import type { RequestHeaders } from '../../routes/types.ts';
import { ApplicationError } from '../../errors/index.ts';
import { HTTP_STATUS_CODE_BAD_REQUEST } from '../../constants.ts';

/** RFC 9110 field-name token grammar. */
const HEADER_NAME_PATTERN = /^[!#$%&'*+\-.^_`|~0-9A-Za-z]+$/;

/**
 * Validate and normalize request headers at a transport boundary.
 *
 * Lowercase names give HTTP and WebSocket routes identical lookup semantics.
 * Case-insensitive duplicates are rejected instead of selecting an ambiguous
 * authentication value. Header values may not contain line breaks.
 */
export const normalizeRequestHeaders = (headers: Readonly<Record<string, string>>): RequestHeaders => {
    const normalizedEntries: Array<[string, string]> = [];
    const names = new Set<string>();

    for (const [ name, value ] of Object.entries(headers)) {
        if (!HEADER_NAME_PATTERN.test(name)) {
            throw new ApplicationError(HTTP_STATUS_CODE_BAD_REQUEST, 'Invalid request header name');
        }

        if (value.includes('\r') || value.includes('\n')) {
            throw new ApplicationError(HTTP_STATUS_CODE_BAD_REQUEST, 'Invalid request header value');
        }

        const normalizedName = name.toLowerCase();
        if (names.has(normalizedName)) {
            throw new ApplicationError(HTTP_STATUS_CODE_BAD_REQUEST, 'Duplicate request header name');
        }

        names.add(normalizedName);
        normalizedEntries.push([ normalizedName, value ]);
    }

    return Object.freeze(Object.fromEntries(normalizedEntries));
};