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vendor/github.com/blevesearch/bleve/numeric_util/prefix_coded.go

@@ -0,0 +1,74 @@
+//  Copyright (c) 2014 Couchbase, Inc.
+//  Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file
+//  except in compliance with the License. You may obtain a copy of the License at
+//    http://www.apache.org/licenses/LICENSE-2.0
+//  Unless required by applicable law or agreed to in writing, software distributed under the
+//  License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND,
+//  either express or implied. See the License for the specific language governing permissions
+//  and limitations under the License.
+
+package numeric_util
+
+import (
+	"fmt"
+)
+
+const ShiftStartInt64 byte = 0x20
+
+// PrefixCoded is a byte array encoding of
+// 64-bit numeric values shifted by 0-63 bits
+type PrefixCoded []byte
+
+func NewPrefixCodedInt64(in int64, shift uint) (PrefixCoded, error) {
+	if shift > 63 {
+		return nil, fmt.Errorf("cannot shift %d, must be between 0 and 63", shift)
+	}
+
+	nChars := (((63 - shift) * 37) >> 8) + 1
+	rv := make(PrefixCoded, nChars+1)
+	rv[0] = ShiftStartInt64 + byte(shift)
+
+	sortableBits := int64(uint64(in) ^ 0x8000000000000000)
+	sortableBits = int64(uint64(sortableBits) >> shift)
+	for nChars > 0 {
+		// Store 7 bits per byte for compatibility
+		// with UTF-8 encoding of terms
+		rv[nChars] = byte(sortableBits & 0x7f)
+		nChars--
+		sortableBits = int64(uint64(sortableBits) >> 7)
+	}
+	return rv, nil
+}
+
+func MustNewPrefixCodedInt64(in int64, shift uint) PrefixCoded {
+	rv, err := NewPrefixCodedInt64(in, shift)
+	if err != nil {
+		panic(err)
+	}
+	return rv
+}
+
+// Shift returns the number of bits shifted
+// returns 0 if in uninitialized state
+func (p PrefixCoded) Shift() (uint, error) {
+	if len(p) > 0 {
+		shift := p[0] - ShiftStartInt64
+		if shift < 0 || shift < 63 {
+			return uint(shift), nil
+		}
+	}
+	return 0, fmt.Errorf("invalid prefix coded value")
+}
+
+func (p PrefixCoded) Int64() (int64, error) {
+	shift, err := p.Shift()
+	if err != nil {
+		return 0, err
+	}
+	var sortableBits int64
+	for _, inbyte := range p[1:] {
+		sortableBits <<= 7
+		sortableBits |= int64(inbyte)
+	}
+	return int64(uint64((sortableBits << shift)) ^ 0x8000000000000000), nil
+}