//! Regex compiler back-end: transforms IR into a CompiledRegex

use crate::bytesearch::{AsciiBitmap, ByteArraySet};
use crate::folds;
use crate::insn::{CompiledRegex, Insn, LoopFields, MAX_BYTE_SEQ_LENGTH, MAX_CHAR_SET_LENGTH};
use crate::ir;
use crate::ir::Node;
use crate::startpredicate;
use crate::types::{BracketContents, CaptureGroupID};
use std::convert::TryInto;

/// \return an anchor instruction for a given IR anchor.
fn make_anchor(anchor_type: ir::AnchorType) -> Insn {
    match anchor_type {
        ir::AnchorType::StartOfLine => Insn::StartOfLine,
        ir::AnchorType::EndOfLine => Insn::EndOfLine,
    }
}

/// Weirdly placed optimization.
/// If the given bracket can be represented as ASCII contents, return the
/// bitmap. Otherwise nothing.
fn bracket_as_ascii(bc: &BracketContents) -> Option<AsciiBitmap> {
    let mut result = AsciiBitmap::default();
    // We just assume that inverted brackets contain non-ASCII characters.
    if bc.invert {
        return None;
    }
    for r in bc.cps.intervals() {
        debug_assert!(r.first <= r.last);
        if r.last >= 128 {
            return None;
        }
        for bit in r.first..=r.last {
            result.set(bit as u8)
        }
    }
    Some(result)
}

/// Type which wraps up the context needed to emit a CompiledRegex.
struct Emitter {
    result: CompiledRegex,

    // Number of loops seen so far.
    next_loop_id: u32,
}

impl Emitter {
    /// Emit a ByteSet instruction.
    /// We awkwardly optimize it like so.
    fn make_byte_set_insn(&self, bytes: &[u8]) -> Insn {
        match bytes.len() {
            0 => Insn::JustFail,
            1 => Insn::ByteSeq1(bytes.try_into().unwrap()),
            2 => Insn::ByteSet2(ByteArraySet(bytes.try_into().unwrap())),
            3 => Insn::ByteSet3(ByteArraySet(bytes.try_into().unwrap())),
            4 => Insn::ByteSet4(ByteArraySet(bytes.try_into().unwrap())),
            _ => panic!("Byte set is too long"),
        }
    }

    /// Emit an instruction.
    /// Return the "instruction" as an index.
    fn emit_insn(&mut self, insn: Insn) {
        self.result.insns.push(insn);
    }

    /// Get an instruction at a given index.
    fn get_insn(&mut self, idx: u32) -> &mut Insn {
        &mut self.result.insns[idx as usize]
    }

    /// \return the offset of the next instruction emitted.
    fn next_offset(&self) -> u32 {
        self.result.insns.len() as u32
    }

    fn emit_insn_offset(&mut self, insn: Insn) -> u32 {
        let ret = self.next_offset();
        self.emit_insn(insn);
        ret
    }

    /// Emit instructions corresponding to a given node.
    /// TODO: make this non-recursive to avoid stack overflow.
    fn emit_node(&mut self, node: &Node) {
        match node {
            Node::Empty => {}
            Node::Goal => self.emit_insn(Insn::Goal),
            Node::Char { c, icase } => {
                let c = *c;
                if !*icase {
                    self.emit_insn(Insn::Char(c))
                } else {
                    std::debug_assert!(folds::fold(c) == c, "Char should be folded");
                    self.emit_insn(Insn::CharICase(c))
                }
            }
            Node::Cat(children) => {
                for nn in children {
                    self.emit_node(nn)
                }
            }
            Node::Alt(left, right) => {
                // Alternation is followed by the primary branch and has a jump to secondary
                // branch. After primary branch, jump to the continuation.
                let alt_insn = self.emit_insn_offset(Insn::Alt { secondary: 0 });
                self.emit_node(left);
                let jump_insn = self.emit_insn_offset(Insn::Jump { target: 0 });
                let right_branch = self.next_offset();
                self.emit_node(right);
                let exit = self.next_offset();

                // Fix up our jump targets.
                match self.get_insn(alt_insn) {
                    Insn::Alt { secondary } => *secondary = right_branch,
                    _ => panic!("Should be an Alt instruction"),
                }
                match self.get_insn(jump_insn) {
                    Insn::Jump { target } => *target = exit,
                    _ => panic!("Should be a Jump instruction"),
                }
            }
            Node::Bracket(contents) => {
                if let Some(ascii_contents) = bracket_as_ascii(contents) {
                    self.emit_insn(Insn::AsciiBracket(ascii_contents))
                } else {
                    self.emit_insn(Insn::Bracket(contents.clone()))
                }
            }
            Node::MatchAny => self.emit_insn(Insn::MatchAny),
            Node::MatchAnyExceptLineTerminator => {
                self.emit_insn(Insn::MatchAnyExceptLineTerminator)
            }
            Node::Anchor(anchor_type) => self.emit_insn(make_anchor(*anchor_type)),
            Node::Loop {
                loopee,
                quant,
                enclosed_groups,
            } => {
                let loop_id = self.next_loop_id;
                self.next_loop_id += 1;
                let loop_insn = self.emit_insn_offset(Insn::EnterLoop(LoopFields {
                    loop_id,
                    min_iters: quant.min,
                    max_iters: quant.max,
                    greedy: quant.greedy,
                    exit: 0,
                }));
                self.result.loops += 1;
                // Emit a sequence of ResetCaptureGroup for any contained groups.
                for gid in enclosed_groups.start..enclosed_groups.end {
                    self.emit_insn(Insn::ResetCaptureGroup(gid))
                }
                self.emit_node(loopee);
                self.emit_insn(Insn::LoopAgain { begin: loop_insn });
                // Fix up our loop exit.
                let exit = self.next_offset();
                match self.get_insn(loop_insn) {
                    Insn::EnterLoop(fields) => fields.exit = exit,
                    _ => panic!("Should be an EnterLoop instruction"),
                }
            }
            Node::Loop1CharBody { loopee, quant } => {
                self.emit_insn(Insn::Loop1CharBody {
                    min_iters: quant.min,
                    max_iters: quant.max,
                    greedy: quant.greedy,
                });
                self.emit_node(loopee);
            }
            Node::CaptureGroup(contents, group) => {
                let group = *group as CaptureGroupID;
                self.result.groups += 1;
                self.emit_insn(Insn::BeginCaptureGroup(group));
                self.emit_node(contents);
                self.emit_insn(Insn::EndCaptureGroup(group));
            }
            Node::LookaroundAssertion {
                negate,
                backwards,
                start_group,
                end_group,
                contents,
            } => {
                let lookaround = if *backwards {
                    self.emit_insn_offset(Insn::LookbehindInsn {
                        negate: *negate,
                        start_group: *start_group,
                        end_group: *end_group,
                        continuation: 0,
                    })
                } else {
                    self.emit_insn_offset(Insn::LookaheadInsn {
                        negate: *negate,
                        start_group: *start_group,
                        end_group: *end_group,
                        continuation: 0,
                    })
                };

                self.emit_node(contents);
                self.emit_insn(Insn::Goal);

                // Fix up the continuation.
                let next_insn = self.next_offset();
                match self.get_insn(lookaround) {
                    Insn::LookbehindInsn {
                        ref mut continuation,
                        ..
                    } => *continuation = next_insn,
                    Insn::LookaheadInsn {
                        ref mut continuation,
                        ..
                    } => *continuation = next_insn,
                    _ => panic!("Should be a Lookaround instruction"),
                }
            }
            Node::WordBoundary { invert } => self.emit_insn(Insn::WordBoundary { invert: *invert }),
            &Node::BackRef(group) => {
                debug_assert!(group >= 1, "Group should not be zero");
                // -1 because \1 matches the first capture group, which has index 0.
                self.emit_insn(Insn::BackRef(group - 1))
            }

            Node::ByteSet(bytes) => self.emit_insn(self.make_byte_set_insn(bytes)),

            Node::CharSet(chars) => {
                debug_assert!(!chars.is_empty() && chars.len() <= MAX_CHAR_SET_LENGTH);
                let mut arr = [chars[0]; MAX_CHAR_SET_LENGTH];
                arr[..chars.len()].copy_from_slice(chars.as_slice());
                self.emit_insn(Insn::CharSet(arr))
            }

            #[allow(clippy::assertions_on_constants)]
            Node::ByteSequence(bytes) => {
                assert!(
                    MAX_BYTE_SEQ_LENGTH == 16,
                    "Need to update our emitting logic"
                );
                for chunk in bytes.as_slice().chunks(MAX_BYTE_SEQ_LENGTH) {
                    let insn = match chunk.len() {
                        1 => Insn::ByteSeq1(chunk.try_into().unwrap()),
                        2 => Insn::ByteSeq2(chunk.try_into().unwrap()),
                        3 => Insn::ByteSeq3(chunk.try_into().unwrap()),
                        4 => Insn::ByteSeq4(chunk.try_into().unwrap()),
                        5 => Insn::ByteSeq5(chunk.try_into().unwrap()),
                        6 => Insn::ByteSeq6(chunk.try_into().unwrap()),
                        7 => Insn::ByteSeq7(chunk.try_into().unwrap()),
                        8 => Insn::ByteSeq8(chunk.try_into().unwrap()),
                        9 => Insn::ByteSeq9(chunk.try_into().unwrap()),
                        10 => Insn::ByteSeq10(chunk.try_into().unwrap()),
                        11 => Insn::ByteSeq11(chunk.try_into().unwrap()),
                        12 => Insn::ByteSeq12(chunk.try_into().unwrap()),
                        13 => Insn::ByteSeq13(chunk.try_into().unwrap()),
                        14 => Insn::ByteSeq14(chunk.try_into().unwrap()),
                        15 => Insn::ByteSeq15(chunk.try_into().unwrap()),
                        16 => Insn::ByteSeq16(chunk.try_into().unwrap()),
                        _ => panic!("Unexpected chunk size"),
                    };
                    self.emit_insn(insn)
                }
            }
        }
    }
}

/// Compile the given IR to a CompiledRegex.
pub fn emit(n: &ir::Regex) -> CompiledRegex {
    let mut emitter = Emitter {
        next_loop_id: 0,
        result: CompiledRegex {
            insns: Vec::new(),
            loops: 0,
            groups: 0,
            flags: n.flags,
            start_pred: startpredicate::predicate_for_re(n),
        },
    };
    emitter.emit_node(&n.node);
    emitter.result
}