Line data Source code
1 : // Copyright 2011 The LevelDB-Go and Pebble Authors. All rights reserved. Use
2 : // of this source code is governed by a BSD-style license that can be found in
3 : // the LICENSE file.
4 :
5 : package base
6 :
7 : import (
8 : "bytes"
9 : "encoding/binary"
10 : "fmt"
11 : "strconv"
12 : "unicode/utf8"
13 : )
14 :
15 : // Compare returns -1, 0, or +1 depending on whether a is 'less than', 'equal
16 : // to' or 'greater than' b. The empty slice must be 'less than' any non-empty
17 : // slice.
18 : //
19 : // Compare is used to compare user keys, such as those passed as arguments to
20 : // the various DB methods, as well as those returned from Separator, Successor,
21 : // and Split. It is also used to compare key suffixes, i.e. the remainder of the
22 : // key after Split.
23 : type Compare func(a, b []byte) int
24 :
25 : // Equal returns true if a and b are equivalent.
26 : //
27 : // For a given Compare, Equal(a,b)=true iff Compare(a,b)=0; that is, Equal is a
28 : // (potentially faster) specialization of Compare.
29 : type Equal func(a, b []byte) bool
30 :
31 : // AbbreviatedKey returns a fixed length prefix of a user key such that
32 : //
33 : // AbbreviatedKey(a) < AbbreviatedKey(b) implies a < b, and
34 : // AbbreviatedKey(a) > AbbreviatedKey(b) implies a > b.
35 : //
36 : // If AbbreviatedKey(a) == AbbreviatedKey(b), an additional comparison is
37 : // required to determine if the two keys are actually equal.
38 : //
39 : // This helps optimize indexed batch comparisons for cache locality. If a Split
40 : // function is specified, AbbreviatedKey usually returns the first eight bytes
41 : // of the user key prefix in the order that gives the correct ordering.
42 : type AbbreviatedKey func(key []byte) uint64
43 :
44 : // FormatKey returns a formatter for the user key.
45 : type FormatKey func(key []byte) fmt.Formatter
46 :
47 : // DefaultFormatter is the default implementation of user key formatting:
48 : // non-ASCII data is formatted as escaped hexadecimal values.
49 1 : var DefaultFormatter FormatKey = func(key []byte) fmt.Formatter {
50 1 : return FormatBytes(key)
51 1 : }
52 :
53 : // FormatValue returns a formatter for the user value. The key is also specified
54 : // for the value formatter in order to support value formatting that is
55 : // dependent on the key.
56 : type FormatValue func(key, value []byte) fmt.Formatter
57 :
58 : // Separator is used to construct SSTable index blocks. A trivial implementation
59 : // is `return append(dst, a...)`, but appending fewer bytes leads to smaller
60 : // SSTables.
61 : //
62 : // Given keys a, b for which Compare(a, b) < 0, Separator produces a key k such
63 : // that:
64 : //
65 : // 1. Compare(a, k) <= 0, and
66 : // 2. Compare(k, b) < 0.
67 : //
68 : // For example, if a and b are the []byte equivalents of the strings "black" and
69 : // "blue", then the function may append "blb" to dst.
70 : type Separator func(dst, a, b []byte) []byte
71 :
72 : // Successor appends to dst a shortened key k given a key a such that
73 : // Compare(a, k) <= 0. A simple implementation may return a unchanged.
74 : // The appended key k must be valid to pass to Compare.
75 : type Successor func(dst, a []byte) []byte
76 :
77 : // ImmediateSuccessor is invoked with a prefix key ([Split(a) == len(a)]) and
78 : // appends to dst the smallest prefix key that is larger than the given prefix a.
79 : //
80 : // ImmediateSuccessor must generate a prefix key k such that:
81 : //
82 : // Split(k) == len(k) and Compare(a, k) < 0
83 : //
84 : // and there exists no representable prefix key k2 such that:
85 : //
86 : // Split(k2) == len(k2) and Compare(a, k2) < 0 and Compare(k2, k) < 0
87 : //
88 : // As an example, an implementation built on the natural byte ordering using
89 : // bytes.Compare could append a `\0` to `a`.
90 : //
91 : // The appended key must be valid to pass to Compare.
92 : type ImmediateSuccessor func(dst, a []byte) []byte
93 :
94 : // Split returns the length of the prefix of the user key that corresponds to
95 : // the key portion of an MVCC encoding scheme to enable the use of prefix bloom
96 : // filters.
97 : //
98 : // The method will only ever be called with valid MVCC keys, that is, keys that
99 : // the user could potentially store in the database. Pebble does not know which
100 : // keys are MVCC keys and which are not, and may call Split on both MVCC keys
101 : // and non-MVCC keys.
102 : //
103 : // A trivial MVCC scheme is one in which Split() returns len(a). This
104 : // corresponds to assigning a constant version to each key in the database. For
105 : // performance reasons, it is preferable to use a `nil` split in this case.
106 : //
107 : // Let prefix(a) = a[:Split(a)] and suffix(a) = a[Split(a):]. The following
108 : // properties must hold:
109 : //
110 : // 1. A key consisting of just a prefix must sort before all other keys with
111 : // that prefix:
112 : //
113 : // If len(suffix(a)) > 0, then Compare(prefix(a), a) < 0.
114 : //
115 : // 2. Prefixes must be used to order keys before suffixes:
116 : //
117 : // If Compare(a, b) <= 0, then Compare(prefix(a), prefix(b)) <= 0.
118 : // If Compare(prefix(a), prefix(b)) < 0, then Compare(a, b) < 0
119 : //
120 : // 3. Suffixes themselves must be valid keys and comparable, respecting the same
121 : // ordering as within a key:
122 : //
123 : // If Compare(prefix(a), prefix(b)) = 0, then Compare(a, b) = Compare(suffix(a), suffix(b)).
124 : type Split func(a []byte) int
125 :
126 : // DefaultSplit is a trivial implementation of Split which always returns the
127 : // full key.
128 0 : var DefaultSplit Split = func(key []byte) int { return len(key) }
129 :
130 : // Comparer defines a total ordering over the space of []byte keys: a 'less
131 : // than' relationship.
132 : type Comparer struct {
133 : // These fields must always be specified.
134 : Compare Compare
135 : AbbreviatedKey AbbreviatedKey
136 : Separator Separator
137 : Successor Successor
138 :
139 : // ImmediateSuccessor must be specified if range keys are used.
140 : ImmediateSuccessor ImmediateSuccessor
141 :
142 : // Equal defaults to using Compare() == 0 if it is not specified.
143 : Equal Equal
144 : // FormatKey defaults to the DefaultFormatter if it is not specified.
145 : FormatKey FormatKey
146 : // Split defaults to a trivial implementation that returns the full key length
147 : // if it is not specified.
148 : Split Split
149 :
150 : // FormatValue is optional.
151 : FormatValue FormatValue
152 :
153 : // Name is the name of the comparer.
154 : //
155 : // The on-disk format stores the comparer name, and opening a database with a
156 : // different comparer from the one it was created with will result in an
157 : // error.
158 : Name string
159 : }
160 :
161 : // EnsureDefaults ensures that all non-optional fields are set.
162 : //
163 : // If c is nil, returns DefaultComparer.
164 : //
165 : // If any fields need to be set, returns a modified copy of c.
166 1 : func (c *Comparer) EnsureDefaults() *Comparer {
167 1 : if c == nil {
168 0 : return DefaultComparer
169 0 : }
170 1 : if c.Compare == nil || c.AbbreviatedKey == nil || c.Separator == nil || c.Successor == nil || c.Name == "" {
171 0 : panic("invalid Comparer: mandatory field not set")
172 : }
173 1 : if c.Equal != nil && c.Split != nil && c.FormatKey != nil {
174 1 : return c
175 1 : }
176 0 : n := &Comparer{}
177 0 : *n = *c
178 0 : if n.Equal == nil {
179 0 : cmp := n.Compare
180 0 : n.Equal = func(a, b []byte) bool {
181 0 : return cmp(a, b) == 0
182 0 : }
183 : }
184 0 : if n.Split == nil {
185 0 : n.Split = DefaultSplit
186 0 : }
187 0 : if n.FormatKey == nil {
188 0 : n.FormatKey = DefaultFormatter
189 0 : }
190 0 : return n
191 : }
192 :
193 : // DefaultComparer is the default implementation of the Comparer interface.
194 : // It uses the natural ordering, consistent with bytes.Compare.
195 : var DefaultComparer = &Comparer{
196 : Compare: bytes.Compare,
197 : Equal: bytes.Equal,
198 :
199 1 : AbbreviatedKey: func(key []byte) uint64 {
200 1 : if len(key) >= 8 {
201 1 : return binary.BigEndian.Uint64(key)
202 1 : }
203 1 : var v uint64
204 1 : for _, b := range key {
205 1 : v <<= 8
206 1 : v |= uint64(b)
207 1 : }
208 1 : return v << uint(8*(8-len(key)))
209 : },
210 :
211 : Split: DefaultSplit,
212 :
213 : FormatKey: DefaultFormatter,
214 :
215 1 : Separator: func(dst, a, b []byte) []byte {
216 1 : i, n := SharedPrefixLen(a, b), len(dst)
217 1 : dst = append(dst, a...)
218 1 :
219 1 : min := len(a)
220 1 : if min > len(b) {
221 1 : min = len(b)
222 1 : }
223 1 : if i >= min {
224 1 : // Do not shorten if one string is a prefix of the other.
225 1 : return dst
226 1 : }
227 :
228 1 : if a[i] >= b[i] {
229 0 : // b is smaller than a or a is already the shortest possible.
230 0 : return dst
231 0 : }
232 :
233 1 : if i < len(b)-1 || a[i]+1 < b[i] {
234 1 : i += n
235 1 : dst[i]++
236 1 : return dst[:i+1]
237 1 : }
238 :
239 1 : i += n + 1
240 1 : for ; i < len(dst); i++ {
241 1 : if dst[i] != 0xff {
242 1 : dst[i]++
243 1 : return dst[:i+1]
244 1 : }
245 : }
246 0 : return dst
247 : },
248 :
249 1 : Successor: func(dst, a []byte) (ret []byte) {
250 1 : for i := 0; i < len(a); i++ {
251 1 : if a[i] != 0xff {
252 1 : dst = append(dst, a[:i+1]...)
253 1 : dst[len(dst)-1]++
254 1 : return dst
255 1 : }
256 : }
257 : // a is a run of 0xffs, leave it alone.
258 0 : return append(dst, a...)
259 : },
260 :
261 0 : ImmediateSuccessor: func(dst, a []byte) (ret []byte) {
262 0 : return append(append(dst, a...), 0x00)
263 0 : },
264 :
265 : // This name is part of the C++ Level-DB implementation's default file
266 : // format, and should not be changed.
267 : Name: "leveldb.BytewiseComparator",
268 : }
269 :
270 : // SharedPrefixLen returns the largest i such that a[:i] equals b[:i].
271 : // This function can be useful in implementing the Comparer interface.
272 1 : func SharedPrefixLen(a, b []byte) int {
273 1 : i, n := 0, len(a)
274 1 : if n > len(b) {
275 1 : n = len(b)
276 1 : }
277 1 : asUint64 := func(c []byte, i int) uint64 {
278 1 : return binary.LittleEndian.Uint64(c[i:])
279 1 : }
280 1 : for i < n-7 && asUint64(a, i) == asUint64(b, i) {
281 1 : i += 8
282 1 : }
283 1 : for i < n && a[i] == b[i] {
284 1 : i++
285 1 : }
286 1 : return i
287 : }
288 :
289 : // FormatBytes formats a byte slice using hexadecimal escapes for non-ASCII
290 : // data.
291 : type FormatBytes []byte
292 :
293 : const lowerhex = "0123456789abcdef"
294 :
295 : // Format implements the fmt.Formatter interface.
296 1 : func (p FormatBytes) Format(s fmt.State, c rune) {
297 1 : buf := make([]byte, 0, len(p))
298 1 : for _, b := range p {
299 1 : if b < utf8.RuneSelf && strconv.IsPrint(rune(b)) {
300 1 : buf = append(buf, b)
301 1 : continue
302 : }
303 0 : buf = append(buf, `\x`...)
304 0 : buf = append(buf, lowerhex[b>>4])
305 0 : buf = append(buf, lowerhex[b&0xF])
306 : }
307 1 : s.Write(buf)
308 : }
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