluster support

go.mod

@@ -0,0 +1,3 @@
+module github.com/liggitt/tabwriter
+
+go 1.21.4

tabwriter.go

@@ -14,6 +14,7 @@
 package tabwriter
 
 import (
+	"github.com/liggitt/tabwriter/unicode"
 	"io"
 	"unicode/utf8"
 )
@@ -199,6 +200,9 @@ const (
 
 	// Remember maximum widths seen per column even after Flush() is called.
 	RememberWidths
+
+	// Combine multiple runes into individual user-perceived characters ("grapheme clusters" in Unicode specification)
+	GraphemeClusterWidth
 )
 
 // A Writer must be initialized with a call to Init. The first parameter (output)
@@ -439,7 +443,11 @@ func (b *Writer) append(text []byte) {
 
 // Update the cell width.
 func (b *Writer) updateWidth() {
-	b.cell.width += utf8.RuneCount(b.buf[b.pos:])
+	if b.flags&GraphemeClusterWidth != 0 {
+		b.cell.width += unicode.MonospaceWidth(b.buf[b.pos:])
+	} else {
+		b.cell.width += utf8.RuneCount(b.buf[b.pos:])
+	}
 	b.pos = len(b.buf)
 }
 

tabwriter_test.go

@@ -361,6 +361,45 @@ var tests = []struct {
 		"1....2....3....4....\n",
 	},
 
+	{
+		"10c",
+		5, 0, 0, '.', GraphemeClusterWidth,
+		"1\t2\t3\t4\t\n",
+		"1....2....3....4....\n",
+	},
+
+	// Emoji Variation Selector - all width of 1
+	{
+		"10d",
+		5, 0, 0, '.', GraphemeClusterWidth,
+		"\u2716\t\u2716\ufe0e\t\u2716\ufe0f\t\n\u263a\t\u263a\ufe0e\t\u263a\ufe0f\t\n",
+		"✖....✖︎....✖️....\n☺....☺︎....☺️....\n",
+	},
+
+	// Emoji Flags - pirate flag is width 2, rest are width 1
+	{
+		"10e",
+		5, 0, 0, '.', GraphemeClusterWidth,
+		"\U0001F3F4\u200D\u2620\t\U0001F3F3\u200D\U0001F308\t\U0001F1E9\U0001F1EA\t\n",
+		"🏴‍☠️...🏳️‍🌈....🇩🇪....\n",
+	},
+
+	// Emoji Speedboat - width 1, rest are width 2
+	{
+		"10e",
+		5, 0, 0, '.', GraphemeClusterWidth,
+		"\U0001F6E5\t\U0001F4A3\t\U0001F468\u200D\U0001F467\u200D\U0001F466\t\n",
+		"🛥....💣...👨‍👧‍👦...\n",
+	},
+
+	// East Asian characters should be width 2
+	{
+		"10f",
+		5, 0, 0, '.', GraphemeClusterWidth,
+		"容器\t容器\t容器\t\n",
+		"容器.容器.容器.\n",
+	},
+
 	{
 		"11",
 		8, 0, 1, '.', 0,

unicode/grapheme.go

@@ -0,0 +1,120 @@
+// From https://github.com/rivo/uniseg/blob/master/grapheme.go
+//
+// Licensed under MIT License
+//
+// Copyright (c) 2019 Oliver Kuederle
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all
+// copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+// SOFTWARE.
+
+package unicode
+
+import "unicode/utf8"
+
+// The number of bits the grapheme property must be shifted to make place for
+// grapheme states.
+const shiftGraphemePropState = 4
+
+// The bit mask used to extract the state returned by the [Step] function, after
+// shifting. These values must correspond to the shift constants.
+const maskGraphemeState = 0xf
+
+// FirstGraphemeCluster returns the first grapheme cluster found in the given
+// byte slice according to the rules of [Unicode Standard Annex #29, Grapheme
+// Cluster Boundaries]. This function can be called continuously to extract all
+// grapheme clusters from a byte slice, as illustrated in the example below.
+//
+// If you don't know the current state, for example when calling the function
+// for the first time, you must pass -1. For consecutive calls, pass the state
+// and rest slice returned by the previous call.
+//
+// The "rest" slice is the sub-slice of the original byte slice "b" starting
+// after the last byte of the identified grapheme cluster. If the length of the
+// "rest" slice is 0, the entire byte slice "b" has been processed. The
+// "cluster" byte slice is the sub-slice of the input slice containing the
+// identified grapheme cluster.
+//
+// The returned width is the width of the grapheme cluster for most monospace
+// fonts where a value of 1 represents one character cell.
+//
+// Given an empty byte slice "b", the function returns nil values.
+//
+// While slightly less convenient than using the Graphemes class, this function
+// has much better performance and makes no allocations. It lends itself well to
+// large byte slices.
+//
+// [Unicode Standard Annex #29, Grapheme Cluster Boundaries]: http://unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries
+func FirstGraphemeCluster(b []byte, state int) (cluster, rest []byte, width, newState int) {
+	// An empty byte slice returns nothing.
+	if len(b) == 0 {
+		return
+	}
+
+	// Extract the first rune.
+	r, length := utf8.DecodeRune(b)
+	if len(b) <= length { // If we're already past the end, there is nothing else to parse.
+		var prop int
+		if state < 0 {
+			prop = property(graphemeCodePoints, r)
+		} else {
+			prop = state >> shiftGraphemePropState
+		}
+		return b, nil, runeWidth(r, prop), grAny | (prop << shiftGraphemePropState)
+	}
+
+	// If we don't know the state, determine it now.
+	var firstProp int
+	if state < 0 {
+		state, firstProp, _ = transitionGraphemeState(state, r)
+	} else {
+		firstProp = state >> shiftGraphemePropState
+	}
+	width += runeWidth(r, firstProp)
+
+	// Transition until we find a boundary.
+	for {
+		var (
+			prop     int
+			boundary bool
+		)
+
+		r, l := utf8.DecodeRune(b[length:])
+		state, prop, boundary = transitionGraphemeState(state&maskGraphemeState, r)
+
+		if boundary {
+			return b[:length], b[length:], width, state | (prop << shiftGraphemePropState)
+		}
+
+		if r == vs16 {
+			width = 2
+		} else if firstProp != prExtendedPictographic && firstProp != prRegionalIndicator && firstProp != prL {
+			width += runeWidth(r, prop)
+		} else if firstProp == prExtendedPictographic {
+			if r == vs15 {
+				width = 1
+			} else {
+				width = 2
+			}
+		}
+
+		length += l
+		if len(b) <= length {
+			return b, nil, width, grAny | (prop << shiftGraphemePropState)
+		}
+	}
+}