Daylon Wilkins
112 lines
3.7 KiB
Go
112 lines
3.7 KiB
Go
// Copyright 2023 Dolthub, Inc.
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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// Copyright 2014 The Cockroach Authors.
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//
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// Use of this software is governed by the Business Source License
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// included in the file licenses/BSL.txt.
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//
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// As of the Change Date specified in that file, in accordance with
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// the Business Source License, use of this software will be governed
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// by the Apache License, Version 2.0, included in the file
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// licenses/APL.txt.
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package encoding
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import (
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"math"
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"github.com/cockroachdb/errors"
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)
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// EncodeFloatAscending returns the resulting byte slice with the encoded float64
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// appended to b. The encoded format for a float64 value f is, for positive f, the
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// encoding of the 64 bits (in IEEE 754 format) re-interpreted as an int64 and
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// encoded using EncodeUint64Ascending. For negative f, we keep the sign bit and
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// invert all other bits, encoding this value using EncodeUint64Descending. This
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// approach was inspired by in github.com/google/orderedcode/orderedcode.go.
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//
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// One of five single-byte prefix tags are appended to the front of the encoding.
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// These tags enforce logical ordering of keys for both ascending and descending
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// encoding directions. The tags split the encoded floats into five categories:
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// - NaN for an ascending encoding direction
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// - Negative valued floats
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// - Zero (positive and negative)
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// - Positive valued floats
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// - NaN for a descending encoding direction
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// This ordering ensures that NaNs are always sorted first in either encoding
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// direction, and that after them a logical ordering is followed.
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func EncodeFloatAscending(b []byte, f float64) []byte {
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// Handle the simplistic cases first.
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switch {
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case math.IsNaN(f):
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return append(b, floatNaN)
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case f == 0:
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// This encodes both positive and negative zero the same. Negative zero uses
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// composite indexes to decode itself correctly.
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return append(b, floatZero)
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}
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u := math.Float64bits(f)
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if u&(1<<63) != 0 {
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u = ^u
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b = append(b, floatNeg)
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} else {
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b = append(b, floatPos)
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}
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return EncodeUint64Ascending(b, u)
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}
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// EncodeFloatDescending is the descending version of EncodeFloatAscending.
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func EncodeFloatDescending(b []byte, f float64) []byte {
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if math.IsNaN(f) {
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return append(b, floatNaNDesc)
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}
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return EncodeFloatAscending(b, -f)
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}
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// DecodeFloatAscending returns the remaining byte slice after decoding and the decoded
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// float64 from buf.
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func DecodeFloatAscending(buf []byte) ([]byte, float64, error) {
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if PeekType(buf) != Float {
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return buf, 0, errors.Errorf("did not find marker")
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}
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switch buf[0] {
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case floatNaN, floatNaNDesc:
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return buf[1:], math.NaN(), nil
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case floatNeg:
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b, u, err := DecodeUint64Ascending(buf[1:])
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if err != nil {
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return b, 0, err
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}
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u = ^u
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return b, math.Float64frombits(u), nil
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case floatZero:
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return buf[1:], 0, nil
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case floatPos:
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b, u, err := DecodeUint64Ascending(buf[1:])
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if err != nil {
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return b, 0, err
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}
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return b, math.Float64frombits(u), nil
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default:
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return nil, 0, errors.Errorf("unknown prefix of the encoded byte slice: %q", buf)
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}
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}
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// DecodeFloatDescending decodes floats encoded with EncodeFloatDescending.
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func DecodeFloatDescending(buf []byte) ([]byte, float64, error) {
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b, r, err := DecodeFloatAscending(buf)
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return b, -r, err
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}
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