rss-tools/vendor/golang.org/x/net/html/atom/atom.go (view raw)
| 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 | } |