export interface ThreadOptions {
    maxChars: number;
    addNumbering: boolean;
}

const DEFAULT_OPTIONS: ThreadOptions = {
    maxChars: 280,
    addNumbering: true,
};

const NUMBERING_RESERVE = 8;

function countChars(text: string): number {
    let count = 0;
    let lastIndex = 0;
    const urlRe = /https?:\/\/\S+/g;
    let match: RegExpExecArray | null;

    while ((match = urlRe.exec(text)) !== null) {
        count += match.index - lastIndex;
        count += 23;
        lastIndex = urlRe.lastIndex;
    }
    count += text.length - lastIndex;
    return count;
}

function textFits(text: string, max: number): boolean {
    return countChars(text) <= max;
}

function splitByWords(text: string, max: number): string[] {
    const words = text.split(/\s+/);
    const chunks: string[] = [];
    let current = "";

    for (const word of words) {
        if (!current) {
            current = word;
            continue;
        }
        const candidate = current + " " + word;
        if (countChars(candidate) <= max - 3) {
            current = candidate;
        } else {
            chunks.push(current + "...");
            current = word;
        }
    }

    if (current) chunks.push(current);
    return chunks;
}

function splitBySentences(text: string, max: number): string[] {
    const sentences = text.match(/[^.!?\s][^.!?]*(?:[.!?]+|$)/g) || [text];
    const chunks: string[] = [];
    let current = "";

    for (const sentence of sentences) {
        if (!current) {
            if (textFits(sentence, max)) {
                current = sentence;
            } else {
                chunks.push(...splitByWords(sentence, max));
            }
        } else {
            const candidate = current + sentence;
            if (textFits(candidate, max)) {
                current = candidate;
            } else {
                chunks.push(current);
                if (textFits(sentence, max)) {
                    current = sentence;
                } else {
                    chunks.push(...splitByWords(sentence, max));
                    current = "";
                }
            }
        }
    }

    if (current) chunks.push(current);
    return chunks;
}

function splitBlock(text: string, max: number): string[] {
    const lines = text.split("\n").filter(l => l.trim());
    const chunks: string[] = [];
    let current = "";

    for (const line of lines) {
        const trimmed = line.trim();
        if (!current) {
            if (textFits(trimmed, max)) {
                current = trimmed;
            } else {
                chunks.push(...splitBySentences(trimmed, max));
            }
        } else {
            const candidate = current + "\n" + trimmed;
            if (textFits(candidate, max)) {
                current = candidate;
            } else {
                chunks.push(current);
                if (textFits(trimmed, max)) {
                    current = trimmed;
                } else {
                    chunks.push(...splitBySentences(trimmed, max));
                    current = "";
                }
            }
        }
    }

    if (current) chunks.push(current);
    return chunks.length > 0 ? chunks : [text];
}

export function splitIntoThreads(text: string, options?: Partial<ThreadOptions>): string[] {
    const opts: ThreadOptions = Object.assign({}, DEFAULT_OPTIONS, options);
    const effectiveMax = opts.addNumbering ? opts.maxChars - NUMBERING_RESERVE : opts.maxChars;
    const paragraphs = text.split(/\n{2,}/).filter(p => p.trim());

    if (paragraphs.length === 0) return [];

    const rawTweets: string[] = [];
    let current = "";

    for (const para of paragraphs) {
        const trimmed = para.trim();

        if (!current) {
            if (textFits(trimmed, effectiveMax)) {
                current = trimmed;
            } else {
                const chunks = splitBlock(trimmed, effectiveMax);
                if (chunks.length > 1) {
                    rawTweets.push(...chunks.slice(0, -1));
                    current = chunks[chunks.length - 1]!;
                } else {
                    current = chunks[0]!;
                }
            }
        } else {
            const candidate = current + "\n\n" + trimmed;
            if (textFits(candidate, effectiveMax)) {
                current = candidate;
            } else {
                rawTweets.push(current);
                if (textFits(trimmed, effectiveMax)) {
                    current = trimmed;
                } else {
                    const chunks = splitBlock(trimmed, effectiveMax);
                    if (chunks.length > 1) {
                        rawTweets.push(...chunks.slice(0, -1));
                        current = chunks[chunks.length - 1]!;
                    } else {
                        current = chunks[0]!;
                    }
                }
            }
        }
    }

    if (current) rawTweets.push(current);

    if (opts.addNumbering && rawTweets.length > 1) {
        const total = rawTweets.length;
        return rawTweets.map((tweet, i) => `${i + 1}/${total} ${tweet}`);
    }

    return rawTweets;
}