github.com/cockroachdb/cockroachdb-parser@v0.23.3-0.20240213214944-911057d40c9a/pkg/util/encoding/float.go (about)

     1  // Copyright 2014 The Cockroach Authors.
     2  //
     3  // Use of this software is governed by the Business Source License
     4  // included in the file licenses/BSL.txt.
     5  //
     6  // As of the Change Date specified in that file, in accordance with
     7  // the Business Source License, use of this software will be governed
     8  // by the Apache License, Version 2.0, included in the file
     9  // licenses/APL.txt.
    10  
    11  package encoding
    12  
    13  import (
    14  	"math"
    15  
    16  	"github.com/cockroachdb/errors"
    17  )
    18  
    19  // EncodeFloatAscending returns the resulting byte slice with the encoded float64
    20  // appended to b. The encoded format for a float64 value f is, for positive f, the
    21  // encoding of the 64 bits (in IEEE 754 format) re-interpreted as an int64 and
    22  // encoded using EncodeUint64Ascending. For negative f, we keep the sign bit and
    23  // invert all other bits, encoding this value using EncodeUint64Descending. This
    24  // approach was inspired by in github.com/google/orderedcode/orderedcode.go.
    25  //
    26  // One of five single-byte prefix tags are appended to the front of the encoding.
    27  // These tags enforce logical ordering of keys for both ascending and descending
    28  // encoding directions. The tags split the encoded floats into five categories:
    29  // - NaN for an ascending encoding direction
    30  // - Negative valued floats
    31  // - Zero (positive and negative)
    32  // - Positive valued floats
    33  // - NaN for a descending encoding direction
    34  // This ordering ensures that NaNs are always sorted first in either encoding
    35  // direction, and that after them a logical ordering is followed.
    36  func EncodeFloatAscending(b []byte, f float64) []byte {
    37  	// Handle the simplistic cases first.
    38  	switch {
    39  	case math.IsNaN(f):
    40  		return append(b, floatNaN)
    41  	case f == 0:
    42  		// This encodes both positive and negative zero the same. Negative zero uses
    43  		// composite indexes to decode itself correctly.
    44  		return append(b, floatZero)
    45  	}
    46  	u := math.Float64bits(f)
    47  	if u&(1<<63) != 0 {
    48  		u = ^u
    49  		b = append(b, floatNeg)
    50  	} else {
    51  		b = append(b, floatPos)
    52  	}
    53  	return EncodeUint64Ascending(b, u)
    54  }
    55  
    56  // EncodeFloatDescending is the descending version of EncodeFloatAscending.
    57  func EncodeFloatDescending(b []byte, f float64) []byte {
    58  	if math.IsNaN(f) {
    59  		return append(b, floatNaNDesc)
    60  	}
    61  	return EncodeFloatAscending(b, -f)
    62  }
    63  
    64  // DecodeFloatAscending returns the remaining byte slice after decoding and the decoded
    65  // float64 from buf.
    66  func DecodeFloatAscending(buf []byte) ([]byte, float64, error) {
    67  	if PeekType(buf) != Float {
    68  		return buf, 0, errors.Errorf("did not find marker")
    69  	}
    70  	switch buf[0] {
    71  	case floatNaN, floatNaNDesc:
    72  		return buf[1:], math.NaN(), nil
    73  	case floatNeg:
    74  		b, u, err := DecodeUint64Ascending(buf[1:])
    75  		if err != nil {
    76  			return b, 0, err
    77  		}
    78  		u = ^u
    79  		return b, math.Float64frombits(u), nil
    80  	case floatZero:
    81  		return buf[1:], 0, nil
    82  	case floatPos:
    83  		b, u, err := DecodeUint64Ascending(buf[1:])
    84  		if err != nil {
    85  			return b, 0, err
    86  		}
    87  		return b, math.Float64frombits(u), nil
    88  	default:
    89  		return nil, 0, errors.Errorf("unknown prefix of the encoded byte slice: %q", buf)
    90  	}
    91  }
    92  
    93  // DecodeFloatDescending decodes floats encoded with EncodeFloatDescending.
    94  func DecodeFloatDescending(buf []byte) ([]byte, float64, error) {
    95  	b, r, err := DecodeFloatAscending(buf)
    96  	if r != 0 && !math.IsNaN(r) {
    97  		// All values except for 0 and NaN were negated in EncodeFloatDescending, so
    98  		// we have to negate them back. Negative zero uses composite indexes to
    99  		// decode itself correctly.
   100  		r = -r
   101  	}
   102  	return b, r, err
   103  }