//! The diff view: a unified, syntax-highlighted rendering of two versions of
//! a file, in the style of the Pierre diff engine used by tangled.org: change
//! bars instead of markers, tinted number gutters, word-level intra-line
//! highlights and hunk separators. Rows reuse the code view's highlighting;
//! consumers that already hold a git diff can build [`DiffRow`]s themselves
//! instead of calling [`diff_rows`].

use gpui::prelude::FluentBuilder as _;
use gpui::{
    App, Context, ElementId, HighlightStyle, InteractiveElement as _, IntoElement,
    ListHorizontalSizingBehavior, ParentElement as _, Pixels, Render, ScrollStrategy, SharedString,
    StatefulInteractiveElement as _, Styled as _, TextStyle, UniformListScrollHandle, Window, div,
    point, uniform_list,
};
use gpui_native_core::scrollbar::{Scrollbar, ScrollbarHandle, ScrollbarVisibility};
use gpui_native_theme::ActiveTheme as _;
use similar::{ChangeTag, TextDiff};

use std::ops::Range;

use crate::highlight::{Language, Line, Role, highlight};
use crate::utils::{gutter_cell, line_row, role_color, wash, wash_gutter, wash_strong};

/// What a diff row did to the file.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub enum DiffRowKind {
    /// Present in both versions.
    Context,
    /// Only in the new version.
    Added,
    /// Only in the old version.
    Removed,
    /// A hunk separator introducing the rows after it.
    Hunk,
}

/// One row of a unified diff: a diff line or a hunk separator.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct DiffRow {
    pub kind: DiffRowKind,
    /// The highlighted line content; empty for hunk separators.
    pub content: Line,
    /// 1-based line number in the old version, when the row exists there.
    pub old_number: Option<usize>,
    /// 1-based line number in the new version, when the row exists there.
    pub new_number: Option<usize>,
    /// Byte ranges of intra-line word changes, tinted on changed rows.
    pub changed_ranges: Vec<Range<usize>>,
    /// The `@@ -a,b +c,d @@` header, for hunk separator rows.
    pub header: SharedString,
    /// Unchanged lines collapsed into this separator row; zero elsewhere.
    pub hidden_lines: usize,
}

/// How changed rows are marked.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub enum DiffIndicators {
    /// A two-pixel bar on the row's left edge.
    #[default]
    Bars,
    /// Classic `+`/`-` glyphs in a marker column.
    Classic,
    /// Only the background washes.
    None,
}

/// How collapsed regions between hunks render.
#[derive(Clone, Copy, Debug, Default, PartialEq, Eq)]
pub enum HunkSeparator {
    /// The collapsed unchanged line count, centered.
    #[default]
    LineInfo,
    /// The patch-style `@@ -a,b +c,d @@` header.
    Metadata,
    /// A minimal `···` bar.
    Simple,
}

/// Presentation options for the diff view.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct DiffViewOptions {
    pub indicators: DiffIndicators,
    pub separator: HunkSeparator,
    /// Whether the line-number gutter renders.
    pub line_numbers: bool,
}

impl Default for DiffViewOptions {
    fn default() -> Self {
        Self {
            indicators: DiffIndicators::Bars,
            separator: HunkSeparator::LineInfo,
            line_numbers: true,
        }
    }
}

impl DiffRow {
    /// The row's text length, in characters.
    fn char_count(&self) -> usize {
        self.content
            .spans
            .iter()
            .map(|span| span.text.chars().count())
            .sum()
    }
}

/// Computes unified rows between two texts, highlighting every line with `language`.
pub fn diff_rows(old: &str, new: &str, language: Language) -> Vec<DiffRow> {
    let old_code = highlight(old, language);
    let new_code = highlight(new, language);
    let diff = TextDiff::from_lines(old, new);

    let mut rows = Vec::new();
    let mut collapsed_before = 0usize;

    for group in diff.grouped_ops(3) {
        let first = group.first().expect("hunk groups are never empty");
        let last = group.last().expect("hunk groups are never empty");

        let old_range = first.old_range().start..last.old_range().end;
        let new_range = first.new_range().start..last.new_range().end;

        // git prints the line before a pure insertion when the count is zero.
        let old_start = if old_range.is_empty() {
            old_range.start
        } else {
            old_range.start + 1
        };

        let new_start = if new_range.is_empty() {
            new_range.start
        } else {
            new_range.start + 1
        };

        let collapsed = first.old_range().start - collapsed_before;
        if collapsed > 0 {
            rows.push(DiffRow {
                kind: DiffRowKind::Hunk,
                content: Line::default(),
                old_number: None,
                new_number: None,
                changed_ranges: Vec::new(),
                header: format!(
                    "@@ -{},{} +{},{} @@",
                    old_start,
                    old_range.len(),
                    new_start,
                    new_range.len()
                )
                .into(),
                hidden_lines: collapsed,
            });
        }
        collapsed_before = last.old_range().end;

        for op in &group {
            let changes: Vec<_> = diff.iter_changes(op).collect();
            let mut ix = 0;

            while ix < changes.len() {
                if changes[ix].tag() == ChangeTag::Equal {
                    rows.push(line_row(&changes[ix], &new_code, Vec::new()));
                    ix += 1;
                    continue;
                }

                let deletions = changes[ix..]
                    .iter()
                    .take_while(|change| change.tag() == ChangeTag::Delete)
                    .count();

                let insertions = changes[ix + deletions..]
                    .iter()
                    .take_while(|change| change.tag() == ChangeTag::Insert)
                    .count();

                let mut deletion_ranges = vec![Vec::new(); deletions];
                let mut insertion_ranges = vec![Vec::new(); insertions];

                for pair in 0..deletions.min(insertions) {
                    let old_text = changes[ix + pair].value().trim_end_matches('\n');
                    let new_text = changes[ix + deletions + pair]
                        .value()
                        .trim_end_matches('\n');
                    let words = TextDiff::from_words(old_text, new_text);

                    let (mut old_offset, mut new_offset) = (0usize, 0usize);

                    for word in words.iter_all_changes() {
                        let len = word.value().len();

                        match word.tag() {
                            ChangeTag::Delete => {
                                deletion_ranges[pair].push(old_offset..old_offset + len);
                                old_offset += len;
                            }
                            ChangeTag::Insert => {
                                insertion_ranges[pair].push(new_offset..new_offset + len);
                                new_offset += len;
                            }
                            ChangeTag::Equal => {
                                old_offset += len;
                                new_offset += len;
                            }
                        }
                    }
                }

                for pair in 0..deletions {
                    rows.push(line_row(
                        &changes[ix + pair],
                        &old_code,
                        deletion_ranges[pair].clone(),
                    ));
                }

                for pair in 0..insertions {
                    rows.push(line_row(
                        &changes[ix + deletions + pair],
                        &new_code,
                        insertion_ranges[pair].clone(),
                    ));
                }

                ix += deletions + insertions;
            }
        }
    }

    rows
}

/// Retained state for a diff view.
pub struct DiffViewState {
    id: ElementId,
    rows: Vec<DiffRow>,
    options: DiffViewOptions,
    /// Drives both scroll axes of the virtualized row list.
    handle: UniformListScrollHandle,
    a11y_label: Option<SharedString>,
}

impl DiffViewState {
    /// Builds the state, diffing `old` against `new`.
    pub fn new<I, O, N>(id: I, old: O, new: N, language: Language) -> Self
    where
        I: Into<ElementId>,
        O: AsRef<str>,
        N: AsRef<str>,
    {
        Self {
            id: id.into(),
            rows: diff_rows(old.as_ref(), new.as_ref(), language),
            options: DiffViewOptions::default(),
            handle: UniformListScrollHandle::new(),
            a11y_label: None,
        }
    }

    /// Sets how changed rows are marked.
    pub fn indicators(mut self, indicators: DiffIndicators) -> Self {
        self.options.indicators = indicators;
        self
    }

    /// Sets how collapsed regions between hunks render.
    pub fn hunk_separator(mut self, separator: HunkSeparator) -> Self {
        self.options.separator = separator;
        self
    }

    /// Shows or hides the line-number gutter.
    pub fn line_numbers(mut self, show: bool) -> Self {
        self.options.line_numbers = show;
        self
    }

    /// Replaces the versions and re-diffs them.
    pub fn set_versions<O, N>(&mut self, old: O, new: N, language: Language, cx: &mut Context<Self>)
    where
        O: AsRef<str>,
        N: AsRef<str>,
    {
        self.rows = diff_rows(old.as_ref(), new.as_ref(), language);
        self.handle.scroll_to_item(0, ScrollStrategy::Top);
        // The replaced versions rarely share the old width; start at the left.
        let offset = ScrollbarHandle::offset(&self.handle);
        ScrollbarHandle::set_offset(&self.handle, point(Pixels::ZERO, offset.y));
        cx.notify();
    }

    /// The diff rows.
    pub fn rows(&self) -> &[DiffRow] {
        &self.rows
    }

    /// Scrolls the first added or removed row to the top of the view.
    pub fn scroll_to_first_change(&self) {
        let Some(row) = self
            .rows
            .iter()
            .position(|row| matches!(row.kind, DiffRowKind::Added | DiffRowKind::Removed))
        else {
            return;
        };
        self.handle.scroll_to_item(row, ScrollStrategy::Top);
    }

    /// Overrides the accessibility label of the view.
    pub fn accessibility_label<L>(mut self, label: L) -> Self
    where
        L: Into<SharedString>,
    {
        self.a11y_label = Some(label.into());
        self
    }
}

impl Render for DiffViewState {
    fn render(&mut self, _window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
        let theme = cx.theme();
        let text_size = theme.sizes.small.text_size;

        let id = self.id.clone();
        let state = cx.entity();
        let row_count = self.rows.len();
        let options = self.options;

        let widest_row = self
            .rows
            .iter()
            .enumerate()
            .filter(|(_, row)| row.kind != DiffRowKind::Hunk)
            .max_by_key(|(_, row)| row.char_count())
            .map(|(ix, _)| ix);

        let mut rows = uniform_list((id.clone(), "rows"), row_count, {
            move |range, _window, cx| {
                let diff = state.read(cx);
                range
                    .map(|row_ix| render_row(&diff.rows[row_ix], row_ix, options, cx))
                    .collect()
            }
        })
        .with_horizontal_sizing_behavior(ListHorizontalSizingBehavior::Unconstrained)
        .with_width_from_item(widest_row)
        .track_scroll(&self.handle)
        .h_full();

        // Lock each gesture to its initial axis
        rows.interactivity().base_style.restrict_scroll_to_axis = Some(true);

        div()
            .id((id.clone(), "view"))
            .relative()
            .size_full()
            .text_size(text_size)
            .when_some(self.a11y_label.clone(), |view, label| {
                view.aria_label(label)
            })
            .child(rows)
            .child(
                Scrollbar::vertical(self.handle.clone())
                    .key("v-scrollbar")
                    .visibility(ScrollbarVisibility::Platform),
            )
            .child(
                Scrollbar::horizontal(self.handle.clone())
                    .key("h-scrollbar")
                    .visibility(ScrollbarVisibility::Platform),
            )
    }
}

fn render_row(
    row: &DiffRow,
    row_ix: usize,
    options: DiffViewOptions,
    cx: &App,
) -> gpui::AnyElement {
    let theme = cx.theme();
    let mono_font = theme.mono_font;
    let colors = theme.colors;
    let ramps = theme.ramps;
    let text_size = theme.sizes.small.text_size;
    let line_height = text_size * 1.5;
    let char_width = text_size * 0.62;

    let frame = div()
        .debug_selector(move || format!("diff-row-{row_ix}"))
        .id(("diff-row", row_ix))
        .w_full()
        .h(line_height)
        .font_family(mono_font)
        .text_size(text_size)
        .overflow_hidden();

    if row.kind == DiffRowKind::Hunk {
        let content = match options.separator {
            HunkSeparator::LineInfo => {
                SharedString::from(format!("{} unchanged lines", row.hidden_lines))
            }
            HunkSeparator::Metadata => row.header.clone(),
            HunkSeparator::Simple => SharedString::from("···"),
        };

        return frame
            .flex()
            .items_center()
            .text_color(colors.muted_foreground)
            .when(
                options.indicators == DiffIndicators::Classic,
                |row_element| row_element.child(div().w_10().flex_none()),
            )
            .when(options.line_numbers, |row_element| {
                row_element.child(div().w_10().flex_none())
            })
            .when(
                options.indicators == DiffIndicators::Bars && !options.line_numbers,
                |row_element| {
                    row_element.child(
                        div()
                            .flex_none()
                            .border_l_4()
                            .border_color(gpui::transparent_black()),
                    )
                },
            )
            .child(div().flex_none().ml_2().child(content))
            .into_any_element();
    }

    let change_color = match row.kind {
        DiffRowKind::Added => ramps.success.base,
        DiffRowKind::Removed => ramps.destructive.base,
        DiffRowKind::Context | DiffRowKind::Hunk => gpui::transparent_black(),
    };

    let wash = match row.kind {
        DiffRowKind::Added => Some(wash(ramps.success.base)),
        DiffRowKind::Removed => Some(wash(ramps.destructive.base)),
        DiffRowKind::Context | DiffRowKind::Hunk => None,
    };

    let mut text = String::new();
    let mut spans: Vec<(Range<usize>, Role)> = Vec::new();

    for span in &row.content.spans {
        let start = text.len();
        text.push_str(&span.text);
        if span.role != Role::Text {
            spans.push((start..text.len(), span.role));
        }
    }

    let mut boundaries: Vec<usize> = spans
        .iter()
        .flat_map(|(range, _)| [range.start, range.end])
        .chain(
            row.changed_ranges
                .iter()
                .flat_map(|range| [range.start, range.end]),
        )
        .collect();

    boundaries.sort_unstable();
    boundaries.dedup();

    let mut highlights: Vec<(std::ops::Range<usize>, HighlightStyle)> = Vec::new();
    let mut span_ix = 0;

    for window in boundaries.windows(2) {
        let (start, end) = (window[0], window[1]);
        while span_ix < spans.len() && spans[span_ix].0.end <= start {
            span_ix += 1;
        }

        let mut segment = HighlightStyle::default();
        if let Some((range, role)) = spans.get(span_ix)
            && range.contains(&start)
            && *role != Role::Text
        {
            segment = segment.highlight(HighlightStyle {
                color: Some(role_color(*role, colors, ramps)),
                ..Default::default()
            });
        }

        if row
            .changed_ranges
            .iter()
            .any(|range| range.contains(&start))
        {
            segment = segment.highlight(HighlightStyle {
                background_color: Some(wash_strong(change_color)),
                ..Default::default()
            });
        }

        highlights.push((start..end, segment));
    }

    let style = TextStyle {
        color: colors.foreground,
        font_family: SharedString::from(mono_font),
        font_size: text_size.into(),
        line_height: line_height.into(),
        ..Default::default()
    };

    frame
        .flex()
        .items_center()
        .when_some(wash, |row_element, tint| row_element.bg(tint))
        .when(
            options.indicators == DiffIndicators::Classic,
            |row_element| {
                row_element.child(gutter_cell(
                    cx,
                    if row.kind == DiffRowKind::Context {
                        gpui::transparent_black()
                    } else {
                        change_color
                    },
                    match row.kind {
                        DiffRowKind::Added => "+",
                        DiffRowKind::Removed => "-",
                        _ => "",
                    },
                ))
            },
        )
        .when(options.line_numbers, |row_element| {
            let mut number = gutter_cell(
                cx,
                colors.muted_foreground,
                row.new_number
                    .or(row.old_number)
                    .map(|number| number.to_string())
                    .unwrap_or_default(),
            )
            .w_10()
            .whitespace_nowrap()
            .bg(match row.kind {
                DiffRowKind::Context => wash_gutter(&colors),
                _ => wash_strong(change_color),
            });
            if options.indicators == DiffIndicators::Bars {
                number = number.border_l_4().border_color(change_color);
            }
            row_element.child(number)
        })
        .when(
            options.indicators == DiffIndicators::Bars && !options.line_numbers,
            |row_element| {
                row_element.child(
                    div()
                        .h_full()
                        .flex_none()
                        .border_l_4()
                        .border_color(change_color),
                )
            },
        )
        .child(
            div()
                .flex_none()
                .ml_2()
                .w(char_width * row.char_count() as f32)
                .whitespace_nowrap()
                .child(gpui::StyledText::new(text).with_default_highlights(&style, highlights)),
        )
        .into_any_element()
}