pheidippides.hpp

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/*
 * The MIT License (MIT)
 *
 * Copyright (c) 2025 Yaroslav Riabtsev <yaroslav.riabtsev@rwth-aachen.de>
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included
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#ifndef ARACHNE_PHEIDIPPIDES_HPP
#define ARACHNE_PHEIDIPPIDES_HPP
#include "http_client.hpp"
 
#include <nlohmann/json.hpp>
#include <unordered_set>
 
namespace arachnespace {
/**
 * @class pheidippides
 * @brief Batch courier for Wikidata/Commons: collects IDs, issues HTTP
 * requests, and returns a merged JSON payload.
 *
 * Responsibilities:
 *  - Pick the endpoint based on entity kind:
 *      * Q/P/L/E -> https://www.wikidata.org/w/api.php
 *      * M (mediainfo) -> https://commons.wikimedia.org/w/api.php
 *  - Build request parameters:
 *      * for E (EntitySchema): `action=query`, `titles=EntitySchema:<id>`,
 *        `prop=<joined opt.prop>`
 *      * for others: `action=wbgetentities`, `ids=<id>|<id>...`,
 *        `props=<joined opt.props>`
 *  - Split the input set into chunks up to `batch_threshold`.
 *  - Filter IDs by expected kind using `arachne::identify(id)`.
 *  - Merge per-chunk JSON responses using `merge_patch`.
 *
 * Thread-safety:
 *  - Not thread-safe; the instance owns a reusable `http_client` (single easy
 *    handle). Use one instance per calling thread.
 *
 * @note The implementation currently issues requests even when a chunk becomes
 *       empty after filtering (for example when `kind == entity_kind::any`).
 *       The server response for an empty identifier list is merged as-is.
 */
class pheidippides {
public:
    /**
     * @brief Fetch metadata for a set of entity IDs and return a merged JSON
     * object.
     *
     * Behavior:
     *  - Empty `batch` results in an empty JSON object.
     *  - For `kind == entity_kind::entity_schema`, IDs are prefixed with
     *    `EntitySchema:` and fields come from `opt.prop`.
     *  - For other kinds, fields come from `opt.props`.
     *  - Only elements where `arachne::identify(id) == kind` are included in a
     *    request chunk; if the filter removes every element the request still
     *    executes with an empty identifier list and the response is merged.
     *  - Chunk responses are merged into a single object via `merge_patch`.
     *
     * Errors:
     *  - Transport or HTTP errors are handled by the internal `http_client`
     *    retry policy; terminal failures throw `std::runtime_error`.
     *  - Invalid JSON payloads propagate `nlohmann::json::parse_error` from
     *    `nlohmann::json::parse`.
     *
     * @param batch Set of full IDs (e.g., "Q123", "L7-F1", "E42").
     * @param kind  Target entity kind (selects API, fields, and filtering).
     * @return Merged JSON object with fetched data.
     */
    nlohmann::json fetch_json(
        const std::unordered_set<std::string>& batch,
        corespace::entity_kind kind = corespace::entity_kind::any
    );
 
    /**
     * @brief Access aggregated network metrics of the underlying client.
     * @return Const reference to metrics snapshot.
     */
    [[nodiscard]] const corespace::network_metrics& metrics_info() const;
    /**
     * @brief Join a span of strings with a separator (no encoding or
     *        validation).
     *
     * Edge cases:
     *  - Empty input yields an empty string.
     *  - Separator defaults to "|" (useful for MediaWiki multi-ID parameters).
     *
     * @param ids        Input strings to join.
     * @param separator  Separator between elements (default: "|").
     * @return Concatenated string.
     */
    static std::string join_str(
        std::span<const std::string> ids, std::string_view separator = "|"
    );
 
private:
    corespace::options
        opt {}; ///< Request shaping parameters (chunking, fields, base params).
    corespace::http_client
        client {}; ///< Reused HTTP client (not thread-safe across threads).
};
}
#endif // ARACHNE_PHEIDIPPIDES_HPP