# Longitude-crossing searches

Find longitude crossings under a sample budget and handle a refusal as a whole result.

Engine 1.0.0: On npm.
Archive identity and release status: https://zodiacs.org/developers/engine/

## Bound the work

Use searchLongitudeCrossings to cap longitude evaluations. A refused search contains no partial crossings: treat it as a failure to complete, then shorten the window or deliberately choose a different budget.

~~~javascript
import { searchLongitudeCrossings } from '@zodiacs/engine';
const result = searchLongitudeCrossings(
  'Mercury', 0, '2026-01-01T00:00:00Z', '2027-01-01T00:00:00Z',
  { stepDays: 1, maxSamples: 5000 },
);
if (result.status === 'refused') {
  console.log({ reason: result.reason, samples: result.samples });
} else {
  console.log(result.crossings.map(({ at, retrograde }) =>
    ({ utc: at.toISOString(), retrograde })));
}
~~~

## Completeness

The solver is tested and assumes smooth motion with a bounded pattern of stations between samples. It is not a proof that every event was found. A plain longitude scan and an interval-certified search have different guarantees; retain the declared method.

## Install the release

~~~sh
npm install @zodiacs/engine@1.0.0
~~~

npm's 1.0.0 tarball is byte-identical to the archive the engine repository carries. To install those bytes from the carrier, with their digest checked before npm sees them, run this in the project's directory instead:

~~~sh
( set -eu
# POSIX shell — macOS, Linux or WSL. Not PowerShell. It runs in a subshell, so
# a failure stops the install without closing your terminal.
BASE=$(pwd)
FILE='zodiacs-engine-1.0.0.tgz'
# Run it in your project's directory. Without a package.json here, npm would
# install into the nearest parent directory that has one.
if test ! -f package.json; then
  echo "Stop: there is no package.json here. Run this in your project's directory, or create one first with: npm init -y" >&2
  exit 1
fi
if test -e "$FILE" || test -L "$FILE"; then
  echo "Stop: $FILE already exists here. Move or delete it, then run this again." >&2
  exit 1
fi
# A download that fails verification is removed, so the guard above does not
# then block the retry. Once npm takes over the archive stays: npm reports its
# own failures, and the file is what you would retry with.
trap 'status=$?; if test "$status" -ne 0; then rm -f "$BASE/$FILE"; fi; exit $status' EXIT

curl --disable --fail --silent --show-error --location --proto '=https' --max-time 120 \
  'https://raw.githubusercontent.com/zodiacs-org/engine/38d31854e5716454921b6ce8a211788a0984328e/artifacts/zodiacs-engine-1.0.0.tgz' -o "$FILE"

node --input-type=module <<'JS'
import { readFileSync } from 'node:fs';
import { createHash } from 'node:crypto';
const file = 'zodiacs-engine-1.0.0.tgz';
const expected = 'c4d3754de899fb98882331e6d7b9ce3ce77fb2072baeb002aa0ef8c77b0ab0f6';
const bytes = readFileSync(file);
const actual = createHash('sha256').update(bytes).digest('hex');
if (actual !== expected) {
  console.error(`Stop: this is not the published archive.\n  expected ${expected}\n  got      ${actual}`);
  console.error('Nothing was installed.');
  process.exit(1);
}
console.log(`Archive verified: ${actual}`);
JS

trap - EXIT
npm install --ignore-scripts "./$FILE"
echo "Installed @zodiacs/engine@1.0.0 from the verified archive."
)
~~~

## References

- [Engine source reference](https://github.com/zodiacs-org/engine/blob/ca2c8cefbaa1a9d6f5f78ee86713e813633942ca/docs/calc.md)
- [Engine source reference](https://github.com/zodiacs-org/engine/blob/ca2c8cefbaa1a9d6f5f78ee86713e813633942ca/src/crossings.ts)