use std::ops::Range;

use nostr::prelude::*;

const BECH32_SEPARATOR: u8 = b'1';
const SCHEME_WITH_COLON: &str = "nostr:";
const NEVENT_PREFIX: &str = "nevent1";

/// A `nevent` reference found in a content string.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct NeventRef {
    /// Range of the reference, including the optional `nostr:` scheme
    pub range: Range<usize>,
    /// Compact display label, like `https://njump.me/nevent1...d9sp`
    pub label: String,
}

/// Extracts valid `nevent1...` and `nostr:nevent1...` references from content.
pub fn nevent_refs(content: &str) -> Vec<NeventRef> {
    let mut references = Vec::new();

    for (match_start, _) in content.match_indices(NEVENT_PREFIX) {
        // Only lowercase references are matched, matching NIP-19 encodings in practice
        let start = if content[..match_start].ends_with(SCHEME_WITH_COLON) {
            match_start - SCHEME_WITH_COLON.len()
        } else {
            match_start
        };

        // Skip matches glued to word characters, like the `nevent1` in `xnevent1...`
        if content[..start].ends_with(char::is_alphanumeric) {
            continue;
        }

        let end = match_start
            + content[match_start..]
                .bytes()
                .take_while(u8::is_ascii_alphanumeric)
                .count();

        // The bech32 token excludes the optional `nostr:` scheme
        let bech32 = &content[match_start..end];
        if Nip19Event::from_bech32(bech32).is_err() {
            continue;
        }

        references.push(NeventRef {
            range: start..end,
            label: nevent_label(bech32),
        });
    }

    references
}

/// Compact display of a nevent link, like `https://njump.me/nevent1...d9sp`.
fn nevent_label(bech32: &str) -> String {
    match bech32.len() {
        0..=4 => format!("https://njump.me/{bech32}"),
        _ => format!("https://njump.me/nevent1...{}", &bech32[bech32.len() - 4..]),
    }
}

/// Nostr parsed token with its range in the original text
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Token {
    /// The parsed NIP-21 URI
    ///
    /// <https://github.com/nostr-protocol/nips/blob/master/21.md>
    pub value: Nip21,
    /// The range of this token in the original text
    pub range: Range<usize>,
}

#[derive(Debug, Clone, Copy)]
struct Match {
    start: usize,
    end: usize,
}

/// Nostr parser
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct NostrParser;

impl NostrParser {
    /// Create new parser
    pub const fn new() -> Self {
        Self
    }

    /// Parse text
    pub fn parse<'a>(&self, text: &'a str) -> NostrParserIter<'a> {
        NostrParserIter::new(text)
    }
}

impl Default for NostrParser {
    fn default() -> Self {
        Self::new()
    }
}

struct FindMatches<'a> {
    bytes: &'a [u8],
    pos: usize,
}

impl<'a> FindMatches<'a> {
    fn new(text: &'a str) -> Self {
        Self {
            bytes: text.as_bytes(),
            pos: 0,
        }
    }

    fn try_parse_nostr_uri(&mut self) -> Option<Match> {
        let start = self.pos;
        let bytes = self.bytes;
        let len = bytes.len();

        // Check if we have "nostr:" prefix
        if len - start < SCHEME_WITH_COLON.len() {
            return None;
        }

        // Check for "nostr:" prefix (case-insensitive)
        let scheme_prefix = &bytes[start..start + SCHEME_WITH_COLON.len()];
        if !scheme_prefix.eq_ignore_ascii_case(SCHEME_WITH_COLON.as_bytes()) {
            return None;
        }

        // Skip the scheme
        let pos = start + SCHEME_WITH_COLON.len();

        // Parse bech32 entity
        let mut has_separator = false;
        let mut end = pos;

        while end < len {
            let byte = bytes[end];

            // Check for bech32 separator
            if byte == BECH32_SEPARATOR && !has_separator {
                has_separator = true;
                end += 1;
                continue;
            }

            // Check if character is valid for bech32
            if !byte.is_ascii_alphanumeric() {
                break;
            }

            end += 1;
        }

        // Must have at least one character after separator
        if !has_separator || end <= pos + 1 {
            return None;
        }

        // Update position
        self.pos = end;

        Some(Match { start, end })
    }
}

impl Iterator for FindMatches<'_> {
    type Item = Match;

    fn next(&mut self) -> Option<Self::Item> {
        while self.pos < self.bytes.len() {
            // Try to parse nostr URI
            if let Some(mat) = self.try_parse_nostr_uri() {
                return Some(mat);
            }

            // Skip one character if no match found
            self.pos += 1;
        }

        None
    }
}

pub struct NostrParserIter<'a> {
    /// The original text
    text: &'a str,
    /// Matches found
    matches: FindMatches<'a>,
}

impl<'a> NostrParserIter<'a> {
    fn new(text: &'a str) -> Self {
        Self {
            text,
            matches: FindMatches::new(text),
        }
    }

    fn handle_match(&self, mat: Match) -> Option<Token> {
        // Extract the matched string
        let data: &str = &self.text[mat.start..mat.end];

        // Parse NIP-21 URI
        match Nip21::parse(data) {
            Ok(uri) => Some(Token {
                value: uri,
                range: mat.start..mat.end,
            }),
            // If the nostr URI parsing is invalid, skip it
            Err(_) => None,
        }
    }
}

impl Iterator for NostrParserIter<'_> {
    type Item = Token;

    fn next(&mut self) -> Option<Self::Item> {
        // Iterative, not recursive: message text controls the match count
        loop {
            let mat = self.matches.next()?;

            if let Some(token) = self.handle_match(mat) {
                return Some(token);
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    // A long run of scanner matches that fail `Nip21::parse`: the count is
    // attacker-controlled, so it must be consumed iteratively.
    #[test]
    fn skips_long_run_of_invalid_uris() {
        let content = "nostr:1q".repeat(50_000);

        let tokens: Vec<Token> = NostrParser::new().parse(&content).collect();

        assert!(tokens.is_empty());
    }

    #[test]
    fn yields_valid_uri_after_invalid_run() {
        let public_key = Keys::new(SecretKey::generate()).public_key();
        let npub = public_key.to_bech32().unwrap();
        let content = format!("{}nostr:{npub}", "nostr:1q".repeat(50_000));

        let tokens: Vec<Token> = NostrParser::new().parse(&content).collect();

        assert_eq!(tokens.len(), 1);
        assert_eq!(tokens[0].range.start, 400_000);
        assert_eq!(tokens[0].value, Nip21::Pubkey(public_key));
    }
}