// 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 ).

//! # ZeroTrie Builder

//!

//! There are two implementations of the ZeroTrie Builder:

//!

//! - [konst::ZeroTrieBuilderConst] allows for human-readable const construction

//! - [nonconst::ZeroTrieBuilder] has the full feaure set but requires `alloc`

//!

//! The two builders follow the same algorithm but have different capabilities.

//!

//! ## Builder Algorithm Overview

//!

//! The tries are built backwards, from the last node to the first node. The key step of the

//! algorithm is **determining what is the next node to prepend.**

//!

//! In the simple case of [`ZeroTrieSimpleAscii`], all nodes are binary-search, so if the input

//! strings are provided in lexicographic order, there is a simple, deterministic method for

//! identifying the next node. This insight is what enables us to make the const builder.

//!

//! The builder works with the following intermediate state variables:

//!

//! - `prefix_len` indicates the byte index we are currently processing.

//! - `i` and `j` bracket a window of strings in the input that share the same prefix.

//! - `current_len` is the length in bytes of the current self-contained trie.

//! - `lengths_stack` contains metadata for branch nodes.

//!

//! What follows is a verbal explanation of the build steps for a trie containing:

//!

//! - "" → 11

//! - "ad" → 22

//! - "adef" → 33

//! - "adghk" → 44

//!

//! When a node is prepended, it is shown in **boldface**.

//!

//! 1. Initialize the builder by setting `i=3`, `j=4`, `prefix_len=5` (the last string),

//!    `current_len=0`, and `lengths_stack` empty. Start the main loop.

//! 2. Top of loop. The string at `i` is equal in length to `prefix_len`, so we prepend

//!    our first node: a **value node 44**, which requires a 2-byte varint. Increase

//!    `current_len` to 2.

//! 3. Reduce `prefix_len` to 4, read our `key_ascii="k"`, and recalculate `i` and `j`

//!    _(this calculation is a long chunk of code in the builder impls)_. Since there is no

//!    other string with the prefix "adgh", `i` and `j` stay the same, we prepend an

//!    **ASCII node "k"**, increase `current_len` to 3, and continue the main loop.

//! 4. Top of loop. The string at `i` is of length 5, but `prefix_len` is 4, so there is

//!    no value node to prepend.

//! 5. Reduce `prefix_len` to 3, read our `key_ascii="h"`, and recalculate `i` and `j`.

//!    There are no other strings sharing the prefix "abg", so we prepend an

//!    **ASCII node "h"**, increase `current_len` to 4, and continue the main loop.

//! 6. Top of loop. There is still no value node to prepend.

//! 7. Reduce `prefix_len` to 2, read our `key_ascii="g"`, and recalculate `i` and `j`.

//!    We find that `i=1` and `j=4`, the range of strings sharing the prefix "ad". Since

//!    `i` or `j` changed, proceed to evaluate the branch node.

//! 8. The last branch byte `ascii_j` for this prefix is "g", which is the same as `key_ascii`,

//!    so we are the _last_ target of a branch node. Push an entry onto `lengths_stack`:

//!    `BranchMeta { ascii: "g", cumulative_length: 4, local_length: 4, count: 1 }`.

//! 9. The first branch byte `ascii_i` for this prefix is "e", which is NOT equal to `key_ascii`,

//!    so we are _not the first_ target of a branch node. We therefore start evaluating the

//!    string preceding where we were at the top of the current loop. We set `i=2`, `j=3`,

//!    `prefix_len=4` (length of the string at `i`), and continue the main loop.

//! 10. Top of loop. Since the string at `i` is equal in length to `prefix_len`, we prepend a

//!     **value node 33** (which requires a 2-byte varint) and increase `current_len` to 2.

//! 11. Reduce `prefix_len` to 3, read our `key_ascii="f"`, and recalculate `i` and `j`.

//!     They stay the same, so we prepend an **ASCII node "f"**, increase `current_len` to 3,

//!     and continue the main loop.

//! 12. Top of loop. No value node this time.

//! 13. Reduce `prefix_len` to 2, read our `key_ascii="e"`, and recalculate `i` and `j`.

//!     They go back to `i=1` and `j=4`.

//! 14. The last branch byte `ascii_j` for this prefix is "g", which is NOT equal to `key_ascii`,

//!     so we are _not the last_ target of a branch node. We peek at the entry at the front of

//!     the lengths stack and use it to push another entry onto the stack:

//!     `BranchMeta { ascii: "e", cumulative_length: 7, local_length: 3, count: 2 }`

//! 15. The first branch byte `ascii_i` for this prefix is "e", which is the same as `key_ascii`,

//!     wo we are the _first_ target of a branch node. We can therefore proceed to prepend the

//!     metadata for the branch node. We peek at the top of the stack and find that there are 2

//!     tries reachable from this branch and they have a total byte length of 5. We then pull off

//!     2 entries from the stack into a local variable `branch_metas`. From here, we write out

//!     the **offset table**, **lookup table**, and **branch head node**, which are determined

//!     from the metadata entries. We set `current_len` to the length of the two tries plus the

//!     metadata, which happens to be 11. Then we return to the top of the main loop.

//! 16. Top of loop. The string at `i` is length 2, which is the same as `prefix_len`, so we

//!     prepend a **value node 22** (2-byte varint) and increase `current_len` to 13.

//! 17. Reduce `prefix_len` to 1, read our `key_ascii="d"`, and recalculate `i` and `j`.

//!     They stay the same, so we prepend an **ASCII node "d"**, increase `current_len` to 14,

//!     and continue the main loop.

//! 18. Top of loop. No value node this time.

//! 19. Reduce `prefix_len` to 0, read our `key_ascii="a"`, and recalculate `i` and `j`.

//!     They change to `i=0` and `j=4`, since all strings have the empty string as a prefix.

//!     However, `ascii_i` and `ascii_j` both equal `key_ascii`, so we prepend **ASCII node "a"**,

//!     increase `current_len` to 15, and continue the main loop.

//! 16. Top of loop. The string at `i` is length 0, which is the same as `prefix_len`, so we

//!     prepend a **value node 11** and increase `current_len` to 16.

//! 17. We can no longer reduce `prefix_len`, so our trie is complete.

//!

//! ## Perfect Hash Reordering

//!

//! When the PHF is added to the mix, the main change is that the strings are no longer in sorted

//! order when they are in the trie. To resolve this issue, when adding a branch node, the target

//! tries are rearranged in-place in the buffer to be in the correct order for the PHF.

//!

//! ## Example

//!

//! Here is the output of the trie described above.

//!

//! ```

//! use zerotrie::ZeroTrieSimpleAscii;

//!

//! const DATA: [(&str, usize); 4] =

//!     [("", 11), ("ad", 22), ("adef", 33), ("adghk", 44)];

//!

//! // As demonstrated above, the required capacity for this trie is 16 bytes

//! const TRIE: ZeroTrieSimpleAscii<[u8; 16]> =

//!     ZeroTrieSimpleAscii::from_sorted_str_tuples(&DATA);

//!

//! assert_eq!(

//!     TRIE.as_bytes(),

//!     &[

//!         0x8B, // value node 11

//!         b'a', // ASCII node 'a'

//!         b'd', // ASCII node 'd'

//!         0x90, // value node 22 lead byte

//!         0x06, // value node 22 trail byte

//!         0xC2, // branch node 2

//!         b'e', // first target of branch

//!         b'g', // second target of branch

//!         3,    // offset

//!         b'f', // ASCII node 'f'

//!         0x90, // value node 33 lead byte

//!         0x11, // value node 33 trail byte

//!         b'h', // ASCII node 'h'

//!         b'k', // ASCII node 'k'

//!         0x90, // value node 44 lead byte

//!         0x1C, // value node 44 trail byte

//!     ]

//! );

//!

//! assert_eq!(TRIE.get(b""), Some(11));

//! assert_eq!(TRIE.get(b"ad"), Some(22));

//! assert_eq!(TRIE.get(b"adef"), Some(33));

//! assert_eq!(TRIE.get(b"adghk"), Some(44));

//! assert_eq!(TRIE.get(b"unknown"), None);

//! ```

mod branch_meta;

pub(crate) mod bytestr;

pub(crate) mod konst;

#[cfg(feature = "litemap")]

mod litemap;

#[cfg(feature = "alloc")]

pub(crate) mod nonconst;

use bytestr::ByteStr;

use super::ZeroTrieSimpleAscii;

impl<const N: usize> ZeroTrieSimpleAscii<[u8; N]> {

    /// **Const Constructor:** Creates an [`ZeroTrieSimpleAscii`] from a sorted slice of keys and values.

    ///

    /// This function needs to know the exact length of the resulting trie at compile time. To

    /// figure out `N`, first set `N` to be too large (say 0xFFFF), then look at the resulting

    /// compile error which will tell you how to set `N`, like this:

    ///

    /// > the evaluated program panicked at 'Buffer too large. Size needed: 17'

    ///

    /// That error message says you need to set `N` to 17.

    ///

    /// Also see [`Self::from_sorted_str_tuples`].

    ///

    /// # Panics

    ///

    /// Panics if `items` is not sorted or if `N` is not correct.

    ///

    /// # Examples

    ///

    /// Create a `const` ZeroTrieSimpleAscii at compile time:

    ///

    /// ```

    /// use zerotrie::ZeroTrieSimpleAscii;

    ///

    /// // The required capacity for this trie happens to be 17 bytes

    /// const TRIE: ZeroTrieSimpleAscii<[u8; 17]> =

    ///     ZeroTrieSimpleAscii::from_sorted_u8_tuples(&[

    ///         (b"bar", 2),

    ///         (b"bazzoo", 3),

    ///         (b"foo", 1),

    ///     ]);

    ///

    /// assert_eq!(TRIE.get(b"foo"), Some(1));

    /// assert_eq!(TRIE.get(b"bar"), Some(2));

    /// assert_eq!(TRIE.get(b"bazzoo"), Some(3));

    /// assert_eq!(TRIE.get(b"unknown"), None);

    /// ```

    ///

    /// Panics if strings are not sorted:

    ///

    /// ```compile_fail

    /// # use zerotrie::ZeroTrieSimpleAscii;

    /// const TRIE: ZeroTrieSimpleAscii<[u8; 17]> = ZeroTrieSimpleAscii::from_sorted_u8_tuples(&[

    ///     (b"foo", 1),

    ///     (b"bar", 2),

    ///     (b"bazzoo", 3),

    /// ]);

    /// ```

    ///

    /// Panics if capacity is too small:

    ///

    /// ```compile_fail

    /// # use zerotrie::ZeroTrieSimpleAscii;

    /// const TRIE: ZeroTrieSimpleAscii<[u8; 15]> = ZeroTrieSimpleAscii::from_sorted_u8_tuples(&[

    ///     (b"bar", 2),

    ///     (b"bazzoo", 3),

    ///     (b"foo", 1),

    /// ]);

    /// ```

    ///

    /// Panics if capacity is too large:

    ///

    /// ```compile_fail

    /// # use zerotrie::ZeroTrieSimpleAscii;

    /// const TRIE: ZeroTrieSimpleAscii<[u8; 20]> = ZeroTrieSimpleAscii::from_sorted_u8_tuples(&[

    ///     (b"bar", 2),

    ///     (b"bazzoo", 3),

    ///     (b"foo", 1),

    /// ]);

    /// ```

    pub const fn from_sorted_u8_tuples(tuples: &[(&[u8], usize)]) -> Self {

        use konst::*;

        let byte_str_slice = ByteStr::from_byte_slice_with_value(tuples);

        let result = ZeroTrieBuilderConst::<N>::from_tuple_slice::<100>(byte_str_slice);

        match result {

            Ok(s) => Self::from_store(s.build_or_panic()),

            Err(_) => panic!("Failed to build ZeroTrie"),

        }

    }

    /// **Const Constructor:** Creates an [`ZeroTrieSimpleAscii`] from a sorted slice of keys and values.

    ///

    /// This function needs to know the exact length of the resulting trie at compile time. To

    /// figure out `N`, first set `N` to be too large (say 0xFFFF), then look at the resulting

    /// compile error which will tell you how to set `N`, like this:

    ///

    /// > the evaluated program panicked at 'Buffer too large. Size needed: 17'

    ///

    /// That error message says you need to set `N` to 17.

    ///

    /// Also see [`Self::from_sorted_u8_tuples`].

    ///

    /// # Panics

    ///

    /// Panics if `items` is not sorted, if `N` is not correct, or if any of the strings contain

    /// non-ASCII characters.

    ///

    /// # Examples

    ///

    /// Create a `const` ZeroTrieSimpleAscii at compile time:

    ///

    /// ```

    /// use zerotrie::ZeroTrieSimpleAscii;

    ///

    /// // The required capacity for this trie happens to be 17 bytes

    /// const TRIE: ZeroTrieSimpleAscii<[u8; 17]> =

    ///     ZeroTrieSimpleAscii::from_sorted_str_tuples(&[

    ///         ("bar", 2),

    ///         ("bazzoo", 3),

    ///         ("foo", 1),

    ///     ]);

    ///

    /// assert_eq!(TRIE.get(b"foo"), Some(1));

    /// assert_eq!(TRIE.get(b"bar"), Some(2));

    /// assert_eq!(TRIE.get(b"bazzoo"), Some(3));

    /// assert_eq!(TRIE.get(b"unknown"), None);

    /// ```

    ///

    /// Panics if the strings are not ASCII:

    ///

    /// ```compile_fail

    /// # use zerotrie::ZeroTrieSimpleAscii;

    /// const TRIE: ZeroTrieSimpleAscii<[u8; 100]> = ZeroTrieSimpleAscii::from_sorted_str_tuples(&[

    ///     ("bár", 2),

    ///     ("båzzöo", 3),

    ///     ("foo", 1),

    /// ]);

    /// ```

    pub const fn from_sorted_str_tuples(tuples: &[(&str, usize)]) -> Self {

        use konst::*;

        let byte_str_slice = ByteStr::from_str_slice_with_value(tuples);

        // 100 is the value of `K`, the size of the lengths stack. If compile errors are

        // encountered, this number may need to be increased.

        let result = ZeroTrieBuilderConst::<N>::from_tuple_slice::<100>(byte_str_slice);

        match result {

            Ok(s) => Self::from_store(s.build_or_panic()),

            Err(_) => panic!("Failed to build ZeroTrie"),

        }

    }

}