Source file src/fmt/print.go

     1  // Copyright 2009 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 fmt
     6  
     7  import (
     8  	"internal/fmtsort"
     9  	"io"
    10  	"os"
    11  	"reflect"
    12  	"strconv"
    13  	"sync"
    14  	"unicode/utf8"
    15  )
    16  
    17  // Strings for use with buffer.WriteString.
    18  // This is less overhead than using buffer.Write with byte arrays.
    19  const (
    20  	commaSpaceString  = ", "
    21  	nilAngleString    = "<nil>"
    22  	nilParenString    = "(nil)"
    23  	nilString         = "nil"
    24  	mapString         = "map["
    25  	percentBangString = "%!"
    26  	missingString     = "(MISSING)"
    27  	badIndexString    = "(BADINDEX)"
    28  	panicString       = "(PANIC="
    29  	extraString       = "%!(EXTRA "
    30  	badWidthString    = "%!(BADWIDTH)"
    31  	badPrecString     = "%!(BADPREC)"
    32  	noVerbString      = "%!(NOVERB)"
    33  	invReflectString  = "<invalid reflect.Value>"
    34  )
    35  
    36  // State represents the printer state passed to custom formatters.
    37  // It provides access to the [io.Writer] interface plus information about
    38  // the flags and options for the operand's format specifier.
    39  type State interface {
    40  	// Write is the function to call to emit formatted output to be printed.
    41  	Write(b []byte) (n int, err error)
    42  	// Width returns the value of the width option and whether it has been set.
    43  	Width() (wid int, ok bool)
    44  	// Precision returns the value of the precision option and whether it has been set.
    45  	Precision() (prec int, ok bool)
    46  
    47  	// Flag reports whether the flag c, a character, has been set.
    48  	Flag(c int) bool
    49  }
    50  
    51  // Formatter is implemented by any value that has a Format method.
    52  // The implementation controls how [State] and rune are interpreted,
    53  // and may call [Sprint] or [Fprint](f) etc. to generate its output.
    54  type Formatter interface {
    55  	Format(f State, verb rune)
    56  }
    57  
    58  // Stringer is implemented by any value that has a String method,
    59  // which defines the “native” format for that value.
    60  // The String method is used to print values passed as an operand
    61  // to any format that accepts a string or to an unformatted printer
    62  // such as [Print].
    63  type Stringer interface {
    64  	String() string
    65  }
    66  
    67  // GoStringer is implemented by any value that has a GoString method,
    68  // which defines the Go syntax for that value.
    69  // The GoString method is used to print values passed as an operand
    70  // to a %#v format.
    71  type GoStringer interface {
    72  	GoString() string
    73  }
    74  
    75  // FormatString returns a string representing the fully qualified formatting
    76  // directive captured by the [State], followed by the argument verb. ([State] does not
    77  // itself contain the verb.) The result has a leading percent sign followed by any
    78  // flags, the width, and the precision. Missing flags, width, and precision are
    79  // omitted. This function allows a [Formatter] to reconstruct the original
    80  // directive triggering the call to Format.
    81  func FormatString(state State, verb rune) string {
    82  	var tmp [16]byte // Use a local buffer.
    83  	b := append(tmp[:0], '%')
    84  	for _, c := range " +-#0" { // All known flags
    85  		if state.Flag(int(c)) { // The argument is an int for historical reasons.
    86  			b = append(b, byte(c))
    87  		}
    88  	}
    89  	if w, ok := state.Width(); ok {
    90  		b = strconv.AppendInt(b, int64(w), 10)
    91  	}
    92  	if p, ok := state.Precision(); ok {
    93  		b = append(b, '.')
    94  		b = strconv.AppendInt(b, int64(p), 10)
    95  	}
    96  	b = utf8.AppendRune(b, verb)
    97  	return string(b)
    98  }
    99  
   100  // Use simple []byte instead of bytes.Buffer to avoid large dependency.
   101  type buffer []byte
   102  
   103  func (b *buffer) write(p []byte) {
   104  	*b = append(*b, p...)
   105  }
   106  
   107  func (b *buffer) writeString(s string) {
   108  	*b = append(*b, s...)
   109  }
   110  
   111  func (b *buffer) writeByte(c byte) {
   112  	*b = append(*b, c)
   113  }
   114  
   115  func (b *buffer) writeRune(r rune) {
   116  	*b = utf8.AppendRune(*b, r)
   117  }
   118  
   119  // pp is used to store a printer's state and is reused with sync.Pool to avoid allocations.
   120  type pp struct {
   121  	buf buffer
   122  
   123  	// arg holds the current item, as an interface{}.
   124  	arg any
   125  
   126  	// value is used instead of arg for reflect values.
   127  	value reflect.Value
   128  
   129  	// fmt is used to format basic items such as integers or strings.
   130  	fmt fmt
   131  
   132  	// reordered records whether the format string used argument reordering.
   133  	reordered bool
   134  	// goodArgNum records whether the most recent reordering directive was valid.
   135  	goodArgNum bool
   136  	// panicking is set by catchPanic to avoid infinite panic, recover, panic, ... recursion.
   137  	panicking bool
   138  	// erroring is set when printing an error string to guard against calling handleMethods.
   139  	erroring bool
   140  	// wrapErrs is set when the format string may contain a %w verb.
   141  	wrapErrs bool
   142  	// wrappedErrs records the targets of the %w verb.
   143  	wrappedErrs []int
   144  }
   145  
   146  var ppFree = sync.Pool{
   147  	New: func() any { return new(pp) },
   148  }
   149  
   150  // newPrinter allocates a new pp struct or grabs a cached one.
   151  func newPrinter() *pp {
   152  	p := ppFree.Get().(*pp)
   153  	p.panicking = false
   154  	p.erroring = false
   155  	p.wrapErrs = false
   156  	p.fmt.init(&p.buf)
   157  	return p
   158  }
   159  
   160  // free saves used pp structs in ppFree; avoids an allocation per invocation.
   161  func (p *pp) free() {
   162  	// Proper usage of a sync.Pool requires each entry to have approximately
   163  	// the same memory cost. To obtain this property when the stored type
   164  	// contains a variably-sized buffer, we add a hard limit on the maximum
   165  	// buffer to place back in the pool. If the buffer is larger than the
   166  	// limit, we drop the buffer and recycle just the printer.
   167  	//
   168  	// See https://golang.org/issue/23199.
   169  	if cap(p.buf) > 64*1024 {
   170  		p.buf = nil
   171  	} else {
   172  		p.buf = p.buf[:0]
   173  	}
   174  	if cap(p.wrappedErrs) > 8 {
   175  		p.wrappedErrs = nil
   176  	}
   177  
   178  	p.arg = nil
   179  	p.value = reflect.Value{}
   180  	p.wrappedErrs = p.wrappedErrs[:0]
   181  	ppFree.Put(p)
   182  }
   183  
   184  func (p *pp) Width() (wid int, ok bool) { return p.fmt.wid, p.fmt.widPresent }
   185  
   186  func (p *pp) Precision() (prec int, ok bool) { return p.fmt.prec, p.fmt.precPresent }
   187  
   188  func (p *pp) Flag(b int) bool {
   189  	switch b {
   190  	case '-':
   191  		return p.fmt.minus
   192  	case '+':
   193  		return p.fmt.plus || p.fmt.plusV
   194  	case '#':
   195  		return p.fmt.sharp || p.fmt.sharpV
   196  	case ' ':
   197  		return p.fmt.space
   198  	case '0':
   199  		return p.fmt.zero
   200  	}
   201  	return false
   202  }
   203  
   204  // Write implements [io.Writer] so we can call [Fprintf] on a pp (through [State]), for
   205  // recursive use in custom verbs.
   206  func (p *pp) Write(b []byte) (ret int, err error) {
   207  	p.buf.write(b)
   208  	return len(b), nil
   209  }
   210  
   211  // WriteString implements [io.StringWriter] so that we can call [io.WriteString]
   212  // on a pp (through state), for efficiency.
   213  func (p *pp) WriteString(s string) (ret int, err error) {
   214  	p.buf.writeString(s)
   215  	return len(s), nil
   216  }
   217  
   218  // These routines end in 'f' and take a format string.
   219  
   220  // Fprintf formats according to a format specifier and writes to w.
   221  // It returns the number of bytes written and any write error encountered.
   222  func Fprintf(w io.Writer, format string, a ...any) (n int, err error) {
   223  	p := newPrinter()
   224  	p.doPrintf(format, a)
   225  	n, err = w.Write(p.buf)
   226  	p.free()
   227  	return
   228  }
   229  
   230  // Printf formats according to a format specifier and writes to standard output.
   231  // It returns the number of bytes written and any write error encountered.
   232  func Printf(format string, a ...any) (n int, err error) {
   233  	return Fprintf(os.Stdout, format, a...)
   234  }
   235  
   236  // Sprintf formats according to a format specifier and returns the resulting string.
   237  func Sprintf(format string, a ...any) string {
   238  	p := newPrinter()
   239  	p.doPrintf(format, a)
   240  	s := string(p.buf)
   241  	p.free()
   242  	return s
   243  }
   244  
   245  // Appendf formats according to a format specifier, appends the result to the byte
   246  // slice, and returns the updated slice.
   247  func Appendf(b []byte, format string, a ...any) []byte {
   248  	p := newPrinter()
   249  	p.doPrintf(format, a)
   250  	b = append(b, p.buf...)
   251  	p.free()
   252  	return b
   253  }
   254  
   255  // These routines do not take a format string
   256  
   257  // Fprint formats using the default formats for its operands and writes to w.
   258  // Spaces are added between operands when neither is a string.
   259  // It returns the number of bytes written and any write error encountered.
   260  func Fprint(w io.Writer, a ...any) (n int, err error) {
   261  	p := newPrinter()
   262  	p.doPrint(a)
   263  	n, err = w.Write(p.buf)
   264  	p.free()
   265  	return
   266  }
   267  
   268  // Print formats using the default formats for its operands and writes to standard output.
   269  // Spaces are added between operands when neither is a string.
   270  // It returns the number of bytes written and any write error encountered.
   271  func Print(a ...any) (n int, err error) {
   272  	return Fprint(os.Stdout, a...)
   273  }
   274  
   275  // Sprint formats using the default formats for its operands and returns the resulting string.
   276  // Spaces are added between operands when neither is a string.
   277  func Sprint(a ...any) string {
   278  	p := newPrinter()
   279  	p.doPrint(a)
   280  	s := string(p.buf)
   281  	p.free()
   282  	return s
   283  }
   284  
   285  // Append formats using the default formats for its operands, appends the result to
   286  // the byte slice, and returns the updated slice.
   287  func Append(b []byte, a ...any) []byte {
   288  	p := newPrinter()
   289  	p.doPrint(a)
   290  	b = append(b, p.buf...)
   291  	p.free()
   292  	return b
   293  }
   294  
   295  // These routines end in 'ln', do not take a format string,
   296  // always add spaces between operands, and add a newline
   297  // after the last operand.
   298  
   299  // Fprintln formats using the default formats for its operands and writes to w.
   300  // Spaces are always added between operands and a newline is appended.
   301  // It returns the number of bytes written and any write error encountered.
   302  func Fprintln(w io.Writer, a ...any) (n int, err error) {
   303  	p := newPrinter()
   304  	p.doPrintln(a)
   305  	n, err = w.Write(p.buf)
   306  	p.free()
   307  	return
   308  }
   309  
   310  // Println formats using the default formats for its operands and writes to standard output.
   311  // Spaces are always added between operands and a newline is appended.
   312  // It returns the number of bytes written and any write error encountered.
   313  func Println(a ...any) (n int, err error) {
   314  	return Fprintln(os.Stdout, a...)
   315  }
   316  
   317  // Sprintln formats using the default formats for its operands and returns the resulting string.
   318  // Spaces are always added between operands and a newline is appended.
   319  func Sprintln(a ...any) string {
   320  	p := newPrinter()
   321  	p.doPrintln(a)
   322  	s := string(p.buf)
   323  	p.free()
   324  	return s
   325  }
   326  
   327  // Appendln formats using the default formats for its operands, appends the result
   328  // to the byte slice, and returns the updated slice. Spaces are always added
   329  // between operands and a newline is appended.
   330  func Appendln(b []byte, a ...any) []byte {
   331  	p := newPrinter()
   332  	p.doPrintln(a)
   333  	b = append(b, p.buf...)
   334  	p.free()
   335  	return b
   336  }
   337  
   338  // getField gets the i'th field of the struct value.
   339  // If the field itself is a non-nil interface, return a value for
   340  // the thing inside the interface, not the interface itself.
   341  func getField(v reflect.Value, i int) reflect.Value {
   342  	val := v.Field(i)
   343  	if val.Kind() == reflect.Interface && !val.IsNil() {
   344  		val = val.Elem()
   345  	}
   346  	return val
   347  }
   348  
   349  // tooLarge reports whether the magnitude of the integer is
   350  // too large to be used as a formatting width or precision.
   351  func tooLarge(x int) bool {
   352  	const max int = 1e6
   353  	return x > max || x < -max
   354  }
   355  
   356  // parsenum converts ASCII to integer.  num is 0 (and isnum is false) if no number present.
   357  func parsenum(s string, start, end int) (num int, isnum bool, newi int) {
   358  	if start >= end {
   359  		return 0, false, end
   360  	}
   361  	for newi = start; newi < end && '0' <= s[newi] && s[newi] <= '9'; newi++ {
   362  		if tooLarge(num) {
   363  			return 0, false, end // Overflow; crazy long number most likely.
   364  		}
   365  		num = num*10 + int(s[newi]-'0')
   366  		isnum = true
   367  	}
   368  	return
   369  }
   370  
   371  func (p *pp) unknownType(v reflect.Value) {
   372  	if !v.IsValid() {
   373  		p.buf.writeString(nilAngleString)
   374  		return
   375  	}
   376  	p.buf.writeByte('?')
   377  	p.buf.writeString(v.Type().String())
   378  	p.buf.writeByte('?')
   379  }
   380  
   381  func (p *pp) badVerb(verb rune) {
   382  	p.erroring = true
   383  	p.buf.writeString(percentBangString)
   384  	p.buf.writeRune(verb)
   385  	p.buf.writeByte('(')
   386  	switch {
   387  	case p.arg != nil:
   388  		p.buf.writeString(reflect.TypeOf(p.arg).String())
   389  		p.buf.writeByte('=')
   390  		p.printArg(p.arg, 'v')
   391  	case p.value.IsValid():
   392  		p.buf.writeString(p.value.Type().String())
   393  		p.buf.writeByte('=')
   394  		p.printValue(p.value, 'v', 0)
   395  	default:
   396  		p.buf.writeString(nilAngleString)
   397  	}
   398  	p.buf.writeByte(')')
   399  	p.erroring = false
   400  }
   401  
   402  func (p *pp) fmtBool(v bool, verb rune) {
   403  	switch verb {
   404  	case 't', 'v':
   405  		p.fmt.fmtBoolean(v)
   406  	default:
   407  		p.badVerb(verb)
   408  	}
   409  }
   410  
   411  // fmt0x64 formats a uint64 in hexadecimal and prefixes it with 0x or
   412  // not, as requested, by temporarily setting the sharp flag.
   413  func (p *pp) fmt0x64(v uint64, leading0x bool) {
   414  	sharp := p.fmt.sharp
   415  	p.fmt.sharp = leading0x
   416  	p.fmt.fmtInteger(v, 16, unsigned, 'v', ldigits)
   417  	p.fmt.sharp = sharp
   418  }
   419  
   420  // fmtInteger formats a signed or unsigned integer.
   421  func (p *pp) fmtInteger(v uint64, isSigned bool, verb rune) {
   422  	switch verb {
   423  	case 'v':
   424  		if p.fmt.sharpV && !isSigned {
   425  			p.fmt0x64(v, true)
   426  		} else {
   427  			p.fmt.fmtInteger(v, 10, isSigned, verb, ldigits)
   428  		}
   429  	case 'd':
   430  		p.fmt.fmtInteger(v, 10, isSigned, verb, ldigits)
   431  	case 'b':
   432  		p.fmt.fmtInteger(v, 2, isSigned, verb, ldigits)
   433  	case 'o', 'O':
   434  		p.fmt.fmtInteger(v, 8, isSigned, verb, ldigits)
   435  	case 'x':
   436  		p.fmt.fmtInteger(v, 16, isSigned, verb, ldigits)
   437  	case 'X':
   438  		p.fmt.fmtInteger(v, 16, isSigned, verb, udigits)
   439  	case 'c':
   440  		p.fmt.fmtC(v)
   441  	case 'q':
   442  		p.fmt.fmtQc(v)
   443  	case 'U':
   444  		p.fmt.fmtUnicode(v)
   445  	default:
   446  		p.badVerb(verb)
   447  	}
   448  }
   449  
   450  // fmtFloat formats a float. The default precision for each verb
   451  // is specified as last argument in the call to fmt_float.
   452  func (p *pp) fmtFloat(v float64, size int, verb rune) {
   453  	switch verb {
   454  	case 'v':
   455  		p.fmt.fmtFloat(v, size, 'g', -1)
   456  	case 'b', 'g', 'G', 'x', 'X':
   457  		p.fmt.fmtFloat(v, size, verb, -1)
   458  	case 'f', 'e', 'E':
   459  		p.fmt.fmtFloat(v, size, verb, 6)
   460  	case 'F':
   461  		p.fmt.fmtFloat(v, size, 'f', 6)
   462  	default:
   463  		p.badVerb(verb)
   464  	}
   465  }
   466  
   467  // fmtComplex formats a complex number v with
   468  // r = real(v) and j = imag(v) as (r+ji) using
   469  // fmtFloat for r and j formatting.
   470  func (p *pp) fmtComplex(v complex128, size int, verb rune) {
   471  	// Make sure any unsupported verbs are found before the
   472  	// calls to fmtFloat to not generate an incorrect error string.
   473  	switch verb {
   474  	case 'v', 'b', 'g', 'G', 'x', 'X', 'f', 'F', 'e', 'E':
   475  		oldPlus := p.fmt.plus
   476  		p.buf.writeByte('(')
   477  		p.fmtFloat(real(v), size/2, verb)
   478  		// Imaginary part always has a sign.
   479  		p.fmt.plus = true
   480  		p.fmtFloat(imag(v), size/2, verb)
   481  		p.buf.writeString("i)")
   482  		p.fmt.plus = oldPlus
   483  	default:
   484  		p.badVerb(verb)
   485  	}
   486  }
   487  
   488  func (p *pp) fmtString(v string, verb rune) {
   489  	switch verb {
   490  	case 'v':
   491  		if p.fmt.sharpV {
   492  			p.fmt.fmtQ(v)
   493  		} else {
   494  			p.fmt.fmtS(v)
   495  		}
   496  	case 's':
   497  		p.fmt.fmtS(v)
   498  	case 'x':
   499  		p.fmt.fmtSx(v, ldigits)
   500  	case 'X':
   501  		p.fmt.fmtSx(v, udigits)
   502  	case 'q':
   503  		p.fmt.fmtQ(v)
   504  	default:
   505  		p.badVerb(verb)
   506  	}
   507  }
   508  
   509  func (p *pp) fmtBytes(v []byte, verb rune, typeString string) {
   510  	switch verb {
   511  	case 'v', 'd':
   512  		if p.fmt.sharpV {
   513  			p.buf.writeString(typeString)
   514  			if v == nil {
   515  				p.buf.writeString(nilParenString)
   516  				return
   517  			}
   518  			p.buf.writeByte('{')
   519  			for i, c := range v {
   520  				if i > 0 {
   521  					p.buf.writeString(commaSpaceString)
   522  				}
   523  				p.fmt0x64(uint64(c), true)
   524  			}
   525  			p.buf.writeByte('}')
   526  		} else {
   527  			p.buf.writeByte('[')
   528  			for i, c := range v {
   529  				if i > 0 {
   530  					p.buf.writeByte(' ')
   531  				}
   532  				p.fmt.fmtInteger(uint64(c), 10, unsigned, verb, ldigits)
   533  			}
   534  			p.buf.writeByte(']')
   535  		}
   536  	case 's':
   537  		p.fmt.fmtBs(v)
   538  	case 'x':
   539  		p.fmt.fmtBx(v, ldigits)
   540  	case 'X':
   541  		p.fmt.fmtBx(v, udigits)
   542  	case 'q':
   543  		p.fmt.fmtQ(string(v))
   544  	default:
   545  		p.printValue(reflect.ValueOf(v), verb, 0)
   546  	}
   547  }
   548  
   549  func (p *pp) fmtPointer(value reflect.Value, verb rune) {
   550  	var u uintptr
   551  	switch value.Kind() {
   552  	case reflect.Chan, reflect.Func, reflect.Map, reflect.Pointer, reflect.Slice, reflect.UnsafePointer:
   553  		u = uintptr(value.UnsafePointer())
   554  	default:
   555  		p.badVerb(verb)
   556  		return
   557  	}
   558  
   559  	switch verb {
   560  	case 'v':
   561  		if p.fmt.sharpV {
   562  			p.buf.writeByte('(')
   563  			p.buf.writeString(value.Type().String())
   564  			p.buf.writeString(")(")
   565  			if u == 0 {
   566  				p.buf.writeString(nilString)
   567  			} else {
   568  				p.fmt0x64(uint64(u), true)
   569  			}
   570  			p.buf.writeByte(')')
   571  		} else {
   572  			if u == 0 {
   573  				p.fmt.padString(nilAngleString)
   574  			} else {
   575  				p.fmt0x64(uint64(u), !p.fmt.sharp)
   576  			}
   577  		}
   578  	case 'p':
   579  		p.fmt0x64(uint64(u), !p.fmt.sharp)
   580  	case 'b', 'o', 'd', 'x', 'X':
   581  		p.fmtInteger(uint64(u), unsigned, verb)
   582  	default:
   583  		p.badVerb(verb)
   584  	}
   585  }
   586  
   587  func (p *pp) catchPanic(arg any, verb rune, method string) {
   588  	if err := recover(); err != nil {
   589  		// If it's a nil pointer, just say "<nil>". The likeliest causes are a
   590  		// Stringer that fails to guard against nil or a nil pointer for a
   591  		// value receiver, and in either case, "<nil>" is a nice result.
   592  		if v := reflect.ValueOf(arg); v.Kind() == reflect.Pointer && v.IsNil() {
   593  			p.buf.writeString(nilAngleString)
   594  			return
   595  		}
   596  		// Otherwise print a concise panic message. Most of the time the panic
   597  		// value will print itself nicely.
   598  		if p.panicking {
   599  			// Nested panics; the recursion in printArg cannot succeed.
   600  			panic(err)
   601  		}
   602  
   603  		oldFlags := p.fmt.fmtFlags
   604  		// For this output we want default behavior.
   605  		p.fmt.clearflags()
   606  
   607  		p.buf.writeString(percentBangString)
   608  		p.buf.writeRune(verb)
   609  		p.buf.writeString(panicString)
   610  		p.buf.writeString(method)
   611  		p.buf.writeString(" method: ")
   612  		p.panicking = true
   613  		p.printArg(err, 'v')
   614  		p.panicking = false
   615  		p.buf.writeByte(')')
   616  
   617  		p.fmt.fmtFlags = oldFlags
   618  	}
   619  }
   620  
   621  func (p *pp) handleMethods(verb rune) (handled bool) {
   622  	if p.erroring {
   623  		return
   624  	}
   625  	if verb == 'w' {
   626  		// It is invalid to use %w other than with Errorf or with a non-error arg.
   627  		_, ok := p.arg.(error)
   628  		if !ok || !p.wrapErrs {
   629  			p.badVerb(verb)
   630  			return true
   631  		}
   632  		// If the arg is a Formatter, pass 'v' as the verb to it.
   633  		verb = 'v'
   634  	}
   635  
   636  	// Is it a Formatter?
   637  	if formatter, ok := p.arg.(Formatter); ok {
   638  		handled = true
   639  		defer p.catchPanic(p.arg, verb, "Format")
   640  		formatter.Format(p, verb)
   641  		return
   642  	}
   643  
   644  	// If we're doing Go syntax and the argument knows how to supply it, take care of it now.
   645  	if p.fmt.sharpV {
   646  		if stringer, ok := p.arg.(GoStringer); ok {
   647  			handled = true
   648  			defer p.catchPanic(p.arg, verb, "GoString")
   649  			// Print the result of GoString unadorned.
   650  			p.fmt.fmtS(stringer.GoString())
   651  			return
   652  		}
   653  	} else {
   654  		// If a string is acceptable according to the format, see if
   655  		// the value satisfies one of the string-valued interfaces.
   656  		// Println etc. set verb to %v, which is "stringable".
   657  		switch verb {
   658  		case 'v', 's', 'x', 'X', 'q':
   659  			// Is it an error or Stringer?
   660  			// The duplication in the bodies is necessary:
   661  			// setting handled and deferring catchPanic
   662  			// must happen before calling the method.
   663  			switch v := p.arg.(type) {
   664  			case error:
   665  				handled = true
   666  				defer p.catchPanic(p.arg, verb, "Error")
   667  				p.fmtString(v.Error(), verb)
   668  				return
   669  
   670  			case Stringer:
   671  				handled = true
   672  				defer p.catchPanic(p.arg, verb, "String")
   673  				p.fmtString(v.String(), verb)
   674  				return
   675  			}
   676  		}
   677  	}
   678  	return false
   679  }
   680  
   681  func (p *pp) printArg(arg any, verb rune) {
   682  	p.arg = arg
   683  	p.value = reflect.Value{}
   684  
   685  	if arg == nil {
   686  		switch verb {
   687  		case 'T', 'v':
   688  			p.fmt.padString(nilAngleString)
   689  		default:
   690  			p.badVerb(verb)
   691  		}
   692  		return
   693  	}
   694  
   695  	// Special processing considerations.
   696  	// %T (the value's type) and %p (its address) are special; we always do them first.
   697  	switch verb {
   698  	case 'T':
   699  		p.fmt.fmtS(reflect.TypeOf(arg).String())
   700  		return
   701  	case 'p':
   702  		p.fmtPointer(reflect.ValueOf(arg), 'p')
   703  		return
   704  	}
   705  
   706  	// Some types can be done without reflection.
   707  	switch f := arg.(type) {
   708  	case bool:
   709  		p.fmtBool(f, verb)
   710  	case float32:
   711  		p.fmtFloat(float64(f), 32, verb)
   712  	case float64:
   713  		p.fmtFloat(f, 64, verb)
   714  	case complex64:
   715  		p.fmtComplex(complex128(f), 64, verb)
   716  	case complex128:
   717  		p.fmtComplex(f, 128, verb)
   718  	case int:
   719  		p.fmtInteger(uint64(f), signed, verb)
   720  	case int8:
   721  		p.fmtInteger(uint64(f), signed, verb)
   722  	case int16:
   723  		p.fmtInteger(uint64(f), signed, verb)
   724  	case int32:
   725  		p.fmtInteger(uint64(f), signed, verb)
   726  	case int64:
   727  		p.fmtInteger(uint64(f), signed, verb)
   728  	case uint:
   729  		p.fmtInteger(uint64(f), unsigned, verb)
   730  	case uint8:
   731  		p.fmtInteger(uint64(f), unsigned, verb)
   732  	case uint16:
   733  		p.fmtInteger(uint64(f), unsigned, verb)
   734  	case uint32:
   735  		p.fmtInteger(uint64(f), unsigned, verb)
   736  	case uint64:
   737  		p.fmtInteger(f, unsigned, verb)
   738  	case uintptr:
   739  		p.fmtInteger(uint64(f), unsigned, verb)
   740  	case string:
   741  		p.fmtString(f, verb)
   742  	case []byte:
   743  		p.fmtBytes(f, verb, "[]byte")
   744  	case reflect.Value:
   745  		// Handle extractable values with special methods
   746  		// since printValue does not handle them at depth 0.
   747  		if f.IsValid() && f.CanInterface() {
   748  			p.arg = f.Interface()
   749  			if p.handleMethods(verb) {
   750  				return
   751  			}
   752  		}
   753  		p.printValue(f, verb, 0)
   754  	default:
   755  		// If the type is not simple, it might have methods.
   756  		if !p.handleMethods(verb) {
   757  			// Need to use reflection, since the type had no
   758  			// interface methods that could be used for formatting.
   759  			p.printValue(reflect.ValueOf(f), verb, 0)
   760  		}
   761  	}
   762  }
   763  
   764  // printValue is similar to printArg but starts with a reflect value, not an interface{} value.
   765  // It does not handle 'p' and 'T' verbs because these should have been already handled by printArg.
   766  func (p *pp) printValue(value reflect.Value, verb rune, depth int) {
   767  	// Handle values with special methods if not already handled by printArg (depth == 0).
   768  	if depth > 0 && value.IsValid() && value.CanInterface() {
   769  		p.arg = value.Interface()
   770  		if p.handleMethods(verb) {
   771  			return
   772  		}
   773  	}
   774  	p.arg = nil
   775  	p.value = value
   776  
   777  	switch f := value; value.Kind() {
   778  	case reflect.Invalid:
   779  		if depth == 0 {
   780  			p.buf.writeString(invReflectString)
   781  		} else {
   782  			switch verb {
   783  			case 'v':
   784  				p.buf.writeString(nilAngleString)
   785  			default:
   786  				p.badVerb(verb)
   787  			}
   788  		}
   789  	case reflect.Bool:
   790  		p.fmtBool(f.Bool(), verb)
   791  	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   792  		p.fmtInteger(uint64(f.Int()), signed, verb)
   793  	case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   794  		p.fmtInteger(f.Uint(), unsigned, verb)
   795  	case reflect.Float32:
   796  		p.fmtFloat(f.Float(), 32, verb)
   797  	case reflect.Float64:
   798  		p.fmtFloat(f.Float(), 64, verb)
   799  	case reflect.Complex64:
   800  		p.fmtComplex(f.Complex(), 64, verb)
   801  	case reflect.Complex128:
   802  		p.fmtComplex(f.Complex(), 128, verb)
   803  	case reflect.String:
   804  		p.fmtString(f.String(), verb)
   805  	case reflect.Map:
   806  		if p.fmt.sharpV {
   807  			p.buf.writeString(f.Type().String())
   808  			if f.IsNil() {
   809  				p.buf.writeString(nilParenString)
   810  				return
   811  			}
   812  			p.buf.writeByte('{')
   813  		} else {
   814  			p.buf.writeString(mapString)
   815  		}
   816  		sorted := fmtsort.Sort(f)
   817  		for i, m := range sorted {
   818  			if i > 0 {
   819  				if p.fmt.sharpV {
   820  					p.buf.writeString(commaSpaceString)
   821  				} else {
   822  					p.buf.writeByte(' ')
   823  				}
   824  			}
   825  			p.printValue(m.Key, verb, depth+1)
   826  			p.buf.writeByte(':')
   827  			p.printValue(m.Value, verb, depth+1)
   828  		}
   829  		if p.fmt.sharpV {
   830  			p.buf.writeByte('}')
   831  		} else {
   832  			p.buf.writeByte(']')
   833  		}
   834  	case reflect.Struct:
   835  		if p.fmt.sharpV {
   836  			p.buf.writeString(f.Type().String())
   837  		}
   838  		p.buf.writeByte('{')
   839  		for i := 0; i < f.NumField(); i++ {
   840  			if i > 0 {
   841  				if p.fmt.sharpV {
   842  					p.buf.writeString(commaSpaceString)
   843  				} else {
   844  					p.buf.writeByte(' ')
   845  				}
   846  			}
   847  			if p.fmt.plusV || p.fmt.sharpV {
   848  				if name := f.Type().Field(i).Name; name != "" {
   849  					p.buf.writeString(name)
   850  					p.buf.writeByte(':')
   851  				}
   852  			}
   853  			p.printValue(getField(f, i), verb, depth+1)
   854  		}
   855  		p.buf.writeByte('}')
   856  	case reflect.Interface:
   857  		value := f.Elem()
   858  		if !value.IsValid() {
   859  			if p.fmt.sharpV {
   860  				p.buf.writeString(f.Type().String())
   861  				p.buf.writeString(nilParenString)
   862  			} else {
   863  				p.buf.writeString(nilAngleString)
   864  			}
   865  		} else {
   866  			p.printValue(value, verb, depth+1)
   867  		}
   868  	case reflect.Array, reflect.Slice:
   869  		switch verb {
   870  		case 's', 'q', 'x', 'X':
   871  			// Handle byte and uint8 slices and arrays special for the above verbs.
   872  			t := f.Type()
   873  			if t.Elem().Kind() == reflect.Uint8 {
   874  				var bytes []byte
   875  				if f.Kind() == reflect.Slice || f.CanAddr() {
   876  					bytes = f.Bytes()
   877  				} else {
   878  					// We have an array, but we cannot Bytes() a non-addressable array,
   879  					// so we build a slice by hand. This is a rare case but it would be nice
   880  					// if reflection could help a little more.
   881  					bytes = make([]byte, f.Len())
   882  					for i := range bytes {
   883  						bytes[i] = byte(f.Index(i).Uint())
   884  					}
   885  				}
   886  				p.fmtBytes(bytes, verb, t.String())
   887  				return
   888  			}
   889  		}
   890  		if p.fmt.sharpV {
   891  			p.buf.writeString(f.Type().String())
   892  			if f.Kind() == reflect.Slice && f.IsNil() {
   893  				p.buf.writeString(nilParenString)
   894  				return
   895  			}
   896  			p.buf.writeByte('{')
   897  			for i := 0; i < f.Len(); i++ {
   898  				if i > 0 {
   899  					p.buf.writeString(commaSpaceString)
   900  				}
   901  				p.printValue(f.Index(i), verb, depth+1)
   902  			}
   903  			p.buf.writeByte('}')
   904  		} else {
   905  			p.buf.writeByte('[')
   906  			for i := 0; i < f.Len(); i++ {
   907  				if i > 0 {
   908  					p.buf.writeByte(' ')
   909  				}
   910  				p.printValue(f.Index(i), verb, depth+1)
   911  			}
   912  			p.buf.writeByte(']')
   913  		}
   914  	case reflect.Pointer:
   915  		// pointer to array or slice or struct? ok at top level
   916  		// but not embedded (avoid loops)
   917  		if depth == 0 && f.UnsafePointer() != nil {
   918  			switch a := f.Elem(); a.Kind() {
   919  			case reflect.Array, reflect.Slice, reflect.Struct, reflect.Map:
   920  				p.buf.writeByte('&')
   921  				p.printValue(a, verb, depth+1)
   922  				return
   923  			}
   924  		}
   925  		fallthrough
   926  	case reflect.Chan, reflect.Func, reflect.UnsafePointer:
   927  		p.fmtPointer(f, verb)
   928  	default:
   929  		p.unknownType(f)
   930  	}
   931  }
   932  
   933  // intFromArg gets the argNumth element of a. On return, isInt reports whether the argument has integer type.
   934  func intFromArg(a []any, argNum int) (num int, isInt bool, newArgNum int) {
   935  	newArgNum = argNum
   936  	if argNum < len(a) {
   937  		num, isInt = a[argNum].(int) // Almost always OK.
   938  		if !isInt {
   939  			// Work harder.
   940  			switch v := reflect.ValueOf(a[argNum]); v.Kind() {
   941  			case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
   942  				n := v.Int()
   943  				if int64(int(n)) == n {
   944  					num = int(n)
   945  					isInt = true
   946  				}
   947  			case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
   948  				n := v.Uint()
   949  				if int64(n) >= 0 && uint64(int(n)) == n {
   950  					num = int(n)
   951  					isInt = true
   952  				}
   953  			default:
   954  				// Already 0, false.
   955  			}
   956  		}
   957  		newArgNum = argNum + 1
   958  		if tooLarge(num) {
   959  			num = 0
   960  			isInt = false
   961  		}
   962  	}
   963  	return
   964  }
   965  
   966  // parseArgNumber returns the value of the bracketed number, minus 1
   967  // (explicit argument numbers are one-indexed but we want zero-indexed).
   968  // The opening bracket is known to be present at format[0].
   969  // The returned values are the index, the number of bytes to consume
   970  // up to the closing paren, if present, and whether the number parsed
   971  // ok. The bytes to consume will be 1 if no closing paren is present.
   972  func parseArgNumber(format string) (index int, wid int, ok bool) {
   973  	// There must be at least 3 bytes: [n].
   974  	if len(format) < 3 {
   975  		return 0, 1, false
   976  	}
   977  
   978  	// Find closing bracket.
   979  	for i := 1; i < len(format); i++ {
   980  		if format[i] == ']' {
   981  			width, ok, newi := parsenum(format, 1, i)
   982  			if !ok || newi != i {
   983  				return 0, i + 1, false
   984  			}
   985  			return width - 1, i + 1, true // arg numbers are one-indexed and skip paren.
   986  		}
   987  	}
   988  	return 0, 1, false
   989  }
   990  
   991  // argNumber returns the next argument to evaluate, which is either the value of the passed-in
   992  // argNum or the value of the bracketed integer that begins format[i:]. It also returns
   993  // the new value of i, that is, the index of the next byte of the format to process.
   994  func (p *pp) argNumber(argNum int, format string, i int, numArgs int) (newArgNum, newi int, found bool) {
   995  	if len(format) <= i || format[i] != '[' {
   996  		return argNum, i, false
   997  	}
   998  	p.reordered = true
   999  	index, wid, ok := parseArgNumber(format[i:])
  1000  	if ok && 0 <= index && index < numArgs {
  1001  		return index, i + wid, true
  1002  	}
  1003  	p.goodArgNum = false
  1004  	return argNum, i + wid, ok
  1005  }
  1006  
  1007  func (p *pp) badArgNum(verb rune) {
  1008  	p.buf.writeString(percentBangString)
  1009  	p.buf.writeRune(verb)
  1010  	p.buf.writeString(badIndexString)
  1011  }
  1012  
  1013  func (p *pp) missingArg(verb rune) {
  1014  	p.buf.writeString(percentBangString)
  1015  	p.buf.writeRune(verb)
  1016  	p.buf.writeString(missingString)
  1017  }
  1018  
  1019  func (p *pp) doPrintf(format string, a []any) {
  1020  	end := len(format)
  1021  	argNum := 0         // we process one argument per non-trivial format
  1022  	afterIndex := false // previous item in format was an index like [3].
  1023  	p.reordered = false
  1024  formatLoop:
  1025  	for i := 0; i < end; {
  1026  		p.goodArgNum = true
  1027  		lasti := i
  1028  		for i < end && format[i] != '%' {
  1029  			i++
  1030  		}
  1031  		if i > lasti {
  1032  			p.buf.writeString(format[lasti:i])
  1033  		}
  1034  		if i >= end {
  1035  			// done processing format string
  1036  			break
  1037  		}
  1038  
  1039  		// Process one verb
  1040  		i++
  1041  
  1042  		// Do we have flags?
  1043  		p.fmt.clearflags()
  1044  	simpleFormat:
  1045  		for ; i < end; i++ {
  1046  			c := format[i]
  1047  			switch c {
  1048  			case '#':
  1049  				p.fmt.sharp = true
  1050  			case '0':
  1051  				p.fmt.zero = true
  1052  			case '+':
  1053  				p.fmt.plus = true
  1054  			case '-':
  1055  				p.fmt.minus = true
  1056  			case ' ':
  1057  				p.fmt.space = true
  1058  			default:
  1059  				// Fast path for common case of ascii lower case simple verbs
  1060  				// without precision or width or argument indices.
  1061  				if 'a' <= c && c <= 'z' && argNum < len(a) {
  1062  					switch c {
  1063  					case 'w':
  1064  						p.wrappedErrs = append(p.wrappedErrs, argNum)
  1065  						fallthrough
  1066  					case 'v':
  1067  						// Go syntax
  1068  						p.fmt.sharpV = p.fmt.sharp
  1069  						p.fmt.sharp = false
  1070  						// Struct-field syntax
  1071  						p.fmt.plusV = p.fmt.plus
  1072  						p.fmt.plus = false
  1073  					}
  1074  					p.printArg(a[argNum], rune(c))
  1075  					argNum++
  1076  					i++
  1077  					continue formatLoop
  1078  				}
  1079  				// Format is more complex than simple flags and a verb or is malformed.
  1080  				break simpleFormat
  1081  			}
  1082  		}
  1083  
  1084  		// Do we have an explicit argument index?
  1085  		argNum, i, afterIndex = p.argNumber(argNum, format, i, len(a))
  1086  
  1087  		// Do we have width?
  1088  		if i < end && format[i] == '*' {
  1089  			i++
  1090  			p.fmt.wid, p.fmt.widPresent, argNum = intFromArg(a, argNum)
  1091  
  1092  			if !p.fmt.widPresent {
  1093  				p.buf.writeString(badWidthString)
  1094  			}
  1095  
  1096  			// We have a negative width, so take its value and ensure
  1097  			// that the minus flag is set
  1098  			if p.fmt.wid < 0 {
  1099  				p.fmt.wid = -p.fmt.wid
  1100  				p.fmt.minus = true
  1101  				p.fmt.zero = false // Do not pad with zeros to the right.
  1102  			}
  1103  			afterIndex = false
  1104  		} else {
  1105  			p.fmt.wid, p.fmt.widPresent, i = parsenum(format, i, end)
  1106  			if afterIndex && p.fmt.widPresent { // "%[3]2d"
  1107  				p.goodArgNum = false
  1108  			}
  1109  		}
  1110  
  1111  		// Do we have precision?
  1112  		if i+1 < end && format[i] == '.' {
  1113  			i++
  1114  			if afterIndex { // "%[3].2d"
  1115  				p.goodArgNum = false
  1116  			}
  1117  			argNum, i, afterIndex = p.argNumber(argNum, format, i, len(a))
  1118  			if i < end && format[i] == '*' {
  1119  				i++
  1120  				p.fmt.prec, p.fmt.precPresent, argNum = intFromArg(a, argNum)
  1121  				// Negative precision arguments don't make sense
  1122  				if p.fmt.prec < 0 {
  1123  					p.fmt.prec = 0
  1124  					p.fmt.precPresent = false
  1125  				}
  1126  				if !p.fmt.precPresent {
  1127  					p.buf.writeString(badPrecString)
  1128  				}
  1129  				afterIndex = false
  1130  			} else {
  1131  				p.fmt.prec, p.fmt.precPresent, i = parsenum(format, i, end)
  1132  				if !p.fmt.precPresent {
  1133  					p.fmt.prec = 0
  1134  					p.fmt.precPresent = true
  1135  				}
  1136  			}
  1137  		}
  1138  
  1139  		if !afterIndex {
  1140  			argNum, i, afterIndex = p.argNumber(argNum, format, i, len(a))
  1141  		}
  1142  
  1143  		if i >= end {
  1144  			p.buf.writeString(noVerbString)
  1145  			break
  1146  		}
  1147  
  1148  		verb, size := rune(format[i]), 1
  1149  		if verb >= utf8.RuneSelf {
  1150  			verb, size = utf8.DecodeRuneInString(format[i:])
  1151  		}
  1152  		i += size
  1153  
  1154  		switch {
  1155  		case verb == '%': // Percent does not absorb operands and ignores f.wid and f.prec.
  1156  			p.buf.writeByte('%')
  1157  		case !p.goodArgNum:
  1158  			p.badArgNum(verb)
  1159  		case argNum >= len(a): // No argument left over to print for the current verb.
  1160  			p.missingArg(verb)
  1161  		case verb == 'w':
  1162  			p.wrappedErrs = append(p.wrappedErrs, argNum)
  1163  			fallthrough
  1164  		case verb == 'v':
  1165  			// Go syntax
  1166  			p.fmt.sharpV = p.fmt.sharp
  1167  			p.fmt.sharp = false
  1168  			// Struct-field syntax
  1169  			p.fmt.plusV = p.fmt.plus
  1170  			p.fmt.plus = false
  1171  			fallthrough
  1172  		default:
  1173  			p.printArg(a[argNum], verb)
  1174  			argNum++
  1175  		}
  1176  	}
  1177  
  1178  	// Check for extra arguments unless the call accessed the arguments
  1179  	// out of order, in which case it's too expensive to detect if they've all
  1180  	// been used and arguably OK if they're not.
  1181  	if !p.reordered && argNum < len(a) {
  1182  		p.fmt.clearflags()
  1183  		p.buf.writeString(extraString)
  1184  		for i, arg := range a[argNum:] {
  1185  			if i > 0 {
  1186  				p.buf.writeString(commaSpaceString)
  1187  			}
  1188  			if arg == nil {
  1189  				p.buf.writeString(nilAngleString)
  1190  			} else {
  1191  				p.buf.writeString(reflect.TypeOf(arg).String())
  1192  				p.buf.writeByte('=')
  1193  				p.printArg(arg, 'v')
  1194  			}
  1195  		}
  1196  		p.buf.writeByte(')')
  1197  	}
  1198  }
  1199  
  1200  func (p *pp) doPrint(a []any) {
  1201  	prevString := false
  1202  	for argNum, arg := range a {
  1203  		isString := arg != nil && reflect.TypeOf(arg).Kind() == reflect.String
  1204  		// Add a space between two non-string arguments.
  1205  		if argNum > 0 && !isString && !prevString {
  1206  			p.buf.writeByte(' ')
  1207  		}
  1208  		p.printArg(arg, 'v')
  1209  		prevString = isString
  1210  	}
  1211  }
  1212  
  1213  // doPrintln is like doPrint but always adds a space between arguments
  1214  // and a newline after the last argument.
  1215  func (p *pp) doPrintln(a []any) {
  1216  	for argNum, arg := range a {
  1217  		if argNum > 0 {
  1218  			p.buf.writeByte(' ')
  1219  		}
  1220  		p.printArg(arg, 'v')
  1221  	}
  1222  	p.buf.writeByte('\n')
  1223  }
  1224  

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