54 lines
1.8 KiB
TypeScript
54 lines
1.8 KiB
TypeScript
// Copyright 2021-2022 Signal Messenger, LLC
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// SPDX-License-Identifier: AGPL-3.0-only
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// We ["MUST NOT generate more than three digits after the decimal point"][0]. We use a
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// space-efficient algorithm that runs out of digits after 28 languages. This should be
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// fine for most users and [the server doesn't parse more than 15 languages, at least
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// in some cases][1].
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//
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// [0]: https://httpwg.org/specs/rfc7231.html#quality.values
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// [1]: https://github.com/signalapp/Signal-Server/blob/bf6d3aa32407ff52b5547ed6ce2e7a2f2bbb0f03/service/src/main/java/org/signal/i18n/HeaderControlledResourceBundleLookup.java#L19
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const MAX_LANGUAGES_TO_FORMAT = 28;
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export function formatAcceptLanguageHeader(
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languages: ReadonlyArray<string>
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): string {
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if (languages.length === 0) {
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return '*';
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}
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const result: Array<string> = [];
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const length = Math.min(languages.length, MAX_LANGUAGES_TO_FORMAT);
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for (let i = 0; i < length; i += 1) {
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const language = languages[i];
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// ["If no 'q' parameter is present, the default weight is 1."][1]
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//
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// [1]: https://httpwg.org/specs/rfc7231.html#quality.values
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if (i === 0) {
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result.push(language);
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continue;
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}
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// These values compute a descending sequence with minimal bytes. See the tests for
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// examples.
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const magnitude = 1 / 10 ** (Math.ceil(i / 9) - 1);
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const subtractor = (((i - 1) % 9) + 1) * (magnitude / 10);
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const q = magnitude - subtractor;
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const formattedQ = q.toFixed(3).replace(/0+$/, '');
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result.push(`${language};q=${formattedQ}`);
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}
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return result.join(', ');
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}
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export function getUserLanguages(
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defaults: undefined | ReadonlyArray<string>,
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fallback: string
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): ReadonlyArray<string> {
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const result = defaults || [];
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return result.length ? result : [fallback];
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}
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