do not fail if lexing empty input

Author: exaexa

Project.toml

@@ -1,7 +1,7 @@
 name = "PikaParser"
 uuid = "3bbf5609-3e7b-44cd-8549-7c69f321e792"
 authors = ["The developers of PikaParser.jl"]
-version = "0.5.0"
+version = "0.5.1"
 
 [deps]
 DocStringExtensions = "ffbed154-4ef7-542d-bbb7-c09d3a79fcae"

src/parse.jl

@@ -208,8 +208,11 @@ parsing terminals where another terminal was already parsed successfully.
 """
 function lex(g::Grammar{G,T}, input::I)::Vector{Vector{Tuple{G,Int}}} where {G,T,I}
     q = PikaQueue(lastindex(input))
-    push!(q, firstindex(input))
-    res = [Vector{Tuple{G,Int}}() for _ = firstindex(input):lastindex(input)] # tricky: do not fill()!
+
+    # tricky: both use of eachindex() and of fill() are actually wrong here
+    res = [Vector{Tuple{G,Int}}() for _ = firstindex(input):lastindex(input)]
+    firstindex(input) <= lastindex(input) && push!(q, firstindex(input))
+
     while !isempty(q)
         pos = pop!(q)
         for tidx in g.terminals

test/fastmatch.jl

@@ -41,6 +41,7 @@ end
 
     # lexing shouldn't make too much tokens
     @test sum(length.(P.lex(g, input))) == 5
+    @test isempty(P.lex(g, ""))
 
     p = P.parse_lex(g, input)