1	# rule fuel: a call to a def of the same file, its name then `(`, passes a
2	# fixed number where that def takes its fuel: a Nat literal (`1000n`: digits,
3	# then `n`), or `U32.to_nat` of a U32 literal (`U32.to_nat(100000)`: digits
4	# alone), the form a big fuel is written in. A fuel parameter is known by its
5	# name alone (its first lowercase name before the colon): `fuel`, `gas`,
6	# `steps` or `budget`, or any name starting with `fuel`. The fuel-0 arm
7	# returns what it has, so input past the number comes out cut short, with no
8	# error (a JSON printer that stopped at 1000 tasks). A law is no safer: a law
9	# proved over every fuel and then used at a big fixed one, against a goal
10	# written another way, overflows the checker ("the machine stack overflowed",
11	# bend 2.0.33 and 2.0.34; ez's lock walk at `U32.to_nat(100000)` did). Before
12	# 2.0.29 the checker unrolled the fixed fuel and hung; 2.0.29 made a checked
13	# recursion on a Nat literal linear. Derive the fuel from the input's size
14	# (`Nat.mul(size, 4n)`) or take it as a parameter. A number of any size
15	# counts, an exact repeat count such as `3n` included. Only an argument that
16	# is the literal alone, or `U32.to_nat(` the U32 literal alone `)`, counts: a
17	# let-bound literal and a parenthesized `(7n)` are not seen. A def's own calls
18	# are exempt: its step passes `fuel - 1`, not a literal. So is a call to a
19	# def that returns an effect: its header holds a top-level `->` token with
20	# the capitalized name token `IO` right after it (`-> IO(Unit):`, parameters
21	# wrapped over lines or not), or, when the `->` is the header's last token,
22	# first on the line under it (`->` then `IO(Unit):`). An effect loop's fuel
23	# bounds its reads, frames or retries, a budget the outside world sets, not
24	# the size of an input it was handed (a netcode drain of 256 datagrams a
25	# tick, a read of 100000 chunks).
26	import Base
27	import ../../src.bend as Src
28	import ../../lazy/lazy.bend as Lazy
29	import ../../finding.bend as F
30	import ../../syntax/lex.bend as Lex
31	import ../../syntax/tree.bend as Tree
32	import ../../syntax/bind.bend as Bind
33	import ../calls.bend as Calls
34	import ../tokens.bend as T
35	
36	# a def of the file and the position of a fuel parameter
37	type Fuel is Data:
38	  Fuel{name: String, at: Nat}
39	
40	# is it a fuel parameter's name?
41	def is_fuel(+nn: String) -> Bool:
42	  Bool.or(String.starts_with(nn, "fuel"),
43	    List.contains(~String, ~String.eq, ["gas", "steps", "budget"], nn))
44	
45	# the fuel parameters among a def's parameters, from position i on
46	def fuels.params(ps: List<&2, Tree.Node>, +name: String, +ii: Nat) -> List<&2, Fuel>:
47	  match ps:
48	    case Nil{}:
49	      Nil{}
50	    case Con{p, rest}:
51	      +more = fuels.params(rest, name, 1n+ii)
52	      Bool.pick(List<&2, Fuel>, is_fuel(Calls.param_name(p)), Fuel{name, ii} <> more, more)
53	
54	# is the chain's first node the capitalized name `IO`?
55	def effect.io(nn: Tree.Node) -> Bool:
56	  match nn:
57	    case Tree.NCons{Tree.Leaf{Lex.Tok{Lex.TUpper{}, +t, l, c}}, rest}:
58	      String.eq(t, "IO")
59	    case other:
60	      False{}
61	
62	# the kids of the first statement under a def: the line a header wrapped
63	# after its `->` continues on (`->` then `IO(Unit):` on the next line)
64	def under(body: Tree.Node) -> Tree.Node:
65	  match body:
66	    case Tree.NCons{Tree.Stmt{kind, kids, inner}, rest}:
67	      kids
68	    case other:
69	      Tree.NNil{}
70	
71	# what follows an arrow: the rest of the header, or the line under it (below)
72	# when the arrow ends the header; is it `IO`?
73	def effect.next(rest: Tree.Node, +below: Tree.Node) -> Bool:
74	  match rest:
75	    case Tree.NCons{h, tail}:
76	      effect.io(Tree.NCons{h, tail})
77	    case other:
78	      effect.io(below)
79	
80	# does a def's header return an effect: a top-level `->` with the
81	# capitalized name `IO` right after it, on the next line (below) when the
82	# arrow ends the header?
83	def effect(sig: Tree.Node, +below: Tree.Node) -> Bool:
84	  match sig:
85	    case Tree.NCons{h, +rest}:
86	      +more = effect(rest, below)
87	      Bool.or(Bool.and(Calls.kind.leaf(~Calls.kind.arrow, h), effect.next(rest, below)), more)
88	    case other:
89	      False{}
90	
91	# the parameters a def's fuel may sit in: none when it returns an effect
92	def slots(+sig: Tree.Node, body: Tree.Node) -> List<&2, Tree.Node>:
93	  Bool.pick(List<&2, Tree.Node>, effect(sig, under(body)), [], Calls.params(sig))
94	
95	# every fuel parameter of every def of the file that returns no effect
96	def fuels(ds: List<&2, Calls.Def>) -> List<&2, Fuel>:
97	  match ds:
98	    case Nil{}:
99	      Nil{}
100	    case Con{Calls.Def{name, sig, body}, rest}:
101	      List.append(&2, Fuel, fuels.params(slots(sig, body), name, 0n), fuels(rest))
102	
103	# the group of a call at line ll, column cc, as a finding when it holds one
104	# U32 literal alone (digits) and ok says the call is `U32.to_nat(`
105	def converted(kids: Tree.Node, ok: Bool, +ll: U32, +cc: U32, +path: String) -> List<&2, F.Finding>:
106	  match kids:
107	    case Tree.NCons{Tree.Leaf{Lex.Tok{Lex.TNum{}, +t, l, c}}, Tree.NNil{}}:
108	      Bool.pick(List<&2, F.Finding>, Bool.and(ok, T.digits_ok(String.to_list(t))),
109	        [F.Finding{path, ll, cc, U32.from_nat(Nat.add(String.length(t), 12n)), "fuel",
110	          "The fuel is fixed at U32.to_nat(" ++ t
111	            ++ "), so longer input is silently cut short; derive the fuel from the input size."}],
112	        [])
113	    case other:
114	      Nil{}
115	
116	# an argument that is a Nat literal alone (digits, then `n`), or
117	# `U32.to_nat` of a U32 literal alone, as a finding
118	def literal(aa: Tree.Node, +path: String) -> List<&2, F.Finding>:
119	  match aa:
120	    case Tree.NCons{Tree.Leaf{Lex.Tok{Lex.TNum{}, +t, l, c}}, Tree.NNil{}}:
121	      Bool.pick(List<&2, F.Finding>, T.nat_ok(String.to_list(t)),
122	        [F.Finding{path, l, c, U32.from_nat(String.length(t)), "fuel",
123	          "The fuel is fixed at " ++ t ++ ", so longer input is silently cut short; derive the fuel from the input size."}],
124	        [])
125	    case Tree.NCons{Tree.Leaf{Lex.Tok{Lex.TDotted{}, +t, l, c}},
126	      Tree.NCons{Tree.Group{Lex.Tok{ok, +o, ol, oc}, kids, cl}, Tree.NNil{}}}:
127	      converted(kids, Bool.and(String.eq(t, "U32.to_nat"), String.eq(o, "(")), l, c, path)
128	    case other:
129	      Nil{}
130	
131	# a call to name with these arguments, against every fuel parameter
132	def call(fs: List<&2, Fuel>, +name: String, +as: List<&2, Tree.Node>, +path: String) -> List<&2, F.Finding>:
133	  match fs:
134	    case Nil{}:
135	      Nil{}
136	    case Con{Fuel{+fn, at}, rest}:
137	      +more = call(rest, name, as, path)
138	      Lazy.stop(List<&2, F.Finding>, Bool.not(String.eq(fn, name)), more,
139	        _u => List.append(&2, F.Finding, literal(Calls.arg(as, at), path), more))
140	
141	# every call at any depth of a chain, but the def's own (self)
142	def calls(nn: Tree.Node, +fs: List<&2, Fuel>, +self: String, +path: String) -> List<&2, F.Finding>:
143	  match nn:
144	    case Tree.NCons{Tree.Leaf{Lex.Tok{k, +t, l, c}}, Tree.NCons{Tree.Group{Lex.Tok{_, +o, _, _}, +kids, _}, rest}}:
145	      +more = List.concat(&2, F.Finding, [calls(kids, fs, self, path), calls(rest, fs, self, path)])
146	      +hit = Bool.and(Bool.and(Lex.is_name(k), String.eq(o, "(")), Bool.not(String.eq(t, self)))
147	      Lazy.stop(List<&2, F.Finding>, Bool.not(hit), more,
148	        _u => List.append(&2, F.Finding, call(fs, t, Calls.args(kids), path), more))
149	    case Tree.NCons{Tree.Group{open, kids, close}, rest}:
150	      List.concat(&2, F.Finding, [calls(kids, fs, self, path), calls(rest, fs, self, path)])
151	    case Tree.NCons{Tree.Stmt{kind, kids, body}, rest}:
152	      List.concat(&2, F.Finding, [calls(kids, fs, self, path), calls(body, fs, self, path),
153	        calls(rest, fs, self, path)])
154	    case Tree.NCons{h, rest}:
155	      calls(rest, fs, self, path)
156	    case other:
157	      Nil{}
158	
159	# a def's (or a law's) name, "" when it has none
160	def own(mm: Maybe<&2, String>) -> String:
161	  match mm:
162	    case None{}:
163	      ""
164	    case Some{n}:
165	      n
166	
167	# each top-level statement, walked as its own def
168	def check.go(root: Tree.Node, +fs: List<&2, Fuel>, +path: String) -> List<&2, F.Finding>:
169	  match root:
170	    case Tree.NCons{Tree.Stmt{kind, +kids, body}, rest}:
171	      +self = own(Bind.declared(kids))
172	      +head = calls(kids, fs, self, path)
173	      +inner = calls(body, fs, self, path)
174	      List.concat(&2, F.Finding, [head, inner, check.go(rest, fs, path)])
175	    case Tree.NCons{h, rest}:
176	      check.go(rest, fs, path)
177	    case other:
178	      Nil{}
179	
180	# the rule
181	def check(ss: Src.Src) -> List<&2, F.Finding>:
182	  Src.Src{path, text, toks, +tree, bound, items} = ss
183	  check.go(tree, fuels(Calls.defs(tree)), path)