use axum::{
	body::Body,
	extract::{Query, State},
	http::{HeaderMap, StatusCode, header},
	response::{IntoResponse, Response},
};
use pile_config::Label;
use pile_value::value::{AsyncReader, AsyncSeekReader};
use serde::Deserialize;
use std::{io::SeekFrom, sync::Arc, time::Instant};
use tokio::sync::mpsc;
use tokio_stream::wrappers::ReceiverStream;
use tracing::debug;
use utoipa::ToSchema;

use crate::Datasets;

#[derive(Deserialize, ToSchema)]
pub struct ItemQuery {
	source: String,
	key: String,
}

/// Parse a `Range: bytes=...` header value.
/// Returns `(start, end)` where either may be `None` (suffix form has `None` start).
fn parse_byte_range(s: &str) -> Option<(Option<u64>, Option<u64>)> {
	let spec = s.strip_prefix("bytes=")?;
	if spec.contains(',') {
		return None; // multiple ranges not supported
	}
	if let Some(suffix) = spec.strip_prefix('-') {
		return Some((None, Some(suffix.parse().ok()?)));
	}
	let mut parts = spec.splitn(2, '-');
	let start: u64 = parts.next()?.parse().ok()?;
	let end = parts
		.next()
		.and_then(|e| if e.is_empty() { None } else { e.parse().ok() });
	Some((Some(start), end))
}

/// Fetch the raw bytes of an item by source and key
#[utoipa::path(
	get,
	path = "/item",
	params(
		("source" = String, Query, description = "Source label"),
		("key" = String, Query, description = "Item key"),
	),
	responses(
		(status = 200, description = "Raw item bytes"),
		(status = 206, description = "Partial content"),
		(status = 400, description = "Invalid source label"),
		(status = 404, description = "Item not found"),
		(status = 416, description = "Range not satisfiable"),
		(status = 500, description = "Internal server error"),
	)
)]
pub async fn item_get(
	State(state): State<Arc<Datasets>>,
	Query(params): Query<ItemQuery>,
	headers: HeaderMap,
) -> Response {
	let start = Instant::now();
	debug!(
		message = "Serving /item",
		source = params.source,
		key = params.key
	);

	let label = match Label::try_from(params.source.clone()) {
		Ok(l) => l,
		Err(e) => return (StatusCode::BAD_REQUEST, format!("{e:?}")).into_response(),
	};

	let Some(item) = state.get(&label, &params.key).await else {
		return StatusCode::NOT_FOUND.into_response();
	};

	let mime = item.mime().to_string();

	let mut reader = match item.read().await {
		Ok(r) => r,
		Err(e) => return (StatusCode::INTERNAL_SERVER_ERROR, format!("{e:?}")).into_response(),
	};

	let total = match reader.seek(SeekFrom::End(0)).await {
		Ok(n) => n,
		Err(e) => return (StatusCode::INTERNAL_SERVER_ERROR, format!("{e:?}")).into_response(),
	};

	let range = headers
		.get(header::RANGE)
		.and_then(|v| v.to_str().ok())
		.and_then(parse_byte_range);

	// Resolve (byte_start, byte_end, content_length, is_range)
	let (byte_start, byte_end, length, is_range) = match range {
		Some((Some(s), e)) => {
			let e = e
				.unwrap_or(total.saturating_sub(1))
				.min(total.saturating_sub(1));
			if s >= total || s > e {
				return (
					StatusCode::RANGE_NOT_SATISFIABLE,
					[(header::CONTENT_RANGE, format!("bytes */{total}"))],
				)
					.into_response();
			}
			(s, e, e - s + 1, true)
		}
		Some((None, Some(suffix))) => {
			let s = total.saturating_sub(suffix);
			let e = total.saturating_sub(1);
			(s, e, total.saturating_sub(s), true)
		}
		_ => (0, total.saturating_sub(1), total, false),
	};

	if let Err(e) = reader.seek(SeekFrom::Start(byte_start)).await {
		return (StatusCode::INTERNAL_SERVER_ERROR, format!("{e:?}")).into_response();
	}

	debug!(
		message = "Served /item",
		source = params.source,
		key = params.key,
		time_ms = start.elapsed().as_millis()
	);

	let (tx, rx) = mpsc::channel::<Result<Vec<u8>, std::io::Error>>(8);

	tokio::spawn(async move {
		let mut buf = vec![0u8; 65536];
		let mut remaining = length;
		loop {
			if remaining == 0 {
				break;
			}
			let to_read = (buf.len() as u64).min(remaining) as usize;
			match reader.read(&mut buf[..to_read]).await {
				Ok(0) => break,
				Ok(n) => {
					remaining -= n as u64;
					if tx.send(Ok(buf[..n].to_vec())).await.is_err() {
						break;
					}
				}
				Err(e) => {
					let _ = tx.send(Err(e)).await;
					break;
				}
			}
		}
	});

	let body = Body::from_stream(ReceiverStream::new(rx));
	let status = if is_range {
		StatusCode::PARTIAL_CONTENT
	} else {
		StatusCode::OK
	};

	let mut builder = axum::http::Response::builder()
		.status(status)
		.header(header::CONTENT_TYPE, mime)
		.header(header::ACCEPT_RANGES, "bytes")
		.header(header::CONTENT_LENGTH, length);

	if is_range {
		builder = builder.header(
			header::CONTENT_RANGE,
			format!("bytes {byte_start}-{byte_end}/{total}"),
		);
	}

	builder
		.body(body)
		.map(IntoResponse::into_response)
		.unwrap_or_else(|e| (StatusCode::INTERNAL_SERVER_ERROR, format!("{e:?}")).into_response())
}