a7p-js

Wrapper for .a7p files

.a7p is the most common ballistic profile format for the latest Archer thermal vision devices

Table of Contents

• a7p-js
  • Table of Contents
  • Installation
  • Usage
    • In a Node.js / TypeScript project
    • In a browser, via CDN
  • Dimensions
    • Build notes

Installation

yarn add a7p-js
# or
npm install a7p-js

Usage

In a Node.js / TypeScript project

import { readFile } from 'fs/promises';
import { decode, encode, ValidationError } from 'a7p-js';

const bytes = await readFile('./example.a7p');

try {
  // decode a .a7p file into a plain Payload object
  const payload = decode(bytes);
  console.log(payload.profile.profileName);
  console.log(payload.profile.switches);

  // encode a Payload back into .a7p bytes
  const buffer = encode(payload);
} catch (error) {
  if (error instanceof ValidationError) {
    error.errors.forEach((message) => console.log(message));
  } else {
    throw error;
  }
}

In a browser, via CDN

a7p-js ships as an ES module, so it can be imported directly from a CDN such as esm.sh or jsDelivr with no build step:

<script type="module">
  import { decode } from "https://esm.sh/[email protected]";

  const input = document.querySelector('input[type="file"]');
  input.addEventListener('change', async () => {
    const file = input.files[0];
    const buffer = await file.arrayBuffer();
    const payload = decode(buffer);
    console.log(payload.profile);
  });
</script>

See it in action in this live example, which combines a7p-js with js-ballistics to compare trajectories from .a7p profiles entirely in the browser.

Dimensions

To obtain values from an .a7p profile in the desired units, you need to divide them by the multiplier. For the reverse operation, you need to perform the inverse operation and convert to an integer.

| key | unit | multiplier | desc | |--------------------------|----------------|------------|---------------------------------------------| | scHeight | mm | 1 | sight height in mm | | rTwist | inch | 100 | positive twist value | | cZeroTemperature | C | 1 | temperature at cMuzzleVelocity | | cMuzzleVelocity | mps | 10 | muzzle velocity at cZeroTemperature | | cTCoeff | %/15C | 1000 | temperature sensitivity | | cZeroDistanceIdx | <int> | 10 | index of zero distance from distances table | | cZeroAirTemperature | C | 1 | air temperature at zero | | cZeroAirPressure | hPa | 10 | air pressure at zero | | cZeroAirHumidity | % | 1 | air humidity at zero | | cZeroPTemperature | C | 1 | powder temperature at zero | | cZeroWPitch | deg | 1 | zeroing look angle | | bDiameter | inch | 1000 | bullet diameter | | bWeight | grain | 10 | bullet weight | | bLength | inch | 1000 | bullet length | | twistDir | RIGHT|LEFT | | twist direction | | bcType | G1|G7|CUSTOM | | g-func type | | distances | m | 100 | distances table in m | | zeroX | <int> | -1000 | zeroing h-clicks for specific device | | zeroY | <int> | 1000 | zeroing v-clicks for specific device | | coefRows[].bcCd (G1/G7) | | 10000 | bc coefficient for mv | | coefRows[].mv (G1/G7) | mps | 10 | mv for bc provided | | coefRows[].bcCd (CUSTOM) | | 10000 | drag coefficient (Cd) | | coefRows[].mv (CUSTOM) | mach | 10 | speed in mach |

Build notes

yarn install
yarn build:proto   # regenerate protobuf bindings after editing ../proto/profedit.proto
yarn build:schema  # regenerate the ajv validator after editing ../schema/a7p.schema.json
yarn build         # tsc + copy the generated schema validator into dist/

build:proto/build:schema only need re-running when the .proto/schema source changes — both write generated, checked-in files under src/ (src/profedit.ts, src/generated/a7p_schema_validator.cjs), so a plain yarn build is enough day-to-day.