use std::borrow::Cow;
use std::cmp::Ordering;
use std::iter::FusedIterator;

use ruff_formatter::FormatContext;
use ruff_python_ast::visitor::source_order::SourceOrderVisitor;
use ruff_python_ast::{
    str::Quote, AnyStringFlags, BytesLiteral, FString, StringFlags, StringLikePart, StringLiteral,
};
use ruff_text_size::{Ranged, TextRange};

use crate::context::FStringState;
use crate::prelude::*;
use crate::preview::is_f_string_formatting_enabled;
use crate::string::{Quoting, StringQuotes};
use crate::QuoteStyle;

pub(crate) struct StringNormalizer<'a, 'src> {
    quoting: Quoting,
    preferred_quote_style: Option<QuoteStyle>,
    context: &'a PyFormatContext<'src>,
}

impl<'a, 'src> StringNormalizer<'a, 'src> {
    pub(crate) fn from_context(context: &'a PyFormatContext<'src>) -> Self {
        Self {
            quoting: Quoting::default(),
            preferred_quote_style: None,
            context,
        }
    }

    pub(crate) fn with_preferred_quote_style(mut self, quote_style: QuoteStyle) -> Self {
        self.preferred_quote_style = Some(quote_style);
        self
    }

    pub(crate) fn with_quoting(mut self, quoting: Quoting) -> Self {
        self.quoting = quoting;
        self
    }

    /// Determines the preferred quote style for `string`.
    /// The formatter should use the preferred quote style unless
    /// it can't because the string contains the preferred quotes OR
    /// it leads to more escaping.
    pub(super) fn preferred_quote_style(&self, string: StringLikePart) -> QuoteStyle {
        match self.quoting {
            Quoting::Preserve => QuoteStyle::Preserve,
            Quoting::CanChange => {
                let preferred_quote_style = self
                    .preferred_quote_style
                    .unwrap_or(self.context.options().quote_style());

                if preferred_quote_style.is_preserve() {
                    return QuoteStyle::Preserve;
                }

                // There are two cases where it's necessary to preserve the quotes
                // if the target version is pre 3.12 and the part is an f-string.
                if !self.context.options().target_version().supports_pep_701() {
                    if let StringLikePart::FString(fstring) = string {
                        // An f-string expression contains a debug text with a quote character
                        // because the formatter will emit the debug expression **exactly** the same as in the source text.
                        if is_fstring_with_quoted_debug_expression(fstring, self.context) {
                            return QuoteStyle::Preserve;
                        }

                        // An f-string expression that contains a triple quoted string literal expression
                        // that contains a quote.
                        if is_fstring_with_triple_quoted_literal_expression_containing_quotes(
                            fstring,
                            self.context,
                        ) {
                            return QuoteStyle::Preserve;
                        }
                    }
                }

                // For f-strings prefer alternating the quotes unless The outer string is triple quoted and the inner isn't.
                if let FStringState::InsideExpressionElement(parent_context) =
                    self.context.f_string_state()
                {
                    let parent_flags = parent_context.f_string().flags();

                    if !parent_flags.is_triple_quoted() || string.flags().is_triple_quoted() {
                        return QuoteStyle::from(parent_flags.quote_style().opposite());
                    }
                }

                // Per PEP 8, always prefer double quotes for triple-quoted strings.
                if string.flags().is_triple_quoted() {
                    // ... unless we're formatting a code snippet inside a docstring,
                    // then we specifically want to invert our quote style to avoid
                    // writing out invalid Python.
                    //
                    // It's worth pointing out that we can actually wind up being
                    // somewhat out of sync with PEP8 in this case. Consider this
                    // example:
                    //
                    //     def foo():
                    //         '''
                    //         Something.
                    //
                    //         >>> """tricksy"""
                    //         '''
                    //         pass
                    //
                    // Ideally, this would be reformatted as:
                    //
                    //     def foo():
                    //         """
                    //         Something.
                    //
                    //         >>> '''tricksy'''
                    //         """
                    //         pass
                    //
                    // But the logic here results in the original quoting being
                    // preserved. This is because the quoting style of the outer
                    // docstring is determined, in part, by looking at its contents. In
                    // this case, it notices that it contains a `"""` and thus infers
                    // that using `'''` would overall read better because it avoids
                    // the need to escape the interior `"""`. Except... in this case,
                    // the `"""` is actually part of a code snippet that could get
                    // reformatted to using a different quoting style itself.
                    //
                    // Fixing this would, I believe, require some fairly seismic
                    // changes to how formatting strings works. Namely, we would need
                    // to look for code snippets before normalizing the docstring, and
                    // then figure out the quoting style more holistically by looking
                    // at the various kinds of quotes used in the code snippets and
                    // what reformatting them might look like.
                    //
                    // Overall this is a bit of a corner case and just inverting the
                    // style from what the parent ultimately decided upon works, even
                    // if it doesn't have perfect alignment with PEP8.
                    if let Some(quote) = self.context.docstring() {
                        QuoteStyle::from(quote.opposite())
                    } else {
                        QuoteStyle::Double
                    }
                } else {
                    preferred_quote_style
                }
            }
        }
    }

    /// Computes the strings preferred quotes.
    pub(crate) fn choose_quotes(&self, string: StringLikePart) -> QuoteSelection {
        let raw_content = self.context.locator().slice(string.content_range());
        let first_quote_or_normalized_char_offset = raw_content
            .bytes()
            .position(|b| matches!(b, b'\\' | b'"' | b'\'' | b'\r' | b'{'));
        let string_flags = string.flags();
        let preferred_style = self.preferred_quote_style(string);

        let new_kind = match (
            Quote::try_from(preferred_style),
            first_quote_or_normalized_char_offset,
        ) {
            // The string contains no quotes so it's safe to use the preferred quote style
            (Ok(preferred_quote), None) => string_flags.with_quote_style(preferred_quote),

            // The preferred quote style is single or double quotes, and the string contains a quote or
            // another character that may require escaping
            (Ok(preferred_quote), Some(first_quote_or_normalized_char_offset)) => {
                let metadata = if string.is_fstring() {
                    QuoteMetadata::from_part(string, self.context, preferred_quote)
                } else {
                    QuoteMetadata::from_str(
                        &raw_content[first_quote_or_normalized_char_offset..],
                        string.flags(),
                        preferred_quote,
                    )
                };

                let quote = metadata.choose(preferred_quote);

                string_flags.with_quote_style(quote)
            }

            // The preferred quote style is to preserve the quotes, so let's do that.
            (Err(()), _) => string_flags,
        };

        QuoteSelection {
            flags: new_kind,
            first_quote_or_normalized_char_offset,
        }
    }

    /// Computes the strings preferred quotes and normalizes its content.
    pub(crate) fn normalize(&self, string: StringLikePart) -> NormalizedString<'src> {
        let raw_content = self.context.locator().slice(string.content_range());
        let quote_selection = self.choose_quotes(string);

        let normalized = if let Some(first_quote_or_escape_offset) =
            quote_selection.first_quote_or_normalized_char_offset
        {
            normalize_string(
                raw_content,
                first_quote_or_escape_offset,
                quote_selection.flags,
                // TODO: Remove the `b'{'` in `choose_quotes` when promoting the
                // `format_fstring` preview style
                is_f_string_formatting_enabled(self.context),
            )
        } else {
            Cow::Borrowed(raw_content)
        };

        NormalizedString {
            flags: quote_selection.flags,
            content_range: string.content_range(),
            text: normalized,
        }
    }
}

#[derive(Debug)]
pub(crate) struct QuoteSelection {
    flags: AnyStringFlags,

    /// Offset to the first quote character or character that needs special handling in [`normalize_string`].
    first_quote_or_normalized_char_offset: Option<usize>,
}

impl QuoteSelection {
    pub(crate) fn flags(&self) -> AnyStringFlags {
        self.flags
    }
}

#[derive(Clone, Debug)]
pub(crate) struct QuoteMetadata {
    kind: QuoteMetadataKind,

    /// The quote style in the source.
    source_style: Quote,
}

/// Tracks information about the used quotes in a string which is used
/// to choose the quotes for a part.
impl QuoteMetadata {
    pub(crate) fn from_part(
        part: StringLikePart,
        context: &PyFormatContext,
        preferred_quote: Quote,
    ) -> Self {
        match part {
            StringLikePart::String(_) | StringLikePart::Bytes(_) => {
                let text = context.locator().slice(part.content_range());

                Self::from_str(text, part.flags(), preferred_quote)
            }
            StringLikePart::FString(fstring) => {
                if is_f_string_formatting_enabled(context) {
                    // For f-strings, only consider the quotes inside string-literals but ignore
                    // quotes inside expressions. This allows both the outer and the nested literals
                    // to make the optimal local-choice to reduce the total number of quotes necessary.
                    // This doesn't require any pre 312 special handling because an expression
                    // can never contain the outer quote character, not even escaped:
                    // ```python
                    // f"{'escaping a quote like this \" is a syntax error pre 312'}"
                    // ```
                    let mut literals = fstring.elements.literals();

                    let Some(first) = literals.next() else {
                        return QuoteMetadata::from_str("", part.flags(), preferred_quote);
                    };

                    let mut metadata = QuoteMetadata::from_str(
                        context.locator().slice(first.range()),
                        fstring.flags.into(),
                        preferred_quote,
                    );

                    for literal in literals {
                        metadata = metadata
                            .merge(&QuoteMetadata::from_str(
                                context.locator().slice(literal.range()),
                                fstring.flags.into(),
                                preferred_quote,
                            ))
                            .expect("Merge to succeed because all parts have the same flags");
                    }

                    metadata
                } else {
                    let text = context.locator().slice(part.content_range());

                    Self::from_str(text, part.flags(), preferred_quote)
                }
            }
        }
    }

    pub(crate) fn from_str(text: &str, flags: AnyStringFlags, preferred_quote: Quote) -> Self {
        let kind = if flags.is_raw_string() {
            QuoteMetadataKind::raw(text, preferred_quote, flags.is_triple_quoted())
        } else if flags.is_triple_quoted() {
            QuoteMetadataKind::triple_quoted(text, preferred_quote)
        } else {
            QuoteMetadataKind::regular(text)
        };

        Self {
            kind,
            source_style: flags.quote_style(),
        }
    }

    pub(super) fn choose(&self, preferred_quote: Quote) -> Quote {
        match self.kind {
            QuoteMetadataKind::Raw { contains_preferred } => {
                if contains_preferred {
                    self.source_style
                } else {
                    preferred_quote
                }
            }
            QuoteMetadataKind::Triple { contains_preferred } => {
                if contains_preferred {
                    self.source_style
                } else {
                    preferred_quote
                }
            }
            QuoteMetadataKind::Regular {
                single_quotes,
                double_quotes,
            } => match single_quotes.cmp(&double_quotes) {
                Ordering::Less => Quote::Single,
                Ordering::Equal => preferred_quote,
                Ordering::Greater => Quote::Double,
            },
        }
    }

    /// Merges the quotes metadata of different literals.
    ///
    /// ## Raw and triple quoted strings
    /// Merging raw and triple quoted strings is only correct if all literals are from the same part.
    /// E.g. it's okay to merge triple and raw strings from a single `FString` part's literals
    /// but it isn't safe to merge raw and triple quoted strings from different parts of an implicit
    /// concatenated string. Where safe means, it may lead to incorrect results.
    pub(super) fn merge(self, other: &QuoteMetadata) -> Option<QuoteMetadata> {
        let kind = match (self.kind, other.kind) {
            (
                QuoteMetadataKind::Regular {
                    single_quotes: self_single,
                    double_quotes: self_double,
                },
                QuoteMetadataKind::Regular {
                    single_quotes: other_single,
                    double_quotes: other_double,
                },
            ) => QuoteMetadataKind::Regular {
                single_quotes: self_single + other_single,
                double_quotes: self_double + other_double,
            },

            // Can't merge quotes from raw strings (even when both strings are raw)
            (
                QuoteMetadataKind::Raw {
                    contains_preferred: self_contains_preferred,
                },
                QuoteMetadataKind::Raw {
                    contains_preferred: other_contains_preferred,
                },
            ) => QuoteMetadataKind::Raw {
                contains_preferred: self_contains_preferred || other_contains_preferred,
            },

            (
                QuoteMetadataKind::Triple {
                    contains_preferred: self_contains_preferred,
                },
                QuoteMetadataKind::Triple {
                    contains_preferred: other_contains_preferred,
                },
            ) => QuoteMetadataKind::Triple {
                contains_preferred: self_contains_preferred || other_contains_preferred,
            },

            (_, _) => return None,
        };

        Some(Self {
            kind,
            source_style: self.source_style,
        })
    }
}

#[derive(Copy, Clone, Debug)]
enum QuoteMetadataKind {
    /// A raw string.
    ///
    /// For raw strings it's only possible to change the quotes if the preferred quote style
    /// isn't used inside the string.
    Raw { contains_preferred: bool },

    /// Regular (non raw) triple quoted string.
    ///
    /// For triple quoted strings it's only possible to change the quotes if no
    /// triple of the preferred quotes is used inside the string.
    Triple { contains_preferred: bool },

    /// A single quoted string that uses either double or single quotes.
    ///
    /// For regular strings it's desired to pick the quote style that requires the least escaping.
    /// E.g. pick single quotes for `'A "dog"'` because using single quotes would require escaping
    /// the two `"`.
    Regular {
        single_quotes: u32,
        double_quotes: u32,
    },
}

impl QuoteMetadataKind {
    /// For triple quoted strings, the preferred quote style can't be used if the string contains
    /// a tripled of the quote character (e.g., if double quotes are preferred, double quotes will be
    /// used unless the string contains `"""`).
    fn triple_quoted(content: &str, preferred_quote: Quote) -> Self {
        // True if the string contains a triple quote sequence of the configured quote style.
        let mut uses_triple_quotes = false;
        let mut chars = content.chars().peekable();

        while let Some(c) = chars.next() {
            let preferred_quote_char = preferred_quote.as_char();
            match c {
                '\\' => {
                    if matches!(chars.peek(), Some('"' | '\\')) {
                        chars.next();
                    }
                }
                // `"` or `'`
                c if c == preferred_quote_char => {
                    match chars.peek().copied() {
                        Some(c) if c == preferred_quote_char => {
                            // `""` or `''`
                            chars.next();

                            match chars.peek().copied() {
                                Some(c) if c == preferred_quote_char => {
                                    // `"""` or `'''`
                                    chars.next();
                                    uses_triple_quotes = true;
                                    break;
                                }
                                Some(_) => {}
                                None => {
                                    // Handle `''' ""'''`. At this point we have consumed both
                                    // double quotes, so on the next iteration the iterator is empty
                                    // and we'd miss the string ending with a preferred quote
                                    uses_triple_quotes = true;
                                    break;
                                }
                            }
                        }
                        Some(_) => {
                            // A single quote char, this is ok
                        }
                        None => {
                            // Trailing quote at the end of the comment
                            uses_triple_quotes = true;
                            break;
                        }
                    }
                }
                _ => continue,
            }
        }

        Self::Triple {
            contains_preferred: uses_triple_quotes,
        }
    }

    /// For single quoted strings, the preferred quote style is used, unless the alternative quote style
    /// would require fewer escapes.
    fn regular(text: &str) -> Self {
        let mut single_quotes = 0u32;
        let mut double_quotes = 0u32;

        for c in text.chars() {
            match c {
                '\'' => {
                    single_quotes += 1;
                }

                '"' => {
                    double_quotes += 1;
                }

                _ => continue,
            }
        }

        Self::Regular {
            single_quotes,
            double_quotes,
        }
    }

    /// Computes if a raw string uses the preferred quote. If it does, then it's not possible
    /// to change the quote style because it would require escaping which isn't possible in raw strings.
    fn raw(text: &str, preferred: Quote, triple_quoted: bool) -> Self {
        let mut chars = text.chars().peekable();
        let preferred_quote_char = preferred.as_char();

        let contains_unescaped_configured_quotes = loop {
            match chars.next() {
                Some('\\') => {
                    // Ignore escaped characters
                    chars.next();
                }
                // `"` or `'`
                Some(c) if c == preferred_quote_char => {
                    if !triple_quoted {
                        break true;
                    }

                    match chars.peek() {
                        // We can't turn `r'''\""'''` into `r"""\"""""`, this would confuse the parser
                        // about where the closing triple quotes start
                        None => break true,
                        Some(next) if *next == preferred_quote_char => {
                            // `""` or `''`
                            chars.next();

                            // We can't turn `r'''""'''` into `r""""""""`, nor can we have
                            // `"""` or `'''` respectively inside the string
                            if chars.peek().is_none() || chars.peek() == Some(&preferred_quote_char)
                            {
                                break true;
                            }
                        }
                        _ => {}
                    }
                }
                Some(_) => continue,
                None => break false,
            }
        };

        Self::Raw {
            contains_preferred: contains_unescaped_configured_quotes,
        }
    }
}

#[derive(Debug)]
pub(crate) struct NormalizedString<'a> {
    /// Holds data about the quotes and prefix of the string
    flags: AnyStringFlags,

    /// The range of the string's content in the source (minus prefix and quotes).
    content_range: TextRange,

    /// The normalized text
    text: Cow<'a, str>,
}

impl<'a> NormalizedString<'a> {
    pub(crate) fn text(&self) -> &Cow<'a, str> {
        &self.text
    }

    pub(crate) fn flags(&self) -> AnyStringFlags {
        self.flags
    }
}

impl Ranged for NormalizedString<'_> {
    fn range(&self) -> TextRange {
        self.content_range
    }
}

impl Format<PyFormatContext<'_>> for NormalizedString<'_> {
    fn fmt(&self, f: &mut Formatter<PyFormatContext<'_>>) -> FormatResult<()> {
        let quotes = StringQuotes::from(self.flags);
        ruff_formatter::write!(f, [self.flags.prefix(), quotes])?;
        match &self.text {
            Cow::Borrowed(_) => source_text_slice(self.range()).fmt(f)?,
            Cow::Owned(normalized) => text(normalized).fmt(f)?,
        }

        quotes.fmt(f)
    }
}

pub(crate) fn normalize_string(
    input: &str,
    start_offset: usize,
    new_flags: AnyStringFlags,
    format_f_string: bool,
) -> Cow<str> {
    // The normalized string if `input` is not yet normalized.
    // `output` must remain empty if `input` is already normalized.
    let mut output = String::new();
    // Tracks the last index of `input` that has been written to `output`.
    // If `last_index` is `0` at the end, then the input is already normalized and can be returned as is.
    let mut last_index = 0;

    let quote = new_flags.quote_style();
    let preferred_quote = quote.as_char();
    let opposite_quote = quote.opposite().as_char();

    let mut chars = CharIndicesWithOffset::new(input, start_offset).peekable();

    let is_raw = new_flags.is_raw_string();

    let is_fstring = !format_f_string && new_flags.is_f_string();
    let mut formatted_value_nesting = 0u32;

    while let Some((index, c)) = chars.next() {
        if matches!(c, '{' | '}') && is_fstring {
            if chars.peek().copied().is_some_and(|(_, next)| next == c) {
                // Skip over the second character of the double braces
                chars.next();
            } else if c == '{' {
                formatted_value_nesting += 1;
            } else {
                // Safe to assume that `c == '}'` here because of the matched pattern above
                formatted_value_nesting = formatted_value_nesting.saturating_sub(1);
            }
            continue;
        }

        if c == '\r' {
            output.push_str(&input[last_index..index]);

            // Skip over the '\r' character, keep the `\n`
            if chars.peek().copied().is_some_and(|(_, next)| next == '\n') {
                chars.next();
            }
            // Replace the `\r` with a `\n`
            else {
                output.push('\n');
            }

            last_index = index + '\r'.len_utf8();
        } else if !is_raw {
            if c == '\\' {
                if let Some((_, next)) = chars.clone().next() {
                    if next == '\\' {
                        // Skip over escaped backslashes
                        chars.next();
                    } else {
                        // Length of the `\` plus the length of the escape sequence character (`u` | `U` | `x`)
                        let escape_start_len = '\\'.len_utf8() + next.len_utf8();
                        if let Some(normalised) =
                            UnicodeEscape::new(next, !new_flags.is_byte_string()).and_then(
                                |escape| escape.normalize(&input[index + escape_start_len..]),
                            )
                        {
                            let escape_start_offset = index + escape_start_len;
                            if let Cow::Owned(normalised) = &normalised {
                                output.push_str(&input[last_index..escape_start_offset]);
                                output.push_str(normalised);
                                last_index = escape_start_offset + normalised.len();
                            };

                            // Move the `chars` iterator passed the escape sequence.
                            // Simply reassigning `chars` doesn't work because the indices` would
                            // then be off.
                            for _ in 0..next.len_utf8() + normalised.len() {
                                chars.next();
                            }
                        }
                    }

                    if !new_flags.is_triple_quoted() {
                        #[allow(clippy::if_same_then_else)]
                        if next == opposite_quote && formatted_value_nesting == 0 {
                            // Remove the escape by ending before the backslash and starting again with the quote
                            chars.next();
                            output.push_str(&input[last_index..index]);
                            last_index = index + '\\'.len_utf8();
                        } else if next == preferred_quote {
                            // Quote is already escaped, skip over it.
                            chars.next();
                        }
                    }
                }
            } else if !new_flags.is_triple_quoted()
                && c == preferred_quote
                && formatted_value_nesting == 0
            {
                // Escape the quote
                output.push_str(&input[last_index..index]);
                output.push('\\');
                output.push(c);
                last_index = index + preferred_quote.len_utf8();
            }
        }
    }

    if last_index == 0 {
        Cow::Borrowed(input)
    } else {
        output.push_str(&input[last_index..]);
        Cow::Owned(output)
    }
}

#[derive(Clone, Debug)]
struct CharIndicesWithOffset<'str> {
    chars: std::str::Chars<'str>,
    next_offset: usize,
}

impl<'str> CharIndicesWithOffset<'str> {
    fn new(input: &'str str, start_offset: usize) -> Self {
        Self {
            chars: input[start_offset..].chars(),
            next_offset: start_offset,
        }
    }
}

impl<'str> Iterator for CharIndicesWithOffset<'str> {
    type Item = (usize, char);

    fn next(&mut self) -> Option<Self::Item> {
        self.chars.next().map(|c| {
            let index = self.next_offset;
            self.next_offset += c.len_utf8();
            (index, c)
        })
    }
}

impl FusedIterator for CharIndicesWithOffset<'_> {}

#[derive(Copy, Clone, Debug, PartialEq, Eq)]
enum UnicodeEscape {
    /// A hex escape sequence of either 2 (`\x`), 4 (`\u`) or 8 (`\U`) hex characters.
    Hex(usize),

    /// An escaped unicode name (`\N{name}`)
    CharacterName,
}

impl UnicodeEscape {
    fn new(first: char, allow_unicode: bool) -> Option<UnicodeEscape> {
        Some(match first {
            'x' => UnicodeEscape::Hex(2),
            'u' if allow_unicode => UnicodeEscape::Hex(4),
            'U' if allow_unicode => UnicodeEscape::Hex(8),
            'N' if allow_unicode => UnicodeEscape::CharacterName,
            _ => return None,
        })
    }

    /// Normalises `\u..`, `\U..`, `\x..` and `\N{..}` escape sequences to:
    ///
    /// * `\u`, `\U'` and `\x`: To use lower case for the characters `a-f`.
    /// * `\N`: To use uppercase letters
    fn normalize(self, input: &str) -> Option<Cow<str>> {
        let mut normalised = String::new();

        let len = match self {
            UnicodeEscape::Hex(len) => {
                // It's not a valid escape sequence if the input string has fewer characters
                // left than required by the escape sequence.
                if input.len() < len {
                    return None;
                }

                for (index, c) in input.char_indices().take(len) {
                    match c {
                        '0'..='9' | 'a'..='f' => {
                            if !normalised.is_empty() {
                                normalised.push(c);
                            }
                        }
                        'A'..='F' => {
                            if normalised.is_empty() {
                                normalised.reserve(len);
                                normalised.push_str(&input[..index]);
                                normalised.push(c.to_ascii_lowercase());
                            } else {
                                normalised.push(c.to_ascii_lowercase());
                            }
                        }
                        _ => {
                            // not a valid escape sequence
                            return None;
                        }
                    }
                }

                len
            }
            UnicodeEscape::CharacterName => {
                let mut char_indices = input.char_indices();

                if !matches!(char_indices.next(), Some((_, '{'))) {
                    return None;
                }

                loop {
                    if let Some((index, c)) = char_indices.next() {
                        match c {
                            '}' => {
                                if !normalised.is_empty() {
                                    normalised.push('}');
                                }

                                // Name must be at least two characters long.
                                if index < 3 {
                                    return None;
                                }

                                break index + '}'.len_utf8();
                            }
                            '0'..='9' | 'A'..='Z' | ' ' | '-' => {
                                if !normalised.is_empty() {
                                    normalised.push(c);
                                }
                            }
                            'a'..='z' => {
                                if normalised.is_empty() {
                                    normalised.reserve(c.len_utf8() + '}'.len_utf8());
                                    normalised.push_str(&input[..index]);
                                    normalised.push(c.to_ascii_uppercase());
                                } else {
                                    normalised.push(c.to_ascii_uppercase());
                                }
                            }
                            _ => {
                                // Seems like an invalid escape sequence, don't normalise it.
                                return None;
                            }
                        }
                    } else {
                        // Unterminated escape sequence, don't normalise it.
                        return None;
                    }
                }
            }
        };

        Some(if normalised.is_empty() {
            Cow::Borrowed(&input[..len])
        } else {
            Cow::Owned(normalised)
        })
    }
}

/// Returns `true` if `string` is an f-string part that contains a debug expression that uses quotes
/// and the format target is pre Python 312
/// We can't join f-strings where:
///
/// ```python
/// f"{10 + len('bar')=}"
/// f'{10 + len("bar")=}'
/// f""""{10 + len('''bar''')=}"""
/// ```
pub(super) fn is_fstring_with_quoted_debug_expression(
    fstring: &FString,
    context: &PyFormatContext,
) -> bool {
    if fstring.elements.expressions().any(|expression| {
        if expression.debug_text.is_some() {
            let content = context.locator().slice(expression.range());
            match fstring.flags.quote_style() {
                Quote::Single => {
                    if fstring.flags.is_triple_quoted() {
                        content.contains(r#"""""#)
                    } else {
                        content.contains('"')
                    }
                }
                Quote::Double => {
                    if fstring.flags.is_triple_quoted() {
                        content.contains("'''")
                    } else {
                        content.contains('\'')
                    }
                }
            }
        } else {
            false
        }
    }) {
        return true;
    }

    false
}

/// Tests if the `fstring` contains any triple quoted string, byte, or f-string literal that
/// contains a quote character opposite to its own quote character.
///
/// ```python
/// f'{"""other " """}'
/// ```
///
/// We can't flip the quote of the outer f-string because it would result in invalid syntax:
/// ```python
/// f"{'''other " '''}'
/// ```
pub(super) fn is_fstring_with_triple_quoted_literal_expression_containing_quotes(
    fstring: &FString,
    context: &PyFormatContext,
) -> bool {
    struct Visitor<'a> {
        context: &'a PyFormatContext<'a>,
        found: bool,
    }

    impl Visitor<'_> {
        fn visit_string_like_part(&mut self, part: StringLikePart) {
            if !part.flags().is_triple_quoted() || self.found {
                return;
            }

            let contains_quotes = match part {
                StringLikePart::String(_) | StringLikePart::Bytes(_) => {
                    self.contains_quote(part.content_range(), part.flags())
                }
                StringLikePart::FString(fstring) => {
                    let mut contains_quotes = false;
                    for literal in fstring.elements.literals() {
                        if self.contains_quote(literal.range(), fstring.flags.into()) {
                            contains_quotes = true;
                            break;
                        }
                    }

                    contains_quotes
                }
            };

            if contains_quotes {
                self.found = true;
            }
        }

        fn contains_quote(&self, range: TextRange, flags: AnyStringFlags) -> bool {
            self.context
                .locator()
                .slice(range)
                .contains(flags.quote_style().as_char())
        }
    }

    impl SourceOrderVisitor<'_> for Visitor<'_> {
        fn visit_f_string(&mut self, f_string: &FString) {
            self.visit_string_like_part(StringLikePart::FString(f_string));
        }

        fn visit_string_literal(&mut self, string_literal: &StringLiteral) {
            self.visit_string_like_part(StringLikePart::String(string_literal));
        }

        fn visit_bytes_literal(&mut self, bytes_literal: &BytesLiteral) {
            self.visit_string_like_part(StringLikePart::Bytes(bytes_literal));
        }
    }

    let mut visitor = Visitor {
        context,
        found: false,
    };

    ruff_python_ast::visitor::source_order::walk_f_string(&mut visitor, fstring);

    visitor.found
}

#[cfg(test)]
mod tests {
    use std::borrow::Cow;

    use ruff_python_ast::{
        str::Quote,
        str_prefix::{AnyStringPrefix, ByteStringPrefix},
        AnyStringFlags,
    };

    use crate::string::normalize_string;

    use super::UnicodeEscape;

    #[test]
    fn normalize_32_escape() {
        let escape_sequence = UnicodeEscape::new('U', true).unwrap();

        assert_eq!(
            Some(Cow::Owned("0001f60e".to_string())),
            escape_sequence.normalize("0001F60E")
        );
    }

    #[test]
    fn normalize_hex_in_byte_string() {
        let input = r"\x89\x50\x4E\x47\x0D\x0A\x1A\x0A";

        let normalized = normalize_string(
            input,
            0,
            AnyStringFlags::new(
                AnyStringPrefix::Bytes(ByteStringPrefix::Regular),
                Quote::Double,
                false,
            ),
            true,
        );

        assert_eq!(r"\x89\x50\x4e\x47\x0d\x0a\x1a\x0a", &normalized);
    }
}