Function init_delta

subprojects/llama.cpp/tests/test-chat.cpp:265–313

}

static delta_data init_delta(const struct common_chat_templates * tmpls, const std::vector<std::string> & end_tokens,
                             const common_chat_msg & user_message,
                             const common_chat_msg & delta_message,
                             const std::vector<common_chat_tool> & tools,
                             const common_chat_tool_choice & tool_choice) {
    common_chat_templates_inputs inputs;
    inputs.parallel_tool_calls = true;
    inputs.messages.push_back(user_message);
    inputs.tools       = tools;
    inputs.tool_choice = tool_choice;
    auto params_prefix = common_chat_templates_apply(tmpls, inputs);

    inputs.messages.push_back(delta_message);
    inputs.add_generation_prompt = false;
    auto params_full             = common_chat_templates_apply(tmpls, inputs);

    std::string prefix = params_prefix.prompt;
    std::string full   = params_full.prompt;

    if (full == prefix) {
        throw std::runtime_error("Full message is the same as the prefix");
    }

    size_t common_prefix_length = 0;
    for (size_t i = 0; i < prefix.size() && i < full.size(); ++i) {
        if (prefix[i] != full[i]) {
            break;
        }
        if (prefix[i] == '<') {
            // DeepSeek R1's template (as of 20250209) adds a trailing <think> if add_generation_prompt,
            // but it removes thinking tags for past messages.
            // The prefix and full strings diverge at <think> vs. <｜tool▁calls▁begin｜>, we avoid consuming the leading <.
            continue;
        }
        common_prefix_length = i + 1;
    }
    auto delta = full.substr(common_prefix_length);

    // Strip end tokens
    for (const auto & end_token : end_tokens) {
        // rfind to find the last occurrence
        auto pos = delta.rfind(end_token);
        if (pos != std::string::npos) {
            delta = delta.substr(0, pos);
            break;
        }
    }
    return { delta, params_full };
}

/*
  Applies the template to 1 user message w/ add_generation_prompt=true, then w/ the test message w/ add_generation_prompt=false,