From 9d1c8e72300338bc2e5068f0e9699af386caf8ce Mon Sep 17 00:00:00 2001
From: Melody Horn <melody@boringcactus.com>
Date: Thu, 1 Apr 2021 11:55:57 -0600
Subject: ideally, but clearly not actually, implement NFA-to-DFA

---
 bird-machine-macros/src/dfa.rs | 156 ++++++++++++++++++++++++++++++++++++
 bird-machine-macros/src/lib.rs |  34 ++++----
 bird-machine-macros/src/nfa.rs | 175 +++++++++++++++++++++++++++++++++++++++--
 3 files changed, 345 insertions(+), 20 deletions(-)
 create mode 100644 bird-machine-macros/src/dfa.rs

diff --git a/bird-machine-macros/src/dfa.rs b/bird-machine-macros/src/dfa.rs
new file mode 100644
index 0000000..0aec059
--- /dev/null
+++ b/bird-machine-macros/src/dfa.rs
@@ -0,0 +1,156 @@
+use std::collections::{BTreeMap, BTreeSet, VecDeque};
+
+use proc_macro2::TokenStream;
+use quote::{quote, ToTokens};
+
+use super::nfa::{Range, NFA};
+
+/// A nondeterministic finite automaton.
+/// By convention, always starts in state 0.
+pub struct DFA {
+    transition_table: BTreeMap<usize, BTreeMap<Range, usize>>,
+    accept_states: BTreeSet<usize>,
+}
+
+struct LabeledDFA<Label: Ord> {
+    start_state: Label,
+    transition_table: BTreeMap<Label, BTreeMap<Range, Label>>,
+    accept_states: BTreeSet<Label>,
+}
+
+impl From<NFA> for LabeledDFA<BTreeSet<usize>> {
+    fn from(nfa: NFA) -> Self {
+        type StateSet = BTreeSet<usize>;
+
+        let mut transition_table: BTreeMap<StateSet, BTreeMap<Range, StateSet>> = BTreeMap::new();
+
+        let mut queue: VecDeque<StateSet> = VecDeque::new();
+
+        let start_state = nfa.epsilon_closure(0);
+        queue.push_back(start_state.clone());
+
+        while let Some(state_set) = queue.pop_front() {
+            let state_set = nfa.epsilon_closure_all(state_set);
+
+            let mut next_table: BTreeMap<Range, StateSet> = BTreeMap::new();
+            for &state in &state_set {
+                let table_entry = nfa.transition_table.get(&state);
+                let non_epsilon_table_entries = table_entry
+                    .iter()
+                    .flat_map(|x| x.iter())
+                    .filter_map(|(c, s)| c.as_ref().map(|c| (c.clone(), s)));
+                for (c, next_states) in non_epsilon_table_entries {
+                    // TODO check for non-disjoint ranges
+                    next_table
+                        .entry(c)
+                        .or_default()
+                        .extend(nfa.epsilon_closure_all(next_states.iter().copied()))
+                }
+            }
+
+            for states in next_table.values() {
+                if !transition_table.contains_key(states) {
+                    queue.push_back(states.clone());
+                }
+            }
+            transition_table.insert(state_set, next_table);
+        }
+
+        let accept_states = transition_table
+            .keys()
+            .chain(transition_table.values().flat_map(|v| v.values()))
+            .filter(|s| s.iter().any(|s| nfa.accept_states.contains(s)))
+            .cloned()
+            .collect();
+
+        dbg!(&accept_states);
+
+        Self {
+            start_state,
+            transition_table,
+            accept_states,
+        }
+    }
+}
+
+impl<Label: Ord> From<LabeledDFA<Label>> for DFA {
+    fn from(dfa: LabeledDFA<Label>) -> Self {
+        let mut state_names = BTreeMap::new();
+        state_names.insert(&dfa.start_state, 0usize);
+        let mut next_name = 1usize;
+
+        for reachable_state in dfa.transition_table.values().flat_map(|x| x.values()) {
+            state_names.entry(reachable_state).or_insert_with(|| {
+                let result = next_name;
+                next_name = next_name.saturating_add(1);
+                result
+            });
+        }
+
+        let transition_table = dfa
+            .transition_table
+            .iter()
+            .map(|(state, table)| {
+                (
+                    state_names[&state],
+                    table
+                        .iter()
+                        .map(|(c, s)| (c.clone(), state_names[&s]))
+                        .collect(),
+                )
+            })
+            .collect();
+        let accept_states = state_names
+            .iter()
+            .filter(|(s, _)| dfa.accept_states.contains(s))
+            .map(|(_, n)| n)
+            .copied()
+            .collect();
+
+        Self {
+            transition_table,
+            accept_states,
+        }
+    }
+}
+
+impl From<NFA> for DFA {
+    fn from(a: NFA) -> Self {
+        let intermediate = LabeledDFA::from(a);
+        Self::from(intermediate)
+    }
+}
+
+impl ToTokens for Range {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let start = self.0.start();
+        let end = self.0.end();
+        let result = quote!(#start..=#end);
+        result.to_tokens(tokens);
+    }
+}
+
+impl ToTokens for DFA {
+    fn to_tokens(&self, tokens: &mut TokenStream) {
+        let transition = self
+            .transition_table
+            .iter()
+            .flat_map(|(current_state, table)| {
+                table
+                    .iter()
+                    .map(move |(ch, next_state)| quote!((#current_state, #ch) => #next_state,))
+            });
+        let accepts = self.accept_states.iter();
+        let result = quote! {
+            let mut state = 0usize;
+            for c in input.chars() {
+                state = match (state, c) {
+                    #(#transition)*
+                    _ => return false,
+                };
+            }
+            matches!(state, #(#accepts)|*)
+        };
+        result.to_tokens(tokens);
+    }
+}
diff --git a/bird-machine-macros/src/lib.rs b/bird-machine-macros/src/lib.rs
index 628ee91..c6df41c 100644
--- a/bird-machine-macros/src/lib.rs
+++ b/bird-machine-macros/src/lib.rs
@@ -1,9 +1,11 @@
 extern crate proc_macro;
 use proc_macro::TokenStream;
 
+use proc_macro2::Ident;
+use quote::{format_ident, quote, ToTokens};
 use syn::{parse_macro_input, DeriveInput, LitStr};
-use quote::{quote, ToTokens};
 
+mod dfa;
 mod nfa;
 
 #[proc_macro_attribute]
@@ -14,13 +16,14 @@ pub fn bird_machine(args: TokenStream, input: TokenStream) -> TokenStream {
     let input_type_name = &input.ident;
     let input_lifetimes: Vec<_> = input.generics.lifetimes().collect();
     let lifetime = match input_lifetimes.as_slice() {
-        [] => quote!{ 'unrestricted },
-        [lt] => quote!{ #lt },
+        [] => quote! { 'unrestricted },
+        [lt] => quote! { #lt },
         _ => panic!("multiple lifetime generics, what is this, pls to halp"),
     };
 
-    let machine = build_machine(&input_regex);
-    dbg!(&machine);
+    let machine_fn = format_ident!("bird_machine_for_{}", input_type_name);
+    let machine = build_machine(&input_regex, &machine_fn);
+    eprintln!("{}", &machine);
 
     let (_, ty_generics, where_clause) = input.generics.split_for_impl();
 
@@ -59,7 +62,7 @@ pub fn bird_machine(args: TokenStream, input: TokenStream) -> TokenStream {
     };
     let is_match = quote! {
         fn is_match(text: &#lifetime str) -> bool {
-            todo!()
+            #machine_fn(text)
         }
     };
     let is_match_at = quote! {
@@ -162,18 +165,21 @@ pub fn bird_machine(args: TokenStream, input: TokenStream) -> TokenStream {
     tokens.into()
 }
 
-fn build_machine(regex: &LitStr) -> proc_macro2::TokenStream {
+fn build_machine(regex: &LitStr, fn_name: &Ident) -> proc_macro2::TokenStream {
     let regex_text = regex.value();
-    let regex_ir = regex_syntax::Parser::new()
-        .parse(&regex_text);
+    let regex_ir = regex_syntax::Parser::new().parse(&regex_text);
     let regex_ir = match regex_ir {
         Ok(x) => x,
-        Err(err ) => panic!("error compiling regex {}: {}", regex.to_token_stream(), err),
+        Err(err) => panic!("error compiling regex {}: {}", regex.to_token_stream(), err),
     };
-    dbg!(&regex_ir);
 
-    // shout out to all the professors who've taught me how to do this
-    let mut built_nfa = nfa::NFA::default();
+    let built_nfa = nfa::NFA::from(&regex_ir);
+    dbg!("built the NFA");
+    let dfa = dfa::DFA::from(built_nfa);
 
-    todo!()
+    quote! {
+        fn #fn_name(input: &str) -> bool {
+            #dfa
+        }
+    }
 }
diff --git a/bird-machine-macros/src/nfa.rs b/bird-machine-macros/src/nfa.rs
index b7b3612..833353e 100644
--- a/bird-machine-macros/src/nfa.rs
+++ b/bird-machine-macros/src/nfa.rs
@@ -1,15 +1,178 @@
-use std::collections::{HashMap, HashSet};
+use std::cmp;
+use std::collections::{BTreeMap, BTreeSet, VecDeque};
+use std::iter::repeat_with;
+use std::ops::RangeInclusive;
 
+use regex_syntax::hir::{Class, Hir, HirKind, Literal, RepetitionKind, RepetitionRange};
+
+#[derive(Eq, PartialEq, Clone)]
+pub struct Range(pub RangeInclusive<char>);
+
+impl PartialOrd for Range {
+    fn partial_cmp(&self, other: &Self) -> Option<cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
+impl Ord for Range {
+    fn cmp(&self, other: &Self) -> cmp::Ordering {
+        let start_order = self.0.start().cmp(other.0.start());
+        start_order.then_with(|| self.0.end().cmp(other.0.end()))
+    }
+}
+
+/// A nondeterministic finite automaton.
+/// By convention, always starts in state 0.
 #[derive(Default)]
 pub struct NFA {
-    state_count: usize,
-    transition_table: HashMap<Option<char>, HashSet<usize>>,
+    pub state_count: usize,
+    pub transition_table: BTreeMap<usize, BTreeMap<Option<Range>, BTreeSet<usize>>>,
+    pub accept_states: BTreeSet<usize>,
+}
+
+impl From<&Hir> for NFA {
+    fn from(hir: &Hir) -> Self {
+        match hir.kind() {
+            HirKind::Empty => {
+                // Conventionally, this matches only the empty string,
+                // but the regex crate has implicit `.*?` at the front and `.*`
+                // at the back of every regex.
+                // TODO figure out what to do with that
+                let mut result = NFA::default();
+                result.state_count = 1;
+                result.accept_states.insert(0);
+                result
+            }
+            HirKind::Literal(literal) => match literal {
+                Literal::Unicode(ch) => {
+                    let ch = *ch;
+                    let mut result = NFA::default();
+                    result.state_count = 2;
+                    result.accept_states.insert(1);
+                    let range = Range(ch..=ch);
+                    result
+                        .transition_table
+                        .entry(0)
+                        .or_default()
+                        .entry(Some(range))
+                        .or_default()
+                        .insert(1);
+                    result
+                }
+                l => todo!("{:?}", l),
+            },
+            HirKind::Class(class) => match class {
+                Class::Unicode(class) => {
+                    let mut result = NFA::default();
+                    result.state_count = 2;
+                    result.accept_states.insert(1);
+                    for range in class.iter() {
+                        let range = Range(range.start()..=range.end());
+                        result
+                            .transition_table
+                            .entry(0)
+                            .or_default()
+                            .entry(Some(range))
+                            .or_default()
+                            .insert(1);
+                    }
+                    result
+                }
+                c => todo!("{:?}", c),
+            },
+            HirKind::Anchor(_) => {
+                // TODO actually fucking implement this
+                let mut result = NFA::default();
+                result.state_count = 1;
+                result.accept_states.insert(0);
+                result
+            }
+            HirKind::Repetition(rep) => match &rep.kind {
+                RepetitionKind::Range(range) => match range {
+                    RepetitionRange::Exactly(n) => repeat_with(|| NFA::from(rep.hir.as_ref()))
+                        .take(*n as usize)
+                        .reduce(NFA::concat)
+                        .expect("exactly 0 of something"),
+                    r => todo!("{:?}", r),
+                },
+                k => todo!("{:?}", k),
+            },
+            HirKind::Concat(elements) => elements
+                .iter()
+                .map(NFA::from)
+                .reduce(NFA::concat)
+                .expect("empty HirKind::Concat"),
+            k => todo!("{:?}", k),
+        }
+    }
 }
 
 impl NFA {
-    pub fn new_state(&mut self) -> usize {
-        let result = self.state_count;
-        self.state_count += 1;
+    fn concat(mut self, other: NFA) -> NFA {
+        // absorb the other automaton's states
+        let other_state_offset = self.state_count;
+        self.state_count += other.state_count;
+        self.transition_table
+            .extend(
+                other
+                    .transition_table
+                    .into_iter()
+                    .map(|(state, table_for_state)| {
+                        (
+                            state + other_state_offset,
+                            table_for_state
+                                .into_iter()
+                                .map(|(char_range, next_states)| {
+                                    (
+                                        char_range,
+                                        next_states
+                                            .into_iter()
+                                            .map(|s| s + other_state_offset)
+                                            .collect(),
+                                    )
+                                })
+                                .collect(),
+                        )
+                    }),
+            );
+        // for all the accept states of this machine...
+        for &self_accept in &self.accept_states {
+            // add an ε-transition from that state to the start of the other machine
+            self.transition_table
+                .entry(self_accept)
+                .or_default()
+                .entry(None)
+                .or_default()
+                .insert(other_state_offset);
+        }
+        // steal the other machine's accept states
+        self.accept_states = other.accept_states;
+        self
+    }
+
+    pub fn epsilon_closure(&self, state: usize) -> BTreeSet<usize> {
+        let mut set = BTreeSet::new();
+        set.insert(state);
+        self.epsilon_closure_all(set)
+    }
+
+    pub fn epsilon_closure_all(&self, states: impl IntoIterator<Item = usize>) -> BTreeSet<usize> {
+        let mut result = BTreeSet::new();
+        let mut queue: VecDeque<usize> = VecDeque::new();
+        queue.extend(states);
+        while let Some(next_state) = queue.pop_front() {
+            result.insert(next_state);
+            let states_after_that = self
+                .transition_table
+                .get(&next_state)
+                .and_then(|state_table| state_table.get(&None))
+                .map(|next_states| next_states.iter())
+                .into_iter()
+                .flatten()
+                .copied()
+                .filter(|s| !result.contains(s));
+            queue.extend(states_after_that);
+        }
         result
     }
 }
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