#include "TextReader.hpp"

#include <spider/compiler/text/utf8.hpp>

#include <stdexcept>

namespace spider {

    // Text Reader //

    TextReader::TextReader() : err(false), eof(false), bufferIndex(0) {
        // Prime the buffer with the first character
        // so current() is immediately valid
        fillBufferTo(0);
    }

    TextReader::~TextReader() {}

    bool TextReader::eat(u32 _char) {
        if(current() == _char) {
            nextChar();
            return true;
        }
        return false;
    }
    
    bool TextReader::eat(char _char) {
        return this->eat(u32(_char));
    }

    bool TextReader::eat(const std::string& chars) {
        // instead of whatever that was, convert to UTF-32
        // and then do an easy compare!
        std::u32string str;
        if(!utf8::toUTF32(chars, str)) throw std::runtime_error("Specified invalid UTF-8 string!");
        return eat(str);
    }

    bool TextReader::eat(const std::u32string& str) {
        // compare now
        isize index;
        for(index = 0; index < str.size(); index++) {
            if(str[index] != peekChar(index)) return false;
        }

        // success!
        nextChar(index);
        return true;
    }

    char TextReader::readByte() {
        if (err) return 0;
        auto& s = getStream();

        if(s.bad()) {
            err = true;
            errmsg = "Stream raised bad bit.";
            return 0;
        }

        int ch = s.get();
        if (ch == std::istream::traits_type::eof()) {
            eof = true;
            return 0;
        }

        at.byteoff++;
        return char(ch);
    }

    /**
     * Returns the current character.
     */
    optional<u32> TextReader::current() {
        if (bufferIndex < buffer.size()) {
            return buffer[bufferIndex];
        }
        return {};
    }

    /**
     * Reads the next character and advances the position tracker.
     */
    optional<u32> TextReader::nextChar(isize n) {
        // Ensure the character we are moving TO exists
        if (fillBufferTo(n)) {
            // advance n characters
            while(n--) {
                advance(buffer[bufferIndex]);
                bufferIndex++;
            }
            return current();
        }
        return {};
    }

    /**
     * Keeps the next n-th character (n = 0 is current).
     */
    optional<u32> TextReader::peekChar(isize n) {
        if (fillBufferTo(n)) return buffer[bufferIndex + n];
        return {};
    }

    void TextReader::commit() {
        if (bufferIndex > 0) {
            // Erase everything before the current buffer index
            buffer.erase(buffer.begin(), buffer.begin() + std::ptrdiff_t(bufferIndex));
            bufferIndex = 0;
        }
    }

    isize TextReader::push() {
        return bufferIndex;
    }

    void TextReader::pop(isize index) {
        bufferIndex = std::min(index, bufferIndex);
    }

    /**
     * Updates track position metrics based on the processed character.
     */
    void TextReader::advance(u32 ch) {
        if (ch == '\n') {
            at.line++;
            at.col = 1;
        } else {
            at.col++;
        }
    }

    bool TextReader::readChar() {
        // Read one byte
        char bytes[4];
        isize bindex = 0;

        bytes[bindex] = readByte();
        if (err) return false;
        if (eof) return false;

        isize chsize = utf8::seqlen(u8(bytes[bindex]));
        if(chsize == 0) {
            err = true;
            errmsg = "Invalid start of UTF-8 sequence.";
            return false;
        }
        bindex++;

        while (bindex < chsize) {
            bytes[bindex] = readByte();
            if (err) return false;
            if (eof) return false;
        }

        u32 decodedChar;
        if(!utf8::decodeArr(bytes, chsize, decodedChar)) {
            err = true;
            errmsg = "Invalid UTF-8 sequence.";
            return false;
        }
        buffer.push_back(decodedChar);
        return true;
    }

    /**
     * Fills the buffer sequentially until it contains at least up
     * to (bufferIndex + targetOffset).
     */
    bool TextReader::fillBufferTo(isize targetOffset) {
        isize targetSize = bufferIndex + targetOffset + 1;
        while (buffer.size() < targetSize) {
            if(readChar()) continue;
            return false;
        }
        return true;
    }

    pos TextReader::getPosition() const {
        return at;
    }

    bool TextReader::isEOF() const{
        return eof && bufferIndex >= buffer.size();
    }

    bool TextReader::hasError() const{
        return err;
    }

    TextReader::operator bool() const {
        return !isEOF() && !hasError();
    }

    std::string TextReader::getError() const {
        return errmsg;
    }

    // File Reader //

    FileTextReader::FileTextReader(const std::string& filename)
        : fileStream(filename, std::ios::binary) {
        if (!fileStream.is_open()) {
            throw std::runtime_error("Failed to open file: " + filename);
        }
    }

    std::istream& FileTextReader::getStream() {
        return fileStream;
    }

    // String Reader //

    StringTextReader::StringTextReader(std::string initialText)
        : buffer(std::move(initialText)),
        stringStream(std::make_unique<std::istringstream>(buffer)) {
    }

    std::istream& StringTextReader::getStream() {
        return *stringStream;
    }

    void StringTextReader::set(const std::string& newText) {
        buffer = newText;
        stringStream = std::make_unique<std::istringstream>(buffer);
    }

    void StringTextReader::append(const std::string& extraText) {
        std::streampos pos = stringStream->tellg();
        buffer += extraText;
        stringStream = std::make_unique<std::istringstream>(buffer);
        stringStream->seekg(pos);
    }

}