//! JSON depth calculations on byte streams.
//!
//! Used for iterating over the document while keeping track of depth.
//! This is heavily optimized for skipping irrelevant parts of a JSON document.
//! For example, to quickly skip to the end of the currently opened object one can set the
//! depth to `1` and then advance until it reaches `0`.
//!
//! It also supports stopping and resuming with [`ResumeClassifierState`].
//!
//! # Examples
//!
//! Illustrating how the skipping works:
//! ```rust
//! use rsonpath_lib::classification::quotes::classify_quoted_sequences;
//! use rsonpath_lib::classification::depth::{
//!     classify_depth, DepthIterator, DepthBlock
//! };
//! use aligners::AlignedBytes;
//!
//! let input = AlignedBytes::new_padded(r#"[42, {"b":[[]],"c":{}}, 44]}"#.as_bytes());
//! // Name a few places in the input:                  ^^            ^
//! //                                                  AB            C
//! let quote_classifier = classify_quoted_sequences(&input);
//! // Goal: skip through the document until the end of the current list.
//! // We pass b'[' as the opening to tell the classifier to consider only '[' and ']' characters.
//! let mut depth_classifier = classify_depth(quote_classifier, b'[');
//! let mut depth_block = depth_classifier.next().unwrap();
//!
//! assert_eq!(depth_block.get_depth(), 0);
//! assert!(depth_block.advance_to_next_depth_decrease()); // Advances to A.
//! assert_eq!(depth_block.get_depth(), 2);
//! assert!(depth_block.advance_to_next_depth_decrease()); // Advances to B.
//! assert_eq!(depth_block.get_depth(), 1);
//! assert!(depth_block.advance_to_next_depth_decrease()); // Advances to C.
//! assert_eq!(depth_block.get_depth(), 0);
//! // Skipping complete.
//! ```
//!
//! Idiomatic usage for a high-performance skipping loop:
//! ```rust
//! use rsonpath_lib::classification::depth::{classify_depth, DepthBlock, DepthIterator};
//! use rsonpath_lib::classification::quotes::classify_quoted_sequences;
//! use aligners::AlignedBytes;
//!
//! let json = r#"
//!     "a": [
//!         42,
//!         {
//!             "b": {},
//!             "c": {}
//!         },
//!         44
//!     ],
//!     "b": {
//!         "c": "value"
//!     }
//! }{"target":true}"#;
//! let input = AlignedBytes::new_padded(json.as_bytes());
//! // We expect to reach the newline before the opening brace of the second object.
//! let expected_idx = json.len() - 15;
//! let quote_classifier = classify_quoted_sequences(&input);
//! let mut depth_classifier = classify_depth(quote_classifier, b'{');
//! let mut current_depth = 1;
//!
//! while let Some(mut vector) = depth_classifier.next() {
//!     vector.add_depth(current_depth);
//!
//!     if vector.estimate_lowest_possible_depth() <= 0 {
//!         while vector.advance_to_next_depth_decrease() {
//!             if vector.get_depth() == 0 {
//!                 let stop_state = depth_classifier.stop(Some(vector));
//!                 assert_eq!(stop_state.get_idx(), expected_idx);
//!                 return;
//!             }
//!         }
//!     }
//!
//!     current_depth = vector.depth_at_end();
//! }
//! unreachable!();
//! ```
//!
use crate::classification::{quotes::QuoteClassifiedIterator, ResumeClassifierState};
use cfg_if::cfg_if;

/// Common trait for structs that enrich a byte block with JSON depth information.
#[allow(clippy::len_without_is_empty)]
pub trait DepthBlock<'a>: Sized {
    /// Add depth to the block.
    /// This is usually done at the start of a block to carry any accumulated
    /// depth over.
    fn add_depth(&mut self, depth: isize);

    /// Returns depth at the current position.
    fn get_depth(&self) -> isize;

    /// A lower bound on the depth that can be reached when advancing.
    ///
    /// It is guaranteed that [`get_depth`](`DepthBlock::get_depth`)
    /// will always return something greater or equal to this return value, but it is not guaranteed to be a depth that
    /// is actually achievable within the block. In particular, an implementation always returning
    /// [`isize::MIN`] is a correct implementation. This is meant to be a tool for performance improvements,
    /// not reliably checking the actual minimal depth within the block.
    fn estimate_lowest_possible_depth(&self) -> isize;

    /// Returns exact depth at the end of the decorated slice.
    fn depth_at_end(&self) -> isize;

    /// Advance to the next position at which depth may decrease.
    ///
    /// # Returns
    /// `false` if the end of the block was reached without any depth decrease,
    /// `true` otherwise.
    fn advance_to_next_depth_decrease(&mut self) -> bool;
}

/// Trait for depth iterators, i.e. finite iterators returning depth information
/// about JSON documents.
pub trait DepthIterator<'a, I: QuoteClassifiedIterator<'a>>:
    Iterator<Item = Self::Block> + 'a
{
    /// Type of the [`DepthBlock`] implementation used by this iterator.
    type Block: DepthBlock<'a>;

    /// Resume classification from a state retrieved by a previous
    /// [`DepthIterator::stop`] or [`StructuralIterator::stop`](`crate::classification::structural::StructuralIterator::stop`) invocation.
    fn resume(state: ResumeClassifierState<'a, I>, opening: u8) -> (Option<Self::Block>, Self);

    /// Stop classification and return a state object that can be used to resume
    /// a classifier from the place in which the current one was stopped.
    fn stop(self, block: Option<Self::Block>) -> ResumeClassifierState<'a, I>;
}

/// The result of resuming a [`DepthIterator`] &ndash; the first block and the rest of the iterator.
pub struct DepthIteratorResumeOutcome<'a, I: QuoteClassifiedIterator<'a>, D: DepthIterator<'a, I>>(
    pub Option<D::Block>,
    pub D,
);

cfg_if! {
    if #[cfg(any(doc, not(feature = "simd")))] {
        mod nosimd;

        /// Enrich quote classified blocks with depth information.
        #[inline(always)]
        pub fn classify_depth<'a, I: QuoteClassifiedIterator<'a>>(iter: I, opening: u8) -> impl DepthIterator<'a, I> {
            nosimd::VectorIterator::new(iter, opening)
        }

        /// Resume classification using a state retrieved from a previously
        /// used classifier via the `stop` function.
        #[inline(always)]
        pub fn resume_depth_classification<'a, I: QuoteClassifiedIterator<'a>>(
            state: ResumeClassifierState<'a, I>, opening: u8
        ) -> DepthIteratorResumeOutcome<'a, I, impl DepthIterator<'a, I>> {
            let (first_block, iter) = nosimd::VectorIterator::resume(state, opening);
            DepthIteratorResumeOutcome(first_block, iter)
        }
    }
    else if #[cfg(simd = "avx2")] {
        mod avx2;

        /// Enrich quote classified blocks with depth information.
        #[inline(always)]
        pub fn classify_depth<'a, I: QuoteClassifiedIterator<'a>>(iter: I, opening: u8) -> impl DepthIterator<'a, I> {
            avx2::VectorIterator::new(iter, opening)
        }

        /// Resume classification using a state retrieved from a previously
        /// used classifier via the `stop` function.
        #[inline(always)]
        pub fn resume_depth_classification<'a, I: QuoteClassifiedIterator<'a>>(
            state: ResumeClassifierState<'a, I>, opening: u8
        ) -> DepthIteratorResumeOutcome<'a, I, impl DepthIterator<'a, I>> {
            let (first_block, iter) = avx2::VectorIterator::resume(state, opening);
            DepthIteratorResumeOutcome(first_block, iter)
        }
    }
    else {
        compile_error!("Target architecture is not supported by SIMD features of this crate. Disable the default `simd` feature.");
    }
}