Keep the case of a noun when making it singular

src/3-three/nouns/api/toSingular.js

@@ -1,5 +1,16 @@
 const keep = { tags: true }
 
+// give 'dane' the case of the word it replaced: 'Danes' -> 'Dane', 'DANES' -> 'DANE'
+const matchCase = function (original, str) {
+  if (original.length > 1 && original === original.toUpperCase() && original !== original.toLowerCase()) {
+    return str.toUpperCase()
+  }
+  if (/^\p{Lu}/u.test(original)) {
+    return str.charAt(0).toUpperCase() + str.slice(1)
+  }
+  return str
+}
+
 const nounToSingular = function (m, parsed) {
   // already singular?
   if (parsed.isPlural === false) {
@@ -10,7 +21,15 @@ const nounToSingular = function (m, parsed) {
   // inflect the root noun
   const str = parsed.root.text('normal')
   const single = toSingular(str, model)
-  m.replace(parsed.root, single, keep).tag('Singular', 'toPlural')
+  const before = (parsed.root.docs[0] || []).map(term => term.text)
+  const after = m.replace(parsed.root, single, keep).tag('Singular', 'toPlural')
+  // the conversion works on the normalized text, so put the original case back
+  const terms = after.docs[0] || []
+  if (terms.length === before.length) {
+    terms.forEach((term, i) => {
+      term.text = matchCase(before[i], term.text)
+    })
+  }
   // should we change the determiner/article?
   // m.debug()
   return m

tests/three/nouns/toSingular.test.js

@@ -257,3 +257,17 @@ test('toSingular - longer:', function (t) {
   })
   t.end()
 })
+test('toSingular - keeps case:', function (t) {
+  const arr = [
+    [`Those are Great Danes`, `Those are Great Dane`],
+    [`The Children played`, `The Child played`],
+    [`I saw Mice`, `I saw Mouse`],
+    [`I saw MICE`, `I saw MOUSE`],
+  ]
+  arr.forEach(function (a) {
+    const doc = nlp(a[0])
+    doc.nouns().toSingular()
+    t.equal(doc.text(), a[1], here + '[case] ' + a[0])
+  })
+  t.end()
+})