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text/tabwriter: don't mimic previous lines on flush

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\f triggers a flush.

This is used (by gofmt, among others) to indicate that
the current aligned segment has ended.

When flushed, it is unlikely that the previous line is
in fact a good predictor of the upcoming line,
so stop treating it as such.

No performance impact on the existing benchmarks,
which do not perform any flushes.

Change-Id: Ifdf3e6d4600713c90db7b51a10e429d9260dc08c
Reviewed-on: https://go-review.googlesource.com/111644
Reviewed-by: Robert Griesemer <gri@golang.org>
This commit is contained in:
Josh Bleecher Snyder 2018-05-05 21:26:23 -07:00 committed by Robert Griesemer
parent 3fbfc83db2
commit a33c595753
1 changed files with 15 additions and 10 deletions
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25
src/text/tabwriter/tabwriter.go
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@ -106,7 +106,10 @@ type Writer struct {
widths []int // list of column widths in runes - re-used during formatting
}
func (b *Writer) addLine() {
// addLine adds a new line.
// flushed is a hint indicating whether the underlying writer was just flushed.
// If so, the previous line is not likely to be a good indicator of the new line's cells.
func (b *Writer) addLine(flushed bool) {
// Grow slice instead of appending,
// as that gives us an opportunity
// to re-use an existing []cell.
@ -117,13 +120,15 @@ func (b *Writer) addLine() {
b.lines = append(b.lines, nil)
}
// The previous line is probably a good indicator
// of how many cells the current line will have.
// If the current line's capacity is smaller than that,
// abandon it and make a new one.
if n := len(b.lines); n >= 2 {
if prev := len(b.lines[n-2]); prev > cap(b.lines[n-1]) {
b.lines[n-1] = make([]cell, 0, prev)
if !flushed {
// The previous line is probably a good indicator
// of how many cells the current line will have.
// If the current line's capacity is smaller than that,
// abandon it and make a new one.
if n := len(b.lines); n >= 2 {
if prev := len(b.lines[n-2]); prev > cap(b.lines[n-1]) {
b.lines[n-1] = make([]cell, 0, prev)
}
}
}
}
@ -136,7 +141,7 @@ func (b *Writer) reset() {
b.endChar = 0
b.lines = b.lines[0:0]
b.widths = b.widths[0:0]
b.addLine()
b.addLine(true)
}
// Internal representation (current state):
@ -527,7 +532,7 @@ func (b *Writer) Write(buf []byte) (n int, err error) {
ncells := b.terminateCell(ch == '\t')
if ch == '\n' || ch == '\f' {
// terminate line
b.addLine()
b.addLine(ch == '\f')
if ch == '\f' || ncells == 1 {
// A '\f' always forces a flush. Otherwise, if the previous
// line has only one cell which does not have an impact on