v2_indent.go

     1  // Copyright 2010 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  //go:build goexperiment.jsonv2
     6  
     7  package json
     8  
     9  import (
    10  	"bytes"
    11  	"strings"
    12  
    13  	"encoding/json/jsontext"
    14  )
    15  
    16  // HTMLEscape appends to dst the JSON-encoded src with <, >, &, U+2028 and U+2029
    17  // characters inside string literals changed to \u003c, \u003e, \u0026, \u2028, \u2029
    18  // so that the JSON will be safe to embed inside HTML <script> tags.
    19  // For historical reasons, web browsers don't honor standard HTML
    20  // escaping within <script> tags, so an alternative JSON encoding must be used.
    21  func HTMLEscape(dst *bytes.Buffer, src []byte) {
    22  	dst.Grow(len(src))
    23  	dst.Write(appendHTMLEscape(dst.AvailableBuffer(), src))
    24  }
    25  
    26  func appendHTMLEscape(dst, src []byte) []byte {
    27  	const hex = "0123456789abcdef"
    28  	// The characters can only appear in string literals,
    29  	// so just scan the string one byte at a time.
    30  	start := 0
    31  	for i, c := range src {
    32  		if c == '<' || c == '>' || c == '&' {
    33  			dst = append(dst, src[start:i]...)
    34  			dst = append(dst, '\\', 'u', '0', '0', hex[c>>4], hex[c&0xF])
    35  			start = i + 1
    36  		}
    37  		// Convert U+2028 and U+2029 (E2 80 A8 and E2 80 A9).
    38  		if c == 0xE2 && i+2 < len(src) && src[i+1] == 0x80 && src[i+2]&^1 == 0xA8 {
    39  			dst = append(dst, src[start:i]...)
    40  			dst = append(dst, '\\', 'u', '2', '0', '2', hex[src[i+2]&0xF])
    41  			start = i + len("\u2029")
    42  		}
    43  	}
    44  	return append(dst, src[start:]...)
    45  }
    46  
    47  // Compact appends to dst the JSON-encoded src with
    48  // insignificant space characters elided.
    49  func Compact(dst *bytes.Buffer, src []byte) error {
    50  	dst.Grow(len(src))
    51  	b := dst.AvailableBuffer()
    52  	b, err := jsontext.AppendFormat(b, src,
    53  		jsontext.AllowDuplicateNames(true),
    54  		jsontext.AllowInvalidUTF8(true),
    55  		jsontext.PreserveRawStrings(true))
    56  	if err != nil {
    57  		return transformSyntacticError(err)
    58  	}
    59  	dst.Write(b)
    60  	return nil
    61  }
    62  
    63  // indentGrowthFactor specifies the growth factor of indenting JSON input.
    64  // Empirically, the growth factor was measured to be between 1.4x to 1.8x
    65  // for some set of compacted JSON with the indent being a single tab.
    66  // Specify a growth factor slightly larger than what is observed
    67  // to reduce probability of allocation in appendIndent.
    68  // A factor no higher than 2 ensures that wasted space never exceeds 50%.
    69  const indentGrowthFactor = 2
    70  
    71  // Indent appends to dst an indented form of the JSON-encoded src.
    72  // Each element in a JSON object or array begins on a new,
    73  // indented line beginning with prefix followed by one or more
    74  // copies of indent according to the indentation nesting.
    75  // The data appended to dst does not begin with the prefix nor
    76  // any indentation, to make it easier to embed inside other formatted JSON data.
    77  // Although leading space characters (space, tab, carriage return, newline)
    78  // at the beginning of src are dropped, trailing space characters
    79  // at the end of src are preserved and copied to dst.
    80  // For example, if src has no trailing spaces, neither will dst;
    81  // if src ends in a trailing newline, so will dst.
    82  func Indent(dst *bytes.Buffer, src []byte, prefix, indent string) error {
    83  	dst.Grow(indentGrowthFactor * len(src))
    84  	b := dst.AvailableBuffer()
    85  	b, err := appendIndent(b, src, prefix, indent)
    86  	dst.Write(b)
    87  	return err
    88  }
    89  
    90  func appendIndent(dst, src []byte, prefix, indent string) ([]byte, error) {
    91  	// In v2, trailing whitespace is discarded, while v1 preserved it.
    92  	dstLen := len(dst)
    93  	if n := len(src) - len(bytes.TrimRight(src, " \n\r\t")); n > 0 {
    94  		// Append the trailing whitespace afterwards.
    95  		defer func() {
    96  			if len(dst) > dstLen {
    97  				dst = append(dst, src[len(src)-n:]...)
    98  			}
    99  		}()
   100  	}
   101  	// In v2, only spaces and tabs are allowed, while v1 allowed any character.
   102  	if len(strings.Trim(prefix, " \t"))+len(strings.Trim(indent, " \t")) > 0 {
   103  		// Use placeholder spaces of correct length, and replace afterwards.
   104  		invalidPrefix, invalidIndent := prefix, indent
   105  		prefix = strings.Repeat(" ", len(prefix))
   106  		indent = strings.Repeat(" ", len(indent))
   107  		defer func() {
   108  			b := dst[dstLen:]
   109  			for i := bytes.IndexByte(b, '\n'); i >= 0; i = bytes.IndexByte(b, '\n') {
   110  				b = b[i+len("\n"):]
   111  				n := len(b) - len(bytes.TrimLeft(b, " ")) // len(prefix)+n*len(indent)
   112  				spaces := b[:n]
   113  				spaces = spaces[copy(spaces, invalidPrefix):]
   114  				for len(spaces) > 0 {
   115  					spaces = spaces[copy(spaces, invalidIndent):]
   116  				}
   117  				b = b[n:]
   118  			}
   119  		}()
   120  	}
   121  
   122  	dst, err := jsontext.AppendFormat(dst, src,
   123  		jsontext.AllowDuplicateNames(true),
   124  		jsontext.AllowInvalidUTF8(true),
   125  		jsontext.PreserveRawStrings(true),
   126  		jsontext.Multiline(true),
   127  		jsontext.WithIndentPrefix(prefix),
   128  		jsontext.WithIndent(indent))
   129  	if err != nil {
   130  		return dst[:dstLen], transformSyntacticError(err)
   131  	}
   132  	return dst, nil
   133  }
   134
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