Initial commit of Ur/Web Markdown libraryHEADmaster

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+(* © 2004 John Gruber <http://daringfireball.net/projects/markdown/>
+© 2015 Benjamin Barenblat
+
+Licensed under the Apache License, Version 2.0 (the "License"); you may not
+use this file except in compliance with the License.  You may obtain a copy
+of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software distributed
+under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR
+CONDITIONS OF ANY KIND, either express or implied.  See the License for the
+specific language governing permissions and limitations under the License. *)
+
+fun lines (text : string) : list string =
+  case text of
+      "" => []
+    | s => case String.split s #"\n" of
+               None => s :: []
+             | Some (before, after) => before :: lines after
+
+fun unlines (lines : list string) : string =
+  List.foldr (fn x y => x ^ "\n" ^ y) "" lines
+
+fun spaces n =
+  case n of
+      0 => ""
+    | x => spaces (n - 1) ^ " "
+
+(* Standardize line endings. *)
+fun standardize_line_endings text =
+  Regex.replace "\r" "\n"  (* Mac to Unix *)
+    (Regex.replace "\r\n" "\n"  (* DOS to Unix *)
+       text)
+
+(* Make sure the text ends with a couple of newlines. *)
+fun ensure_ends_with_newlines text = text ^ "\n\n"
+
+(* Convert tabs to spaces. *)
+fun detab text =
+  let
+    fun detab_line line =
+      case String.split line #"\t" of
+          None => line
+        | Some (before, after) =>
+          detab_line (before ^ spaces (8 - String.length before % 8) ^ after)
+  in
+    unlines (List.mp detab_line (lines text))
+  end
+
+(* Strip any lines consisting of only spaces and tabs.  This makes subsequent
+regexes easier to write, because we can match consecutive blank lines with
+/\n+/ instead of something contorted like /[ \t]*\n+/. *)
+fun strip_blank text =
+  Regex.replace "(^|\n) +($|\n)" "\n" text
+
+fun do_headers text =
+  (* TODO(bbaren): run_span_gamut on the inner data of the header tags. *)
+  let
+    (* As of 30 July 2015, the compiler generates incorrect C if I don't
+    specify 'text' as a parameter and instead partially apply
+    'Regex.replace'. *)
+    fun atx_header level text =
+      let
+        val level_string = show level
+      in
+        Regex.replace ("(?:^|\n)\#{" ^ level_string ^ "} *(.+?) *\#*\n+")
+                      ("<h" ^ level_string ^ ">$1</h" ^ level_string ^ ">\n\n")
+                      text
+      end
+  in
+    (atx_header 1
+       (atx_header 2
+          (atx_header 3
+             (atx_header 4
+                (atx_header 5
+                   (atx_header 6
+                      (Regex.replace "(?:^|\n)(.+) *\n-+ *\n+" "<h2>$1</h2>\n\n"
+                         (Regex.replace "(?:^|\n)(.+) *\n=+ *\n+"
+                                        "<h1>$1</h1>\n\n"
+                                        text))))))))
+  end
+
+fun run_block_gamut text =
+  do_headers text
+
+fun render markdown =
+  run_block_gamut
+    (strip_blank
+       (detab
+         (ensure_ends_with_newlines
+           (standardize_line_endings markdown))))