//! `comments <number> [--limit N | --tail N]` — list comments on an
//! issue or PR. Closes the curl-fallback gap (#418); `--tail`
//! closes the third of the four #694 gaps (paging-for-long-threads
//! awkwardness).
//!
//! - `--limit N` (default 50, Forgejo's cap) returns the first N
//!   comments — same shape this verb has always had.
//! - `--tail N` returns the *last* N comments in chronological
//!   order. Reads the issue's `comments` count first to compute
//!   which page contains the tail, then fetches only `ceil(N/50) +
//!   1` pages. No upstream cap — the work is bounded by `N`, not
//!   by the thread's length, so it stays cheap even on threads with
//!   thousands of comments. Use this for "what was the conclusion
//!   on this long thread?" without scrolling through the whole
//!   history.
//!
//! Use the global `--json` flag for JSON output (#421).

use anyhow::Result;
use clap::Args as ClapArgs;
use serde_json::{Value, json};

use crate::client::Client;
use crate::verbs::print_json;

/// Forgejo's per-page comment cap. The API caps `limit` at 50 even
/// if a higher value is requested; pin it explicitly so the math
/// downstream doesn't depend on a hidden default.
const PAGE_SIZE: usize = 50;

#[derive(ClapArgs)]
pub struct Args {
    /// Issue or PR number.
    number: u64,
    /// Page size for the head-of-thread shape (Forgejo caps at 50).
    /// Mutually exclusive with `--tail`.
    #[arg(long, default_value_t = 50, conflicts_with = "tail")]
    limit: u64,
    /// Return the last `N` comments in chronological order. Reads
    /// the issue's `comments` count first, then fetches only the
    /// `ceil(N/50) + 1` pages that contain the tail — work is
    /// bounded by N, not by thread length. Mutually exclusive with
    /// `--limit`.
    #[arg(long)]
    tail: Option<usize>,
}

pub fn run(client: &Client, args: Args) -> Result<()> {
    let repo = client.repo();
    let comments = match args.tail {
        Some(n) => fetch_tail(client, repo, args.number, n)?,
        None => fetch_head(client, repo, args.number, args.limit)?,
    };
    if client.json_mode() {
        let trimmed: Vec<Value> = comments
            .iter()
            .map(|c| {
                json!({
                    "id": c.get("id"),
                    "user": c.get("user").and_then(|u| u.get("login")),
                    "created_at": c.get("created_at"),
                    "updated_at": c.get("updated_at"),
                    "body": c.get("body"),
                    "url": c.get("html_url"),
                })
            })
            .collect();
        print_json(&Value::Array(trimmed))
    } else {
        for c in &comments {
            let user = c
                .get("user")
                .and_then(|u| u.get("login"))
                .and_then(Value::as_str)
                .unwrap_or("?");
            let ts = c.get("created_at").and_then(Value::as_str).unwrap_or("?");
            let body = c.get("body").and_then(Value::as_str).unwrap_or("");
            println!("**{user} @ {ts}**: {body}");
            println!();
        }
        Ok(())
    }
}

/// Fetch the first page's worth of comments (existing behaviour).
fn fetch_head(client: &Client, repo: &str, number: u64, limit: u64) -> Result<Vec<Value>> {
    let v = client.get_json(&format!(
        "/repos/{repo}/issues/{number}/comments?limit={limit}"
    ))?;
    Ok(v.as_array().cloned().unwrap_or_default())
}

/// Fetch the last `n` comments on an issue/PR in chronological order.
///
/// Forgejo orders `/issues/<n>/comments` oldest-first and has no
/// `direction=desc` knob, so naive "page everything and slice" pages
/// from the WRONG end on long threads — the first 1000 comments
/// instead of the last `n`. Fix: read the issue's `comments` count
/// first to know how many exist, then start paginating from the
/// page that contains item `total - n`. Work is bounded by
/// `ceil(n/50) + 1` page fetches, regardless of thread length.
#[allow(
    clippy::cast_possible_truncation,
    reason = "the forge `comments` count cast to usize is a small issue-thread length, never anywhere near usize::MAX even on a 32-bit target, so it cannot truncate in practice"
)]
fn fetch_tail(client: &Client, repo: &str, number: u64, n: usize) -> Result<Vec<Value>> {
    if n == 0 {
        return Ok(Vec::new());
    }
    let issue = client.get_json(&format!("/repos/{repo}/issues/{number}"))?;
    let total = issue.get("comments").and_then(Value::as_u64).unwrap_or(0) as usize;
    if total == 0 {
        return Ok(Vec::new());
    }
    // Cap `n` at the actual total so the math below stays in range
    // when the caller asks for more comments than exist.
    let n = n.min(total);
    let page_size = PAGE_SIZE;
    // 0-based index of the first comment we want; integer-divide to
    // get the 1-based page that contains it.
    let start_idx = total - n;
    let start_page = (start_idx / page_size) + 1;
    let last_page = (total - 1) / page_size + 1;
    let mut merged: Vec<Value> = Vec::with_capacity(n + page_size);
    for page in start_page..=last_page {
        let v = client.get_json(&format!(
            "/repos/{repo}/issues/{number}/comments?limit={PAGE_SIZE}&page={page}"
        ))?;
        let arr = v.as_array().cloned().unwrap_or_default();
        if arr.is_empty() {
            // Page came back empty — either we miscounted (comments
            // deleted between the issue GET and now) or upstream's
            // playing tricks. Stop rather than spin.
            break;
        }
        merged.extend(arr);
    }
    // The first fetched page contains items from `start_page` × 50
    // back; we overshoot by `start_idx % 50` items. Slice the tail
    // to exactly `n` (or fewer if the count shrank under us).
    let overshoot = merged.len().saturating_sub(n);
    Ok(merged.into_iter().skip(overshoot).collect())
}

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

    /// Pure helper mirroring the page-arithmetic in `fetch_tail`:
    /// given a total comment count + requested tail size, return
    /// the (`start_page`, `last_page`) pair the network loop would
    /// walk. Lets us pin the pagination plan — the part that's
    /// easy to off-by-one — without touching the network.
    fn tail_plan(total: usize, n: usize) -> (usize, usize) {
        let n = n.min(total);
        let page_size = PAGE_SIZE;
        let start_idx = total - n;
        let start_page = (start_idx / page_size) + 1;
        let last_page = (total - 1) / page_size + 1;
        (start_page, last_page)
    }

    #[test]
    fn tail_plan_small_thread() {
        // 5 total, tail 2 → page 1 covers everything; trim happens
        // via the merged.len() - n overshoot calculation, not pages.
        assert_eq!(tail_plan(5, 2), (1, 1));
    }

    #[test]
    fn tail_plan_exact_page_boundary() {
        // 50 total, tail 3 → all on page 1 (items 1..50).
        assert_eq!(tail_plan(50, 3), (1, 1));
    }

    #[test]
    fn tail_plan_crosses_page_boundary() {
        // 51 total, tail 3 → start_idx=48 lives on page 1, item 51
        // lives on page 2; fetch both.
        assert_eq!(tail_plan(51, 3), (1, 2));
    }

    #[test]
    fn tail_plan_large_thread_bounded_pages() {
        // 5000 total, tail 3 → start_idx=4997 lives on page 100,
        // last_page=100. ONE page fetch on a 5000-comment thread —
        // the whole point of swapping to count-then-page (vs the
        // old "page everything, then slice the wrong end").
        assert_eq!(tail_plan(5000, 3), (100, 100));
    }

    #[test]
    fn tail_plan_large_thread_spans_two_pages() {
        // 5000 total, tail 60 → start_idx=4940 on page 99, item 5000
        // on page 100. Two fetches even for n > PAGE_SIZE.
        assert_eq!(tail_plan(5000, 60), (99, 100));
    }

    #[test]
    fn tail_plan_n_exceeds_total() {
        // 5 total, tail 100 → cap n at total; same plan as the
        // small-thread case above.
        assert_eq!(tail_plan(5, 100), (1, 1));
    }
}