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node_modules/indefinite/lib/rules/acronyms.js

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+const { startsWithVowel } = require('../helpers');
+const ACRONYM = /^[A-Z]+$/;
+const IRREGULAR_ACRONYM = /^[UFHLMNRSX]/;
+
+const isIrregularAcronym = word => IRREGULAR_ACRONYM.test(word.charAt(0));
+
+/**
+ * Both = a && b
+ * Neither = !a && !b
+ * In the case of Booleans, this means
+ * either both true or both false, so
+ * we can just compare the equality of
+ * a and b.
+ */
+const bothOrNeither = (a, b) => a === b;
+
+/**
+ * If the entirety of the first word is capital letters
+ * and case insensitivity is off, it's an acronym.
+ */
+exports.check = (word, { caseInsensitive }) => caseInsensitive ? false : ACRONYM.test(word.split(' ')[0]);
+
+exports.run = (word) => {
+  let isIrregular = isIrregularAcronym(word);
+  let initialVowel = startsWithVowel(word);
+  /*
+   * If it starts with U: "a"
+   * If it starts with any other vowel: "an"
+   * If it starts with F, H, L, M, N, R, S, or X: "an"
+   * If it starts with any other consonant: "a"
+   */
+  let article = bothOrNeither(initialVowel, isIrregular) ? 'a' : 'an';
+  return article;
+};
+