// vdm/net/text_codec.cpp

#include "net/text_codec.hpp"

#include <array>
#include <cctype>

namespace vdm::net::detail {
namespace {

int hex_val(char c) noexcept {
    if (c >= '0' && c <= '9')
        return c - '0';
    if (c >= 'a' && c <= 'f')
        return c - 'a' + 10;
    if (c >= 'A' && c <= 'F')
        return c - 'A' + 10;
    return -1;
}

void append_utf8(std::string &out, std::uint32_t cp) {
    if (cp <= 0x7F) {
        out.push_back(static_cast<char>(cp));
    } else if (cp <= 0x7FF) {
        out.push_back(static_cast<char>(0xC0 | (cp >> 6)));
        out.push_back(static_cast<char>(0x80 | (cp & 0x3F)));
    } else if (cp <= 0xFFFF) {
        out.push_back(static_cast<char>(0xE0 | (cp >> 12)));
        out.push_back(static_cast<char>(0x80 | ((cp >> 6) & 0x3F)));
        out.push_back(static_cast<char>(0x80 | (cp & 0x3F)));
    } else {
        out.push_back(static_cast<char>(0xF0 | (cp >> 18)));
        out.push_back(static_cast<char>(0x80 | ((cp >> 12) & 0x3F)));
        out.push_back(static_cast<char>(0x80 | ((cp >> 6) & 0x3F)));
        out.push_back(static_cast<char>(0x80 | (cp & 0x3F)));
    }
}

bool charset_is(std::string_view cs, std::string_view want) {
    if (cs.size() != want.size())
        return false;
    for (std::size_t i = 0; i < cs.size(); ++i) {
        char a = cs[i], b = want[i];
        if (a >= 'A' && a <= 'Z')
            a = static_cast<char>(a - 'A' + 'a');
        if (b >= 'A' && b <= 'Z')
            b = static_cast<char>(b - 'A' + 'a');
        if (a != b)
            return false;
    }
    return true;
}

bool is_utf8(std::string_view cs) {
    return charset_is(cs, "utf-8") || charset_is(cs, "utf8");
}
bool is_latin1(std::string_view cs) {
    return charset_is(cs, "iso-8859-1") || charset_is(cs, "latin1") ||
           charset_is(cs, "iso8859-1") || charset_is(cs, "windows-1252");
}

}  // namespace

std::string percent_decode(std::string_view in, bool plus_as_space) {
    std::string out;
    out.reserve(in.size());
    for (std::size_t i = 0; i < in.size(); ++i) {
        char c = in[i];
        if (c == '%' && i + 2 < in.size()) {
            int hi = hex_val(in[i + 1]);
            int lo = hex_val(in[i + 2]);
            if (hi >= 0 && lo >= 0) {
                out.push_back(static_cast<char>((hi << 4) | lo));
                i += 2;
                continue;
            }
        }
        if (c == '+' && plus_as_space) {
            out.push_back(' ');
            continue;
        }
        out.push_back(c);
    }
    return out;
}

bool is_valid_utf8(std::string_view s) noexcept {
    std::size_t i = 0;
    const std::size_t n = s.size();
    auto cont = [&](std::size_t k) {
        return k < n && (static_cast<unsigned char>(s[k]) & 0xC0) == 0x80;
    };
    while (i < n) {
        unsigned char c = static_cast<unsigned char>(s[i]);
        if (c < 0x80) {
            ++i;
        } else if ((c & 0xE0) == 0xC0) {
            if (!cont(i + 1))
                return false;
            std::uint32_t cp = (c & 0x1F) << 6 | (static_cast<unsigned char>(s[i + 1]) & 0x3F);
            if (cp < 0x80)
                return false;  // overlong
            i += 2;
        } else if ((c & 0xF0) == 0xE0) {
            if (!cont(i + 1) || !cont(i + 2))
                return false;
            std::uint32_t cp = (c & 0x0F) << 12 |
                               (static_cast<unsigned char>(s[i + 1]) & 0x3F) << 6 |
                               (static_cast<unsigned char>(s[i + 2]) & 0x3F);
            if (cp < 0x800 || (cp >= 0xD800 && cp <= 0xDFFF))
                return false;
            i += 3;
        } else if ((c & 0xF8) == 0xF0) {
            if (!cont(i + 1) || !cont(i + 2) || !cont(i + 3))
                return false;
            std::uint32_t cp = (c & 0x07) << 18 |
                               (static_cast<unsigned char>(s[i + 1]) & 0x3F) << 12 |
                               (static_cast<unsigned char>(s[i + 2]) & 0x3F) << 6 |
                               (static_cast<unsigned char>(s[i + 3]) & 0x3F);
            if (cp < 0x10000 || cp > 0x10FFFF)
                return false;
            i += 4;
        } else {
            return false;
        }
    }
    return true;
}

std::string latin1_to_utf8(std::string_view s) {
    std::string out;
    out.reserve(s.size() + s.size() / 2);
    for (char ch : s)
        append_utf8(out, static_cast<unsigned char>(ch));
    return out;
}

std::string to_utf8_best_effort(std::string_view s) {
    return is_valid_utf8(s) ? std::string(s) : latin1_to_utf8(s);
}

std::string base64_decode(std::string_view in) {
    auto val = [](char c) -> int {
        if (c >= 'A' && c <= 'Z')
            return c - 'A';
        if (c >= 'a' && c <= 'z')
            return c - 'a' + 26;
        if (c >= '0' && c <= '9')
            return c - '0' + 52;
        if (c == '+')
            return 62;
        if (c == '/')
            return 63;
        return -1;
    };
    std::string out;
    out.reserve(in.size() / 4 * 3 + 3);
    std::uint32_t acc = 0;
    int bits = 0;
    for (char c : in) {
        if (c == '=' || c == '\r' || c == '\n' || c == ' ' || c == '\t')
            continue;
        int v = val(c);
        if (v < 0)
            continue;  // skip stray bytes
        acc = (acc << 6) | static_cast<std::uint32_t>(v);
        bits += 6;
        if (bits >= 8) {
            bits -= 8;
            out.push_back(static_cast<char>((acc >> bits) & 0xFF));
        }
    }
    return out;
}

std::string decode_rfc2047(std::string_view in, bool *had_encoded_word) {
    if (had_encoded_word)
        *had_encoded_word = false;
    std::string out;
    out.reserve(in.size());
    std::size_t i = 0;
    const std::size_t n = in.size();

    while (i < n) {
        auto start = in.find("=?", i);
        if (start == std::string_view::npos) {
            out.append(in.substr(i));
            break;
        }
        out.append(in.substr(i, start - i));

        // =?charset?enc?text?=
        auto q1 = in.find('?', start + 2);
        if (q1 == std::string_view::npos) {
            out.append(in.substr(start));
            break;
        }
        auto q2 = in.find('?', q1 + 1);
        if (q2 == std::string_view::npos || q2 != q1 + 2) {
            out.append("=?");
            i = start + 2;
            continue;
        }
        auto end = in.find("?=", q2 + 1);
        if (end == std::string_view::npos) {
            out.append(in.substr(start));
            break;
        }

        std::string_view charset = in.substr(start + 2, q1 - (start + 2));
        char enc = in[q1 + 1];
        std::string_view text = in.substr(q2 + 1, end - (q2 + 1));

        std::string bytes;
        if (enc == 'B' || enc == 'b') {
            bytes = base64_decode(text);
        } else if (enc == 'Q' || enc == 'q') {
            for (std::size_t k = 0; k < text.size(); ++k) {
                char c = text[k];
                if (c == '_') {
                    bytes.push_back(' ');
                } else if (c == '=' && k + 2 < text.size()) {
                    int hi = hex_val(text[k + 1]);
                    int lo = hex_val(text[k + 2]);
                    if (hi >= 0 && lo >= 0) {
                        bytes.push_back(static_cast<char>((hi << 4) | lo));
                        k += 2;
                    } else {
                        bytes.push_back(c);
                    }
                } else {
                    bytes.push_back(c);
                }
            }
        } else {
            out.append(in.substr(start, end + 2 - start));  // unknown encoding: verbatim
            i = end + 2;
            continue;
        }

        if (is_utf8(charset))
            out.append(to_utf8_best_effort(bytes));
        else if (is_latin1(charset))
            out.append(latin1_to_utf8(bytes));
        else
            out.append(to_utf8_best_effort(bytes));

        if (had_encoded_word)
            *had_encoded_word = true;
        i = end + 2;

        // RFC 2047: whitespace between adjacent encoded words is elided.
        std::size_t j = i;
        while (j < n && (in[j] == ' ' || in[j] == '\t'))
            ++j;
        if (j < n && in.compare(j, 2, "=?") == 0)
            i = j;
    }
    return out;
}

}  // namespace vdm::net::detail