feat: add half-normal PDF

lib/node_modules/@stdlib/stats/base/dists/halfnormal/README.md

@@ -0,0 +1,35 @@
+<!--
+@license Apache-2.0
+Copyright (c) 2025 The Stdlib Authors.
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+
+# Half-Normal Distribution
+
+> Half-normal distribution.
+
+<section class="intro">
+
+The half-normal distribution is a continuous probability distribution whose values are the absolute values of a normally distributed random variable with mean zero.
+
+</section>
+
+<section class="usage">
+
+## Usage
+
+```javascript
+var pdf = require( '@stdlib/stats/base/dists/halfnormal' ).pdf;
+
+var y = pdf( 1.0, 1.0 );
+console.log( y );

lib/node_modules/@stdlib/stats/base/dists/halfnormal/lib/index.js

@@ -0,0 +1,25 @@
+/**
+* @license Apache-2.0
+*
+* Copyright (c) 2025 The Stdlib Authors.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+*    http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+'use strict';
+
+var pdf = require('./pdf.js');
+
+module.exports = {
+  pdf: pdf
+};

lib/node_modules/@stdlib/stats/base/dists/halfnormal/lib/pdf.js

@@ -0,0 +1,37 @@
+/**
+* @license Apache-2.0
+*
+* Copyright (c) 2025 The Stdlib Authors.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+*    http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+'use strict';
+
+var sqrt = require('@stdlib/math/base/special/sqrt');
+var exp = require('@stdlib/math/base/special/exp');
+var pow = require('@stdlib/math/base/special/pow');
+var PI = require('@stdlib/constants/float64/pi');
+const TWO = 2.0;
+
+function pdf(x, sigma) {
+  if (sigma <= 0.0) {
+    return NaN;
+  }
+  if (x < 0.0) {
+    return 0.0;
+  }
+  return sqrt(TWO / (PI * pow(sigma, 2))) * exp(-pow(x, 2) / (TWO * pow(sigma, 2)));
+}
+
+module.exports = pdf;

lib/node_modules/@stdlib/stats/base/dists/halfnormal/test/test.pdf.js

@@ -0,0 +1,54 @@
+/**
+* @license Apache-2.0
+*
+* Copyright (c) 2025 The Stdlib Authors.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+*    http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+
+
+'use strict';
+
+// MODULES //
+var tape = require('tape');
+var pdf = require('../lib/pdf.js');
+var isnan = require('@stdlib/math/base/assert/is-nan');
+
+// TESTS //
+tape('main export is a function', function(t) {
+	t.equal(typeof pdf, 'function', 'main export is a function');
+	t.end();
+});
+
+tape('returns NaN if sigma <= 0', function(t) {
+	t.equal(isnan(pdf(1.0, 0.0)), true, 'sigma = 0 returns NaN');
+	t.equal(isnan(pdf(1.0, -1.0)), true, 'negative sigma returns NaN');
+	t.end();
+});
+
+tape('pdf is zero for x < 0', function(t) {
+	t.equal(pdf(-0.5, 1.0), 0.0, 'x < 0 returns 0');
+	t.end();
+});
+
+tape('pdf evaluates half-normal distribution for valid inputs', function(t) {
+	var sigma = 1.0;
+	var xs = [0.0, 0.5, 1.0, 2.0];
+	for (var i = 0; i < xs.length; i++) {
+		var v = pdf(xs[i], sigma);
+		t.ok(v >= 0, 'pdf(' + xs[i] + ',1.0) = ' + v);
+	}
+	t.end();
+});
+