1 // Copyright 2012 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 // Package atom provides integer codes (also known as atoms) for a fixed set of
6 // frequently occurring HTML strings: tag names and attribute keys such as "p"
7 // and "id".
8 //
9 // Sharing an atom's name between all elements with the same tag can result in
10 // fewer string allocations when tokenizing and parsing HTML. Integer
11 // comparisons are also generally faster than string comparisons.
12 //
13 // The value of an atom's particular code is not guaranteed to stay the same
14 // between versions of this package. Neither is any ordering guaranteed:
15 // whether atom.H1 < atom.H2 may also change. The codes are not guaranteed to
16 // be dense. The only guarantees are that e.g. looking up "div" will yield
17 // atom.Div, calling atom.Div.String will return "div", and atom.Div != 0.
18 package atom // import "golang.org/x/net/html/atom"
19 20 // Atom is an integer code for a string. The zero value maps to "".
21 type Atom uint32
22 23 // String returns the atom's name.
24 func (a Atom) String() string {
25 start := uint32(a >> 8)
26 n := uint32(a & 0xff)
27 if start+n > uint32(len(atomText)) {
28 return ""
29 }
30 return atomText[start : start+n]
31 }
32 33 func (a Atom) string() string {
34 return atomText[a>>8 : a>>8+a&0xff]
35 }
36 37 // fnv computes the FNV hash with an arbitrary starting value h.
38 func fnv(h uint32, s []byte) uint32 {
39 for i := range s {
40 h ^= uint32(s[i])
41 h *= 16777619
42 }
43 return h
44 }
45 46 func match(s string, t []byte) bool {
47 for i, c := range t {
48 if s[i] != c {
49 return false
50 }
51 }
52 return true
53 }
54 55 // Lookup returns the atom whose name is s. It returns zero if there is no
56 // such atom. The lookup is case sensitive.
57 func Lookup(s []byte) Atom {
58 if len(s) == 0 || len(s) > maxAtomLen {
59 return 0
60 }
61 h := fnv(hash0, s)
62 if a := table[h&uint32(len(table)-1)]; int(a&0xff) == len(s) && match(a.string(), s) {
63 return a
64 }
65 if a := table[(h>>16)&uint32(len(table)-1)]; int(a&0xff) == len(s) && match(a.string(), s) {
66 return a
67 }
68 return 0
69 }
70 71 // String returns a string whose contents are equal to s. In that sense, it is
72 // equivalent to string(s) but may be more efficient.
73 func String(s []byte) string {
74 if a := Lookup(s); a != 0 {
75 return a.String()
76 }
77 return string(s)
78 }
79