1	# ezhttp/src/cache: Cache-Control and a small freshness check (RFC 9111). This is
2	# not a shared cache. Expires is recorded; max-age overrides it for freshness.
3	# IMF-fixdate arithmetic is not evaluated.
4	import Base
5	import ./http.bend as Http
6	
7	# the directives this module records
8	type Cc is Data:
9	  Cc{
10	    no_store: Bool
11	    no_cache: Bool
12	    public: Bool
13	    private: Bool
14	    max_age: String
15	    expires: String
16	  }
17	
18	# no directives
19	def cache.empty() -> Cc:
20	  Cc{False{}, False{}, False{}, False{}, "", ""}
21	
22	# set no-store
23	def cache.store(c: Cc) -> Cc:
24	  match c:
25	    case Cc{no_store, no_cache, public, private, max_age, expires}:
26	      Cc{True{}, no_cache, public, private, max_age, expires}
27	
28	# set no-cache
29	def cache.revalidate(c: Cc) -> Cc:
30	  match c:
31	    case Cc{no_store, no_cache, public, private, max_age, expires}:
32	      Cc{no_store, True{}, public, private, max_age, expires}
33	
34	# set public
35	def cache.pub(c: Cc) -> Cc:
36	  match c:
37	    case Cc{no_store, no_cache, public, private, max_age, expires}:
38	      Cc{no_store, no_cache, True{}, private, max_age, expires}
39	
40	# set private
41	def cache.priv(c: Cc) -> Cc:
42	  match c:
43	    case Cc{no_store, no_cache, public, private, max_age, expires}:
44	      Cc{no_store, no_cache, public, True{}, max_age, expires}
45	
46	# set max-age text
47	def cache.age(c: Cc, age: String) -> Cc:
48	  match c:
49	    case Cc{no_store, no_cache, public, private, max_age, expires}:
50	      Cc{no_store, no_cache, public, private, age, expires}
51	
52	# set the Expires header text
53	def cache.expires(c: Cc, when: String) -> Cc:
54	  match c:
55	    case Cc{no_store, no_cache, public, private, max_age, expires}:
56	      Cc{no_store, no_cache, public, private, max_age, when}
57	
58	# private, or leave the directive unrecognized
59	def cache.one.private(c: Cc, _name: String, _value: String, hit: Bool) -> Cc:
60	  match hit:
61	    case True{}:
62	      cache.priv(c)
63	    case False{}:
64	      c
65	
66	# public
67	def cache.one.public(c: Cc, +name: String, value: String, hit: Bool) -> Cc:
68	  match hit:
69	    case True{}:
70	      cache.pub(c)
71	    case False{}:
72	      cache.one.private(c, name, value, String.eq(name, "private"))
73	
74	# max-age, kept only when the argument is digits (delta-seconds)
75	def cache.one.age_ok(c: Cc, +raw: String, m: Maybe<&2, Nat>) -> Cc:
76	  match m:
77	    case None{}:
78	      c
79	    case Some{n}:
80	      cache.age(c, raw)
81	
82	# max-age
83	def cache.one.age(c: Cc, +name: String, +value: String, hit: Bool) -> Cc:
84	  match hit:
85	    case True{}:
86	      cache.one.age_ok(c, value, Http.digits.read(value))
87	    case False{}:
88	      cache.one.public(c, name, value, String.eq(name, "public"))
89	
90	# no-cache
91	def cache.one.cache(c: Cc, +name: String, +value: String, hit: Bool) -> Cc:
92	  match hit:
93	    case True{}:
94	      cache.revalidate(c)
95	    case False{}:
96	      cache.one.age(c, name, value, String.eq(name, "max-age"))
97	
98	# no-store
99	def cache.one.store(c: Cc, +name: String, +value: String, hit: Bool) -> Cc:
100	  match hit:
101	    case True{}:
102	      cache.store(c)
103	    case False{}:
104	      cache.one.cache(c, name, value, String.eq(name, "no-cache"))
105	
106	# a directive with no `=` argument
107	def cache.one.flag(c: Cc, +name: String) -> Cc:
108	  cache.one.store(c, name, "", String.eq(name, "no-store"))
109	
110	# flag or name=value
111	def cache.one.dispatch(c: Cc, +piece: String, cut: Http.Cut) -> Cc:
112	  match cut:
113	    case Http.NoCut{}:
114	      cache.one.flag(c, String.to_lower(String.trim(piece)))
115	    case Http.Cut{+before, after}:
116	      +name = String.to_lower(String.trim(before))
117	      cache.one.store(c, name, String.trim(after), String.eq(name, "no-store"))
118	
119	# one directive text
120	def cache.one(c: Cc, +piece: String) -> Cc:
121	  cache.one.dispatch(c, piece, Http.divide(piece, "="))
122	
123	# walk directives. The list shrinks.
124	def cache.fold(ps: List<&2, String>, c: Cc) -> Cc:
125	  match ps:
126	    case []:
127	      c
128	    case h <> t:
129	      cache.fold(t, cache.one(c, String.trim(h)))
130	
131	# Cache-Control field value (RFC 9111 §5.2)
132	def cache.parse(value: String) -> Cc:
133	  cache.fold(String.split(value, ','), cache.empty())
134	
135	# record an Expires header beside Cache-Control (RFC 9111 §5.3)
136	def cache.note(value: String, expires: String) -> Cc:
137	  cache.expires(cache.parse(value), expires)
138	
139	# true when max-age is present, so it overrides Expires (RFC 9111 §4.2.1)
140	def cache.max_age(c: Cc) -> Bool:
141	  match c:
142	    case Cc{no_store, no_cache, public, private, max_age, expires}:
143	      Bool.not(String.eq(max_age, ""))
144	
145	# freshness from max-age. no-store and no-cache are not fresh.
146	def cache.fresh.go(age: Nat, m: Maybe<&2, Nat>) -> Bool:
147	  match m:
148	    case None{}:
149	      False{}
150	    case Some{n}:
151	      Nat.is_lt(age, n)
152	
153	# no-cache must revalidate
154	def cache.fresh.cache(age: Nat, max_age: String, no_cache: Bool) -> Bool:
155	  match no_cache:
156	    case True{}:
157	      False{}
158	    case False{}:
159	      cache.fresh.go(age, Http.digits.read(max_age))
160	
161	# no-store is not reused
162	def cache.fresh.store(age: Nat, no_cache: Bool, max_age: String, no_store: Bool)
163	  -> Bool:
164	  match no_store:
165	    case True{}:
166	      False{}
167	    case False{}:
168	      cache.fresh.cache(age, max_age, no_cache)
169	
170	# age seconds against the directives. max-age=0 is stale (RFC 9111 §4.2.1).
171	def cache.fresh(c: Cc, age: Nat) -> Bool:
172	  match c:
173	    case Cc{no_store, no_cache, public, private, max_age, expires}:
174	      cache.fresh.store(age, no_cache, max_age, no_store)