builder.go

     1  // Copyright 2017 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  package strings
     6  
     7  import (
     8  	"internal/abi"
     9  	"internal/bytealg"
    10  	"unicode/utf8"
    11  	"unsafe"
    12  )
    13  
    14  // A Builder is used to efficiently build a string using [Builder.Write] methods.
    15  // It minimizes memory copying. The zero value is ready to use.
    16  // Do not copy a non-zero Builder.
    17  type Builder struct {
    18  	addr *Builder // of receiver, to detect copies by value
    19  
    20  	// External users should never get direct access to this buffer, since
    21  	// the slice at some point will be converted to a string using unsafe, also
    22  	// data between len(buf) and cap(buf) might be uninitialized.
    23  	buf []byte
    24  }
    25  
    26  // copyCheck implements a dynamic check to prevent modification after
    27  // copying a non-zero Builder, which would be unsafe (see #25907, #47276).
    28  //
    29  // We cannot add a noCopy field to Builder, to cause vet's copylocks
    30  // check to report copying, because copylocks cannot reliably
    31  // discriminate the zero and nonzero cases.
    32  func (b *Builder) copyCheck() {
    33  	if b.addr == nil {
    34  		// This hack works around a failing of Go's escape analysis
    35  		// that was causing b to escape and be heap allocated.
    36  		// See issue 23382.
    37  		// TODO: once issue 7921 is fixed, this should be reverted to
    38  		// just "b.addr = b".
    39  		b.addr = (*Builder)(abi.NoEscape(unsafe.Pointer(b)))
    40  	} else if b.addr != b {
    41  		panic("strings: illegal use of non-zero Builder copied by value")
    42  	}
    43  }
    44  
    45  // String returns the accumulated string.
    46  func (b *Builder) String() string {
    47  	return unsafe.String(unsafe.SliceData(b.buf), len(b.buf))
    48  }
    49  
    50  // Len returns the number of accumulated bytes; b.Len() == len(b.String()).
    51  func (b *Builder) Len() int { return len(b.buf) }
    52  
    53  // Cap returns the capacity of the builder's underlying byte slice. It is the
    54  // total space allocated for the string being built and includes any bytes
    55  // already written.
    56  func (b *Builder) Cap() int { return cap(b.buf) }
    57  
    58  // Reset resets the [Builder] to be empty.
    59  func (b *Builder) Reset() {
    60  	b.addr = nil
    61  	b.buf = nil
    62  }
    63  
    64  // grow copies the buffer to a new, larger buffer so that there are at least n
    65  // bytes of capacity beyond len(b.buf).
    66  func (b *Builder) grow(n int) {
    67  	buf := bytealg.MakeNoZero(2*cap(b.buf) + n)[:len(b.buf)]
    68  	copy(buf, b.buf)
    69  	b.buf = buf
    70  }
    71  
    72  // Grow grows b's capacity, if necessary, to guarantee space for
    73  // another n bytes. After Grow(n), at least n bytes can be written to b
    74  // without another allocation. If n is negative, Grow panics.
    75  func (b *Builder) Grow(n int) {
    76  	b.copyCheck()
    77  	if n < 0 {
    78  		panic("strings.Builder.Grow: negative count")
    79  	}
    80  	if cap(b.buf)-len(b.buf) < n {
    81  		b.grow(n)
    82  	}
    83  }
    84  
    85  // Write appends the contents of p to b's buffer.
    86  // Write always returns len(p), nil.
    87  func (b *Builder) Write(p []byte) (int, error) {
    88  	b.copyCheck()
    89  	b.buf = append(b.buf, p...)
    90  	return len(p), nil
    91  }
    92  
    93  // WriteByte appends the byte c to b's buffer.
    94  // The returned error is always nil.
    95  func (b *Builder) WriteByte(c byte) error {
    96  	b.copyCheck()
    97  	b.buf = append(b.buf, c)
    98  	return nil
    99  }
   100  
   101  // WriteRune appends the UTF-8 encoding of Unicode code point r to b's buffer.
   102  // It returns the length of r and a nil error.
   103  func (b *Builder) WriteRune(r rune) (int, error) {
   104  	b.copyCheck()
   105  	n := len(b.buf)
   106  	b.buf = utf8.AppendRune(b.buf, r)
   107  	return len(b.buf) - n, nil
   108  }
   109  
   110  // WriteString appends the contents of s to b's buffer.
   111  // It returns the length of s and a nil error.
   112  func (b *Builder) WriteString(s string) (int, error) {
   113  	b.copyCheck()
   114  	b.buf = append(b.buf, s...)
   115  	return len(s), nil
   116  }
   117
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