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use indexmap::IndexSet;
use std::vec::Vec;
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub(crate) 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 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);
}
}
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