From 9f00f50e520bc8155db52b8c4977a429993a754c Mon Sep 17 00:00:00 2001
From: Ryan Kavanagh <rak@rak.ac>
Date: Sat, 30 May 2020 14:01:16 -0400
Subject: Initial import of a text/gemini parser

---
 parse_gemini.ml | 105 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 105 insertions(+)
 create mode 100644 parse_gemini.ml

(limited to 'parse_gemini.ml')

diff --git a/parse_gemini.ml b/parse_gemini.ml
new file mode 100644
index 0000000..2246a90
--- /dev/null
+++ b/parse_gemini.ml
@@ -0,0 +1,105 @@
+(* Copyright (C) 2020 Ryan Kavanagh <rak@rak.ac> *)
+(* Implements a parser for the text/gemini mime-type specified on
+   https://gemini.circumlunar.space/docs/spec-spec.txt .  *)
+
+open Angstrom
+
+type gemini_line =
+  | H1 of string                     (* # header   *)
+  | H2 of string                     (* ## header  *)
+  | H3 of string                     (* ### header *)
+  | Link of string * (string option) (* link with url and optional name *)
+  | Pre of string                    (* preformatted text line *)
+  | Text of string                   (* plain text line *)
+  | Ul of string                     (* unordered list item *)
+
+type gemini = gemini_line list
+
+let is_space = function
+  | ' ' | '\t' -> true
+  | _ -> false
+
+let is_cr = function
+  | '\r' -> true
+  | _ -> false
+
+let skip_spaces = skip_while is_space
+
+let take_till_cr = take_till is_cr <* string "\r\n"
+
+let header =
+  lift (fun x -> H3 x) (string "###" *> skip_spaces *> take_till_cr)
+  <|> lift (fun x -> H2 x) (string "##" *> skip_spaces *> take_till_cr)
+  <|> lift (fun x -> H1 x) (string "#" *> skip_spaces *> take_till_cr)
+  <?> "gemini_header"
+
+let link =
+  let end_of_url b = is_space b || is_cr b in
+  let to_link s = function
+    | "" -> Link (s, None)
+    | n -> Link (s, Some n)
+  in
+  lift2 to_link
+    (string "=>" *> skip_spaces *> take_till end_of_url)
+    (* the optional name, will be "" if there is none *)
+    (skip_spaces *> take_till_cr)
+    <?> "gemini_link"
+
+(* Makes sure a line is actually a plain text line *)
+let text =
+  let take_line = lift (fun s -> Text s) take_till_cr in
+  peek_char >>= function
+  | Some '#' -> fail "text line cannot start with #"
+  | Some '*' -> fail "text line cannot start with *"
+  | Some '`' -> (peek_string 3 >>= function
+    | "```" -> fail "text line cannot start with ```"
+    | _ -> take_line)
+  | Some '=' -> (peek_char >>= function
+    | Some '>' -> fail "text line cannot start with =>"
+    | _ -> take_line)
+  | _ -> take_line
+
+(* The gemini spec says that pre blocks are demarcated by lines that
+   start with "```". It says nothing about what follows on these
+   lines. *)
+let pre_divider = string "```" *> take_till_cr *> return None
+
+let pre_line = peek_string 3
+  >>= function
+  | "```" -> fail "preformatted line cannot start with ```"
+  | _ -> lift (fun s -> Pre s) take_till_cr
+
+let pre_block = pre_divider *> many pre_line <* pre_divider
+
+let list_item = lift (fun x -> Ul x) (string "*" *> skip_spaces *> take_till_cr)
+
+let gemini : gemini Angstrom.t =
+  (* Because many tries each in order, we know that we will have a
+     plain text line whenever we get to the text parser. As a result,
+     we don't need to recheck that it's actually a text line and not
+     some special line. *)
+  let plain_text = lift (fun s -> Text s) take_till_cr
+  in
+  fix (fun gemini ->
+      lift2 List.append pre_block gemini
+      <|> lift2 List.cons (header <|> link <|> list_item <|> plain_text) gemini
+      <|> return [])
+  <?> "gemini"
+
+let gemini_line_to_str = function
+  | H1 s -> "# " ^ s
+  | H2 s -> "## " ^ s
+  | H3 s -> "### " ^ s
+  | Link (s, Some n) -> "=> " ^ s ^ " " ^ n
+  | Link (s, None) -> "=> " ^ s
+  | Pre s -> "P " ^ s
+  | Text s -> "T " ^ s
+  | Ul s -> "* " ^ s
+
+let gemini_to_str l =
+  List.fold_right (fun s -> fun r -> (gemini_line_to_str s) ^ "\n" ^ r) l ""
+
+let test_parse p str =
+  match parse_string ~consume:All (p >>= return) str with
+  | Ok x -> gemini_to_str x
+  | _ -> ""
-- 
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