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// This file is part of ICU4X. For terms of use, please see the file
// called LICENSE at the top level of the ICU4X source tree
// (online at: https://github.com/unicode-org/icu4x/blob/main/LICENSE ).
use super::super::branch_meta::BranchMeta;
use super::super::slice_indices::{self, ByteSliceWithIndices};
use super::store::ConstArrayBuilder;
use super::store::ConstLengthsStack;
use super::store::ConstSlice;
use crate::varint;
/// A low-level builder for [`ZeroTrieSimpleAscii`]. Works in const contexts.
///
/// All methods that grow the trie will panic if the capacity N is not enough.
pub(crate) struct ZeroTrieBuilderConst<const N: usize> {
data: ConstArrayBuilder<N, u8>,
}
impl<const N: usize> ZeroTrieBuilderConst<N> {
/// Non-const function that returns the current trie data as a slice.
#[cfg(feature = "litemap")]
pub fn as_bytes(&self) -> &[u8] {
self.data.as_const_slice().as_slice()
}
/// Returns the trie data, panicking if the buffer is the wrong size.
pub const fn build_or_panic(self) -> [u8; N] {
self.data.const_build_or_panic()
}
/// Creates a new empty builder.
pub const fn new() -> Self {
Self {
data: ConstArrayBuilder::new_empty([0; N], N),
}
}
/// Prepends an ASCII node to the front of the builder. Returns the new builder
/// and the delta in length, which is always 1.
pub const fn prepend_ascii(&mut self, ascii: u8) -> usize {
if ascii >= 128 {
panic!("Non-ASCII not supported in ZeroTrieSimpleAscii");
}
self.data.const_push_front_or_panic(ascii);
1
}
/// Prepends a value node to the front of the builder. Returns the new builder
/// and the delta in length, which depends on the size of the varint.
pub const fn prepend_value(&mut self, value: usize) -> usize {
let varint_array = varint::write_varint_meta3(value);
// Can panic (as documented in class docs):
self.data
.const_extend_front_or_panic(varint_array.as_const_slice());
// Shouldn't panic: index 0 is always a valid index, and the array is nonempty now
self.data.const_bitor_assign_or_panic(0, 0b10000000);
varint_array.len()
}
/// Prepends a branch node to the front of the builder. Returns the new builder
/// and the delta in length, which depends on the size of the varint.
pub const fn prepend_branch(&mut self, value: usize) -> usize {
let varint_array = varint::write_varint_meta2(value);
// Can panic (as documented in type-level docs):
self.data
.const_extend_front_or_panic(varint_array.as_const_slice());
// Shouldn't panic: index 0 is always a valid index, and the array is nonempty now
self.data.const_bitor_assign_or_panic(0, 0b11000000);
varint_array.len()
}
/// Prepends multiple arbitrary bytes to the front of the builder. Returns the new builder
/// and the delta in length, which is the length of the slice.
pub const fn prepend_slice(&mut self, s: ConstSlice<u8>) -> usize {
let mut i = s.len();
while i > 0 {
// Can panic (as documented in type-level docs):
self.data.const_push_front_or_panic(*s.get_or_panic(i - 1));
i -= 1;
}
s.len()
}
/// Prepends multiple zeros to the front of the builder. Returns the new builder.
pub const fn prepend_n_zeros(&mut self, n: usize) {
let mut i = 0;
while i < n {
// Can panic (as documented in type-level docs):
self.data.const_push_front_or_panic(0);
i += 1;
}
}
/// Performs the operation `self[index] |= bits`
pub const fn bitor_assign_at_or_panic(&mut self, index: usize, bits: u8) {
self.data.const_bitor_assign_or_panic(index, bits);
}
/// Creates a new builder containing the elements in the given slice of key/value pairs.
///
/// `K` is the stack size of the lengths stack. If you get an error such as
/// `AsciiTrie Builder: Need more stack`, try increasing `K`.
///
/// # Panics
///
/// Panics if the items are not sorted
pub const fn from_tuple_slice<'a, const K: usize>(items: ByteSliceWithIndices<'a>) -> Self {
assert!(
items.is_sorted(),
"Strings in ZeroTrie constructor are not sorted"
);
Self::from_slice_with_indices::<K>(items)
}
/// Creates a new builder containing the elements in the given slice of key/value pairs.
///
/// Assumes that the items are sorted. If they are not, unexpected behavior may occur.
///
/// `K` is the stack size of the lengths stack. If you get an error such as
/// `AsciiTrie Builder: Need more stack`, try increasing `K`.
pub const fn from_slice_with_indices<const K: usize>(items: ByteSliceWithIndices) -> Self {
let mut result = Self::new();
let total_size = result.create_or_panic::<K>(items);
debug_assert!(total_size == result.data.len());
result
}
/// The actual builder algorithm. For an explanation, see [`crate::builder`].
#[must_use]
#[expect(
clippy::indexing_slicing,
reason = "all call sites are well-commented and will not panic"
)]
const fn create_or_panic<const K: usize>(&mut self, all_items: ByteSliceWithIndices) -> usize {
let mut prefix_len = match all_items.last() {
Some(x) => x.0.len(),
// Empty slice:
None => return 0,
};
// Initialize the main loop to point at the last string.
let mut lengths_stack = ConstLengthsStack::<K>::new();
let mut i = all_items.len() - 1;
let mut j = all_items.len();
let mut current_len = 0;
// Start the main loop.
loop {
let item_i = all_items.get_or_panic(i);
let item_j = all_items.get_or_panic(j - 1);
debug_assert!(slice_indices::prefix_eq_or_panic(
item_i.0, item_j.0, prefix_len
));
// Check if we need to add a value node here.
if item_i.0.len() == prefix_len {
current_len += self.prepend_value(item_i.1);
}
if prefix_len == 0 {
// All done! Leave the main loop.
break;
}
// Reduce the prefix length by 1 and recalculate i and j.
prefix_len -= 1;
let mut new_i = i;
let mut new_j = j;
// `prefix_len` is guaranteed to be in bounds for `item_i` since it is
// instantiated and updated based on the length of `item_i`, and then only decremented.
// This is a bit harder to prove for `item_j`, but it is a loop invariant
// TODO(sffc): Consider proving this or perhaps using a checked get.
let mut ascii_i = item_i.0[prefix_len];
let mut ascii_j = item_j.0[prefix_len];
debug_assert!(ascii_i == ascii_j);
let key_ascii = ascii_i;
loop {
if new_i == 0 {
break;
}
let candidate = all_items.get_or_panic(new_i - 1).0;
if candidate.len() < prefix_len {
// Too short
break;
}
if slice_indices::prefix_eq_or_panic(item_i.0, candidate, prefix_len) {
new_i -= 1;
} else {
break;
}
if candidate.len() == prefix_len {
// A string that equals the prefix does not take part in the branch node.
break;
}
let candidate = candidate[prefix_len];
if candidate != ascii_i {
ascii_i = candidate;
}
}
loop {
if new_j == all_items.len() {
break;
}
let candidate = all_items.get_or_panic(new_j).0;
if candidate.len() < prefix_len {
// Too short
break;
}
if slice_indices::prefix_eq_or_panic(item_j.0, candidate, prefix_len) {
new_j += 1;
} else {
break;
}
if candidate.len() == prefix_len {
panic!("A shorter string should be earlier in the sequence");
}
let candidate = candidate[prefix_len];
if candidate != ascii_j {
ascii_j = candidate;
}
}
// If there are no different bytes at this prefix level, we can add an ASCII or Span
// node and then continue to the next iteration of the main loop.
if ascii_i == key_ascii && ascii_j == key_ascii {
current_len += self.prepend_ascii(ascii_i);
debug_assert!(i == new_i || i == new_i + 1);
i = new_i;
debug_assert!(j == new_j);
continue;
}
// If i and j changed, we are a target of a branch node.
if ascii_j == key_ascii {
// We are the _last_ target of a branch node.
lengths_stack.push_or_panic(BranchMeta {
ascii: key_ascii,
cumulative_length: current_len,
local_length: current_len,
count: 1,
});
} else {
// We are the _not the last_ target of a branch node.
let BranchMeta {
cumulative_length,
count,
..
} = lengths_stack.peek_or_panic();
lengths_stack.push_or_panic(BranchMeta {
ascii: key_ascii,
cumulative_length: cumulative_length + current_len,
local_length: current_len,
count: count + 1,
});
}
if ascii_i != key_ascii {
// We are _not the first_ target of a branch node.
// Set the cursor to the previous string and continue the loop.
j = i;
i -= 1;
prefix_len = all_items.get_or_panic(i).0.len();
current_len = 0;
continue;
}
// Branch (first)
let (total_length, total_count) = {
let BranchMeta {
cumulative_length,
count,
..
} = lengths_stack.peek_or_panic();
(cumulative_length, count)
};
let branch_metas = lengths_stack.pop_many_or_panic(total_count);
let original_keys = branch_metas.map_to_ascii_bytes();
// Write out the offset table
current_len = total_length;
let w = (usize::BITS as usize - (total_length.leading_zeros() as usize) - 1) / 8;
if w > 3 {
panic!("ZeroTrie capacity exceeded");
}
let mut k = 0;
while k <= w {
self.prepend_n_zeros(total_count - 1);
current_len += total_count - 1;
let mut l = 0;
let mut length_to_write = 0;
while l < total_count {
let BranchMeta { local_length, .. } = *branch_metas
.as_const_slice()
.get_or_panic(total_count - l - 1);
let mut adjusted_length = length_to_write;
let mut m = 0;
while m < k {
adjusted_length >>= 8;
m += 1;
}
if l > 0 {
self.bitor_assign_at_or_panic(l - 1, adjusted_length as u8);
}
l += 1;
length_to_write += local_length;
}
k += 1;
}
// Write out the lookup table
assert!(0 < total_count && total_count <= 256);
let branch_value = (w << 8) + (total_count & 0xff);
current_len += self.prepend_slice(original_keys.as_const_slice());
current_len += self.prepend_branch(branch_value);
i = new_i;
j = new_j;
}
assert!(lengths_stack.is_empty());
current_len
}
}