use aws_sdk_s3::{error::SdkError, operation::get_object::GetObjectError};
use mime::Mime;
use std::io::{Error as IoError, Seek, SeekFrom, Write};
use thiserror::Error;

use super::S3Client;
use crate::retry;

#[derive(Debug, Error)]
#[expect(clippy::large_enum_variant)]
pub enum S3ReaderError {
	#[error("sdk error")]
	SdkError(#[from] SdkError<GetObjectError>),

	#[error("byte stream error")]
	ByteStreamError(#[from] aws_sdk_s3::primitives::ByteStreamError),

	#[error("i/o error")]
	IoError(#[from] IoError),
}

/// Provides a [`std::io::Read`]-like interface to an S3 object. \
/// This doesn't actually implement [`std::io::Read`] because Read isn't async.
///
/// Also implements [`std::io::Seek`]
pub struct S3Reader {
	pub(super) client: S3Client,
	pub(super) bucket: String,
	pub(super) key: String,

	pub(super) cursor: u64,
	pub(super) size: u64,
	pub(super) mime: Mime,
}

impl S3Reader {
	pub async fn read(&mut self, mut buf: &mut [u8]) -> Result<usize, S3ReaderError> {
		let len_left = self.size - self.cursor;
		if len_left == 0 || buf.is_empty() {
			return Ok(0);
		}

		#[expect(clippy::unwrap_used)] // TODO: probably fits?
		let start_byte = usize::try_from(self.cursor).unwrap();

		#[expect(clippy::unwrap_used)] // usize fits in u64
		let len_to_read = u64::try_from(buf.len()).unwrap().min(len_left);

		#[expect(clippy::unwrap_used)] // must fit, we called min()
		let len_to_read = usize::try_from(len_to_read).unwrap();

		let end_byte = start_byte + len_to_read - 1;

		let b = retry!(
			self.client.retries,
			self.client
				.client
				.get_object()
				.bucket(self.bucket.as_str())
				.key(self.key.as_str())
				.range(format!("bytes={start_byte}-{end_byte}"))
				.send()
				.await
		)?;

		// Looks like `bytes 31000000-31999999/33921176``
		// println!("{:?}", b.content_range);

		let mut bytes = b.body.collect().await?.into_bytes();
		bytes.truncate(len_to_read);
		let l = bytes.len();

		// Memory to memory writes are infallible
		#[expect(clippy::unwrap_used)]
		buf.write_all(&bytes).unwrap();

		// Cannot fail, usize should always fit into u64
		#[expect(clippy::unwrap_used)]
		{
			self.cursor += u64::try_from(l).unwrap();
		}

		return Ok(len_to_read);
	}

	pub fn is_done(&self) -> bool {
		return self.cursor == self.size;
	}

	pub fn mime(&self) -> &Mime {
		&self.mime
	}

	/// Write the entire contents of this reader to `r`.
	///
	/// This method always downloads the whole object,
	/// and always preserves `self.cursor`.
	pub async fn download<W: Write>(&mut self, r: &mut W) -> Result<(), S3ReaderError> {
		let pos = self.stream_position()?;

		const BUF_LEN: usize = 10_000_000;
		#[expect(clippy::unwrap_used)] // Cannot fail
		let mut buf: Box<[u8; BUF_LEN]> = vec![0u8; BUF_LEN].try_into().unwrap();

		while !self.is_done() {
			let b = self.read(&mut buf[..]).await?;
			r.write_all(&buf[0..b])?;
		}

		self.seek(SeekFrom::Start(pos))?;
		Ok(())
	}
}

impl Seek for S3Reader {
	fn seek(&mut self, pos: SeekFrom) -> std::io::Result<u64> {
		match pos {
			SeekFrom::Start(x) => self.cursor = x.min(self.size - 1),

			// Cannot panic, we handle all cases
			#[expect(clippy::unwrap_used)]
			SeekFrom::Current(x) => {
				if x < 0 {
					if u64::try_from(x.abs()).unwrap() > self.cursor {
						return Err(std::io::Error::new(
							std::io::ErrorKind::InvalidInput,
							"cannot seek past start",
						));
					}
					self.cursor -= u64::try_from(x.abs()).unwrap();
				} else {
					self.cursor += u64::try_from(x).unwrap();
				}
			}

			// Cannot panic, we handle all cases
			#[expect(clippy::unwrap_used)]
			SeekFrom::End(x) => {
				if x < 0 {
					if u64::try_from(x.abs()).unwrap() > self.size {
						return Err(std::io::Error::new(
							std::io::ErrorKind::InvalidInput,
							"cannot seek past start",
						));
					}
					// Cannot fail, is abs
					self.cursor = self.size - u64::try_from(x.abs()).unwrap();
				} else {
					// Cannot fail, is positive
					self.cursor = self.size + u64::try_from(x).unwrap();
				}
			}
		}

		self.cursor = self.cursor.min(self.size - 1);
		return Ok(self.cursor);
	}
}