npm package: @stdlib/number-float64-base-from-int64-bytes
Description: Convert a signed 64-bit integer byte array to a double-precision floating-point number.
Author: The Stdlib Authors
Latest version: 0.2.3

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<details>
  <summary>
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  <p>We believe in a future in which the web is a preferred environment for numerical computation. To help realize this future, we've built stdlib. stdlib is a standard library, with an emphasis on numerical and scientific computation, written in JavaScript (and C) for execution in browsers and in Node.js.</p>
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# fromInt64Bytes

[![NPM version][npm-image]][npm-url] [![Build Status][test-image]][test-url] [![Coverage Status][coverage-image]][coverage-url] <!-- [![dependencies][dependencies-image]][dependencies-url] -->

> Convert a signed 64-bit integer byte array to a [double-precision floating-point number][ieee754].

<section class="installation">

## Installation

```bash
npm install @stdlib/number-float64-base-from-int64-bytes
```

</section>

<section class="usage">

## Usage

```javascript
var fromInt64Bytes = require( '@stdlib/number-float64-base-from-int64-bytes' );
```

#### fromInt64Bytes( bytes, stride, offset )

Converts a signed 64-bit integer byte array to a [double-precision floating-point number][ieee754].

```javascript
var Uint8Array = require( '@stdlib/array-uint8' );

var bytes = new Uint8Array( [ 255, 255, 255, 255, 255, 255, 255, 255 ] );
var out = fromInt64Bytes( bytes, 1, 0 );
// returns -1.0
```

The function supports providing a `stride` and an index `offset` for indexing into a provided byte array.

<!-- eslint-disable max-len -->

```javascript
var Uint8Array = require( '@stdlib/array-uint8' );

var bytes = new Uint8Array( [ 0, 255, 0, 255, 0, 255, 0, 255, 0, 255, 0, 255, 0, 255, 0, 255 ] );
var out = fromInt64Bytes( bytes, 2, 1 );
// returns -1.0
```

</section>

<!-- /.usage -->

<section class="notes">

## Notes

-   The function assumes host byte order (endianness).

</section>

<!-- /.notes -->

<section class="examples">

## Examples

<!-- eslint no-undef: "error" -->

```javascript
var IS_LITTLE_ENDIAN = require( '@stdlib/assert-is-little-endian' );
var discreteUniform = require( '@stdlib/random-base-discrete-uniform' );
var bernoulli = require( '@stdlib/random-base-bernoulli' );
var Uint8Array = require( '@stdlib/array-uint8' );
var fromInt64Bytes = require( '@stdlib/number-float64-base-from-int64-bytes' );

var bytes;
var sgn;
var x;
var b;
var s;
var i;
var j;
var k;

bytes = new Uint8Array( 8 );
if ( IS_LITTLE_ENDIAN ) {
    k = 0;
    s = 1;
} else {
    k = 7;
    s = -1;
}
// Generate random integer-valued doubles on the interval (-2^16, 2^16)...
for ( i = 0; i < 10; i++ ) {
    // Determine the sign:
    sgn = ( bernoulli( 0.5 ) ) ? 0 : 128; // 2^7

    // Set a subset of individual (lower-order) bytes:
    for ( j = 0; j < 2; j++ ) {
        b = discreteUniform( 0, 255 ); // 2^8-1
        bytes[ k+(j*s) ] = b;
    }
    // Set higher-order bytes using two's complement:
    for ( j = 2; j < 8; j++ ) {
        bytes[ k+(j*s) ] = ( sgn ) ? 255 : 0; // 2^8-1
    }
    // Convert the bytes to a double:
    x = fromInt64Bytes( bytes, 1, 0 );
    console.log( bytes + ' => ' + x );
}
```

</section>

<!-- /.examples -->

<!-- Section for related `stdlib` packages. Do not manually edit this section, as it is automatically populated. -->

<section class="related">

* * *

## See Also

-   <span class="package-name">[`@stdlib/number-float64/base/to-int64-bytes`][@stdlib/number/float64/base/to-int64-bytes]</span><span class="delimiter">: </span><span class="description">convert an integer-valued double-precision floating-point number to a signed 64-bit integer byte array according to host byte order.</span>

</section>

<!-- /.related -->

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<section class="main-repo" >

* * *

## Notice

This package is part of [stdlib][stdlib], a standard library for JavaScript and Node.js, with an emphasis on numerical and scientific computing. The library provides a collection of robust, high performance libraries for mathematics, statistics, streams, utilities, and more.

For more information on the project, filing bug reports and feature requests, and guidance on how to develop [stdlib][stdlib], see the main project [repository][stdlib].

#### Community

[![Chat][chat-image]][chat-url]

---

## License

See [LICENSE][stdlib-license].


## Copyright

Copyright &copy; 2016-2026. The Stdlib [Authors][stdlib-authors].

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[umd]: https://github.com/umdjs/umd
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[ieee754]: https://en.wikipedia.org/wiki/IEEE_754-1985

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