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  	"unicode/utf8"
     9  	"unsafe"
    10  )
    11  
    12  // A Builder is used to efficiently build a string using Write methods.
    13  // It minimizes memory copying. The zero value is ready to use.
    14  // Do not copy a non-zero Builder.
    15  type Builder struct {
    16  	addr *Builder // of receiver, to detect copies by value
    17  	buf  []byte
    18  }
    19  
    20  // noescape hides a pointer from escape analysis. It is the identity function
    21  // but escape analysis doesn't think the output depends on the input.
    22  // noescape is inlined and currently compiles down to zero instructions.
    23  // USE CAREFULLY!
    24  // This was copied from the runtime; see issues 23382 and 7921.
    25  //go:nosplit
    26  //go:nocheckptr
    27  func noescape(p unsafe.Pointer) unsafe.Pointer {
    28  	x := uintptr(p)
    29  	return unsafe.Pointer(x ^ 0)
    30  }
    31  
    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)(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 *(*string)(unsafe.Pointer(&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 := make([]byte, len(b.buf), 2*cap(b.buf)+n)
    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  	// Compare as uint32 to correctly handle negative runes.
   106  	if uint32(r) < utf8.RuneSelf {
   107  		b.buf = append(b.buf, byte(r))
   108  		return 1, nil
   109  	}
   110  	l := len(b.buf)
   111  	if cap(b.buf)-l < utf8.UTFMax {
   112  		b.grow(utf8.UTFMax)
   113  	}
   114  	n := utf8.EncodeRune(b.buf[l:l+utf8.UTFMax], r)
   115  	b.buf = b.buf[:l+n]
   116  	return n, nil
   117  }
   118  
   119  // WriteString appends the contents of s to b's buffer.
   120  // It returns the length of s and a nil error.
   121  func (b *Builder) WriteString(s string) (int, error) {
   122  	b.copyCheck()
   123  	b.buf = append(b.buf, s...)
   124  	return len(s), nil
   125  }
   126
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