diff --git a/README.md b/README.md
index e543cf7..f6475e7 100644
--- a/README.md
+++ b/README.md
@@ -74,6 +74,10 @@ randomNIEWith('X', 'E', 1); //=> 'X2080280E'
 
 `better-dni` does a similar job as other libraries like [dni-js](https://github.com/albertfdp/dni-js/), [dni-js-validator](https://github.com/idirouhab/dni-js-validator), and [@willowi/validate-nif](https://github.com/WillowiDev/validate-nif) but `better-dni` is built with optimization and speed in mind. Take a look at these benchmark results:
 
+```js
+yarn build && yarn benchmark
+```
+
 ### isValid
 
 | lib                   | method       | operations/sec |             |
@@ -87,8 +91,8 @@ randomNIEWith('X', 'E', 1); //=> 'X2080280E'
 
 | lib                   | method     | operations/sec |             |
 | --------------------- | ---------- | -------------- | ----------- |
-| better-dni            | #ctrlChar  | **9,328,614**  | 2.3x faster |
-| dni-js                | #getLetter | 3,947,197      |             |
+| better-dni            | #ctrlChar  | **10,874,568** | 5.3x faster |
+| dni-js                | #getLetter | 2,032,845      |             |
 | dni-js-validator      | no method  | -              |             |
 | @willowi/validate-nif | no method  | -              |             |
 
diff --git a/benchmark.js b/benchmark.js
index a538c86..9a07af3 100644
--- a/benchmark.js
+++ b/benchmark.js
@@ -96,7 +96,7 @@ benches.push(
     })
 );
 
-Benchmark.invoke([benches[5]], {
+Benchmark.invoke([...benches], {
   name: 'run',
   args: true,
   queued: true
diff --git a/dist/index.js b/dist/index.js
index c603dd4..0e1d243 100644
--- a/dist/index.js
+++ b/dist/index.js
@@ -45,16 +45,17 @@
 
   // _Random :: https://gist.github.com/blixt/f17b47c62508be59987b#file-prng-js
 
-  const LETTERS = 'TRWAGMYFPDXBNJZSQVHLCKE';
-  const _upper = x => x.toUpperCase();
-
-  const _letter = x => LETTERS[+x % 23];
-  const _randStrLimit = limit => ('' + Math.random()).substr(-limit);
+  const _letter = x => 'trwagmyfpdxbnjzsqvhlcke'[+x % 23];
+  const _randStrLimit = limit => `${Math.random()}`.slice(-limit);
   const _randFloat = seed => (new _Random(seed).next() - 1) / 2147483646;
 
   const _char = y => {
-    const f = { X: '0', Y: '1', Z: '2' }[y[0]] || y[0];
-    const i = f + '' + y.substr(1, 7);
+    // Get a number from 0 - 2 when `y` is a NIE
+    let f = 'xyzXYZ'.indexOf(y[0]) % 3;
+    // Otherwise default to the number (NIF case only)
+    if (f === -1) f = y[0];
+    // Strip the letters
+    const i = `${f}${y.slice(1, 8)}`;
     return _letter(i);
   };
 
@@ -70,7 +71,7 @@
    * ctrlChar("03118880B"); // => 'B'
    * ctrlChar("03118880"); // => 'B'
    */
-  const ctrlChar = x => _char(x.toUpperCase());
+  const ctrlChar = x => _char(x).toUpperCase();
 
   /**
    * Returns true if the string is a NIE
@@ -81,7 +82,9 @@
    * isNIE("X4108613P"); // => true
    */
   const isNIE = value => {
-    return !!value && value.length === 9 && _isNIE(value) && ctrlChar(value) === _upper(value[8]);
+    return (
+      !!value && value.length === 9 && _isNIE(value) && ctrlChar(value) === value[8].toUpperCase()
+    );
   };
 
   /**
@@ -93,7 +96,9 @@
    * isNIF("93375221M"); // => true
    */
   const isNIF = value => {
-    return !!value && value.length === 9 && _isNIF(value) && ctrlChar(value) === _upper(value[8]);
+    return (
+      !!value && value.length === 9 && _isNIF(value) && ctrlChar(value) === value[8].toUpperCase()
+    );
   };
 
   /**
@@ -118,7 +123,8 @@
   const randomNIE = () => {
     const r = Math.floor(Math.random() * 3);
     const nn = _randStrLimit(7);
-    return ['X', 'Y', 'Z'][r] + nn + _letter(+(r + '' + nn));
+    const l = _letter(+`${r}${nn}`);
+    return `${'XYZ'[r]}${nn}${l}`;
   };
 
   /**
@@ -203,3 +209,4 @@
   Object.defineProperty(exports, '__esModule', { value: true });
 
 })));
+//# sourceMappingURL=index.js.map
diff --git a/dist/index.js.map b/dist/index.js.map
new file mode 100644
index 0000000..4d2b7f8
--- /dev/null
+++ b/dist/index.js.map
@@ -0,0 +1 @@
+{"version":3,"file":"index.js","sources":["../src/internal/_isNIE.js","../src/internal/_isNIF.js","../src/isValid.js","../src/internal/_utils.js","../src/internal/_char.js","../src/ctrlChar.js","../src/isNIE.js","../src/isNIF.js","../src/randomNIF.js","../src/randomNIE.js","../src/randomNIFWith.js","../src/randomNIEWith.js"],"sourcesContent":["export const _isNIE = v => /^[XYZ]{1}[0-9]{7}[TRWAGMYFPDXBNJZSQVHLCKE]{1}$/i.test(v);\n","export const _isNIF = v => /^[0-9]{8}[TRWAGMYFPDXBNJZSQVHLCKE]{1}$/i.test(v);\n","import { _isNIE } from './internal/_isNIE';\nimport { _isNIF } from './internal/_isNIF';\n\n/**\n * Returns true if the string is a valid DNI (NIF or NIE)\n * @param {string} value\n * @returns {boolean}\n * @since 1.1.0\n * @example\n * isValid(\"X9464186P\"); // => true\n * isValid(\"03118880B\"); // => true\n */\nconst isValid = value => {\n  const dni = (!value ? '' : value).toLowerCase(); // lowercase is faster\n\n  if (dni.length !== 9 && !_isNIE(dni) && !_isNIF(dni)) return false;\n\n  const f = { x: '0', y: '1', z: '2' }[dni[0]] || dni[0];\n  const dni_1_to_7 = dni.substr(1, 7);\n  const i = +(f + dni_1_to_7) % 23;\n\n  return 'trwagmyfpdxbnjzsqvhlcket'[i] === dni[8];\n};\n\nexport { isValid };\n","function _Random(seed) {\n  this._seed = seed % 2147483647;\n  if (this._seed <= 0) this._seed += 2147483646;\n}\n\n_Random.prototype.next = function() {\n  return (this._seed = (this._seed * 16807) % 2147483647);\n};\n\n// _Random :: https://gist.github.com/blixt/f17b47c62508be59987b#file-prng-js\n\nconst _letter = x => 'trwagmyfpdxbnjzsqvhlcke'[+x % 23];\nconst _randStrLimit = limit => `${Math.random()}`.slice(-limit);\nconst _randFloat = seed => (new _Random(seed).next() - 1) / 2147483646;\n\nexport { _letter, _randStrLimit, _randFloat };\n","import { _letter } from './_utils';\n\nconst _char = y => {\n  // Get a number from 0 - 2 when `y` is a NIE\n  let f = 'xyzXYZ'.indexOf(y[0]) % 3;\n  // Otherwise default to the number (NIF case only)\n  if (f === -1) f = y[0];\n  // Strip the letters\n  const i = `${f}${y.slice(1, 8)}`;\n  return _letter(i);\n};\n\nexport { _char };\n","import { _char } from './internal/_char';\n\n/**\n * Returns the control letter in upper case\n * for a NIF or NIE with or without control letter\n * @param {string} value\n * @returns {string}\n * @since 1.9.1\n * @example\n * ctrlChar(\"X9464186P\"); // => 'P'\n * ctrlChar(\"X9464186\"); // => 'P'\n * ctrlChar(\"03118880B\"); // => 'B'\n * ctrlChar(\"03118880\"); // => 'B'\n */\nconst ctrlChar = x => _char(x).toUpperCase();\n\nexport { ctrlChar };\n","import { _isNIE } from './internal/_isNIE';\nimport { ctrlChar } from './ctrlChar';\n\n/**\n * Returns true if the string is a NIE\n * @param {string} value\n * @returns {boolean}\n * @since 1.0.1\n * @example\n * isNIE(\"X4108613P\"); // => true\n */\nconst isNIE = value => {\n  return (\n    !!value && value.length === 9 && _isNIE(value) && ctrlChar(value) === value[8].toUpperCase()\n  );\n};\n\nexport { isNIE };\n","import { _isNIF } from './internal/_isNIF';\nimport { ctrlChar } from './ctrlChar';\n\n/**\n * Returns true if the string is a NIF\n * @param {string} value\n * @returns {boolean}\n * @since 1.0.1\n * @example\n * isNIF(\"93375221M\"); // => true\n */\nconst isNIF = value => {\n  return (\n    !!value && value.length === 9 && _isNIF(value) && ctrlChar(value) === value[8].toUpperCase()\n  );\n};\n\nexport { isNIF };\n","import { _randStrLimit, _letter } from './internal/_utils';\n\n/**\n * Returns a valid NIF string\n * @returns {string}\n * @since 1.8.1\n * @example\n * randomNIF() // => \"93375221M\"\n */\nconst randomNIF = () => {\n  const nn = _randStrLimit(8);\n  return nn + _letter(nn);\n};\n\nexport { randomNIF };\n","import { _randStrLimit, _letter } from './internal/_utils';\n\n/**\n * Returns a valid NIE string\n * @returns {string}\n * @since 1.8.1\n * @example\n * randomNIE() // => \"X4108613P\"\n */\nconst randomNIE = () => {\n  const r = Math.floor(Math.random() * 3);\n  const nn = _randStrLimit(7);\n  const l = _letter(+`${r}${nn}`);\n  return `${'XYZ'[r]}${nn}${l}`;\n};\n\nexport { randomNIE };\n","import { _randFloat } from './internal/_utils';\n\n/**\n * Returns a random NIF with a specific letter\n * A seed can be passed as a second parameter and\n * it will always return the same value\n * @returns {string | null}\n * @since 1.11.0\n * @example\n * with random seed\n * randomNIFWith('C'); //=> '95652190C'\n * randomNIFWith('G'); //=> '60869550G'\n *\n * with the same seed\n * randomNIFWith('G', 1); //=> '95652174G'\n * randomNIFWith('G', 1); //=> '95652174G'\n */\nconst randomNIFWith = (char, seed = 100000000 * Math.random()) => {\n  const lastNum = 'trwagmyfpdxbnjzsqvhlckeTRWAGMYFPDXBNJZSQVHLCKE'.indexOf(char) % 23;\n\n  if (lastNum === -1) return null;\n\n  // TODO: Better calculation\n  const n = 99999998 - 4347826 * (Math.floor(_randFloat(seed) * 22) + 1);\n\n  const d = Math.max(0, n) % 23;\n  const h = n + (lastNum - d);\n  const s = `0${h}`.slice(-8);\n\n  return `${s}${char}`.toUpperCase();\n};\n\nexport { randomNIFWith };\n","import { _randFloat } from './internal/_utils';\nimport { ctrlChar } from './ctrlChar';\n\n/**\n * Returns a random NIE with a specific letter\n * A seed can be passed as a third parameter and\n * it will always return the same value\n * @param head {string}\n * @param char {string}\n * @returns {string | null}\n * @since 1.12.0\n * @example\n * with random seed\n * randomNIEWith('Y', 'C'); //=> 'Y2098020C'\n * randomNIEWith('Z', 'G'); //=> 'Z5670557G'\n *\n * with the same seed\n * randomNIEWith('X', 'E', 1); //=> 'X2080280E'\n * randomNIEWith('X', 'E', 1); //=> 'X2080280E'\n */\nconst randomNIEWith = (xyz, l, seed = 100000000 * Math.random()) => {\n  const headNum = 'xyzXYZ'.indexOf(xyz) % 3;\n\n  if (headNum === -1) return null;\n\n  const lastNum = 'trwagmyfpdxbnjzsqvhlckeTRWAGMYFPDXBNJZSQVHLCKE'.indexOf(l) % 23;\n\n  if (lastNum === -1) return null;\n\n  const headOne = headNum + 1;\n\n  // TODO: Better calculation\n  const num = Math.floor(1000000 * headOne + (9999999 - 1000000 * headOne - 23) * _randFloat(seed));\n\n  const b = +`${headNum}${num}`;\n  const rest = b % 23;\n  const h = b - rest + lastNum;\n\n  const s = `0${h}${l}`.slice(-8);\n\n  return `${xyz}${s}`.toUpperCase();\n};\n\nexport { randomNIEWith 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\ No newline at end of file
diff --git a/package.json b/package.json
index 6aaa8c8..b7fb103 100644
--- a/package.json
+++ b/package.json
@@ -40,6 +40,7 @@
   "types": "types/index.d.ts",
   "files": [
     "dist/index.js",
+    "dist/index.js.map",
     "types/index.d.ts"
   ]
 }
diff --git a/rollup.config.js b/rollup.config.js
index 1640983..88460a2 100644
--- a/rollup.config.js
+++ b/rollup.config.js
@@ -11,6 +11,7 @@ const banner =
   '//  Better DNI may be freely distributed under the MIT license.\n';
 
 export default {
+  sourcemap: true,
   input: 'src/index.js',
   output: {
     banner,
diff --git a/src/ctrlChar.js b/src/ctrlChar.js
index b4e384e..be85170 100644
--- a/src/ctrlChar.js
+++ b/src/ctrlChar.js
@@ -12,6 +12,6 @@ import { _char } from './internal/_char';
  * ctrlChar("03118880B"); // => 'B'
  * ctrlChar("03118880"); // => 'B'
  */
-const ctrlChar = x => _char(x.toUpperCase());
+const ctrlChar = x => _char(x).toUpperCase();
 
 export { ctrlChar };
diff --git a/src/internal/_char.js b/src/internal/_char.js
index 5956af2..1c4d732 100644
--- a/src/internal/_char.js
+++ b/src/internal/_char.js
@@ -1,8 +1,12 @@
 import { _letter } from './_utils';
 
 const _char = y => {
-  const f = { X: '0', Y: '1', Z: '2' }[y[0]] || y[0];
-  const i = f + '' + y.substr(1, 7);
+  // Get a number from 0 - 2 when `y` is a NIE
+  let f = 'xyzXYZ'.indexOf(y[0]) % 3;
+  // Otherwise default to the number (NIF case only)
+  if (f === -1) f = y[0];
+  // Strip the letters
+  const i = `${f}${y.slice(1, 8)}`;
   return _letter(i);
 };
 
diff --git a/src/internal/_utils.js b/src/internal/_utils.js
index e4a2b7d..a6286b6 100644
--- a/src/internal/_utils.js
+++ b/src/internal/_utils.js
@@ -9,17 +9,8 @@ _Random.prototype.next = function() {
 
 // _Random :: https://gist.github.com/blixt/f17b47c62508be59987b#file-prng-js
 
-const LETTERS = 'TRWAGMYFPDXBNJZSQVHLCKE';
-
-const _idxOf = x => y => x.indexOf(y);
-const _xyzAsNum = _idxOf('xyz');
-const _lastIndex = _idxOf(LETTERS);
-const _upper = x => x.toUpperCase();
-
-const _letter = x => LETTERS[+x % 23];
-const _randStrLimit = limit => ('' + Math.random()).substr(-limit);
+const _letter = x => 'trwagmyfpdxbnjzsqvhlcke'[+x % 23];
+const _randStrLimit = limit => `${Math.random()}`.slice(-limit);
 const _randFloat = seed => (new _Random(seed).next() - 1) / 2147483646;
 
-const compose = (...fns) => x => fns.reduceRight((acc, fn) => fn(acc), x);
-
-export { _letter, _randStrLimit, _Random, _lastIndex, _upper, _randFloat, _xyzAsNum };
+export { _letter, _randStrLimit, _randFloat };
diff --git a/src/isNIE.js b/src/isNIE.js
index a68f9a4..4a9bd58 100644
--- a/src/isNIE.js
+++ b/src/isNIE.js
@@ -1,5 +1,4 @@
 import { _isNIE } from './internal/_isNIE';
-import { _upper } from './internal/_utils';
 import { ctrlChar } from './ctrlChar';
 
 /**
@@ -11,7 +10,9 @@ import { ctrlChar } from './ctrlChar';
  * isNIE("X4108613P"); // => true
  */
 const isNIE = value => {
-  return !!value && value.length === 9 && _isNIE(value) && ctrlChar(value) === _upper(value[8]);
+  return (
+    !!value && value.length === 9 && _isNIE(value) && ctrlChar(value) === value[8].toUpperCase()
+  );
 };
 
 export { isNIE };
diff --git a/src/isNIF.js b/src/isNIF.js
index e3e6735..d62cfc8 100644
--- a/src/isNIF.js
+++ b/src/isNIF.js
@@ -1,5 +1,4 @@
 import { _isNIF } from './internal/_isNIF';
-import { _upper } from './internal/_utils';
 import { ctrlChar } from './ctrlChar';
 
 /**
@@ -11,7 +10,9 @@ import { ctrlChar } from './ctrlChar';
  * isNIF("93375221M"); // => true
  */
 const isNIF = value => {
-  return !!value && value.length === 9 && _isNIF(value) && ctrlChar(value) === _upper(value[8]);
+  return (
+    !!value && value.length === 9 && _isNIF(value) && ctrlChar(value) === value[8].toUpperCase()
+  );
 };
 
 export { isNIF };
diff --git a/src/randomNIE.js b/src/randomNIE.js
index d3a7972..7dad0c5 100644
--- a/src/randomNIE.js
+++ b/src/randomNIE.js
@@ -10,7 +10,8 @@ import { _randStrLimit, _letter } from './internal/_utils';
 const randomNIE = () => {
   const r = Math.floor(Math.random() * 3);
   const nn = _randStrLimit(7);
-  return ['X', 'Y', 'Z'][r] + nn + _letter(+(r + '' + nn));
+  const l = _letter(+`${r}${nn}`);
+  return `${'XYZ'[r]}${nn}${l}`;
 };
 
 export { randomNIE };
diff --git a/src/randomNIEWith.js b/src/randomNIEWith.js
index 90baea4..672c851 100644
--- a/src/randomNIEWith.js
+++ b/src/randomNIEWith.js
@@ -1,4 +1,4 @@
-import { _upper, _randFloat } from './internal/_utils';
+import { _randFloat } from './internal/_utils';
 import { ctrlChar } from './ctrlChar';
 
 /**