//! In-memory stub data mimicking a lore.kernel.org archive.
//!
//! Nothing here performs I/O. The real backend will fetch from
//! <https://lore.kernel.org/git/> or shell out to `b4`/`lei`.

use std::collections::HashMap;

use crate::models::{Author, MailingList, Message, MessageId, Thread, ThreadId};

use super::Backend;

/// Static "mailing list content" the mock rotates through.
struct ThreadSeed {
    /// Series prefix, e.g. `PATCH v4`; empty for plain discussions.
    prefix: &'static str,
    subject: &'static str,
    author_name: &'static str,
    author_email: &'static str,
    /// Total message count, including the cover letter when > 1.
    message_count: usize,
    /// Short display date for the thread list.
    date: &'static str,
}

const THREAD_SEEDS: [ThreadSeed; 10] = [
    ThreadSeed {
        prefix: "PATCH v4",
        subject: "refs: introduce a reftable backend for packed refs",
        author_name: "Patrick Steinhardt",
        author_email: "ps@pks.im",
        message_count: 6,
        date: "Sep 10",
    },
    ThreadSeed {
        prefix: "PATCH v2",
        subject: "t/perf: add benchmarks for ref operations",
        author_name: "Victoria Dye",
        author_email: "vdye@github.com",
        message_count: 3,
        date: "Sep 10",
    },
    ThreadSeed {
        prefix: "RFC PATCH",
        subject: "log --format: add a reftable dump backend",
        author_name: "Karthik Nayak",
        author_email: "karthik.188@gmail.com",
        message_count: 2,
        date: "Sep 09",
    },
    ThreadSeed {
        prefix: "PATCH",
        subject: "tests: convert remaining scripts to test-tool helpers",
        author_name: "Ævar Arnfjörð Bjarmason",
        author_email: "avarab@gmail.com",
        message_count: 4,
        date: "Sep 09",
    },
    ThreadSeed {
        prefix: "",
        subject: "git maintenance: schedule tasks on macOS via launchd",
        author_name: "Derrick Stolee",
        author_email: "stolee@gmail.com",
        message_count: 1,
        date: "Sep 08",
    },
    ThreadSeed {
        prefix: "PATCH v3",
        subject: "bundle-uri: support sparse filters in clones",
        author_name: "Junio C Hamano",
        author_email: "gitster@pobox.com",
        message_count: 5,
        date: "Sep 08",
    },
    ThreadSeed {
        prefix: "PATCH",
        subject: "rebase --update-refs: silence reflog message noise",
        author_name: "Phillip Wood",
        author_email: "phillip.wood123@gmail.com",
        message_count: 2,
        date: "Sep 07",
    },
    ThreadSeed {
        prefix: "",
        subject: "Git v2.48.0 released",
        author_name: "Junio C Hamano",
        author_email: "gitster@pobox.com",
        message_count: 1,
        date: "Sep 07",
    },
    ThreadSeed {
        prefix: "PATCH v6",
        subject: "worktree list: add --porcelain output",
        author_name: "Elijah Newren",
        author_email: "newren@gmail.com",
        message_count: 7,
        date: "Sep 06",
    },
    ThreadSeed {
        prefix: "PATCH",
        subject: "submodule: parallelize fetch with --jobs",
        author_name: "Taylor Blau",
        author_email: "me@ttaylorr.com",
        message_count: 3,
        date: "Sep 06",
    },
];

/// Fixed pool of reviewers assigned as `Cc:` on mock messages.
const REVIEWERS: [(&str, &str); 6] = [
    ("Junio C Hamano", "gitster@pobox.com"),
    ("Taylor Blau", "me@ttaylorr.com"),
    ("Ævar Arnfjörð Bjarmason", "avarab@gmail.com"),
    ("Victoria Dye", "vdye@github.com"),
    ("Elijah Newren", "newren@gmail.com"),
    ("Phillip Wood", "phillip.wood123@gmail.com"),
];

/// In-memory data source. [`Backend::refresh`] rotates the visible window
/// over [`THREAD_SEEDS`] so `r` visibly changes the data.
#[derive(Debug)]
pub struct MockBackend {
    list: MailingList,
    /// Refresh counter: each refresh rotates the thread window.
    generation: usize,
    threads: Vec<Thread>,
    messages: HashMap<ThreadId, Vec<Message>>,
}

impl MockBackend {
    /// Build a backend for `list` with an initial snapshot.
    ///
    /// The starting generation is derived from the list name so different
    /// lists show different (but deterministic) thread windows.
    #[must_use]
    pub fn new(list: &MailingList) -> Self {
        let mut backend = Self {
            list: list.clone(),
            generation: fnv1a(list.name.as_bytes()) % THREAD_SEEDS.len(),
            threads: Vec::new(),
            messages: HashMap::new(),
        };
        backend.rebuild();
        backend
    }

    /// Visible thread count varies with the generation so refreshes differ.
    const fn visible_thread_count(generation: usize) -> usize {
        THREAD_SEEDS.len().saturating_sub(2) + generation % 3
    }

    /// Regenerate the snapshot from the seed pool, rotated by `generation`.
    fn rebuild(&mut self) {
        self.threads.clear();
        self.messages.clear();
        let visible = Self::visible_thread_count(self.generation);
        let start = self.generation % THREAD_SEEDS.len();
        for offset in 0..visible {
            let seed = &THREAD_SEEDS[(start + offset) % THREAD_SEEDS.len()];
            let thread_id = ThreadId(offset);
            self.threads.push(Thread {
                id: thread_id,
                subject: series_subject(seed),
                author: seed_author(seed),
                date: seed.date.to_owned(),
                message_count: seed.message_count,
            });
            let list = (0..seed.message_count)
                .map(|index| message_from_seed(seed, &self.list, offset, index))
                .collect();
            self.messages.insert(thread_id, list);
        }
    }
}

impl Backend for MockBackend {
    fn list_name(&self) -> &str {
        &self.list.name
    }

    fn threads(&self) -> &[Thread] {
        &self.threads
    }

    fn messages(&self, thread_id: ThreadId) -> &[Message] {
        self.messages.get(&thread_id).map_or(&[], Vec::as_slice)
    }

    fn refresh(&mut self) -> anyhow::Result<()> {
        self.generation += 1;
        self.rebuild();
        Ok(())
    }
}

/// Deterministic FNV-1a hash used to vary mock data per list.
fn fnv1a(bytes: &[u8]) -> usize {
    let mut hash: usize = 0x811c_9dc5;
    let mut index = 0;
    while index < bytes.len() {
        hash ^= usize::from(bytes[index]);
        hash = hash.wrapping_mul(0x0100_0193);
        index += 1;
    }
    hash
}

fn seed_author(seed: &ThreadSeed) -> Author {
    Author {
        name: seed.author_name.to_owned(),
        email: seed.author_email.to_owned(),
    }
}

/// Thread subject: `[<prefix> 0/<n>] <subject>` for series, plain otherwise.
fn series_subject(seed: &ThreadSeed) -> String {
    if seed.prefix.is_empty() || seed.message_count <= 1 {
        seed.subject.to_owned()
    } else {
        format!(
            "[{} 0/{}] {}",
            seed.prefix,
            seed.message_count - 1,
            seed.subject
        )
    }
}

fn message_from_seed(
    seed: &ThreadSeed,
    list: &MailingList,
    thread_offset: usize,
    index: usize,
) -> Message {
    let subject = if seed.prefix.is_empty() || seed.message_count <= 1 {
        seed.subject.to_owned()
    } else {
        format!(
            "[{} {}/{}] {}",
            seed.prefix,
            index,
            seed.message_count - 1,
            seed.subject
        )
    };
    let to = vec![Author {
        name: list.name.clone(),
        email: list.address.clone(),
    }];
    let count = 1 + (thread_offset + index) % 2;
    let start = (thread_offset * 7 + index) % REVIEWERS.len();
    let cc = (0..count)
        .map(|i| {
            let (name, email) = REVIEWERS[(start + i) % REVIEWERS.len()];
            Author {
                name: name.to_owned(),
                email: email.to_owned(),
            }
        })
        .collect();
    Message {
        id: MessageId(index),
        subject,
        author: seed_author(seed),
        to,
        cc,
        date: message_date(thread_offset, index),
        body: message_body(seed, index),
    }
}

fn message_date(thread_offset: usize, index: usize) -> String {
    format!(
        "Fri, 11 Sep 2026 {:02}:{:02}:{:02} +0200",
        8 + thread_offset % 9,
        index * 7 % 60,
        (index * 13 + thread_offset) % 60
    )
}

fn message_body(seed: &ThreadSeed, index: usize) -> String {
    let total = seed.message_count.saturating_sub(1);
    let intro = if seed.message_count <= 1 {
        format!(
            "This is a standalone message about {}.\n\nDetails and rationale follow; review and\nfeedback are welcome.",
            seed.subject
        )
    } else if index == 0 {
        format!(
            "Cover letter for the {} series: {}.\n\nThe full series follows. See the individual\npatches for the actual changes.",
            seed.prefix, seed.subject
        )
    } else {
        format!(
            "Patch {index}/{total} of the {} series.\n\nOne focused change:\n- {} (summary stub)\n- tests updated accordingly",
            seed.prefix, seed.subject
        )
    };
    if index == 0 && seed.message_count > 1 {
        // Cover letters carry no diff.
        return intro;
    }
    let diff = format!(
        "---\n diff --git a/refs/reftable/backend.c b/refs/reftable/backend.c\n\
         index abcdef0..fedcba1 100644\n\
         --- a/refs/reftable/backend.c\n\
         +++ b/refs/reftable/backend.c\n\
         @@ -1{index},3 +1{index},3 @@\n\
         -// old behavior (removed)\n\
         +// new behavior (lazymail placeholder diff)"
    );
    format!("{intro}\n\n{diff}")
}