go.chromium.org/luci@v0.0.0-20240309015107-7cdc2e660f33/common/data/text/indented/writer.go (about)

     1  // Copyright 2016 The LUCI Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //      http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  package indented
    16  
    17  import (
    18  	"bytes"
    19  	"io"
    20  )
    21  
    22  // Writer inserts indentation before each line.
    23  type Writer struct {
    24  	io.Writer       // underlying writer.
    25  	Level      int  // number of times \t must be inserted before each line.
    26  	UseSpaces  bool // true iff Level is the number of spaces instead of tabs.
    27  	Width      int  // number of whitespace characters in each level. Defaults to 1.
    28  	insideLine bool
    29  }
    30  
    31  // Limit is the maximum value of Level.
    32  const Limit = 256
    33  
    34  var indentationTabs = bytes.Repeat([]byte{'\t'}, Limit)
    35  var indentationSpaces = bytes.Repeat([]byte{' '}, Limit)
    36  
    37  // prefix returns the indentation prefix.
    38  func (w *Writer) prefix() []byte {
    39  	if w.Level == 0 {
    40  		return nil
    41  	}
    42  
    43  	buf := indentationTabs
    44  	if w.UseSpaces {
    45  		buf = indentationSpaces
    46  	}
    47  
    48  	width := w.Width
    49  	if width <= 0 {
    50  		width = 1
    51  	}
    52  	return buf[:width*w.Level]
    53  }
    54  
    55  // Write writes data inserting a newline before each line.
    56  // Panics if w.Indent is outside of [0, Limit) range.
    57  func (w *Writer) Write(data []byte) (n int, err error) {
    58  	// Do not print indentation if there is no data.
    59  	for len(data) > 0 {
    60  		var printUntil int
    61  		endsWithNewLine := false
    62  
    63  		lineBeginning := !w.insideLine
    64  		if data[0] == '\n' && lineBeginning {
    65  			// This is a blank line. Do not indent it, just print as is.
    66  			printUntil = 1
    67  		} else {
    68  			if lineBeginning {
    69  				// Print indentation.
    70  				w.Writer.Write(w.prefix())
    71  				w.insideLine = true
    72  			}
    73  
    74  			lineEnd := bytes.IndexRune(data, '\n')
    75  			if lineEnd < 0 {
    76  				// Print the whole thing.
    77  				printUntil = len(data)
    78  			} else {
    79  				// Print until the newline inclusive.
    80  				printUntil = lineEnd + 1
    81  				endsWithNewLine = true
    82  			}
    83  		}
    84  		toPrint := data[:printUntil]
    85  		data = data[printUntil:]
    86  
    87  		// Assertion: none of the runes in toPrint
    88  		// can be newline except the last rune.
    89  		// The last rune is newline iff endsWithNewLine==true.
    90  
    91  		m, err := w.Writer.Write(toPrint)
    92  		n += m
    93  
    94  		if m == len(toPrint) && endsWithNewLine {
    95  			// We've printed the newline, so we are the line beginning again.
    96  			w.insideLine = false
    97  		}
    98  		if err != nil {
    99  			return n, err
   100  		}
   101  	}
   102  	return n, nil
   103  }