Add support for threshold as a setting

Closes GH-1.

Author: wooorm

index.js

@@ -89,11 +89,12 @@ function smogToAge(value) {
  *
  * @param {File} file - Virtual file.
  * @param {Node} node - NLCST node.
+ * @param {number} threshold - Target threshold.
  * @param {number} target - Target age.
  * @param {Array.<number>} results - Reading-level in age
  *   for `node` according to several algorithms.
  */
-function report(file, node, target, results) {
+function report(file, node, threshold, target, results) {
     var length = results.length;
     var result = 0;
     var index = -1;
@@ -107,7 +108,7 @@ function report(file, node, target, results) {
 
     result /= length;
 
-    if (result >= SURENESS_THRESHOLD) {
+    if (result >= threshold) {
         if (result >= SURENESS_THRESHOLD_DEFINITELY) {
             level = 'Definitely';
         } else if (result >= SURENESS_THRESHOLD_VERY) {
@@ -128,7 +129,9 @@ function report(file, node, target, results) {
  * @return {Function} - `transformer`.
  */
 function attacher(processor, options) {
-    var targetAge = (options || {}).age || DEFAULT_TARGET_AGE;
+    var settings = options || {};
+    var targetAge = settings.age || DEFAULT_TARGET_AGE;
+    var threshold = settings.threshold || SURENESS_THRESHOLD;
 
     return function (tree, file) {
         /**
@@ -216,7 +219,7 @@ function attacher(processor, options) {
                 'letter': letters
             };
 
-            report(file, sentence, targetAge, [
+            report(file, sentence, threshold, targetAge, [
                 gradeToAge(daleChallFormula.gradeLevel(
                     daleChallFormula(counts)
                 )[1]),

readme.md

@@ -87,6 +87,12 @@ Detect possibly hard to read sentences.
         provide varying results, so your milage might vary with
         people actually that age. :wink:
 
+    *   `threshold` (`number`, default: `4 / 7`)
+        — By default, 4 out of the 7 algorithms need to agree that
+        a sentence is higher that the target age and whether it should
+        be warned about.  This can be modified by passing in a new
+        threshold.
+
 ## License
 
 [MIT][license] © [Titus Wormer][author]

test.js

@@ -54,7 +54,7 @@ test('readability', function (t) {
 
     retext()
         .use(readability, {
-            'age': 18
+            'threshold': 5 / 7
         })
         .process([
             'Oberon, also designated Uranus IV, is the outermost ',
@@ -64,6 +64,25 @@ test('readability', function (t) {
         ].join('\n'), function (err, file) {
             t.ifError(err, 'should not fail (#3)');
 
+            t.deepEqual(
+                file.messages.map(String),
+                [],
+                'should support a threshold'
+            );
+        });
+
+    retext()
+        .use(readability, {
+            'age': 18
+        })
+        .process([
+            'Oberon, also designated Uranus IV, is the outermost ',
+            'major moon of the planet Uranus and quite large',
+            'and massive for a Uranian moon.',
+            ''
+        ].join('\n'), function (err, file) {
+            t.ifError(err, 'should not fail (#4)');
+
             t.deepEqual(
                 file.messages.map(String),
                 [],
@@ -81,7 +100,7 @@ test('readability', function (t) {
             'and massive for a Uranian moon.',
             ''
         ].join('\n'), function (err, file) {
-            t.ifError(err, 'should not fail (#4)');
+            t.ifError(err, 'should not fail (#5)');
 
             t.deepEqual(
                 file.messages.map(String),
@@ -98,7 +117,7 @@ test('readability', function (t) {
             'and second most massive of the Uranian moons.',
             ''
         ].join('\n'), function (err, file) {
-            t.ifError(err, 'should not fail (#5)');
+            t.ifError(err, 'should not fail (#6)');
 
             t.deepEqual(
                 file.messages.map(String),
@@ -118,7 +137,7 @@ test('readability', function (t) {
             'ninth most massive moon in the Solar System.',
             ''
         ].join('\n'), function (err, file) {
-            t.ifError(err, 'should not fail (#6)');
+            t.ifError(err, 'should not fail (#7)');
 
             t.deepEqual(
                 file.messages.map(String),