use std::{cell::Cell, sync::LazyLock};

use fend_core::SpanKind;

use crate::engines::EngineResponse;

use super::regex;

pub fn request(query: &str) -> EngineResponse {
    let query = clean_query(query.to_string());

    let Some(result_html) = evaluate(&query, true) else {
        return EngineResponse::new();
    };

    EngineResponse::answer_html(format!(
        r#"<p class="answer-calc-query">{query} =</p>
<h3><b>{result_html}</b></h3>"#,
        query = html_escape::encode_text(&query),
    ))
}

pub fn request_autocomplete(query: &str) -> Vec<String> {
    let mut results = Vec::new();

    let query = clean_query(query.to_string());

    if let Some(result) = evaluate(&query, false) {
        results.push(format!("= {result}"));
    }

    results
}

fn clean_query(query: String) -> String {
    query.strip_suffix('=').unwrap_or(&query).trim().to_string()
}

#[derive(Debug)]
pub struct Span {
    pub text: String,
    pub kind: SpanKind,
}

fn evaluate(query: &str, html: bool) -> Option<String> {
    let spans = evaluate_into_spans(query, html);

    if spans.is_empty() {
        return None;
    }

    if !html {
        return Some(
            spans
                .iter()
                .map(|span| span.text.clone())
                .collect::<Vec<_>>()
                .join(""),
        );
    }

    let mut result_html = String::new();
    for span in &spans {
        let class = match span.kind {
            fend_core::SpanKind::Number
            | fend_core::SpanKind::Boolean
            | fend_core::SpanKind::Date => "answer-calc-constant",
            fend_core::SpanKind::String => "answer-calc-string",
            _ => "",
        };
        if !class.is_empty() {
            result_html.push_str(&format!(
                r#"<span class="{class}">{text}</span>"#,
                text = html_escape::encode_text(&span.text)
            ));
        } else {
            result_html.push_str(&html_escape::encode_text(&span.text));
        }
    }

    // if the result was a single hex number then we add the decimal equivalent
    // below
    if spans.len() == 1
        && spans[0].kind == fend_core::SpanKind::Number
        && spans[0].text.starts_with("0x")
    {
        let hex = spans[0].text.trim_start_matches("0x");
        if let Ok(num) = u64::from_str_radix(hex, 16) {
            result_html.push_str(&format!(
                r#" <span class="answer-calc-comment">= {num}</span>"#,
                num = num
            ));
        }
    }

    Some(result_html)
}

pub static FEND_CONTEXT: LazyLock<fend_core::Context> = LazyLock::new(|| {
    let mut context = fend_core::Context::new();

    // make lowercase f and c work
    context.define_custom_unit_v1("f", "f", "°F", &fend_core::CustomUnitAttribute::Alias);
    context.define_custom_unit_v1("c", "c", "°C", &fend_core::CustomUnitAttribute::Alias);
    // make random work
    context.set_random_u32_fn(rand::random::<u32>);

    fend_core::evaluate("ord=(x: x to codepoint)", &mut context).unwrap();

    context
});

struct Interrupter {
    invocations_left: Cell<u32>,
}

impl fend_core::Interrupt for Interrupter {
    fn should_interrupt(&self) -> bool {
        if self.invocations_left.get() == 0 {
            return true;
        }
        self.invocations_left.set(self.invocations_left.get() - 1);
        false
    }
}

fn evaluate_into_spans(query: &str, multiline: bool) -> Vec<Span> {
    // match queries like "chr(8831)" or "8831 to char"
    let re = regex!(
        r"^(?:(?:chr|charcode|char|charcode)(?:| for| of)\s*\(?\s*(\d+)\s*\)?)|(?:(\d+) (?:|to |into |as )(?:charcode|char|character))$"
    );
    if let Some(m) = re.captures(query) {
        if let Some(ord) = m
            .get(1)
            .or_else(|| m.get(2))
            .and_then(|m| m.as_str().parse::<u32>().ok())
        {
            let chr = std::char::from_u32(ord);
            if let Some(chr) = chr {
                return vec![Span {
                    text: format!("'{chr}'"),
                    kind: fend_core::SpanKind::String,
                }];
            } else {
                return vec![];
            }
        }
    }

    // fend incorrectly triggers on these often
    {
        // at least 3 characters and not one of the short constants
        if query.len() < 3 && !matches!(query.to_lowercase().as_str(), "pi" | "e" | "c") {
            return vec![];
        }

        // probably a query operator thing or a url, fend evaluates these but it
        // shouldn't
        if regex!("^[a-z]{2,}:").is_match(query) {
            return vec![];
        }

        // if it starts and ends with quotes then the person was just searching in
        // quotes and didn't mean to evaluate a string
        if query.starts_with('"')
            && query.ends_with('"')
            && query.chars().filter(|c| *c == '"').count() == 2
        {
            return vec![];
        }
    }

    let mut context = FEND_CONTEXT.clone();
    if multiline {
        // this makes it generate slightly nicer outputs for some queries like 2d6
        context.set_output_mode_terminal();
    }

    // avoids stackoverflows and queries that take too long
    // examples:
    // - Y = (\f. (\x. f x x)) (\x. f x x); Y(Y)
    // - 10**100000000
    let interrupt = Interrupter {
        invocations_left: Cell::new(1000),
    };
    let Ok(result) = fend_core::evaluate_with_interrupt(query, &mut context, &interrupt) else {
        return vec![];
    };
    let main_result = result.get_main_result();
    if main_result == query {
        return vec![];
    }

    result
        .get_main_result_spans()
        .filter(|span| !span.string().is_empty())
        .map(|span| Span {
            text: span.string().to_string(),
            kind: span.kind(),
        })
        .collect()
}