1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159

use alloc::vec::Vec;

#[cfg(feature = "std")]
type IndexSet<K> = indexmap::IndexSet<K>;
#[cfg(not(feature = "std"))]
type IndexSet<K> = indexmap::IndexSet<K, hashbrown::hash_map::DefaultHashBuilder>;

/// An identifer for an entry in a string table.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct StringId(usize);

#[derive(Debug, Default)]
pub(crate) struct StringTable<'a> {
    strings: IndexSet<&'a [u8]>,
    offsets: Vec<usize>,
}

impl<'a> StringTable<'a> {
    /// Add a string to the string table.
    ///
    /// Panics if the string table has already been written, or
    /// if the string contains a null byte.
    pub fn add(&mut self, string: &'a [u8]) -> StringId {
        assert!(self.offsets.is_empty());
        assert!(!string.contains(&0));
        let id = self.strings.insert_full(string).0;
        StringId(id)
    }

    /// Return the id of the given string.
    ///
    /// Panics if the string is not in the string table.
    pub fn get_id(&self, string: &[u8]) -> StringId {
        let id = self.strings.get_index_of(string).unwrap();
        StringId(id)
    }

    /// Return the string for the given id.
    ///
    /// Panics if the string is not in the string table.
    pub fn get_string(&self, id: StringId) -> &'a [u8] {
        self.strings.get_index(id.0).unwrap()
    }

    /// Return the offset of the given string.
    ///
    /// Panics if the string table has not been written, or
    /// if the string is not in the string table.
    pub fn get_offset(&self, id: StringId) -> usize {
        self.offsets[id.0]
    }

    /// Append the string table to the given `Vec`, and
    /// calculate the list of string offsets.
    ///
    /// `base` is the initial string table offset. For example,
    /// this should be 1 for ELF, to account for the initial
    /// null byte (which must have been written by the caller).
    pub fn write(&mut self, base: usize, w: &mut Vec<u8>) {
        assert!(self.offsets.is_empty());

        let mut ids: Vec<_> = (0..self.strings.len()).collect();
        sort(&mut ids, 1, &self.strings);

        self.offsets = vec![0; ids.len()];
        let mut offset = base;
        let mut previous = &[][..];
        for id in ids {
            let string = self.strings.get_index(id).unwrap();
            if previous.ends_with(string) {
                self.offsets[id] = offset - string.len() - 1;
            } else {
                self.offsets[id] = offset;
                w.extend_from_slice(string);
                w.push(0);
                offset += string.len() + 1;
                previous = string;
            }
        }
    }
}

// Multi-key quicksort.
//
// Ordering is such that if a string is a suffix of at least one other string,
// then it is placed immediately after one of those strings. That is:
// - comparison starts at the end of the string
// - shorter strings come later
//
// Based on the implementation in LLVM.
fn sort(mut ids: &mut [usize], mut pos: usize, strings: &IndexSet<&[u8]>) {
    loop {
        if ids.len() <= 1 {
            return;
        }

        let pivot = byte(ids[0], pos, strings);
        let mut lower = 0;
        let mut upper = ids.len();
        let mut i = 1;
        while i < upper {
            let b = byte(ids[i], pos, strings);
            if b > pivot {
                ids.swap(lower, i);
                lower += 1;
                i += 1;
            } else if b < pivot {
                upper -= 1;
                ids.swap(upper, i);
            } else {
                i += 1;
            }
        }

        sort(&mut ids[..lower], pos, strings);
        sort(&mut ids[upper..], pos, strings);

        if pivot == 0 {
            return;
        }
        ids = &mut ids[lower..upper];
        pos += 1;
    }
}

fn byte(id: usize, pos: usize, strings: &IndexSet<&[u8]>) -> u8 {
    let string = strings.get_index(id).unwrap();
    let len = string.len();
    if len >= pos {
        string[len - pos]
    } else {
        // We know the strings don't contain null bytes.
        0
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn string_table() {
        let mut table = StringTable::default();
        let id0 = table.add(b"");
        let id1 = table.add(b"foo");
        let id2 = table.add(b"bar");
        let id3 = table.add(b"foobar");

        let mut data = Vec::new();
        data.push(0);
        table.write(1, &mut data);
        assert_eq!(data, b"\0foobar\0foo\0");

        assert_eq!(table.get_offset(id0), 11);
        assert_eq!(table.get_offset(id1), 8);
        assert_eq!(table.get_offset(id2), 4);
        assert_eq!(table.get_offset(id3), 1);
    }
}