//! The diff the review classifier reads, built from both sides of each
//! changed file rather than taken from the forge's `.diff`, whose context is
//! fixed at 3 lines. [`CONTEXT_LINES`] of context shows an enclosing gate (a
//! `config = lib.mkIf …`, an option's `default`) only when it is within that
//! many lines of the change; a gate further away is still out of view.

use std::fmt::{self, Write as _};

use std::time::Duration;

use similar::TextDiff;

/// Lines of context around each change. A block's enclosing
/// `config = lib.mkIf …` is in view when it opens within this many lines above
/// the block.
pub const CONTEXT_LINES: usize = 25;

/// The largest wide diff sent, in bytes, about 64k tokens of code. A larger
/// one gives way to the forge's `.diff`.
pub const MAX_BYTES: usize = 256 * 1024;

/// The most changed files a wide diff is read for. Each costs a few forge
/// reads per side, all within the one deadline the PR read runs under.
pub const MAX_FILES: usize = 100;

/// How long one file's diff may search for the smallest edit before settling
/// for a larger one. Without it a rewritten file of tens of thousands of
/// lines takes minutes.
const DIFF_TIMEOUT: Duration = Duration::from_secs(1);

/// One changed file: its path and bytes at the merge base and at the head,
/// `None` on a side where it does not exist.
#[derive(Clone, Debug, PartialEq, Eq)]
pub struct FileChange {
    pub old_path: String,
    pub new_path: String,
    pub old: Option<Vec<u8>>,
    pub new: Option<Vec<u8>>,
}

/// Why a PR of `count` changed files gets the forge's `.diff`, if it does.
pub fn too_many_files(count: usize) -> Option<String> {
    (count > MAX_FILES).then(|| {
        format!("the PR changes {count} files, more than the {MAX_FILES} a wide diff is read for")
    })
}

/// `files` as one diff with [`CONTEXT_LINES`] of context, or why the forge's
/// `.diff` stands in: the result is over [`MAX_BYTES`].
pub fn wide(files: &[FileChange]) -> Result<String, String> {
    render(files, CONTEXT_LINES, MAX_BYTES).map_err(|_| {
        format!(
            "the diff with {CONTEXT_LINES} lines of context is over the {MAX_BYTES} bytes a \
             review sends"
        )
    })
}

/// `files` as one unified diff with `context` lines around each change, each
/// file in `git diff`'s shape: a `diff --git` line, `---`/`+++`, then hunks.
/// A file with a non-UTF-8 or NUL-bearing side is one `Binary files` line.
///
/// # Errors
/// The bytes rendered when the output passed `cap`; rendering stops there.
pub fn render(files: &[FileChange], context: usize, cap: usize) -> Result<String, usize> {
    let mut out = Capped {
        out: String::new(),
        cap,
    };
    for file in files {
        if section(&mut out, file, context).is_err() {
            return Err(out.out.len());
        }
    }
    Ok(out.out)
}

/// A `String` whose writes fail once it holds more than `cap` bytes, which
/// stops a `Display` impl writing into it.
struct Capped {
    out: String,
    cap: usize,
}

impl fmt::Write for Capped {
    fn write_str(&mut self, s: &str) -> fmt::Result {
        self.out.push_str(s);
        if self.out.len() > self.cap {
            Err(fmt::Error)
        } else {
            Ok(())
        }
    }
}

fn section(out: &mut Capped, file: &FileChange, context: usize) -> fmt::Result {
    let (old_path, new_path) = (&file.old_path, &file.new_path);
    writeln!(out, "diff --git a/{old_path} b/{new_path}")?;
    if file.old.is_some() && file.new.is_some() && old_path != new_path {
        writeln!(out, "rename from {old_path}\nrename to {new_path}")?;
    }
    let a = file
        .old
        .as_ref()
        .map_or_else(|| "/dev/null".to_owned(), |_| format!("a/{old_path}"));
    let b = file
        .new
        .as_ref()
        .map_or_else(|| "/dev/null".to_owned(), |_| format!("b/{new_path}"));
    let (Some(old), Some(new)) = (text(file.old.as_deref()), text(file.new.as_deref())) else {
        return writeln!(out, "Binary files {a} and {b} differ");
    };
    let diff = TextDiff::configure()
        .timeout(DIFF_TIMEOUT)
        .diff_lines(old, new);
    write!(
        out,
        "{}",
        diff.unified_diff().context_radius(context).header(&a, &b)
    )
}

/// A side's text: empty when the file is absent, `None` when it is binary.
fn text(bytes: Option<&[u8]>) -> Option<&str> {
    let bytes = bytes.unwrap_or_default();
    if bytes.contains(&0) {
        return None;
    }
    std::str::from_utf8(bytes).ok()
}

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

    fn change(path: &str, old: Option<&str>, new: Option<&str>) -> FileChange {
        FileChange {
            old_path: path.to_owned(),
            new_path: path.to_owned(),
            old: old.map(|s| s.as_bytes().to_vec()),
            new: new.map(|s| s.as_bytes().to_vec()),
        }
    }

    /// A Nix module whose `config = lib.mkIf …` block opens `gap` lines above
    /// a firewall rule on the wireguard interface.
    fn module(gap: usize, with_rule: bool) -> String {
        let mut s =
            String::from("{ lib, config, ... }:\n{\n  config = lib.mkIf deployCfg.nats.enable {\n");
        for i in 0..gap {
            let _ = writeln!(s, "    services.nats.settings.k{i} = {i};");
        }
        if with_rule {
            s.push_str(
                "    networking.firewall.interfaces.wg0 = {\n      allowedTCPPorts = lib.mkIf \
                 deployCfg.wireguard.enable [ 4222 ];\n    };\n",
            );
        }
        for i in 0..5 {
            let _ = writeln!(s, "    systemd.services.nats.environment.E{i} = \"{i}\";");
        }
        s.push_str("  };\n}\n");
        s
    }

    /// The rule moves out of the module into a glue module that restates
    /// both conditions.
    fn move_into_glue() -> Vec<FileChange> {
        let glue = "{ lib, config, ... }:\n{\n  config = lib.mkIf (deployCfg.nats.enable && \
                    deployCfg.wireguard.enable) {\n    networking.firewall.interfaces.wg0.\
                    allowedTCPPorts = [ 4222 ];\n  };\n}\n";
        vec![
            change("nix/host-modules/glue-nats-wireguard.nix", None, Some(glue)),
            change(
                "nix/host-modules/swarm-nats.nix",
                Some(&module(20, true)),
                Some(&module(20, false)),
            ),
        ]
    }

    /// `count` lines, line `i` reading `line(i)`.
    fn numbered(count: usize, line: impl Fn(usize) -> String) -> String {
        (0..count).fold(String::new(), |mut s, i| {
            let _ = writeln!(s, "{}", line(i));
            s
        })
    }

    const GATE: &str = "config = lib.mkIf deployCfg.nats.enable {";

    #[test]
    fn a_moved_block_keeps_its_enclosing_gate_in_view() {
        let diff = wide(&move_into_glue()).expect("fits");
        assert!(diff.contains(&format!("\n   {GATE}\n")), "{diff}");
        assert!(diff.contains("\n-      allowedTCPPorts = lib.mkIf deployCfg.wireguard.enable"));
    }

    #[test]
    fn three_lines_of_context_leave_that_gate_out() {
        let diff = render(&move_into_glue(), 3, usize::MAX).unwrap();
        assert!(!diff.contains(GATE), "{diff}");
    }

    #[test]
    fn context_is_as_wide_as_asked_and_no_wider() {
        let old = numbered(30, |i| format!("line {i}"));
        let new = old.replace("line 15\n", "line fifteen\n");
        let at = |n| render(&[change("f", Some(&old), Some(&new))], n, usize::MAX).unwrap();
        let diff = at(10);
        assert!(diff.contains("@@ -6,21 +6,21 @@"), "{diff}");
        assert!(diff.contains(" line 5\n") && !diff.contains("line 4\n"));
        assert!(at(CONTEXT_LINES).contains("@@ -1,30 +1,30 @@"));
    }

    #[test]
    fn file_headers_follow_git_diff() {
        let diff = render(
            &[
                change("added", None, Some("a\n")),
                change("gone", Some("a\n"), None),
                FileChange {
                    old_path: "old".to_owned(),
                    new_path: "new".to_owned(),
                    old: Some(b"a\n".to_vec()),
                    new: Some(b"b\n".to_vec()),
                },
                change("img", Some("a\n"), Some("\0")),
            ],
            3,
            usize::MAX,
        )
        .unwrap();
        assert_eq!(
            diff,
            "diff --git a/added b/added\n--- /dev/null\n+++ b/added\n@@ -0,0 +1 @@\n+a\n\
             diff --git a/gone b/gone\n--- a/gone\n+++ /dev/null\n@@ -1 +0,0 @@\n-a\n\
             diff --git a/old b/new\nrename from old\nrename to new\n--- a/old\n+++ b/new\n\
             @@ -1 +1 @@\n-a\n+b\n\
             diff --git a/img b/img\nBinary files a/img and b/img differ\n"
        );
    }

    #[test]
    fn a_wide_diff_over_the_limit_gives_way() {
        let big = numbered(MAX_BYTES / 8, |i| format!("{i:07}"));
        let edited = numbered(MAX_BYTES / 8, |i| format!("{i:07}x"));
        let files = [change("big", Some(&big), Some(&edited))];
        let reason = wide(&files).expect_err("over the limit");
        assert!(reason.contains("over the"), "{reason}");
        let small = [change("small", Some("a\n"), Some("b\n"))];
        assert!(wide(&small).is_ok());
    }

    #[test]
    fn rendering_stops_once_past_the_limit() {
        let huge = numbered(MAX_BYTES / 2, |i| format!("{i:07}"));
        let files = [change("huge", None, Some(&huge))];
        let written = render(&files, CONTEXT_LINES, MAX_BYTES).expect_err("over the limit");
        assert!(
            written <= MAX_BYTES + "0000000\n".len(),
            "{written} bytes rendered of a {}-byte file",
            huge.len()
        );
    }

    #[test]
    fn more_files_than_the_limit_give_way() {
        assert!(too_many_files(MAX_FILES).is_none());
        assert!(too_many_files(MAX_FILES + 1).is_some());
    }
}