/// TextIndex provides conversions between byte offsets (used in the core)
/// and LSP positions (line, column in UTF-16 units).
///
/// Notes:
/// - `line_starts` stores byte offsets for the start of each line.
/// - We keep a copy of the text to allow conversions without external dependencies.
/// - The LSP column is counted in UTF-16 units, excluding the end-of-line `\n`.
#[derive(Clone, Debug)]
pub struct TextIndex {
    text: String,
    line_starts: Vec<u32>,
}

impl TextIndex {
    /// Builds the index from the file's current textual content.
    pub fn new(text: &str) -> Self {
        let mut line_starts = Vec::with_capacity(128);
        line_starts.push(0);
        for (byte, ch) in text.char_indices() {
            if ch == '\n' {
                // the start of the next line is the byte after the '\n'
                line_starts.push((byte + 1) as u32);
            }
        }
        Self {
            text: text.to_string(),
            line_starts,
        }
    }

    /// Number of lines (0-based; empty lines count).
    #[inline]
    pub fn line_count(&self) -> u32 {
        self.line_starts.len() as u32
    }

    /// Converts a byte offset (within the file) to (line, UTF-16 column) in LSP format.
    ///
    /// For offsets exactly at end-of-line, the column will be the line's UTF-16 length.
    pub fn byte_to_lsp(&self, byte: u32) -> (u32, u32) {
        let byte = byte.min(self.text.len() as u32);
        let line = match self.line_starts.binary_search(&byte) {
            Ok(i) => i as u32,
            Err(i) => (i.saturating_sub(1)) as u32,
        };

        let (line_start, line_end) = self.line_bounds(line);
        let rel = byte.saturating_sub(line_start as u32) as usize;
        let slice = &self.text[line_start..line_end];

        let mut utf16_col: u32 = 0;
        for (i, ch) in slice.char_indices() {
            if i >= rel { break; }
            utf16_col += ch.len_utf16() as u32;
        }
        (line, utf16_col)
    }

    /// Converts (line, UTF-16 column) to a byte offset.
    ///
    /// - Lines outside the range are clamped to [0, last].
    /// - Columns larger than the line's UTF-16 length return end-of-line.
    pub fn lsp_to_byte(&self, line: u32, utf16_col: u32) -> u32 {
        let line = line.min(self.line_count().saturating_sub(1));
        let (line_start, line_end) = self.line_bounds(line);
        let slice = &self.text[line_start..line_end];

        let mut acc: u32 = 0;
        for (i, ch) in slice.char_indices() {
            if acc >= utf16_col {
                return (line_start + i) as u32;
            }
            acc += ch.len_utf16() as u32;
        }
        // If the target column is after the last character, return end-of-line.
        line_end as u32
    }

    #[inline]
    fn line_bounds(&self, line: u32) -> (usize, usize) {
        let start = *self
            .line_starts
            .get(line as usize)
            .unwrap_or(self.line_starts.last().unwrap());
        let next = self.line_starts.get(line as usize + 1).copied();
        // If there is a next line, `next` points to the byte after the current line's '\n',
        // so the content ends at `next - 1`. Otherwise (last line), the content ends at `text.len()`.
        let end = match next {
            Some(next_start) => next_start.saturating_sub(1),
            None => self.text.len() as u32,
        };
        (start as usize, end as usize)
    }
}

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

    #[test]
    fn line_bounds_basic() {
        let s = "ab\ncd\n";
        let idx = TextIndex::new(s);
        assert_eq!(idx.line_count(), 3);
        // line 0: "ab"
        assert_eq!(idx.line_bounds(0), (0, 2));
        // line 1: "cd"
        assert_eq!(idx.line_bounds(1), (3, 5));
        // line 2: final empty line
        assert_eq!(idx.line_bounds(2), (6, 6));
    }
}