/*
 * Copyright (c) 2019-2020, Sergey Bugaev <bugaevc@serenityos.org>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#include <AK/Debug.h>
#include <AK/ScopeGuard.h>
#include <AK/StringBuilder.h>
#include <LibMarkdown/Text.h>
#include <string.h>

namespace Markdown {

static String unescape(const StringView& text)
{
    StringBuilder builder;
    for (size_t i = 0; i < text.length(); ++i) {
        if (text[i] == '\\' && i != text.length() - 1) {
            builder.append(text[i + 1]);
            i++;
            continue;
        }
        builder.append(text[i]);
    }
    return builder.build();
}

Text::Text(String&& text)
{
    m_spans.append({ move(text), Style {} });
}

String Text::render_to_html() const
{
    StringBuilder builder;

    Vector<String> open_tags;
    Style current_style;

    for (auto& span : m_spans) {
        struct TagAndFlag {
            String tag;
            bool Style::*flag;
        };
        TagAndFlag tags_and_flags[] = {
            { "em", &Style::emph },
            { "b", &Style::strong },
            { "code", &Style::code }
        };
        auto it = open_tags.find_if([&](const String& open_tag) {
            if (open_tag == "a" && current_style.href != span.style.href)
                return true;
            if (open_tag == "img" && current_style.img != span.style.img)
                return true;
            for (auto& tag_and_flag : tags_and_flags) {
                if (open_tag == tag_and_flag.tag && !(span.style.*tag_and_flag.flag))
                    return true;
            }
            return false;
        });

        if (!it.is_end()) {
            // We found an open tag that should
            // not be open for the new span. Close
            // it and all the open tags that follow
            // it.
            for (ssize_t j = open_tags.size() - 1; j >= static_cast<ssize_t>(it.index()); --j) {
                auto& tag = open_tags[j];
                if (tag == "img") {
                    builder.append("\" />");
                    current_style.img = {};
                    continue;
                }
                builder.appendff("</{}>", tag);
                if (tag == "a") {
                    current_style.href = {};
                    continue;
                }
                for (auto& tag_and_flag : tags_and_flags)
                    if (tag == tag_and_flag.tag)
                        current_style.*tag_and_flag.flag = false;
            }
            open_tags.shrink(it.index());
        }
        if (current_style.href.is_null() && !span.style.href.is_null()) {
            open_tags.append("a");
            builder.appendff("<a href=\"{}\">", span.style.href);
        }
        if (current_style.img.is_null() && !span.style.img.is_null()) {
            open_tags.append("img");
            builder.appendff("<img src=\"{}\" alt=\"", span.style.img);
        }
        for (auto& tag_and_flag : tags_and_flags) {
            if (current_style.*tag_and_flag.flag != span.style.*tag_and_flag.flag) {
                open_tags.append(tag_and_flag.tag);
                builder.appendff("<{}>", tag_and_flag.tag);
            }
        }

        current_style = span.style;
        builder.append(escape_html_entities(span.text));
    }

    for (ssize_t i = open_tags.size() - 1; i >= 0; --i) {
        auto& tag = open_tags[i];
        if (tag == "img") {
            builder.append("\" />");
            continue;
        }
        builder.appendff("</{}>", tag);
    }

    return builder.build();
}

String Text::render_for_terminal() const
{
    StringBuilder builder;

    for (auto& span : m_spans) {
        bool needs_styling = span.style.strong || span.style.emph || span.style.code;
        if (needs_styling) {
            builder.append("\033[");
            bool first = true;
            if (span.style.strong || span.style.code) {
                builder.append('1');
                first = false;
            }
            if (span.style.emph) {
                if (!first)
                    builder.append(';');
                builder.append('4');
            }
            builder.append('m');
        }

        if (!span.style.href.is_null()) {
            if (strstr(span.style.href.characters(), "://") != nullptr) {
                builder.append("\033]8;;");
                builder.append(span.style.href);
                builder.append("\033\\");
            }
        }

        builder.append(span.text.characters());

        if (needs_styling)
            builder.append("\033[0m");

        if (!span.style.href.is_null()) {
            // When rendering for the terminal, ignore any
            // non-absolute links, because the user has no
            // chance to follow them anyway.
            if (strstr(span.style.href.characters(), "://") != nullptr) {
                builder.appendff(" <{}>", span.style.href);
                builder.append("\033]8;;\033\\");
            }
        }
        if (!span.style.img.is_null()) {
            if (strstr(span.style.img.characters(), "://") != nullptr) {
                builder.appendff(" <{}>", span.style.img);
            }
        }
    }

    return builder.build();
}

Optional<Text> Text::parse(const StringView& str)
{
    Style current_style;
    size_t current_span_start = 0;
    int first_span_in_the_current_link = -1;
    bool current_link_is_actually_img = false;
    Vector<Span> spans;

    auto append_span_if_needed = [&](size_t offset) {
        VERIFY(current_span_start <= offset);
        if (current_span_start != offset) {
            Span span {
                unescape(str.substring_view(current_span_start, offset - current_span_start)),
                current_style
            };
            spans.append(move(span));
            current_span_start = offset;
        }
    };

    for (size_t offset = 0; offset < str.length(); offset++) {
        char ch = str[offset];

        bool is_escape = ch == '\\';
        if (is_escape && offset != str.length() - 1) {
            offset++;
            continue;
        }

        bool is_special_character = false;
        is_special_character |= ch == '`';
        if (!current_style.code)
            is_special_character |= ch == '*' || ch == '_' || ch == '[' || ch == ']' || (ch == '!' && offset + 1 < str.length() && str[offset + 1] == '[');
        if (!is_special_character)
            continue;

        append_span_if_needed(offset);

        switch (ch) {
        case '`':
            current_style.code = !current_style.code;
            break;
        case '*':
        case '_':
            if (offset + 1 < str.length() && str[offset + 1] == ch) {
                offset++;
                current_style.strong = !current_style.strong;
            } else {
                current_style.emph = !current_style.emph;
            }
            break;
        case '!':
            current_link_is_actually_img = true;
            break;
        case '[':
            if constexpr (MARKDOWN_DEBUG) {
                if (first_span_in_the_current_link != -1)
                    dbgln("Dropping the outer link");
            }
            first_span_in_the_current_link = spans.size();
            break;
        case ']': {
            if (first_span_in_the_current_link == -1) {
                dbgln_if(MARKDOWN_DEBUG, "Unmatched ]");
                continue;
            }
            ScopeGuard guard = [&] {
                first_span_in_the_current_link = -1;
                current_link_is_actually_img = false;
            };
            if (offset + 2 >= str.length() || str[offset + 1] != '(')
                continue;
            offset += 2;
            size_t start_of_href = offset;

            do
                offset++;
            while (offset < str.length() && str[offset] != ')');
            if (offset == str.length())
                offset--;

            const StringView href = str.substring_view(start_of_href, offset - start_of_href);
            for (size_t i = first_span_in_the_current_link; i < spans.size(); i++) {
                if (current_link_is_actually_img)
                    spans[i].style.img = href;
                else
                    spans[i].style.href = href;
            }
            break;
        }
        default:
            VERIFY_NOT_REACHED();
        }

        // We've processed the character as a special, so the next offset will
        // start after it. Note that explicit continue statements skip over this
        // line, effectively treating the character as not special.
        current_span_start = offset + 1;
    }

    append_span_if_needed(str.length());

    return Text(move(spans));
}

}