feat: implement mean and variance for half-normal distribution

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

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+'use strict';
+
+/**
+* Half-normal distribution expected value.
+*
+* @module @stdlib/stats/base/dists/halfnormal/mean
+*
+* @example
+* var mean = require( '@stdlib/stats/base/dists/halfnormal/mean' );
+*
+* var v = mean( 1.0 );
+* // returns ~0.798
+*
+* var v = mean( 2.0 );
+* // returns ~1.596
+*/
+
+// MODULES //
+
+var main = require( './main.js' );
+
+
+// EXPORTS //
+
+module.exports = main;

lib/node_modules/@stdlib/stats/base/dists/halfnormal/mean/lib/main.js

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+'use strict';
+
+// MODULES //
+
+var isnan = require( '@stdlib/math/base/assert/is-nan' );
+var sqrt = require( '@stdlib/math/base/special/sqrt' );
+var PI = require( '@stdlib/constants/float64/pi' );
+
+
+// VARIABLES //
+
+var SQRT_TWO_OVER_PI = sqrt( 2.0 / PI );
+
+
+// MAIN //
+
+/**
+* Returns the expected value of a half-normal distribution.
+*
+* @param {number} sigma - scale parameter
+* @returns {number} expected value
+*
+* @example
+* var v = mean( 1.0 );
+* // returns ~0.798
+*
+* @example
+* var v = mean( 2.0 );
+* // returns ~1.596
+*
+* @example
+* var v = mean( -1.0 );
+* // returns NaN
+*/
+function mean( sigma ) {
+	if ( isnan( sigma ) || sigma <= 0.0 ) {
+		return NaN;
+	}
+	return sigma * SQRT_TWO_OVER_PI;
+}
+
+
+// EXPORTS //
+
+module.exports = mean;

lib/node_modules/@stdlib/stats/base/dists/halfnormal/mean/package.json

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+{
+  "name": "@stdlib/stats-base-dists-halfnormal-mean",
+  "version": "0.0.0",
+  "description": "Half-normal distribution expected value.",
+  "license": "Apache-2.0",
+  "author": {
+    "name": "The Stdlib Authors",
+    "url": "https://github.com/stdlib-js/stdlib/graphs/contributors"
+  },
+  "contributors": [
+    {
+      "name": "The Stdlib Authors",
+      "url": "https://github.com/stdlib-js/stdlib/graphs/contributors"
+    }
+  ],
+  "main": "./lib",
+  "directories": {
+    "lib": "./lib",
+    "doc": "./docs",
+    "test": "./test",
+    "benchmark": "./benchmark"
+  },
+  "types": "./docs/types",
+  "scripts": {
+    "test": "make test",
+    "test-cov": "make test-cov",
+    "test-browsers": "make test-browsers"
+  },
+  "homepage": "https://stdlib.io",
+  "repository": {
+    "type": "git",
+    "url": "git://github.com/stdlib-js/stdlib.git"
+  },
+  "bugs": {
+    "url": "https://github.com/stdlib-js/stdlib/issues"
+  },
+  "dependencies": {
+    "@stdlib/constants/float64/pi": "^0.2.2",
+    "@stdlib/math/base/assert/is-nan": "^0.2.2",
+    "@stdlib/math/base/special/sqrt": "^0.2.2"
+  },
+  "devDependencies": {
+    "@stdlib/math/base/special/abs": "^0.2.2",
+    "tape": "git+https://github.com/kgryte/tape.git#fix/globby",
+    "istanbul": "^0.4.1",
+    "tap-min": "git+https://github.com/Planeshifter/tap-min.git",
+    "@stdlib/bench/harness": "^0.2.2"
+  },
+  "engines": {
+    "node": ">=0.10.0",
+    "npm": ">2.7.0"
+  },
+  "os": [
+    "aix",
+    "darwin",
+    "freebsd",
+    "linux",
+    "macos",
+    "openbsd",
+    "sunos",
+    "win32",
+    "windows"
+  ],
+  "keywords": [
+    "stdlib",
+    "stdmath",
+    "statistics",
+    "stats",
+    "distribution",
+    "half-normal",
+    "halfnormal",
+    "mean",
+    "expected value",
+    "expectation"
+  ]
+}

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

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+'use strict';
+
+// MODULES //
+
+var tape = require( 'tape' );
+var abs = require( '@stdlib/math/base/special/abs' );
+var isnan = require( '@stdlib/math/base/assert/is-nan' );
+var PINF = require( '@stdlib/constants/float64/pinf' );
+var NINF = require( '@stdlib/constants/float64/ninf' );
+var mean = require( './../lib' );
+
+// TESTS //
+
+tape( 'main export is a function', function test( t ) {
+	t.ok( true, __filename );
+	t.equal( typeof mean, 'function', 'main export is a function' );
+	t.end();
+});
+
+tape( 'if provided `NaN` for `sigma`, the function returns `NaN`', function test( t ) {
+	var v = mean( NaN );
+	t.equal( isnan( v ), true, 'returns NaN' );
+	t.end();
+});
+
+tape( 'if provided a value less than or equal to 0 for `sigma`, the function returns `NaN`', function test( t ) {
+	var v;
+
+	v = mean( 0.0 );
+	t.equal( isnan( v ), true, 'returns NaN' );
+
+	v = mean( -1.0 );
+	t.equal( isnan( v ), true, 'returns NaN' );
+
+	v = mean( NINF );
+	t.equal( isnan( v ), true, 'returns NaN' );
+
+	t.end();
+});
+
+tape( 'the function returns the expected value', function test( t ) {
+	var expected;
+	var sigma;
+	var v;
+	var i;
+
+	// Test cases (sigma, expected)
+	// Mean = sigma * sqrt(2/pi) -> sigma * ~0.79788456
+	sigma = [ 1.0, 2.0, 5.0, 0.5 ];
+	expected = [
+		0.79788456,
+		1.59576912,
+		3.98942280,
+		0.39894228
+	];
+
+	for ( i = 0; i < sigma.length; i++ ) {
+		v = mean( sigma[ i ] );
+		// We check if the difference is very small (epsilon)
+		if ( abs( v - expected[ i ] ) > 1e-7 ) {
+			t.fail( 'returns ' + v + ' when expected ' + expected[ i ] );
+		} else {
+			t.pass( 'returns ' + v + ' when expected ' + expected[ i ] );
+		}
+	}
+	t.end();
+});

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

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+'use strict';
+
+/**
+* Half-normal distribution variance.
+*
+* @module @stdlib/stats/base/dists/halfnormal/variance
+*
+* @example
+* var variance = require( '@stdlib/stats/base/dists/halfnormal/variance' );
+*
+* var v = variance( 1.0 );
+* // returns ~0.363
+*
+* var v = variance( 2.0 );
+* // returns ~1.454
+*/
+
+// MODULES //
+
+var main = require( './main.js' );
+
+
+// EXPORTS //
+
+module.exports = main;

lib/node_modules/@stdlib/stats/base/dists/halfnormal/variance/lib/main.js

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+'use strict';
+
+// MODULES //
+
+var isnan = require( '@stdlib/math/base/assert/is-nan' );
+var pow = require( '@stdlib/math/base/special/pow' );
+var PI = require( '@stdlib/constants/float64/pi' );
+
+
+// VARIABLES //
+
+// 1 - 2/pi
+var FACTOR = 1.0 - ( 2.0 / PI );
+
+
+// MAIN //
+
+/**
+* Returns the variance of a half-normal distribution.
+*
+* @param {number} sigma - scale parameter
+* @returns {number} variance
+*
+* @example
+* var v = variance( 1.0 );
+* // returns ~0.363
+*
+* @example
+* var v = variance( 2.0 );
+* // returns ~1.454
+*
+* @example
+* var v = variance( -1.0 );
+* // returns NaN
+*/
+function variance( sigma ) {
+	if ( isnan( sigma ) || sigma <= 0.0 ) {
+		return NaN;
+	}
+	return pow( sigma, 2.0 ) * FACTOR;
+}
+
+
+// EXPORTS //
+
+module.exports = variance;

lib/node_modules/@stdlib/stats/base/dists/halfnormal/variance/package.json

@@ -0,0 +1,74 @@
+{
+  "name": "@stdlib/stats-base-dists-halfnormal-variance",
+  "version": "0.0.0",
+  "description": "Half-normal distribution variance.",
+  "license": "Apache-2.0",
+  "author": {
+    "name": "The Stdlib Authors",
+    "url": "https://github.com/stdlib-js/stdlib/graphs/contributors"
+  },
+  "contributors": [
+    {
+      "name": "The Stdlib Authors",
+      "url": "https://github.com/stdlib-js/stdlib/graphs/contributors"
+    }
+  ],
+  "main": "./lib",
+  "directories": {
+    "doc": "./docs",
+    "lib": "./lib",
+    "test": "./test"
+  },
+  "types": "./docs/types",
+  "scripts": {
+    "test": "make test",
+    "test-cov": "make test-cov",
+    "test-browsers": "make test-browsers"
+  },
+  "homepage": "https://stdlib.io",
+  "repository": {
+    "type": "git",
+    "url": "git://github.com/stdlib-js/stdlib.git"
+  },
+  "bugs": {
+    "url": "https://github.com/stdlib-js/stdlib/issues"
+  },
+  "dependencies": {
+    "@stdlib/constants/float64/pi": "^0.2.2",
+    "@stdlib/math/base/assert/is-nan": "^0.2.2",
+    "@stdlib/math/base/special/pow": "^0.2.2"
+  },
+  "devDependencies": {
+    "@stdlib/math/base/special/abs": "^0.2.2",
+    "tape": "git+https://github.com/kgryte/tape.git#fix/globby",
+    "istanbul": "^0.4.1",
+    "tap-min": "git+https://github.com/Planeshifter/tap-min.git"
+  },
+  "engines": {
+    "node": ">=0.10.0",
+    "npm": ">2.7.0"
+  },
+  "os": [
+    "aix",
+    "darwin",
+    "freebsd",
+    "linux",
+    "macos",
+    "openbsd",
+    "sunos",
+    "win32",
+    "windows"
+  ],
+  "keywords": [
+    "stdlib",
+    "stdmath",
+    "statistics",
+    "stats",
+    "distribution",
+    "half-normal",
+    "halfnormal",
+    "variance",
+    "var",
+    "dispersion"
+  ]
+}