Source file src/math/big/intconv.go

Documentation: math/big

     1  // Copyright 2015 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  // This file implements int-to-string conversion functions.
     6  
     7  package big
     8  
     9  import (
    10  	"errors"
    11  	"fmt"
    12  	"io"
    13  )
    14  
    15  // Text returns the string representation of x in the given base.
    16  // Base must be between 2 and 62, inclusive. The result uses the
    17  // lower-case letters 'a' to 'z' for digit values 10 to 35, and
    18  // the upper-case letters 'A' to 'Z' for digit values 36 to 61.
    19  // No prefix (such as "0x") is added to the string. If x is a nil
    20  // pointer it returns "<nil>".
    21  func (x *Int) Text(base int) string {
    22  	if x == nil {
    23  		return "<nil>"
    24  	}
    25  	return string(x.abs.itoa(x.neg, base))
    26  }
    27  
    28  // Append appends the string representation of x, as generated by
    29  // x.Text(base), to buf and returns the extended buffer.
    30  func (x *Int) Append(buf []byte, base int) []byte {
    31  	if x == nil {
    32  		return append(buf, "<nil>"...)
    33  	}
    34  	return append(buf, x.abs.itoa(x.neg, base)...)
    35  }
    36  
    37  // String returns the decimal representation of x as generated by
    38  // x.Text(10).
    39  func (x *Int) String() string {
    40  	return x.Text(10)
    41  }
    42  
    43  // write count copies of text to s
    44  func writeMultiple(s fmt.State, text string, count int) {
    45  	if len(text) > 0 {
    46  		b := []byte(text)
    47  		for ; count > 0; count-- {
    48  			s.Write(b)
    49  		}
    50  	}
    51  }
    52  
    53  var _ fmt.Formatter = intOne // *Int must implement fmt.Formatter
    54  
    55  // Format implements fmt.Formatter. It accepts the formats
    56  // 'b' (binary), 'o' (octal with 0 prefix), 'O' (octal with 0o prefix),
    57  // 'd' (decimal), 'x' (lowercase hexadecimal), and
    58  // 'X' (uppercase hexadecimal).
    59  // Also supported are the full suite of package fmt's format
    60  // flags for integral types, including '+' and ' ' for sign
    61  // control, '#' for leading zero in octal and for hexadecimal,
    62  // a leading "0x" or "0X" for "%#x" and "%#X" respectively,
    63  // specification of minimum digits precision, output field
    64  // width, space or zero padding, and '-' for left or right
    65  // justification.
    66  //
    67  func (x *Int) Format(s fmt.State, ch rune) {
    68  	// determine base
    69  	var base int
    70  	switch ch {
    71  	case 'b':
    72  		base = 2
    73  	case 'o', 'O':
    74  		base = 8
    75  	case 'd', 's', 'v':
    76  		base = 10
    77  	case 'x', 'X':
    78  		base = 16
    79  	default:
    80  		// unknown format
    81  		fmt.Fprintf(s, "%%!%c(big.Int=%s)", ch, x.String())
    82  		return
    83  	}
    84  
    85  	if x == nil {
    86  		fmt.Fprint(s, "<nil>")
    87  		return
    88  	}
    89  
    90  	// determine sign character
    91  	sign := ""
    92  	switch {
    93  	case x.neg:
    94  		sign = "-"
    95  	case s.Flag('+'): // supersedes ' ' when both specified
    96  		sign = "+"
    97  	case s.Flag(' '):
    98  		sign = " "
    99  	}
   100  
   101  	// determine prefix characters for indicating output base
   102  	prefix := ""
   103  	if s.Flag('#') {
   104  		switch ch {
   105  		case 'b': // binary
   106  			prefix = "0b"
   107  		case 'o': // octal
   108  			prefix = "0"
   109  		case 'x': // hexadecimal
   110  			prefix = "0x"
   111  		case 'X':
   112  			prefix = "0X"
   113  		}
   114  	}
   115  	if ch == 'O' {
   116  		prefix = "0o"
   117  	}
   118  
   119  	digits := x.abs.utoa(base)
   120  	if ch == 'X' {
   121  		// faster than bytes.ToUpper
   122  		for i, d := range digits {
   123  			if 'a' <= d && d <= 'z' {
   124  				digits[i] = 'A' + (d - 'a')
   125  			}
   126  		}
   127  	}
   128  
   129  	// number of characters for the three classes of number padding
   130  	var left int  // space characters to left of digits for right justification ("%8d")
   131  	var zeros int // zero characters (actually cs[0]) as left-most digits ("%.8d")
   132  	var right int // space characters to right of digits for left justification ("%-8d")
   133  
   134  	// determine number padding from precision: the least number of digits to output
   135  	precision, precisionSet := s.Precision()
   136  	if precisionSet {
   137  		switch {
   138  		case len(digits) < precision:
   139  			zeros = precision - len(digits) // count of zero padding
   140  		case len(digits) == 1 && digits[0] == '0' && precision == 0:
   141  			return // print nothing if zero value (x == 0) and zero precision ("." or ".0")
   142  		}
   143  	}
   144  
   145  	// determine field pad from width: the least number of characters to output
   146  	length := len(sign) + len(prefix) + zeros + len(digits)
   147  	if width, widthSet := s.Width(); widthSet && length < width { // pad as specified
   148  		switch d := width - length; {
   149  		case s.Flag('-'):
   150  			// pad on the right with spaces; supersedes '0' when both specified
   151  			right = d
   152  		case s.Flag('0') && !precisionSet:
   153  			// pad with zeros unless precision also specified
   154  			zeros = d
   155  		default:
   156  			// pad on the left with spaces
   157  			left = d
   158  		}
   159  	}
   160  
   161  	// print number as [left pad][sign][prefix][zero pad][digits][right pad]
   162  	writeMultiple(s, " ", left)
   163  	writeMultiple(s, sign, 1)
   164  	writeMultiple(s, prefix, 1)
   165  	writeMultiple(s, "0", zeros)
   166  	s.Write(digits)
   167  	writeMultiple(s, " ", right)
   168  }
   169  
   170  // scan sets z to the integer value corresponding to the longest possible prefix
   171  // read from r representing a signed integer number in a given conversion base.
   172  // It returns z, the actual conversion base used, and an error, if any. In the
   173  // error case, the value of z is undefined but the returned value is nil. The
   174  // syntax follows the syntax of integer literals in Go.
   175  //
   176  // The base argument must be 0 or a value from 2 through MaxBase. If the base
   177  // is 0, the string prefix determines the actual conversion base. A prefix of
   178  // ``0b'' or ``0B'' selects base 2; a ``0'', ``0o'', or ``0O'' prefix selects
   179  // base 8, and a ``0x'' or ``0X'' prefix selects base 16. Otherwise the selected
   180  // base is 10.
   181  //
   182  func (z *Int) scan(r io.ByteScanner, base int) (*Int, int, error) {
   183  	// determine sign
   184  	neg, err := scanSign(r)
   185  	if err != nil {
   186  		return nil, 0, err
   187  	}
   188  
   189  	// determine mantissa
   190  	z.abs, base, _, err = z.abs.scan(r, base, false)
   191  	if err != nil {
   192  		return nil, base, err
   193  	}
   194  	z.neg = len(z.abs) > 0 && neg // 0 has no sign
   195  
   196  	return z, base, nil
   197  }
   198  
   199  func scanSign(r io.ByteScanner) (neg bool, err error) {
   200  	var ch byte
   201  	if ch, err = r.ReadByte(); err != nil {
   202  		return false, err
   203  	}
   204  	switch ch {
   205  	case '-':
   206  		neg = true
   207  	case '+':
   208  		// nothing to do
   209  	default:
   210  		r.UnreadByte()
   211  	}
   212  	return
   213  }
   214  
   215  // byteReader is a local wrapper around fmt.ScanState;
   216  // it implements the ByteReader interface.
   217  type byteReader struct {
   218  	fmt.ScanState
   219  }
   220  
   221  func (r byteReader) ReadByte() (byte, error) {
   222  	ch, size, err := r.ReadRune()
   223  	if size != 1 && err == nil {
   224  		err = fmt.Errorf("invalid rune %#U", ch)
   225  	}
   226  	return byte(ch), err
   227  }
   228  
   229  func (r byteReader) UnreadByte() error {
   230  	return r.UnreadRune()
   231  }
   232  
   233  var _ fmt.Scanner = intOne // *Int must implement fmt.Scanner
   234  
   235  // Scan is a support routine for fmt.Scanner; it sets z to the value of
   236  // the scanned number. It accepts the formats 'b' (binary), 'o' (octal),
   237  // 'd' (decimal), 'x' (lowercase hexadecimal), and 'X' (uppercase hexadecimal).
   238  func (z *Int) Scan(s fmt.ScanState, ch rune) error {
   239  	s.SkipSpace() // skip leading space characters
   240  	base := 0
   241  	switch ch {
   242  	case 'b':
   243  		base = 2
   244  	case 'o':
   245  		base = 8
   246  	case 'd':
   247  		base = 10
   248  	case 'x', 'X':
   249  		base = 16
   250  	case 's', 'v':
   251  		// let scan determine the base
   252  	default:
   253  		return errors.New("Int.Scan: invalid verb")
   254  	}
   255  	_, _, err := z.scan(byteReader{s}, base)
   256  	return err
   257  }
   258  

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