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+/*
+
+`util/bio_metadata`
+===================
+
+>   For more information on the contents of this file, please contact:
+>
+>   - kibigo! [@kibi@glitch.social]
+
+This file provides two functions for dealing with bio metadata. The
+functions are:
+
+ -  __`processBio(content)` :__
+    Processes `content` to extract any frontmatter. The returned
+    object has two properties: `text`, which contains the text of
+    `content` sans-frontmatter, and `metadata`, which is an array
+    of key-value pairs (in two-element array format). If no
+    frontmatter was provided in `content`, then `metadata` will be
+    an empty array.
+
+ -  __`createBio(note, data)` :__
+    Reverses the process in `processBio()`; takes a `note` and an
+    array of two-element arrays (which should give keys and values)
+    and outputs a string containing a well-formed bio with
+    frontmatter.
+
+*/
+
+//  * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
+
+/*********************************************************************\
+
+                                       To my lovely code maintainers,
+
+  The syntax recognized by the Mastodon frontend for its bio metadata
+  feature is a subset of that provided by the YAML 1.2 specification.
+  In particular, Mastodon recognizes metadata which is provided as an
+  implicit YAML map, where each key-value pair takes up only a single
+  line (no multi-line values are permitted). To simplify the level of
+  processing required, Mastodon metadata frontmatter has been limited
+  to only allow those characters in the `c-printable` set, as defined
+  by the YAML 1.2 specification, instead of permitting those from the
+  `nb-json` characters inside double-quoted strings like YAML proper.
+    ¶ It is important to note that Mastodon only borrows the *syntax*
+  of YAML, not its semantics. This is to say, Mastodon won't make any
+  attempt to interpret the data it receives. `true` will not become a
+  boolean; `56` will not be interpreted as a number. Rather, each key
+  and every value will be read as a string, and as a string they will
+  remain. The order of the pairs is unchanged, and any duplicate keys
+  are preserved. However, YAML escape sequences will be replaced with
+  the proper interpretations according to the YAML 1.2 specification.
+    ¶ The implementation provided below interprets `<br>` as `\n` and
+  allows for an open <p> tag at the beginning of the bio. It replaces
+  the escaped character entities `&apos;` and `&quot;` with single or
+  double quotes, respectively, prior to processing. However, no other
+  escaped characters are replaced, not even those which might have an
+  impact on the syntax otherwise. These minor allowances are provided
+  because the Mastodon backend will insert these things automatically
+  into a bio before sending it through the API, so it is important we
+  account for them. Aside from this, the YAML frontmatter must be the
+  very first thing in the bio, leading with three consecutive hyphen-
+  minues (`---`), and ending with the same or, alternatively, instead
+  with three periods (`...`). No limits have been set with respect to
+  the number of characters permitted in the frontmatter, although one
+  should note that only limited space is provided for them in the UI.
+    ¶ The regular expression used to check the existence of, and then
+  process, the YAML frontmatter has been split into a number of small
+  components in the code below, in the vain hope that it will be much
+  easier to read and to maintain. I leave it to the future readers of
+  this code to determine the extent of my successes in this endeavor.
+
+  UPDATE 19 Oct 2017: We no longer allow character escapes inside our
+  double-quoted strings for ease of processing. We now internally use
+  the name "ƔAML" in our code to clarify that this is Not Quite YAML.
+
+                                       Sending love + warmth eternal,
+                                       - kibigo [@kibi@glitch.social]
+
+\*********************************************************************/
+
+/*  "u" FLAG COMPATABILITY  */
+
+let compat_mode = false;
+try {
+  new RegExp('.', 'u');
+} catch (e) {
+  compat_mode = true;
+}
+
+/*  CONVENIENCE FUNCTIONS  */
+
+const unirex = str => compat_mode ? new RegExp(str) : new RegExp(str, 'u');
+const rexstr = exp => '(?:' + exp.source + ')';
+
+/*  CHARACTER CLASSES  */
+
+const DOCUMENT_START    = /^/;
+const DOCUMENT_END      = /$/;
+const ALLOWED_CHAR      =  unirex( //  `c-printable` in the YAML 1.2 spec.
+  compat_mode ? '[\t\n\r\x20-\x7e\x85\xa0-\ufffd]' : '[\t\n\r\x20-\x7e\x85\xa0-\ud7ff\ue000-\ufffd\u{10000}-\u{10FFFF}]'
+);
+const WHITE_SPACE       = /[ \t]/;
+const LINE_BREAK        = /\r?\n|\r|<br\s*\/?>/;
+const INDICATOR         = /[-?:,[\]{}&#*!|>'"%@`]/;
+const FLOW_CHAR         = /[,[\]{}]/;
+
+/*  NEGATED CHARACTER CLASSES  */
+
+const NOT_WHITE_SPACE   = unirex('(?!' + rexstr(WHITE_SPACE) + ')[^]');
+const NOT_LINE_BREAK    = unirex('(?!' + rexstr(LINE_BREAK) + ')[^]');
+const NOT_INDICATOR     = unirex('(?!' + rexstr(INDICATOR) + ')[^]');
+const NOT_FLOW_CHAR     = unirex('(?!' + rexstr(FLOW_CHAR) + ')[^]');
+const NOT_ALLOWED_CHAR  = unirex(
+  '(?!' + rexstr(ALLOWED_CHAR) + ')[^]'
+);
+
+/*  BASIC CONSTRUCTS  */
+
+const ANY_WHITE_SPACE   = unirex(rexstr(WHITE_SPACE) + '*');
+const ANY_ALLOWED_CHARS = unirex(rexstr(ALLOWED_CHAR) + '*');
+const NEW_LINE          = unirex(
+  rexstr(ANY_WHITE_SPACE) + rexstr(LINE_BREAK)
+);
+const SOME_NEW_LINES    = unirex(
+  '(?:' + rexstr(NEW_LINE) + ')+'
+);
+const POSSIBLE_STARTS   = unirex(
+  rexstr(DOCUMENT_START) + rexstr(/<p[^<>]*>/) + '?'
+);
+const POSSIBLE_ENDS     = unirex(
+  rexstr(SOME_NEW_LINES) + '|' +
+  rexstr(DOCUMENT_END) + '|' +
+  rexstr(/<\/p>/)
+);
+const QUOTE_CHAR         = unirex(
+  '(?=' + rexstr(NOT_LINE_BREAK) + ')[^"]'
+);
+const ANY_QUOTE_CHAR    = unirex(
+  rexstr(QUOTE_CHAR) + '*'
+);
+
+const ESCAPED_APOS      = unirex(
+  '(?=' + rexstr(NOT_LINE_BREAK) + ')' + rexstr(/[^']|''/)
+);
+const ANY_ESCAPED_APOS  = unirex(
+  rexstr(ESCAPED_APOS) + '*'
+);
+const FIRST_KEY_CHAR    = unirex(
+  '(?=' + rexstr(NOT_LINE_BREAK) + ')' +
+  '(?=' + rexstr(NOT_WHITE_SPACE) + ')' +
+  rexstr(NOT_INDICATOR) + '|' +
+  rexstr(/[?:-]/) +
+  '(?=' + rexstr(NOT_LINE_BREAK) + ')' +
+  '(?=' + rexstr(NOT_WHITE_SPACE) + ')' +
+  '(?=' + rexstr(NOT_FLOW_CHAR) + ')'
+);
+const FIRST_VALUE_CHAR  = unirex(
+  '(?=' + rexstr(NOT_LINE_BREAK) + ')' +
+  '(?=' + rexstr(NOT_WHITE_SPACE) + ')' +
+  rexstr(NOT_INDICATOR) + '|' +
+  rexstr(/[?:-]/) +
+  '(?=' + rexstr(NOT_LINE_BREAK) + ')' +
+  '(?=' + rexstr(NOT_WHITE_SPACE) + ')'
+  //  Flow indicators are allowed in values.
+);
+const LATER_KEY_CHAR    = unirex(
+  rexstr(WHITE_SPACE) + '|' +
+  '(?=' + rexstr(NOT_LINE_BREAK) + ')' +
+  '(?=' + rexstr(NOT_WHITE_SPACE) + ')' +
+  '(?=' + rexstr(NOT_FLOW_CHAR) + ')' +
+  rexstr(/[^:#]#?/) + '|' +
+  rexstr(/:/) + '(?=' + rexstr(NOT_WHITE_SPACE) + ')'
+);
+const LATER_VALUE_CHAR  = unirex(
+  rexstr(WHITE_SPACE) + '|' +
+  '(?=' + rexstr(NOT_LINE_BREAK) + ')' +
+  '(?=' + rexstr(NOT_WHITE_SPACE) + ')' +
+  //  Flow indicators are allowed in values.
+  rexstr(/[^:#]#?/) + '|' +
+  rexstr(/:/) + '(?=' + rexstr(NOT_WHITE_SPACE) + ')'
+);
+
+/*  YAML CONSTRUCTS  */
+
+const ƔAML_START        = unirex(
+  rexstr(ANY_WHITE_SPACE) + '---'
+);
+const ƔAML_END          = unirex(
+  rexstr(ANY_WHITE_SPACE) + '(?:---|\.\.\.)'
+);
+const ƔAML_LOOKAHEAD    = unirex(
+  '(?=' +
+    rexstr(ƔAML_START) +
+    rexstr(ANY_ALLOWED_CHARS) + rexstr(NEW_LINE) +
+    rexstr(ƔAML_END) + rexstr(POSSIBLE_ENDS) +
+  ')'
+);
+const ƔAML_DOUBLE_QUOTE = unirex(
+  '"' + rexstr(ANY_QUOTE_CHAR) + '"'
+);
+const ƔAML_SINGLE_QUOTE = unirex(
+  '\'' + rexstr(ANY_ESCAPED_APOS) + '\''
+);
+const ƔAML_SIMPLE_KEY   = unirex(
+  rexstr(FIRST_KEY_CHAR) + rexstr(LATER_KEY_CHAR) + '*'
+);
+const ƔAML_SIMPLE_VALUE = unirex(
+  rexstr(FIRST_VALUE_CHAR) + rexstr(LATER_VALUE_CHAR) + '*'
+);
+const ƔAML_KEY          = unirex(
+  rexstr(ƔAML_DOUBLE_QUOTE) + '|' +
+  rexstr(ƔAML_SINGLE_QUOTE) + '|' +
+  rexstr(ƔAML_SIMPLE_KEY)
+);
+const ƔAML_VALUE        = unirex(
+  rexstr(ƔAML_DOUBLE_QUOTE) + '|' +
+  rexstr(ƔAML_SINGLE_QUOTE) + '|' +
+  rexstr(ƔAML_SIMPLE_VALUE)
+);
+const ƔAML_SEPARATOR    = unirex(
+  rexstr(ANY_WHITE_SPACE) +
+  ':' + rexstr(WHITE_SPACE) +
+  rexstr(ANY_WHITE_SPACE)
+);
+const ƔAML_LINE         = unirex(
+  '(' + rexstr(ƔAML_KEY) + ')' +
+  rexstr(ƔAML_SEPARATOR) +
+  '(' + rexstr(ƔAML_VALUE) + ')'
+);
+
+/*  FRONTMATTER REGEX  */
+
+const ƔAML_FRONTMATTER  = unirex(
+  rexstr(POSSIBLE_STARTS) +
+  rexstr(ƔAML_LOOKAHEAD) +
+  rexstr(ƔAML_START) + rexstr(SOME_NEW_LINES) +
+  '(?:' +
+    rexstr(ANY_WHITE_SPACE) + rexstr(ƔAML_LINE) + rexstr(SOME_NEW_LINES) +
+  '){0,5}' +
+  rexstr(ƔAML_END) + rexstr(POSSIBLE_ENDS)
+);
+
+/*  SEARCHES  */
+
+const FIND_ƔAML_LINE    = unirex(
+  rexstr(NEW_LINE) + rexstr(ANY_WHITE_SPACE) + rexstr(ƔAML_LINE)
+);
+
+/*  STRING PROCESSING  */
+
+function processString (str) {
+  switch (str.charAt(0)) {
+  case '"':
+    return str.substring(1, str.length - 1);
+  case '\'':
+    return str
+      .substring(1, str.length - 1)
+      .replace(/''/g, '\'');
+  default:
+    return str;
+  }
+}
+
+/*  BIO PROCESSING  */
+
+export function processBio(content) {
+  content = content.replace(/&quot;/g, '"').replace(/&apos;/g, '\'');
+  let result = {
+    text: content,
+    metadata: [],
+  };
+  let ɣaml = content.match(ƔAML_FRONTMATTER);
+  if (!ɣaml) {
+    return result;
+  } else {
+    ɣaml = ɣaml[0];
+  }
+  const start = content.search(ƔAML_START);
+  const end = start + ɣaml.length - ɣaml.search(ƔAML_START);
+  result.text = content.substr(end);
+  let metadata = null;
+  let query = new RegExp(rexstr(FIND_ƔAML_LINE), 'g');  //  Some browsers don't allow flags unless both args are strings
+  while ((metadata = query.exec(ɣaml))) {
+    result.metadata.push([
+      processString(metadata[1]),
+      processString(metadata[2]),
+    ]);
+  }
+  return result;
+}
+
+/*  BIO CREATION  */
+
+export function createBio(note, data) {
+  if (!note) note = '';
+  let frontmatter = '';
+  if ((data && data.length) || note.match(/^\s*---\s+/)) {
+    if (!data) frontmatter = '---\n...\n';
+    else {
+      frontmatter += '---\n';
+      for (let i = 0; i < data.length; i++) {
+        let key = '' + data[i][0];
+        let val = '' + data[i][1];
+
+        //  Key processing
+        if (key === (key.match(ƔAML_SIMPLE_KEY) || [])[0]) /*  do nothing  */;
+        else if (key === (key.match(ANY_QUOTE_CHAR) || [])[0]) key = '"' + key + '"';
+        else {
+          key = key
+            .replace(/'/g, '\'\'')
+            .replace(new RegExp(rexstr(NOT_ALLOWED_CHAR), compat_mode ? 'g' : 'gu'), '�');
+          key = '\'' + key + '\'';
+        }
+
+        //  Value processing
+        if (val === (val.match(ƔAML_SIMPLE_VALUE) || [])[0]) /*  do nothing  */;
+        else if (val === (val.match(ANY_QUOTE_CHAR) || [])[0]) val = '"' + val + '"';
+        else {
+          key = key
+            .replace(/'/g, '\'\'')
+            .replace(new RegExp(rexstr(NOT_ALLOWED_CHAR), compat_mode ? 'g' : 'gu'), '�');
+          key = '\'' + key + '\'';
+        }
+
+        frontmatter += key + ': ' + val + '\n';
+      }
+      frontmatter += '...\n';
+    }
+  }
+  return frontmatter + note;
+}