ctxstream/src/backend.cpp

// Minimal HTTP/1.1 client + just enough JSON to talk to a local model server.
// No libcurl, no JSON library: the request shape is fixed and only three fields
// are read back, so a dependency would cost more than it saves.
#include "ctxstream.hpp"

#include <netdb.h>
#include <netinet/in.h>
#include <sys/socket.h>
#include <sys/types.h>
#include <unistd.h>

#include <cstring>
#include <sstream>
#include <string>

namespace ctxstream {

static std::string json_escape(const std::string& s) {
    std::string o;
    o.reserve(s.size() + 32);
    for (unsigned char c : s) {
        switch (c) {
            case '"':  o += "\\\""; break;
            case '\\': o += "\\\\"; break;
            case '\n': o += "\\n";  break;
            case '\r': o += "\\r";  break;
            case '\t': o += "\\t";  break;
            case '\b': o += "\\b";  break;
            case '\f': o += "\\f";  break;
            default:
                if (c < 0x20) {
                    char buf[8];
                    std::snprintf(buf, sizeof buf, "\\u%04x", c);
                    o += buf;
                } else {
                    o += static_cast<char>(c);
                }
        }
    }
    return o;
}

// Unescape a JSON string body (the model's reply). Handles the escapes we emit
// and the \uXXXX that servers produce; non-ASCII code points are passed through
// as UTF-8.
static std::string json_unescape(const std::string& s) {
    std::string o;
    o.reserve(s.size());
    for (std::size_t i = 0; i < s.size(); ++i) {
        if (s[i] != '\\' || i + 1 >= s.size()) { o += s[i]; continue; }
        const char n = s[++i];
        switch (n) {
            case 'n': o += '\n'; break;
            case 't': o += '\t'; break;
            case 'r': o += '\r'; break;
            case 'b': o += '\b'; break;
            case 'f': o += '\f'; break;
            case '"': o += '"';  break;
            case '\\': o += '\\'; break;
            case '/': o += '/';  break;
            case 'u': {
                if (i + 4 >= s.size()) break;
                const unsigned cp =
                    std::stoul(s.substr(i + 1, 4), nullptr, 16);
                i += 4;
                if (cp < 0x80) {
                    o += static_cast<char>(cp);
                } else if (cp < 0x800) {
                    o += static_cast<char>(0xC0 | (cp >> 6));
                    o += static_cast<char>(0x80 | (cp & 0x3F));
                } else {
                    o += static_cast<char>(0xE0 | (cp >> 12));
                    o += static_cast<char>(0x80 | ((cp >> 6) & 0x3F));
                    o += static_cast<char>(0x80 | (cp & 0x3F));
                }
                break;
            }
            default: o += n;
        }
    }
    return o;
}

// Extract "key":"..." respecting backslash escapes.
static bool json_string_field(const std::string& body, const std::string& key,
                              std::string* out) {
    const std::string needle = "\"" + key + "\"";
    std::size_t p = body.find(needle);
    if (p == std::string::npos) return false;
    p = body.find(':', p + needle.size());
    if (p == std::string::npos) return false;
    while (p < body.size() && (body[++p] == ' ' || body[p] == '\t')) {}
    if (p >= body.size() || body[p] != '"') return false;
    const std::size_t start = ++p;
    for (; p < body.size(); ++p) {
        if (body[p] == '\\') { ++p; continue; }
        if (body[p] == '"') break;
    }
    if (p >= body.size()) return false;
    *out = json_unescape(body.substr(start, p - start));
    return true;
}

static long json_number_field(const std::string& body, const std::string& key,
                              long fallback = 0) {
    const std::string needle = "\"" + key + "\"";
    std::size_t p = body.find(needle);
    if (p == std::string::npos) return fallback;
    p = body.find(':', p + needle.size());
    if (p == std::string::npos) return fallback;
    ++p;
    while (p < body.size() && (body[p] == ' ' || body[p] == '\t')) ++p;
    const std::size_t start = p;
    while (p < body.size() && (std::isdigit(static_cast<unsigned char>(body[p])) ||
                               body[p] == '-' || body[p] == '.')) ++p;
    if (p == start) return fallback;
    try { return std::stol(body.substr(start, p - start)); }
    catch (...) { return fallback; }
}

static bool http_post(const BackendOptions& opt, const std::string& payload,
                      std::string* response, std::string* err) {
    addrinfo hints{};
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    addrinfo* res = nullptr;
    const std::string port = std::to_string(opt.port);
    if (getaddrinfo(opt.host.c_str(), port.c_str(), &hints, &res) != 0 || !res) {
        *err = "getaddrinfo failed for " + opt.host + ":" + port;
        return false;
    }

    int fd = -1;
    for (addrinfo* a = res; a; a = a->ai_next) {
        fd = ::socket(a->ai_family, a->ai_socktype, a->ai_protocol);
        if (fd < 0) continue;
        timeval tv{opt.timeout_sec, 0};
        setsockopt(fd, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof tv);
        setsockopt(fd, SOL_SOCKET, SO_SNDTIMEO, &tv, sizeof tv);
        if (::connect(fd, a->ai_addr, a->ai_addrlen) == 0) break;
        ::close(fd);
        fd = -1;
    }
    freeaddrinfo(res);
    if (fd < 0) { *err = "connect failed to " + opt.host + ":" + port; return false; }

    std::ostringstream req;
    req << "POST " << opt.path << " HTTP/1.1\r\n"
        << "Host: " << opt.host << ":" << opt.port << "\r\n"
        << "Content-Type: application/json\r\n"
        << "Content-Length: " << payload.size() << "\r\n"
        << "Connection: close\r\n\r\n"
        << payload;
    const std::string raw = req.str();

    for (std::size_t sent = 0; sent < raw.size();) {
        const ssize_t n = ::send(fd, raw.data() + sent, raw.size() - sent, 0);
        if (n <= 0) { ::close(fd); *err = "send failed"; return false; }
        sent += static_cast<std::size_t>(n);
    }

    std::string all;
    char buf[16384];
    for (;;) {
        const ssize_t n = ::recv(fd, buf, sizeof buf, 0);
        if (n < 0) { ::close(fd); *err = "recv timeout/error"; return false; }
        if (n == 0) break;
        all.append(buf, static_cast<std::size_t>(n));
    }
    ::close(fd);

    const std::size_t hdr = all.find("\r\n\r\n");
    if (hdr == std::string::npos) { *err = "malformed HTTP response"; return false; }
    *response = all.substr(hdr + 4);
    return true;
}

Completion complete(const BackendOptions& opt, const std::string& system_prompt,
                    const std::string& user_prompt) {
    Completion c;
    std::ostringstream body;
    body << "{\"model\":\"" << json_escape(opt.model) << "\","
         << "\"stream\":false,"
         << "\"options\":{\"num_ctx\":" << opt.num_ctx
         << ",\"num_predict\":" << opt.num_predict
         << ",\"temperature\":0},"
         << "\"messages\":[";
    if (!system_prompt.empty()) {
        body << "{\"role\":\"system\",\"content\":\"" << json_escape(system_prompt)
             << "\"},";
    }
    body << "{\"role\":\"user\",\"content\":\"" << json_escape(user_prompt)
         << "\"}]}";

    std::string resp;
    if (!http_post(opt, body.str(), &resp, &c.error)) return c;

    std::string err;
    if (json_string_field(resp, "error", &err) && !err.empty()) {
        c.error = "server: " + err;
        return c;
    }
    if (!json_string_field(resp, "content", &c.text)) {
        c.error = "no content field in response: " + resp.substr(0, 200);
        return c;
    }
    c.prompt_tokens = json_number_field(resp, "prompt_eval_count");
    c.output_tokens = json_number_field(resp, "eval_count");
    c.ok = true;
    return c;
}

bool looks_truncated(std::size_t prompt_chars, long prompt_tokens) {
    if (prompt_tokens <= 0) return false;  // server did not report; cannot judge
    // Deliberately loose: no real tokenizer reaches 5 chars/token, so this fires
    // only on gross clipping (measured: 50,000 tokens sent, 16,387 processed),
    // never on tokenizer variance.
    return static_cast<std::size_t>(prompt_tokens) < prompt_chars / 5;
}

}  // namespace ctxstream