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import { toLowerCase } from "./deps.ts";
const SingletonFields: string[] = [  "Access-Control-Allow-Origin",  "Authorization",  "Content-Language",  "Content-Length",  "Content-Location",  "Content-Range",  "Content-Type",  "Date",  "ETag",  "Expect",  "From",  "Host",  "If-Modified-Since",  "If-Range",  "If-Unmodified-Since",  "Last-Modified",  "Location",  "Max-Forwards",  "Origin",  "Range",  "Referer",  "Retry-After",  "Server",  "User-Agent",  "Proxy-Authorization",  "Age",  "Expires",];
/** Weather the field is singleton field or not. * * ```ts * import { isSingletonField } from "https://deno.land/x/http_utils@$VERSION/mod.ts"; * import { assertEquals } from "https://deno.land/std@$VERSION/testing/asserts.ts"; * * assertEquals(isSingletonField("Origin"), true); * assertEquals(isSingletonField("Vary"), false); * ``` */export function isSingletonField(  fieldName: string,): boolean {  return SingletonFields.map(toLowerCase).includes(    toLowerCase(fieldName),  );}
/** Merge two `Headers` object. * The first `Headers` always takes precedence. * When fields conflict, the first `Headers` takes precedence if it is a singleton * field. * If it is a list-based field and not empty, it is appended to the first `Headers` * field. * Invalid field names and field values are ignored. * No destructive operation is performed on the arguments and returns a new * `Headers` object. * * ```ts * import { mergeHeaders } from "https://deno.land/x/http_utils@$VERSION/mod.ts"; * import { assertEquals } from "https://deno.land/std@$VERSION/testing/asserts.ts"; * * assertEquals( *   mergeHeaders( *     new Headers({ accept: "text/html" }), *     new Headers({ accept: "application/json", "content-type": "text/plain" }), *   ), *   new Headers({ *     accept: "text/html, application/json", *     "content-type": "text/plain", *   }), * ); * assertEquals( *   mergeHeaders( *     new Headers({ origin: "http://test.test" }), *     new Headers({ origin: "http://example.test" }), *   ), *   new Headers({ origin: "http://test.test" }), * ); * // origin is singleton field * ``` */export function mergeHeaders(  primaryHeaders: Headers,  newHeaders: Headers,): Headers {  primaryHeaders = new Headers(primaryHeaders);  newHeaders = new Headers(newHeaders);
  newHeaders.forEach((value, key) => {    if (!value) return;    const has = primaryHeaders.has(key);    if (has && isSingletonField(key)) return;
    try {      primaryHeaders.append(key, value);    } catch {      // When the key is invalid header name, throw error.      // But just ignore it.    }  });
  return primaryHeaders;}
/** Check two `Headers` field name and field value equality. * * ```ts * import { equalsHeaders } from "https://deno.land/x/http_utils@$VERSION/mod.ts"; * import { assertEquals } from "https://deno.land/std@$VERSION/testing/asserts.ts"; * * assertEquals( *   equalsHeaders(new Headers({ a: "b" }), new Headers({ a: "b" })), *   true, * ); * assertEquals( *   equalsHeaders(new Headers({ a: "b" }), new Headers({ c: "d" })), *   false, * ); * ``` */export function equalsHeaders(a: Headers, b: Headers): boolean {  const header = [...a, ...b];  for (const [key, value] of header) {    if (!a.has(key) || !b.has(key)) {      return false;    }    if (a.get(key) !== value || b.get(key) !== value) {      return false;    }  }
  return true;}