rust/compiler/rustc_span/src/caching_source_map_view.rs

use crate::source_map::SourceMap;
use crate::{BytePos, SourceFile};
use rustc_data_structures::sync::Lrc;

#[derive(Clone)]
struct CacheEntry {
    time_stamp: usize,
    line_number: usize,
    line_start: BytePos,
    line_end: BytePos,
    file: Lrc<SourceFile>,
    file_index: usize,
}

#[derive(Clone)]
pub struct CachingSourceMapView<'sm> {
    source_map: &'sm SourceMap,
    line_cache: [CacheEntry; 3],
    time_stamp: usize,
}

impl<'sm> CachingSourceMapView<'sm> {
    pub fn new(source_map: &'sm SourceMap) -> CachingSourceMapView<'sm> {
        let files = source_map.files();
        let first_file = files[0].clone();
        let entry = CacheEntry {
            time_stamp: 0,
            line_number: 0,
            line_start: BytePos(0),
            line_end: BytePos(0),
            file: first_file,
            file_index: 0,
        };

        CachingSourceMapView {
            source_map,
            line_cache: [entry.clone(), entry.clone(), entry],
            time_stamp: 0,
        }
    }

    pub fn byte_pos_to_line_and_col(
        &mut self,
        pos: BytePos,
    ) -> Option<(Lrc<SourceFile>, usize, BytePos)> {
        self.time_stamp += 1;

        // Check if the position is in one of the cached lines
        for cache_entry in self.line_cache.iter_mut() {
            if line_contains((cache_entry.line_start, cache_entry.line_end), pos) {
                cache_entry.time_stamp = self.time_stamp;

                return Some((
                    cache_entry.file.clone(),
                    cache_entry.line_number,
                    pos - cache_entry.line_start,
                ));
            }
        }

        // No cache hit ...
        let mut oldest = 0;
        for index in 1..self.line_cache.len() {
            if self.line_cache[index].time_stamp < self.line_cache[oldest].time_stamp {
                oldest = index;
            }
        }

        let cache_entry = &mut self.line_cache[oldest];

        // If the entry doesn't point to the correct file, fix it up
        if !file_contains(&cache_entry.file, pos) {
            let file_valid;
            if self.source_map.files().len() > 0 {
                let file_index = self.source_map.lookup_source_file_idx(pos);
                let file = self.source_map.files()[file_index].clone();

                if file_contains(&file, pos) {
                    cache_entry.file = file;
                    cache_entry.file_index = file_index;
                    file_valid = true;
                } else {
                    file_valid = false;
                }
            } else {
                file_valid = false;
            }

            if !file_valid {
                return None;
            }
        }

        let line_index = cache_entry.file.lookup_line(pos).unwrap();
        let line_bounds = cache_entry.file.line_bounds(line_index);

        cache_entry.line_number = line_index + 1;
        cache_entry.line_start = line_bounds.0;
        cache_entry.line_end = line_bounds.1;
        cache_entry.time_stamp = self.time_stamp;

        Some((cache_entry.file.clone(), cache_entry.line_number, pos - cache_entry.line_start))
    }
}

#[inline]
fn line_contains(line_bounds: (BytePos, BytePos), pos: BytePos) -> bool {
    // This condition will be false in one case where we'd rather it wasn't. Spans often start/end
    // one past something, and when that something is the last character of a file (this can happen
    // when a file doesn't end in a newline, for example), we'd still like for the position to be
    // considered within the last line. However, it isn't according to the exclusive upper bound
    // below. We cannot change the upper bound to be inclusive, because for most lines, the upper
    // bound is the same as the lower bound of the next line, so there would be an ambiguity.
    //
    // Supposing we only use this function to check whether or not the line cache entry contains
    // a position, the only ramification of the above is that we will get cache misses for these
    // rare positions. A line lookup for the position via `SourceMap::lookup_line` after a cache
    // miss will produce the last line number, as desired.
    line_bounds.0 <= pos && pos < line_bounds.1
}

#[inline]
fn file_contains(file: &SourceFile, pos: BytePos) -> bool {
    // `SourceMap::lookup_source_file_idx` and `SourceFile::contains` both consider the position
    // one past the end of a file to belong to it. Normally, that's what we want. But for the
    // purposes of converting a byte position to a line and column number, we can't come up with a
    // line and column number if the file is empty, because an empty file doesn't contain any
    // lines. So for our purposes, we don't consider empty files to contain any byte position.
    file.contains(pos) && !file.is_empty()
}