annotate_snippets/renderer/

source_map.rs

use alloc::borrow::Cow;
use alloc::string::String;
use alloc::{vec, vec::Vec};
use core::cmp::{max, min};
use core::ops::Range;

use crate::renderer::{LineAnnotation, LineAnnotationType, char_width, num_overlap};
use crate::{Annotation, AnnotationKind, Patch};

#[derive(Debug)]
pub(crate) struct SourceMap<'a> {
    lines: Vec<LineInfo<'a>>,
    pub(crate) source: &'a str,
}

impl<'a> SourceMap<'a> {
    pub(crate) fn new(source: &'a str, line_start: usize) -> Self {
        // Empty sources do have a "line", but it is empty, so we need to add
        // a line with an empty string to the source map.
        if source.is_empty() {
            return Self {
                lines: vec![LineInfo {
                    line: "",
                    line_index: line_start,
                    start_byte: 0,
                    end_byte: 0,
                    end_line_size: 0,
                }],
                source,
            };
        }

        let mut current_index = 0;

        let mut mapping = vec![];
        for (idx, (line, end_line)) in CursorLines::new(source).enumerate() {
            let line_length = line.len();
            let line_range = current_index..current_index + line_length;
            let end_line_size = end_line.len();

            mapping.push(LineInfo {
                line,
                line_index: line_start + idx,
                start_byte: line_range.start,
                end_byte: line_range.end + end_line_size,
                end_line_size,
            });

            current_index += line_length + end_line_size;
        }
        Self {
            lines: mapping,
            source,
        }
    }

    pub(crate) fn get_line(&self, idx: usize) -> Option<&'a str> {
        self.lines
            .iter()
            .find(|l| l.line_index == idx)
            .map(|info| info.line)
    }

    pub(crate) fn span_to_locations(&self, span: Range<usize>) -> (Loc, Loc) {
        let start_info = self
            .lines
            .iter()
            .find(|info| span.start >= info.start_byte && span.start < info.end_byte)
            .unwrap_or(self.lines.last().unwrap());
        let (mut start_char_pos, start_display_pos) = start_info.line
            [0..(span.start - start_info.start_byte).min(start_info.line.len())]
            .chars()
            .fold((0, 0), |(char_pos, byte_pos), c| {
                let display = char_width(c);
                (char_pos + 1, byte_pos + display)
            });
        // correct the char pos if we are highlighting the end of a line
        if (span.start - start_info.start_byte).saturating_sub(start_info.line.len()) > 0 {
            start_char_pos += 1;
        }
        let start = Loc {
            line: start_info.line_index,
            char: start_char_pos,
            display: start_display_pos,
            byte: span.start,
        };

        if span.start == span.end {
            return (start, start);
        }

        let end_info = self
            .lines
            .iter()
            .find(|info| span.end >= info.start_byte && span.end < info.end_byte)
            .unwrap_or(self.lines.last().unwrap());
        let (end_char_pos, end_display_pos) = end_info.line
            [0..(span.end - end_info.start_byte).min(end_info.line.len())]
            .chars()
            .fold((0, 0), |(char_pos, byte_pos), c| {
                let display = char_width(c);
                (char_pos + 1, byte_pos + display)
            });

        let mut end = Loc {
            line: end_info.line_index,
            char: end_char_pos,
            display: end_display_pos,
            byte: span.end,
        };
        if start.line != end.line && end.byte > end_info.end_byte - end_info.end_line_size {
            end.char += 1;
            end.display += 1;
        }

        (start, end)
    }

    pub(crate) fn span_to_snippet(&self, span: Range<usize>) -> Option<&str> {
        self.source.get(span)
    }

    pub(crate) fn span_to_lines(&self, span: Range<usize>) -> Vec<&LineInfo<'a>> {
        let mut lines = vec![];
        let start = span.start;
        let end = span.end;
        for line_info in &self.lines {
            if start >= line_info.end_byte {
                continue;
            }
            if end < line_info.start_byte {
                break;
            }
            lines.push(line_info);
        }

        if lines.is_empty() && !self.lines.is_empty() {
            lines.push(self.lines.last().unwrap());
        }

        lines
    }

    pub(crate) fn annotated_lines(
        &self,
        annotations: Vec<Annotation<'a>>,
        fold: bool,
    ) -> (usize, Vec<AnnotatedLineInfo<'a>>) {
        let source_len = self.source.len();
        if let Some(bigger) = annotations.iter().find_map(|x| {
            // Allow highlighting one past the last character in the source.
            if source_len + 1 < x.span.end {
                Some(&x.span)
            } else {
                None
            }
        }) {
            panic!("Annotation range `{bigger:?}` is beyond the end of buffer `{source_len}`")
        }

        let mut annotated_line_infos = self
            .lines
            .iter()
            .map(|info| AnnotatedLineInfo {
                line: info.line,
                line_index: info.line_index,
                annotations: vec![],
                keep: false,
            })
            .collect::<Vec<_>>();
        let mut multiline_annotations = vec![];

        for Annotation {
            span,
            label,
            kind,
            highlight_source,
        } in annotations
        {
            let (lo, mut hi) = self.span_to_locations(span.clone());
            if kind == AnnotationKind::Visible {
                for line_idx in lo.line..=hi.line {
                    self.keep_line(&mut annotated_line_infos, line_idx);
                }
                continue;
            }
            // Watch out for "empty spans". If we get a span like 6..6, we
            // want to just display a `^` at 6, so convert that to
            // 6..7. This is degenerate input, but it's best to degrade
            // gracefully -- and the parser likes to supply a span like
            // that for EOF, in particular.

            if lo.display == hi.display && lo.line == hi.line {
                hi.display += 1;
            }

            if lo.line == hi.line {
                let line_ann = LineAnnotation {
                    start: lo,
                    end: hi,
                    kind,
                    label,
                    annotation_type: LineAnnotationType::Singleline,
                    highlight_source,
                };
                self.add_annotation_to_file(&mut annotated_line_infos, lo.line, line_ann);
            } else {
                multiline_annotations.push(MultilineAnnotation {
                    depth: 1,
                    start: lo,
                    end: hi,
                    kind,
                    label,
                    overlaps_exactly: false,
                    highlight_source,
                });
            }
        }

        let mut primary_spans = vec![];

        // Find overlapping multiline annotations, put them at different depths
        multiline_annotations.sort_by_key(|ml| (ml.start.line, usize::MAX - ml.end.line));
        for (outer_i, ann) in multiline_annotations.clone().into_iter().enumerate() {
            if ann.kind.is_primary() {
                primary_spans.push((ann.start, ann.end));
            }
            for (inner_i, a) in &mut multiline_annotations.iter_mut().enumerate() {
                // Move all other multiline annotations overlapping with this one
                // one level to the right.
                if !ann.same_span(a)
                    && num_overlap(ann.start.line, ann.end.line, a.start.line, a.end.line, true)
                {
                    a.increase_depth();
                } else if ann.same_span(a) && outer_i != inner_i {
                    a.overlaps_exactly = true;
                } else {
                    if primary_spans
                        .iter()
                        .any(|(s, e)| a.start == *s && a.end == *e)
                    {
                        a.kind = AnnotationKind::Primary;
                    }
                    break;
                }
            }
        }

        let mut max_depth = 0; // max overlapping multiline spans
        for ann in &multiline_annotations {
            max_depth = max(max_depth, ann.depth);
        }
        // Change order of multispan depth to minimize the number of overlaps in the ASCII art.
        for a in &mut multiline_annotations {
            a.depth = max_depth - a.depth + 1;
        }
        for ann in multiline_annotations {
            let mut end_ann = ann.as_end();
            if ann.overlaps_exactly {
                end_ann.annotation_type = LineAnnotationType::Singleline;
            } else {
                // avoid output like
                //
                //  |        foo(
                //  |   _____^
                //  |  |_____|
                //  | ||         bar,
                //  | ||     );
                //  | ||      ^
                //  | ||______|
                //  |  |______foo
                //  |         baz
                //
                // and instead get
                //
                //  |       foo(
                //  |  _____^
                //  | |         bar,
                //  | |     );
                //  | |      ^
                //  | |      |
                //  | |______foo
                //  |        baz
                self.add_annotation_to_file(
                    &mut annotated_line_infos,
                    ann.start.line,
                    ann.as_start(),
                );
                // 4 is the minimum vertical length of a multiline span when presented: two lines
                // of code and two lines of underline. This is not true for the special case where
                // the beginning doesn't have an underline, but the current logic seems to be
                // working correctly.
                let middle = min(ann.start.line + 4, ann.end.line);
                // We'll show up to 4 lines past the beginning of the multispan start.
                // We will *not* include the tail of lines that are only whitespace, a comment or
                // a bare delimiter.
                let filter = |s: &str| {
                    let s = s.trim();
                    // Consider comments as empty, but don't consider docstrings to be empty.
                    !(s.starts_with("//") && !(s.starts_with("///") || s.starts_with("//!")))
                        // Consider lines with nothing but whitespace, a single delimiter as empty.
                        && !["", "{", "}", "(", ")", "[", "]"].contains(&s)
                };
                let until = (ann.start.line..middle)
                    .rev()
                    .filter_map(|line| self.get_line(line).map(|s| (line + 1, s)))
                    .find(|(_, s)| filter(s))
                    .map_or(ann.start.line, |(line, _)| line);
                for line in ann.start.line + 1..until {
                    // Every `|` that joins the beginning of the span (`___^`) to the end (`|__^`).
                    self.add_annotation_to_file(&mut annotated_line_infos, line, ann.as_line());
                }
                let line_end = ann.end.line - 1;
                let end_is_empty = self.get_line(line_end).is_some_and(|s| !filter(s));
                if middle < line_end && !end_is_empty {
                    self.add_annotation_to_file(&mut annotated_line_infos, line_end, ann.as_line());
                }
            }
            self.add_annotation_to_file(&mut annotated_line_infos, end_ann.end.line, end_ann);
        }

        if fold {
            annotated_line_infos.retain(|l| !l.annotations.is_empty() || l.keep);
        }

        (max_depth, annotated_line_infos)
    }

    fn add_annotation_to_file(
        &self,
        annotated_line_infos: &mut Vec<AnnotatedLineInfo<'a>>,
        line_index: usize,
        line_ann: LineAnnotation<'a>,
    ) {
        if let Some(line_info) = annotated_line_infos
            .iter_mut()
            .find(|line_info| line_info.line_index == line_index)
        {
            line_info.annotations.push(line_ann);
        } else {
            let info = self
                .lines
                .iter()
                .find(|l| l.line_index == line_index)
                .unwrap();
            annotated_line_infos.push(AnnotatedLineInfo {
                line: info.line,
                line_index,
                annotations: vec![line_ann],
                keep: false,
            });
            annotated_line_infos.sort_by_key(|l| l.line_index);
        }
    }

    fn keep_line(&self, annotated_line_infos: &mut Vec<AnnotatedLineInfo<'a>>, line_index: usize) {
        if let Some(line_info) = annotated_line_infos
            .iter_mut()
            .find(|line_info| line_info.line_index == line_index)
        {
            line_info.keep = true;
        } else {
            let info = self
                .lines
                .iter()
                .find(|l| l.line_index == line_index)
                .unwrap();
            annotated_line_infos.push(AnnotatedLineInfo {
                line: info.line,
                line_index,
                annotations: vec![],
                keep: true,
            });
            annotated_line_infos.sort_by_key(|l| l.line_index);
        }
    }

    pub(crate) fn splice_lines<'b>(
        &'a self,
        mut patches: Vec<Patch<'b>>,
        fold: bool,
    ) -> Option<SplicedLines<'b>> {
        fn push_trailing(buf: &mut String, line_opt: Option<&str>, lo: &Loc, hi_opt: Option<&Loc>) {
            // Convert CharPos to Usize, as CharPose is character offset
            // Extract low index and high index
            let (lo, hi_opt) = (lo.char, hi_opt.map(|hi| hi.char));
            if let Some(line) = line_opt {
                if let Some(lo) = line.char_indices().map(|(i, _)| i).nth(lo) {
                    // Get high index while account for rare unicode and emoji with char_indices
                    let hi_opt = hi_opt.and_then(|hi| line.char_indices().map(|(i, _)| i).nth(hi));
                    match hi_opt {
                        // If high index exist, take string from low to high index
                        Some(hi) if hi > lo => buf.push_str(&line[lo..hi]),
                        Some(_) => (),
                        // If high index absence, take string from low index till end string.len
                        None => buf.push_str(&line[lo..]),
                    }
                }
                // If high index is None
                if hi_opt.is_none() {
                    buf.push('\n');
                }
            }
        }

        let source_len = self.source.len();
        if let Some(bigger) = patches.iter().find_map(|x| {
            // Allow patching one past the last character in the source.
            if source_len + 1 < x.span.end {
                Some(&x.span)
            } else {
                None
            }
        }) {
            panic!("Patch span `{bigger:?}` is beyond the end of buffer `{source_len}`")
        }

        // Assumption: all spans are in the same file, and all spans
        // are disjoint. Sort in ascending order.
        patches.sort_by_key(|p| p.span.start);

        // Find the bounding span.
        let (lo, hi) = if fold {
            let lo = patches.iter().map(|p| p.span.start).min()?;
            let hi = patches.iter().map(|p| p.span.end).max()?;
            (lo, hi)
        } else {
            (0, source_len)
        };

        let lines = self.span_to_lines(lo..hi);
        let (bounding_lo, bounding_hi) = self.span_to_locations(lo..hi);

        let mut highlights = vec![];
        // To build up the result, we do this for each span:
        // - push the line segment trailing the previous span
        //   (at the beginning a "phantom" span pointing at the start of the line)
        // - push lines between the previous and current span (if any)
        // - if the previous and current span are not on the same line
        //   push the line segment leading up to the current span
        // - splice in the span substitution
        //
        // Finally push the trailing line segment of the last span
        let mut prev_hi = bounding_lo;
        prev_hi.char = 0;
        let mut prev_line = lines.first().map(|line| line.line);
        let mut buf = String::new();

        let trimmed_patches = patches
            .into_iter()
            // If this is a replacement of, e.g. `"a"` into `"ab"`, adjust the
            // suggestion and snippet to look as if we just suggested to add
            // `"b"`, which is typically much easier for the user to understand.
            .map(|part| part.trim_trivial_replacements(self.source))
            .collect::<Vec<_>>();
        let mut line_highlight = vec![];
        // We need to keep track of the difference between the existing code and the added
        // or deleted code in order to point at the correct column *after* substitution.
        let mut acc = 0;
        for part in &trimmed_patches {
            let (cur_lo, cur_hi) = self.span_to_locations(part.span.clone());
            if prev_hi.line == cur_lo.line {
                push_trailing(&mut buf, prev_line, &prev_hi, Some(&cur_lo));
            } else {
                acc = 0;
                highlights.push(core::mem::take(&mut line_highlight));
                push_trailing(&mut buf, prev_line, &prev_hi, None);
                // push lines between the previous and current span (if any)
                for idx in prev_hi.line + 1..(cur_lo.line) {
                    if let Some(line) = self.get_line(idx) {
                        buf.push_str(line.as_ref());
                        buf.push('\n');
                        highlights.push(core::mem::take(&mut line_highlight));
                    }
                }
                if let Some(cur_line) = self.get_line(cur_lo.line) {
                    let end = match cur_line.char_indices().nth(cur_lo.char) {
                        Some((i, _)) => i,
                        None => cur_line.len(),
                    };
                    buf.push_str(&cur_line[..end]);
                }
            }
            // Add a whole line highlight per line in the snippet.
            let len: isize = part
                .replacement
                .split('\n')
                .next()
                .unwrap_or(&part.replacement)
                .chars()
                .map(|c| match c {
                    '\t' => 4,
                    _ => 1,
                })
                .sum();
            line_highlight.push(SubstitutionHighlight {
                start: (cur_lo.char.cast_signed() + acc).cast_unsigned(),
                end: (cur_lo.char.cast_signed() + acc + len).cast_unsigned(),
            });
            buf.push_str(&part.replacement);
            // Account for the difference between the width of the current code and the
            // snippet being suggested, so that the *later* suggestions are correctly
            // aligned on the screen. Note that cur_hi and cur_lo can be on different
            // lines, so cur_hi.col can be smaller than cur_lo.col
            acc += len - (cur_hi.char.cast_signed() - cur_lo.char.cast_signed());
            prev_hi = cur_hi;
            prev_line = self.get_line(prev_hi.line);
            for line in part.replacement.split('\n').skip(1) {
                acc = 0;
                highlights.push(core::mem::take(&mut line_highlight));
                let end: usize = line
                    .chars()
                    .map(|c| match c {
                        '\t' => 4,
                        _ => 1,
                    })
                    .sum();
                line_highlight.push(SubstitutionHighlight { start: 0, end });
            }
        }
        highlights.push(core::mem::take(&mut line_highlight));
        if fold {
            // if the replacement already ends with a newline, don't print the next line
            if !buf.ends_with('\n') {
                push_trailing(&mut buf, prev_line, &prev_hi, None);
            }
        } else {
            // Add the trailing part of the source after the last patch
            if let Some(snippet) = self.span_to_snippet(prev_hi.byte..source_len) {
                buf.push_str(snippet);
                for _ in snippet.matches('\n') {
                    highlights.push(core::mem::take(&mut line_highlight));
                }
            }
        }
        // remove trailing newlines
        while buf.ends_with('\n') {
            buf.pop();
        }

        let line_count = bounding_hi.line.saturating_sub(bounding_lo.line) + 1;
        let mut replaced_highlights: Vec<Vec<SubstitutionHighlight>> = vec![Vec::new(); line_count];
        for part in &trimmed_patches {
            let (cur_lo, cur_hi) = self.span_to_locations(part.span.clone());
            for line in cur_lo.line..=cur_hi.line {
                let start = if line == cur_lo.line { cur_lo.char } else { 0 };
                let end = if line == cur_hi.line {
                    cur_hi.char
                } else {
                    self.get_line(line).unwrap_or_default().chars().count()
                };
                replaced_highlights[line - bounding_lo.line]
                    .push(SubstitutionHighlight { start, end });
            }
        }

        if highlights.iter().all(|parts| parts.is_empty()) {
            None
        } else {
            Some(SplicedLines {
                complete: buf,
                patches: trimmed_patches,
                highlights,
                replaced_highlights,
            })
        }
    }
}

#[derive(Clone, Debug, PartialOrd, Ord, PartialEq, Eq)]
pub(crate) struct MultilineAnnotation<'a> {
    pub depth: usize,
    pub start: Loc,
    pub end: Loc,
    pub kind: AnnotationKind,
    pub label: Option<Cow<'a, str>>,
    pub overlaps_exactly: bool,
    pub highlight_source: bool,
}

impl<'a> MultilineAnnotation<'a> {
    pub(crate) fn increase_depth(&mut self) {
        self.depth += 1;
    }

    /// Compare two `MultilineAnnotation`s considering only the `Span` they cover.
    pub(crate) fn same_span(&self, other: &MultilineAnnotation<'_>) -> bool {
        self.start == other.start && self.end == other.end
    }

    pub(crate) fn as_start(&self) -> LineAnnotation<'a> {
        LineAnnotation {
            start: self.start,
            end: Loc {
                line: self.start.line,
                char: self.start.char + 1,
                display: self.start.display + 1,
                byte: self.start.byte + 1,
            },
            kind: self.kind,
            label: None,
            annotation_type: LineAnnotationType::MultilineStart(self.depth),
            highlight_source: self.highlight_source,
        }
    }

    pub(crate) fn as_end(&self) -> LineAnnotation<'a> {
        LineAnnotation {
            start: Loc {
                line: self.end.line,
                char: self.end.char.saturating_sub(1),
                display: self.end.display.saturating_sub(1),
                byte: self.end.byte.saturating_sub(1),
            },
            end: self.end,
            kind: self.kind,
            label: self.label.clone(),
            annotation_type: LineAnnotationType::MultilineEnd(self.depth),
            highlight_source: self.highlight_source,
        }
    }

    pub(crate) fn as_line(&self) -> LineAnnotation<'a> {
        LineAnnotation {
            start: Loc::default(),
            end: Loc::default(),
            kind: self.kind,
            label: None,
            annotation_type: LineAnnotationType::MultilineLine(self.depth),
            highlight_source: self.highlight_source,
        }
    }
}

#[derive(Debug)]
pub(crate) struct LineInfo<'a> {
    pub(crate) line: &'a str,
    pub(crate) line_index: usize,
    pub(crate) start_byte: usize,
    pub(crate) end_byte: usize,
    end_line_size: usize,
}

#[derive(Debug)]
pub(crate) struct AnnotatedLineInfo<'a> {
    pub(crate) line: &'a str,
    pub(crate) line_index: usize,
    pub(crate) annotations: Vec<LineAnnotation<'a>>,
    pub(crate) keep: bool,
}

/// A source code location used for error reporting.
#[derive(Clone, Copy, Debug, Default, PartialOrd, Ord, PartialEq, Eq)]
pub(crate) struct Loc {
    /// The (1-based) line number.
    pub(crate) line: usize,
    /// The (0-based) column offset.
    pub(crate) char: usize,
    /// The (0-based) column offset when displayed.
    pub(crate) display: usize,
    /// The (0-based) byte offset.
    pub(crate) byte: usize,
}

struct CursorLines<'a>(&'a str);

impl CursorLines<'_> {
    fn new(src: &str) -> CursorLines<'_> {
        CursorLines(src)
    }
}

#[derive(Copy, Clone, Debug, PartialEq)]
enum EndLine {
    Eof,
    Lf,
    Crlf,
}

impl EndLine {
    /// The number of characters this line ending occupies in bytes.
    pub(crate) fn len(self) -> usize {
        match self {
            Self::Eof => 0,
            Self::Lf => 1,
            Self::Crlf => 2,
        }
    }
}

impl<'a> Iterator for CursorLines<'a> {
    type Item = (&'a str, EndLine);

    fn next(&mut self) -> Option<Self::Item> {
        if self.0.is_empty() {
            None
        } else {
            self.0
                .find('\n')
                .map(|x| {
                    let ret = if 0 < x {
                        if self.0.as_bytes()[x - 1] == b'\r' {
                            (&self.0[..x - 1], EndLine::Crlf)
                        } else {
                            (&self.0[..x], EndLine::Lf)
                        }
                    } else {
                        ("", EndLine::Lf)
                    };
                    self.0 = &self.0[x + 1..];
                    ret
                })
                .or_else(|| {
                    let ret = Some((self.0, EndLine::Eof));
                    self.0 = "";
                    ret
                })
        }
    }
}

pub(crate) struct SplicedLines<'a> {
    pub(crate) complete: String,
    pub(crate) patches: Vec<TrimmedPatch<'a>>,
    // Char spans to highlight per line of the post-substitution output.
    pub(crate) highlights: Vec<Vec<SubstitutionHighlight>>,
    // Char spans of the replaced (original) code, per original line in the
    // bounding range covered by the splice.
    pub(crate) replaced_highlights: Vec<Vec<SubstitutionHighlight>>,
}

/// Used to translate between `Span`s and byte positions within a single output line in highlighted
/// code of structured suggestions.
#[derive(Debug, Clone, Copy)]
pub(crate) struct SubstitutionHighlight {
    pub(crate) start: usize,
    pub(crate) end: usize,
}

#[derive(Clone, Debug)]
pub(crate) struct TrimmedPatch<'a> {
    pub(crate) original_span: Range<usize>,
    pub(crate) span: Range<usize>,
    pub(crate) replacement: Cow<'a, str>,
}

impl<'a> TrimmedPatch<'a> {
    pub(crate) fn is_addition(&self, sm: &SourceMap<'_>) -> bool {
        !self.replacement.is_empty() && !self.replaces_meaningful_content(sm)
    }

    pub(crate) fn is_deletion(&self, sm: &SourceMap<'_>) -> bool {
        self.replacement.trim().is_empty() && self.replaces_meaningful_content(sm)
    }

    pub(crate) fn is_replacement(&self, sm: &SourceMap<'_>) -> bool {
        !self.replacement.is_empty() && self.replaces_meaningful_content(sm)
    }

    /// Whether this is a replacement that overwrites source with a snippet
    /// in a way that isn't a superset of the original string. For example,
    /// replacing "abc" with "abcde" is not destructive, but replacing it
    /// it with "abx" is, since the "c" character is lost.
    pub(crate) fn is_destructive_replacement(&self, sm: &SourceMap<'_>) -> bool {
        self.is_replacement(sm)
            && sm
                .span_to_snippet(self.span.clone())
                .is_none_or(|s| as_substr(s.trim(), self.replacement.trim()).is_none())
    }

    fn replaces_meaningful_content(&self, sm: &SourceMap<'_>) -> bool {
        sm.span_to_snippet(self.span.clone())
            .map_or(!self.span.is_empty(), |snippet| !snippet.trim().is_empty())
    }
}

/// Given an original string like `AACC`, and a suggestion like `AABBCC`, try to detect
/// the case where a substring of the suggestion is "sandwiched" in the original, like
/// `BB` is. Return the length of the prefix, the "trimmed" suggestion, and the length
/// of the suffix.
pub(crate) fn as_substr<'a>(
    original: &'a str,
    suggestion: &'a str,
) -> Option<(usize, &'a str, usize)> {
    if let Some(stripped) = suggestion.strip_prefix(original) {
        Some((original.len(), stripped, 0))
    } else if let Some(stripped) = suggestion.strip_suffix(original) {
        Some((0, stripped, original.len()))
    } else {
        let common_prefix = original
            .chars()
            .zip(suggestion.chars())
            .take_while(|(c1, c2)| c1 == c2)
            .map(|(c, _)| c.len_utf8())
            .sum();
        let original = &original[common_prefix..];
        let suggestion = &suggestion[common_prefix..];
        if let Some(stripped) = suggestion.strip_suffix(original) {
            let common_suffix = original.len();
            Some((common_prefix, stripped, common_suffix))
        } else {
            None
        }
    }
}