1	# ezmcp/src/sse: the data of a text/event-stream body, as an MCP client
2	# reads a POST's reply when the server streams it
3	# (basic/transports#sending-messages-to-the-server). Only `data:` fields
4	# are read: an event's data is its data lines joined by newlines, and the
5	# reply is the data of the last event that has any.
6	import Base
7	
8	# while reading: the current event's data so far (if it has any), and the
9	# data of the last event that ended
10	type Sse is Data:
11	  Sse{cur: Maybe<&2, String>, last: String}
12	
13	# one char of a line: a `\r` that ends the line is dropped
14	def unr.at(head: Char, rest: Unit -> String, ends: Bool) -> String:
15	  match ends:
16	    case True{}:
17	      ""
18	    case False{}:
19	      SCon{head, rest(Unit{})}
20	
21	# a line's text with one `\r` at its end dropped
22	def unr(text: String) -> String:
23	  match text:
24	    case SNil{}:
25	      SNil{}
26	    case SCon{+head, +tail}:
27	      unr.at(head, _u => unr(tail),
28	        Bool.and(Char.is_eq(head, '\r'), String.is_empty(tail)))
29	
30	# a data field's value: the text after `data:`, less one space
31	def value(text: String) -> String:
32	  match text:
33	    case SNil{}:
34	      SNil{}
35	    case SCon{+h, +t}:
36	      Bool.pick(String, Char.is_eq(h, ' '), t, SCon{h, t})
37	
38	# more data for the current event
39	def more(cur: Maybe<&2, String>, val: String) -> Maybe<&2, String>:
40	  match cur:
41	    case None{}:
42	      Some{val}
43	    case Some{d}:
44	      Some{d ++ "\n" ++ val}
45	
46	# the end of an event: its data, if any, is the latest
47	def end(cur: Maybe<&2, String>, last: String) -> Sse:
48	  match cur:
49	    case None{}:
50	      Sse{None{}, last}
51	    case Some{d}:
52	      Sse{None{}, d}
53	
54	def line.data(
55	  cur: Maybe<&2, String>,
56	  last: String,
57	  +line: String,
58	  data: Bool
59	) -> Sse:
60	  match data:
61	    case True{}:
62	      Sse{more(cur, value(String.drop(line, 5n))), last}
63	    case False{}:
64	      Sse{cur, last}
65	
66	def line.of(
67	  cur: Maybe<&2, String>,
68	  last: String,
69	  +line: String,
70	  blank: Bool
71	) -> Sse:
72	  match blank:
73	    case True{}:
74	      end(cur, last)
75	    case False{}:
76	      line.data(cur, last, line, String.starts_with(line, "data:"))
77	
78	# one line of the stream
79	def line(text: Sse, +line2: String) -> Sse:
80	  match text:
81	    case Sse{cur, last}:
82	      line.of(cur, last, line2, String.is_empty(line2))
83	
84	# every line of the stream, in order
85	def lines(ls: List<&2, String>, text: Sse) -> Sse:
86	  match ls:
87	    case []:
88	      text
89	    case l <> rest:
90	      lines(rest, line(text, unr(l)))
91	
92	# the data the stream ends with: an unfinished event's, else the last one's
93	def fin(text: Sse) -> String:
94	  match text:
95	    case Sse{cur, last}:
96	      Maybe.default(&2, String, cur, last)
97	
98	# the data of the last event that has any; an event the stream cut off
99	# before its blank line still counts
100	def last(body: String) -> String:
101	  fin(lines(String.lines(body), Sse{None{}, ""}))