nitcc: to_minimal_dfa fix transition checks (and document it)

Signed-off-by: Jean Privat <jean@pryen.org>

diff --git a/contrib/nitcc/src/autom.nit b/contrib/nitcc/src/autom.nit
index 5c5667a..3ca56bc 100644 (file)
--- a/contrib/nitcc/src/autom.nit
+++ b/contrib/nitcc/src/autom.nit
@@ -375,12 +375,14 @@ class Automaton
        do
                trim
 
+               # Graph of known distinct states.
                var distincts = new HashMap[State, Set[State]]
                for s in states do
                        distincts[s] = new HashSet[State]
                end
 
-               # split accept states
+               # split accept states.
+               # An accept state is distinct with a non accept state.
                for s1 in states do
                        for s2 in states do
                                if distincts[s1].has(s2) then continue
@@ -398,24 +400,36 @@ class Automaton
                        end
                end
 
+               # Fixed point algorithm.
+               # * Get 2 states s1 and s2 not yet distinguished.
+               # * Get a symbol w.
+               # * If s1.trans(w) and s2.trans(w) are distinguished, then
+               #   distinguish s1 and s2.
                var changed = true
-               var ints = new Array[Int]
+               var ints = new Array[Int] # List of symbols to check
                while changed do
                        changed = false
                        for s1 in states do for s2 in states do
                                if distincts[s1].has(s2) then continue
+
+                               # The transitions use intervals. Therefore, for the states s1 and s2,
+                               # we need to check only the meaningful symbols. They are the `first`
+                               # symbol of each interval and the first one after the interval (`last+1`).
                                ints.clear
+                               # Check only `s1`; `s2` will be checked later when s1 and s2 are switched.
                                for t in s1.outs do
                                        var sym = t.symbol
                                        assert sym != null
                                        ints.add sym.first
                                        var l = sym.last
-                                       if l != null then ints.add l
+                                       if l != null then ints.add l + 1
                                end
+
+                               # Check each symbol
                                for i in ints do
                                        var ds1 = s1.trans(i)
                                        var ds2 = s2.trans(i)
-                                       if ds1 == null and ds2 == null then continue
+                                       if ds1 == ds2 then continue
                                        if ds1 != null and ds2 != null and not distincts[ds1].has(ds2) then continue
                                        distincts[s1].add(s2)
                                        distincts[s2].add(s1)
@@ -425,6 +439,8 @@ class Automaton
                        end
                end
 
+               # We need to unify not-distinguished states.
+               # Just add an epsilon-transition and DFAize the automaton.
                for s1 in states do for s2 in states do
                        if distincts[s1].has(s2) then continue
                        s1.add_trans(s2, null)