// 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 zerofrom::ZeroFrom;
use zerovec::{ZeroSlice, ZeroVec};

// Match-node lead unit values, after masking off intermediate-value bits:

// 00..0f: Branch node. If node!=0 then the length is node+1, otherwise
// the length is one more than the next byte.

// For a branch sub-node with at most this many entries, we drop down
// to a linear search.
const MAX_BRANCH_LINEAR_SUB_NODE_LENGTH: usize = 5;

// 0030..003f: Linear-match node, match 1..16 units and continue reading the next node.
const MIN_LINEAR_MATCH: u16 = 0x30;
const MAX_LINEAR_MATCH_LENGTH: u16 = 0x10;

// Match-node lead unit bits 14..6 for the optional intermediate value.
// If these bits are 0, then there is no intermediate value.
// Otherwise, see the *NodeValue* constants below.
const MIN_VALUE_LEAD: u16 = MIN_LINEAR_MATCH + MAX_LINEAR_MATCH_LENGTH; // 0x40
const NODE_TYPE_MASK: u16 = MIN_VALUE_LEAD - 1; // 0x003f

// A final-value node has bit 15 set.
const VALUE_IS_FINAL: u16 = 0x8000;

// Compact value: After testing bit 0, shift right by 15 and then use the following thresholds.
const MAX_ONE_UNIT_VALUE: u16 = 0x3fff;

const MIN_TWO_UNIT_VALUE_LEAD: u16 = MAX_ONE_UNIT_VALUE + 1; // 0x4000

const MAX_ONE_UNIT_NODE_VALUE: u16 = 0xff;

const MIN_TWO_UNIT_NODE_VALUE_LEAD: u16 = MIN_VALUE_LEAD + ((MAX_ONE_UNIT_NODE_VALUE + 1) << 6); // 0x4040

const THREE_UNIT_NODE_VALUE_LEAD: u16 = 0x7fc0;

const THREE_UNIT_VALUE_LEAD: u16 = 0x7fff;

// Compact delta integers.
const MAX_ONE_UNIT_DELTA: u16 = 0xfbff;
const MIN_TWO_UNIT_DELTA_LEAD: u16 = MAX_ONE_UNIT_DELTA + 1; // 0xfc00
const THREE_UNIT_DELTA_LEAD: u16 = 0xffff;

fn skip_value(pos: usize, lead: u16) -> usize {
    if lead < MIN_TWO_UNIT_VALUE_LEAD {
        pos
    } else if lead < THREE_UNIT_VALUE_LEAD {
        pos + 1
    } else {
        pos + 2
    }
}

fn skip_node_value(pos: usize, lead: u16) -> usize {
    if lead < MIN_TWO_UNIT_NODE_VALUE_LEAD {
        pos
    } else if lead < THREE_UNIT_NODE_VALUE_LEAD {
        pos + 1
    } else {
        pos + 2
    }
}

/// This struct represents a de-serialized `Char16Trie` that was exported from
/// ICU binary data.
///
/// Light-weight, non-const reader class for a `CharsTrie`. Traverses a
/// char-serialized data structure with minimal state, for mapping 16-bit-unit
/// sequences to non-negative integer values.
///
/// For more information:
/// - [ICU4C UCharsTrie](https://unicode-org.github.io/icu-docs/apidoc/released/icu4c/classicu_1_1UCharsTrie.html)
/// - [ICU4J CharsTrie](https://unicode-org.github.io/icu-docs/apidoc/released/icu4j/com/ibm/icu/util/CharsTrie.html) API.
#[cfg_attr(feature = "serde", derive(serde::Deserialize, serde::Serialize))]
#[cfg_attr(feature = "databake", derive(databake::Bake))]
#[cfg_attr(feature = "databake", databake(path = icu_collections::char16trie))]
#[derive(Clone, Debug, PartialEq, Eq, ZeroFrom)]
pub struct Char16Trie<'data> {
    /// An array of u16 containing the trie data.
    #[cfg_attr(feature = "serde", serde(borrow))]
    #[doc(hidden)] // #2417
    pub data: ZeroVec<'data, u16>,
}

impl<'data> Char16Trie<'data> {
    /// Returns a new [`Char16Trie`] with ownership of the provided data.
    pub fn new(data: ZeroVec<'data, u16>) -> Self {
        Self { data }
    }

    /// Returns a new [`Char16TrieIterator`] backed by borrowed data from the `trie` data
    pub fn iter(&self) -> Char16TrieIterator {
        Char16TrieIterator::new(&self.data)
    }
}

/// This struct represents an iterator over a [`Char16Trie`].
#[derive(Clone)]
pub struct Char16TrieIterator<'a> {
    /// A reference to the Char16Trie data to iterate over.
    trie: &'a ZeroSlice<u16>,
    /// Index of next trie unit to read, or `None` if there are no more matches.
    pos: Option<usize>,
    /// Remaining length of a linear-match node, minus 1, or `None` if not in
    /// such a node.
    remaining_match_length: Option<usize>,
}

/// An enum representing the return value from a lookup in [`Char16Trie`].
#[derive(Clone, Copy, Debug, PartialEq)]
pub enum TrieResult {
    /// The input unit(s) did not continue a matching string.
    /// Once `next()` returns `TrieResult::NoMatch`, all further calls to `next()`
    /// will also return `TrieResult::NoMatch`.
    NoMatch,
    /// The input unit(s) matched a string but there is no value for the string
    /// so far.  (It is a prefix of a longer string.)
    NoValue,
    /// The input unit(s) continued a matching string and there is a value for
    /// the string so far. No further input byte/unit can continue a matching
    /// string.
    FinalValue(i32),
    /// The input unit(s) continued a matching string and there is a value for
    /// the string so far.  Another input byte/unit can continue a matching
    /// string.
    Intermediate(i32),
}

// Get the lead surrogate (0xd800..0xdbff) for a
// supplementary code point (0x10000..0x10ffff).
// @param supplementary 32-bit code point (U+10000..U+10ffff)
// @return lead surrogate (U+d800..U+dbff) for supplementary
fn u16_lead(supplementary: i32) -> u16 {
    (((supplementary) >> 10) + 0xd7c0) as u16
}

// Get the trail surrogate (0xdc00..0xdfff) for a
// supplementary code point (0x10000..0x10ffff).
// @param supplementary 32-bit code point (U+10000..U+10ffff)
// @return trail surrogate (U+dc00..U+dfff) for supplementary
fn u16_tail(supplementary: i32) -> u16 {
    (((supplementary) & 0x3ff) | 0xdc00) as u16
}

/// A macro that takes an `Option` argument and either unwraps it if it has a value or
/// causes the function to return `TrieResult::NoMatch` if there is no value.
/// This could perhaps be done with `std::ops::Try` once stabilized.
macro_rules! trie_unwrap {
    ($option:expr) => {
        match $option {
            Some(x) => x,
            None => {
                // Unexpected
                debug_assert!(false);
                return TrieResult::NoMatch;
            }
        }
    };
}

impl<'a> Char16TrieIterator<'a> {
    /// Returns a new [`Char16TrieIterator`] backed by borrowed data for the `trie` array
    pub fn new(trie: &'a ZeroSlice<u16>) -> Self {
        Self {
            trie,
            pos: Some(0),
            remaining_match_length: None,
        }
    }

    /// Traverses the trie from the current state for this input char.
    ///
    /// # Examples
    ///
    /// ```
    /// use icu::collections::char16trie::{Char16Trie, TrieResult};
    /// use zerovec::ZeroVec;
    ///
    /// // A Char16Trie containing the ASCII characters 'a' and 'b'.
    /// let trie_data = [48, 97, 176, 98, 32868];
    /// let trie = Char16Trie::new(ZeroVec::from_slice_or_alloc(&trie_data));
    ///
    /// let mut iter = trie.iter();
    /// let res = iter.next('a');
    /// assert_eq!(res, TrieResult::Intermediate(1));
    /// let res = iter.next('b');
    /// assert_eq!(res, TrieResult::FinalValue(100));
    /// let res = iter.next('c');
    /// assert_eq!(res, TrieResult::NoMatch);
    /// ```
    pub fn next(&mut self, c: char) -> TrieResult {
        if (c as u32) <= 0xffff {
            self.next16(c as u16)
        } else {
            match self.next16(u16_lead(c as i32)) {
                TrieResult::NoValue | TrieResult::Intermediate(_) => {
                    self.next16(u16_tail(c as i32))
                }
                _ => TrieResult::NoMatch,
            }
        }
    }

    /// Traverses the trie from the current state for this input char.
    ///
    /// # Examples
    ///
    /// ```
    /// use icu::collections::char16trie::{Char16Trie, TrieResult};
    /// use zerovec::ZeroVec;
    ///
    /// // A Char16Trie containing the ASCII characters 'a' and 'b'.
    /// let trie_data = [48, 97, 176, 98, 32868];
    /// let trie = Char16Trie::new(ZeroVec::from_slice_or_alloc(&trie_data));
    ///
    /// let mut iter = trie.iter();
    /// let res = iter.next('a');
    /// assert_eq!(res, TrieResult::Intermediate(1));
    /// let res = iter.next('b');
    /// assert_eq!(res, TrieResult::FinalValue(100));
    /// let res = iter.next('c');
    /// assert_eq!(res, TrieResult::NoMatch);
    /// ```
    pub fn next32(&mut self, c: u32) -> TrieResult {
        if c <= 0xffff {
            self.next16(c as u16)
        } else {
            match self.next16(u16_lead(c as i32)) {
                TrieResult::NoValue | TrieResult::Intermediate(_) => {
                    self.next16(u16_tail(c as i32))
                }
                _ => TrieResult::NoMatch,
            }
        }
    }

    /// Traverses the trie from the current state for this input char.
    ///
    /// # Examples
    ///
    /// ```
    /// use icu::collections::char16trie::{Char16Trie, TrieResult};
    /// use zerovec::ZeroVec;
    ///
    /// // A Char16Trie containing the ASCII characters 'a' and 'b'.
    /// let trie_data = [48, 97, 176, 98, 32868];
    /// let trie = Char16Trie::new(ZeroVec::from_slice_or_alloc(&trie_data));
    ///
    /// let mut iter = trie.iter();
    /// let res = iter.next16('a' as u16);
    /// assert_eq!(res, TrieResult::Intermediate(1));
    /// let res = iter.next16('b' as u16);
    /// assert_eq!(res, TrieResult::FinalValue(100));
    /// let res = iter.next16('c' as u16);
    /// assert_eq!(res, TrieResult::NoMatch);
    /// ```
    pub fn next16(&mut self, c: u16) -> TrieResult {
        let mut pos = match self.pos {
            Some(p) => p,
            None => return TrieResult::NoMatch,
        };
        if let Some(length) = self.remaining_match_length {
            // Remaining part of a linear-match node
            if c == trie_unwrap!(self.trie.get(pos)) {
                pos += 1;
                self.pos = Some(pos);
                if length == 0 {
                    self.remaining_match_length = None;
                    let node = trie_unwrap!(self.trie.get(pos));
                    if node >= MIN_VALUE_LEAD {
                        return self.value_result(pos);
                    }
                } else {
                    self.remaining_match_length = Some(length - 1);
                }
                return TrieResult::NoValue;
            }
            self.stop();
            TrieResult::NoMatch
        } else {
            self.next_impl(pos, c)
        }
    }

    fn branch_next(&mut self, pos: usize, length: usize, in_unit: u16) -> TrieResult {
        let mut pos = pos;
        let mut length = length;
        if length == 0 {
            length = trie_unwrap!(self.trie.get(pos)) as usize;
            pos += 1;
        }
        length += 1;

        // The length of the branch is the number of units to select from.
        // The data structure encodes a binary search.
        while length > MAX_BRANCH_LINEAR_SUB_NODE_LENGTH {
            if in_unit < trie_unwrap!(self.trie.get(pos)) {
                length >>= 1;
                pos = trie_unwrap!(self.jump_by_delta(pos + 1));
            } else {
                length = length - (length >> 1);
                pos = trie_unwrap!(self.skip_delta(pos + 1));
            }
        }
        // Drop down to linear search for the last few bytes.
        // length>=2 because the loop body above sees length>kMaxBranchLinearSubNodeLength>=3
        // and divides length by 2.
        loop {
            if in_unit == trie_unwrap!(self.trie.get(pos)) {
                pos += 1;
                let mut node = trie_unwrap!(self.trie.get(pos));
                if node & VALUE_IS_FINAL != 0 {
                    self.pos = Some(pos);
                    return self.value_result(pos);
                }
                // Use the non-final value as the jump delta.
                pos += 1;

                if node < MIN_TWO_UNIT_VALUE_LEAD {
                    pos += node as usize;
                } else if node < THREE_UNIT_VALUE_LEAD {
                    pos += (((node - MIN_TWO_UNIT_VALUE_LEAD) as u32) << 16) as usize
                        | trie_unwrap!(self.trie.get(pos)) as usize;
                    pos += 1;
                } else {
                    pos += (trie_unwrap!(self.trie.get(pos)) as usize) << 16
                        | trie_unwrap!(self.trie.get(pos + 1)) as usize;
                    pos += 2;
                }
                node = trie_unwrap!(self.trie.get(pos));
                self.pos = Some(pos);

                if node >= MIN_VALUE_LEAD {
                    return self.value_result(pos);
                }
                return TrieResult::NoValue;
            }
            length -= 1;
            pos = trie_unwrap!(self.skip_value(pos + 1));
            if length <= 1 {
                break;
            }
        }

        if in_unit == trie_unwrap!(self.trie.get(pos)) {
            pos += 1;
            self.pos = Some(pos);
            let node = trie_unwrap!(self.trie.get(pos));
            if node >= MIN_VALUE_LEAD {
                return self.value_result(pos);
            }
            TrieResult::NoValue
        } else {
            self.stop();
            TrieResult::NoMatch
        }
    }

    fn next_impl(&mut self, pos: usize, in_unit: u16) -> TrieResult {
        let mut node = trie_unwrap!(self.trie.get(pos));
        let mut pos = pos + 1;
        loop {
            if node < MIN_LINEAR_MATCH {
                return self.branch_next(pos, node as usize, in_unit);
            } else if node < MIN_VALUE_LEAD {
                // Match the first of length+1 units.
                let length = node - MIN_LINEAR_MATCH;
                if in_unit == trie_unwrap!(self.trie.get(pos)) {
                    pos += 1;
                    if length == 0 {
                        self.remaining_match_length = None;
                        self.pos = Some(pos);
                        node = trie_unwrap!(self.trie.get(pos));
                        if node >= MIN_VALUE_LEAD {
                            return self.value_result(pos);
                        }
                        return TrieResult::NoValue;
                    }
                    self.remaining_match_length = Some(length as usize - 1);
                    self.pos = Some(pos);
                    return TrieResult::NoValue;
                }
                // No match
                break;
            } else if (node & VALUE_IS_FINAL) != 0 {
                // No further matching units.
                break;
            } else {
                // Skip intermediate value.
                pos = skip_node_value(pos, node);
                node &= NODE_TYPE_MASK;
            }
        }
        self.stop();
        TrieResult::NoMatch
    }

    fn stop(&mut self) {
        self.pos = None;
    }

    #[inline(always)] // 1 call site and we want the Option to go away
    fn jump_by_delta(&self, pos: usize) -> Option<usize> {
        let delta = self.trie.get(pos)?;
        let v = if delta < MIN_TWO_UNIT_DELTA_LEAD {
            // nothing to do
            pos + 1 + delta as usize
        } else if delta == THREE_UNIT_DELTA_LEAD {
            let delta =
                ((self.trie.get(pos + 1)? as usize) << 16) | (self.trie.get(pos + 2)? as usize);
            pos + delta + 3
        } else {
            let delta = ((delta - MIN_TWO_UNIT_DELTA_LEAD) as usize) << 16
                | (self.trie.get(pos + 1)? as usize);
            pos + delta + 2
        };
        Some(v)
    }

    #[inline(always)] // 1 call site and we want the Option to go away
    fn skip_value(&self, pos: usize) -> Option<usize> {
        let lead_unit = self.trie.get(pos)?;
        Some(skip_value(pos + 1, lead_unit & 0x7fff))
    }

    #[inline(always)] // 1 call site and we want the Option to go away
    fn skip_delta(&self, pos: usize) -> Option<usize> {
        let delta = self.trie.get(pos)?;
        let v = if delta < MIN_TWO_UNIT_DELTA_LEAD {
            pos + 1
        } else if delta == THREE_UNIT_DELTA_LEAD {
            pos + 3
        } else {
            pos + 2
        };
        Some(v)
    }

    fn value_result(&self, pos: usize) -> TrieResult {
        match self.get_value(pos) {
            Some(result) => result,
            None => {
                // Unexpected
                debug_assert!(false);
                TrieResult::NoMatch
            }
        }
    }

    #[inline(always)] // 1 call site and we want the Option to go away
    fn get_value(&self, pos: usize) -> Option<TrieResult> {
        let lead_unit = self.trie.get(pos)?;
        if lead_unit & VALUE_IS_FINAL == VALUE_IS_FINAL {
            self.read_value(pos + 1, lead_unit & 0x7fff)
                .map(TrieResult::FinalValue)
        } else {
            self.read_node_value(pos + 1, lead_unit)
                .map(TrieResult::Intermediate)
        }
    }

    #[inline(always)] // 1 call site and we want the Option to go away
    fn read_value(&self, pos: usize, lead_unit: u16) -> Option<i32> {
        let v = if lead_unit < MIN_TWO_UNIT_VALUE_LEAD {
            lead_unit.into()
        } else if lead_unit < THREE_UNIT_VALUE_LEAD {
            ((lead_unit - MIN_TWO_UNIT_VALUE_LEAD) as i32) << 16 | self.trie.get(pos)? as i32
        } else {
            (self.trie.get(pos)? as i32) << 16 | self.trie.get(pos + 1)? as i32
        };
        Some(v)
    }

    #[inline(always)] // 1 call site and we want the Option to go away
    fn read_node_value(&self, pos: usize, lead_unit: u16) -> Option<i32> {
        let v = if lead_unit < (MIN_TWO_UNIT_NODE_VALUE_LEAD) {
            ((lead_unit >> 6) - 1).into()
        } else if lead_unit < THREE_UNIT_NODE_VALUE_LEAD {
            (((lead_unit & 0x7fc0) - MIN_TWO_UNIT_NODE_VALUE_LEAD) as i32) << 10
                | self.trie.get(pos)? as i32
        } else {
            (self.trie.get(pos)? as i32) << 16 | self.trie.get(pos + 1)? as i32
        };
        Some(v)
    }
}