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// Package colwriter provides a write filter that formats
// input lines in multiple columns.
//
// The package is a straightforward translation from
// /src/cmd/draw/mc.c in Plan 9 from User Space.
package colwriter
import (
"bytes"
"io"
"unicode/utf8"
)
const (
tab = 4
)
const (
// Print each input line ending in a colon ':' separately.
BreakOnColon uint = 1 << iota
)
// A Writer is a filter that arranges input lines in as many columns as will
// fit in its width. Tab '\t' chars in the input are translated to sequences
// of spaces ending at multiples of 4 positions.
//
// If BreakOnColon is set, each input line ending in a colon ':' is written
// separately.
//
// The Writer assumes that all Unicode code points have the same width; this
// may not be true in some fonts.
type Writer struct {
w io.Writer
buf []byte
width int
flag uint
}
// NewWriter allocates and initializes a new Writer writing to w.
// Parameter width controls the total number of characters on each line
// across all columns.
func NewWriter(w io.Writer, width int, flag uint) *Writer {
return &Writer{
w: w,
width: width,
flag: flag,
}
}
// Write writes p to the writer w. The only errors returned are ones
// encountered while writing to the underlying output stream.
func (w *Writer) Write(p []byte) (n int, err error) {
var linelen int
var lastWasColon bool
for i, c := range p {
w.buf = append(w.buf, c)
linelen++
if c == '\t' {
w.buf[len(w.buf)-1] = ' '
for linelen%tab != 0 {
w.buf = append(w.buf, ' ')
linelen++
}
}
if w.flag&BreakOnColon != 0 && c == ':' {
lastWasColon = true
} else if lastWasColon {
if c == '\n' {
pos := bytes.LastIndex(w.buf[:len(w.buf)-1], []byte{'\n'})
if pos < 0 {
pos = 0
}
line := w.buf[pos:]
w.buf = w.buf[:pos]
if err = w.columnate(); err != nil {
if len(line) < i {
return i - len(line), err
}
return 0, err
}
if n, err := w.w.Write(line); err != nil {
if r := len(line) - n; r < i {
return i - r, err
}
return 0, err
}
}
lastWasColon = false
}
if c == '\n' {
linelen = 0
}
}
return len(p), nil
}
// Flush should be called after the last call to Write to ensure that any data
// buffered in the Writer is written to output.
func (w *Writer) Flush() error {
return w.columnate()
}
func (w *Writer) columnate() error {
words := bytes.Split(w.buf, []byte{'\n'})
w.buf = nil
if len(words[len(words)-1]) == 0 {
words = words[:len(words)-1]
}
maxwidth := 0
for _, wd := range words {
if n := utf8.RuneCount(wd); n > maxwidth {
maxwidth = n
}
}
maxwidth++ // space char
wordsPerLine := w.width / maxwidth
if wordsPerLine <= 0 {
wordsPerLine = 1
}
nlines := (len(words) + wordsPerLine - 1) / wordsPerLine
for i := 0; i < nlines; i++ {
col := 0
endcol := 0
for j := i; j < len(words); j += nlines {
endcol += maxwidth
_, err := w.w.Write(words[j])
if err != nil {
return err
}
col += utf8.RuneCount(words[j])
if j+nlines < len(words) {
for col < endcol {
_, err := w.w.Write([]byte{' '})
if err != nil {
return err
}
col++
}
}
}
_, err := w.w.Write([]byte{'\n'})
if err != nil {
return err
}
}
return nil
}
|
