// SPDX-FileCopyrightText: Copyright (C) 2024 Roland Csaszar
// SPDX-License-Identifier: MPL-2.0
//
// Project:  token-string
// File:     builder.rs
// Date:     22.Nov.2024
// =============================================================================
//! String builder, concatenation of [`TokenString`]s.

use crate::{
	EMPTY,
	MAX_LENGTH,
	MAX_LENGTH_SMALL,
	MAX_LENGTH_SMALL_ADD1,
	PREFIX_LENGTH,
	StringPtr,
	TkStrError,
	TokenString,
};

extern crate alloc;

use core::{fmt, marker, mem, ptr, slice};

/// A struct to concatenate [`TokenString`]s without allocating unnecessary
/// memory on the heap.
///
/// The const generic parameter `N` should be set to the number of strings to
/// concatenate. See the [`crate::concat!`] macro for a more comfortable way to
/// use a [`Builder`].
///
/// Memory:
///
/// Uses a static array of `N`  pointers to [`TokenString`]s and two 8 byte
/// fields, so (N + 2) * 8 bytes, (N + 2) * 64 bits.
#[derive(Debug, Clone, Copy)]
pub struct Builder<'a, const N: usize> {
	/// The array of strings to concatenate.
	strings: [*const TokenString; N],
	/// The size of the concatenated string.
	total_size: usize,
	/// The number of strings to concatenate.
	num_strings: usize,
	/// Phantom field to hold the lifetime of the [`TokenString`] pointers.
	lifetime: marker::PhantomData<&'a ()>,
}

/// An iterator over the [`TokenString`]s of a [`Builder`].
///
/// A way to iterator over the strings to concatenate in a [`Builder`].
#[derive(Debug)]
pub struct BuilderIter<'a, const N: usize> {
	/// The [`Builder`] to iterate over.
	builder: &'a Builder<'a, N>,
	/// The current index in the array of [`TokenString`]s to concatenate.
	idx: usize,
}

impl<const N: usize> fmt::Display for Builder<'_, N> {
	#[inline]
	fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
		write!(f, "Builder < ")?;
		for idx in 0 .. self.num_strings {
			if idx == 0 {
				write!(
					f,
					"'{}'",
					// SAFETY:
					// The pointers have been constructed from valid
					// references, and are not null.
					unsafe { &*self.strings[idx] }
				)?;
			} else {
				write!(
					f,
					" + '{}'",
					// SAFETY:
					// The pointers have been constructed from valid
					// references, and are not null.
					unsafe { &*self.strings[idx] }
				)?;
			}
		}
		write!(f, " >")
	}
}

impl<'a, const N: usize> IntoIterator for &'a Builder<'a, N> {
	type IntoIter = BuilderIter<'a, N>;
	type Item = &'a TokenString;

	#[inline]
	fn into_iter(self) -> Self::IntoIter {
		Self::IntoIter {
			builder: self,
			idx: 0,
		}
	}
}

impl<'a, const N: usize> Iterator for BuilderIter<'a, N> {
	type Item = &'a TokenString;

	#[inline]
	fn next(&mut self) -> Option<Self::Item> {
		debug_assert!(
			self.idx <= self.builder.num_strings,
			"Builder iterator index is out of bounds"
		);
		self.idx += 1;
		if self.idx - 1 == self.builder.num_strings {
			None
		} else {
			// SAFETY:
			// The pointers have been constructed from valid references, and
			// are not null.
			Some(unsafe { &*self.builder.strings[self.idx - 1] })
		}
	}
}

impl<'a, const N: usize> Builder<'a, N> {
	#[inline]
	#[must_use]
	pub fn iter(&'a self) -> BuilderIter<'a, N> {
		<&Self as IntoIterator>::into_iter(self)
	}

	/// Create a new [`Builder`] from the given [`TokenString`].
	///
	/// Use this to concatenate the given string with other strings.
	///
	/// # Panics
	///
	/// Panics, if the number of strings to concatenate is set to 0.
	#[inline]
	#[must_use]
	pub const fn new(s: &'a TokenString) -> Self {
		assert!(N > 0, "the number of elements must not be 0");
		let mut ret_val = Self {
			total_size: s.len as usize,
			strings: [ptr::null(); N],
			num_strings: 1,
			lifetime: marker::PhantomData,
		};
		ret_val.strings[0] = s;
		ret_val
	}

	/// Concatenate the given string at the end of the builder.
	///
	/// See also [`Builder::concat_checked`] for the version that checks for
	/// errors, or the trait method [`Concat::concat`].
	///
	/// # Panics
	///
	/// Panics, if the number of concatenated strings is bigger than the size
	/// reserved by the constant generic `N` of [`Builder`].
	#[inline]
	#[must_use]
	pub const fn concat_unchecked(&mut self, s: &'a TokenString) -> &mut Self {
		assert!(
			(self.num_strings < N),
			"more strings concatenated than reserved space in Builder"
		);
		self.total_size += s.len as usize;
		self.strings[self.num_strings] = s;
		self.num_strings += 1;

		self
	}

	/// Concatenate another string to the end of the builder.
	///
	/// See also [`Builder::concat_unchecked`] for the version that does not
	/// check for errors, or the trait method [`concat`].
	///
	/// # Errors
	///
	/// This function will return [`TkStrError::TooBig`] if the concatenated
	/// string is greater than [`MAX_LENGTH`].
	/// If more than `N`, the const generic value of the [`Builder`], strings
	/// are being concatenated, [`TkStrError::TooMany`] is returned.
	#[inline]
	pub const fn concat_checked(
		&mut self,
		s: &'a TokenString,
	) -> Result<&mut Self, TkStrError> {
		if self.num_strings == N {
			return Err(TkStrError::TooMany(N));
		}
		if self.total_size + s.len as usize > MAX_LENGTH {
			return Err(TkStrError::TooBig(self.total_size + s.len as usize));
		}
		self.total_size += s.len as usize;
		self.strings[self.num_strings] = s;
		self.num_strings += 1;

		Ok(self)
	}

	/// Collect all strings passed to this [`Builder`].
	///
	/// # Errors
	///
	/// Returns [`TkStrError::TooBig`] if the sum of all string lengths is
	/// greater than [`MAX_LENGTH`].
	#[inline]
	pub fn collect_checked(self) -> Result<TokenString, TkStrError> {
		match self.total_size {
			| 0 => Ok(EMPTY),
			| 1 ..= MAX_LENGTH_SMALL => Ok(self.collect_to_small()),
			| MAX_LENGTH_SMALL_ADD1 ..= MAX_LENGTH =>
				Ok(self.collect_to_alloc()),
			| _ => Err(TkStrError::TooBig(self.total_size)),
		}
	}

	/// Collect all strings passed to this [`Builder`].
	///
	/// # Panics
	///
	/// If the concatenated string would be greater than [`MAX_LENGTH`], this
	/// panics.
	#[inline]
	#[must_use]
	pub fn collect_unchecked(self) -> TokenString {
		match self.total_size {
			| 0 => EMPTY,
			| 1 ..= MAX_LENGTH_SMALL => self.collect_to_small(),
			| MAX_LENGTH_SMALL_ADD1 ..= MAX_LENGTH => self.collect_to_alloc(),
			| _ => panic!(
				"the result of this builder would be bigger than `MAX_LENGTH`!"
			),
		}
	}

	/// Collect the [`TokenString`]s of the builder into a "small string".
	fn collect_to_small(&self) -> TokenString {
		let mut ret_val = EMPTY;
		#[expect(
			clippy::cast_possible_truncation,
			reason = "We checked for overflow in the parent function"
		)]
		let total = self.total_size as u16;
		ret_val.len = total;
		// SAFETY:
		// The two arrays, `prefix` and `u.small`, are guaranteed to be
		// consecutive in memory.
		let dest: &mut [u8] = unsafe {
			slice::from_raw_parts_mut(
				ret_val.prefix.as_mut_ptr(),
				self.total_size,
			)
		};
		let mut curr_end = 0;
		for b in self {
			let len = b.len as usize;
			let idx = curr_end;
			curr_end += len;
			dest[idx .. curr_end].copy_from_slice(&b.as_bytes()[.. len]);
		}

		ret_val
	}

	/// Collect the [`TokenString`]s of the builder into the allocated array.
	fn collect_to_alloc(&self) -> TokenString {
		let mut ret_val = EMPTY;
		#[expect(
			clippy::cast_possible_truncation,
			reason = "We checked for overflow in the parent function"
		)]
		let total = self.total_size as u16;
		ret_val.u.ptr =
			mem::ManuallyDrop::new(StringPtr::alloc_manually(self.total_size));
		ret_val.len = total;
		let mut curr_end = 0;
		for b in self {
			let len = b.len as usize;
			let idx = curr_end;
			curr_end += len;
			// SAFETY:
			// We've checked, that there is a pointer saved in the union.
			unsafe {
				(*ret_val.u.ptr).copy_manually(idx, b.as_bytes());
			}
		}
		// SAFETY:
		// We've checked, that there is a pointer saved in the union.
		ret_val.prefix.copy_from_slice(unsafe {
			&(*ret_val.u.ptr).as_slice_manually_mut(self.total_size)
				[.. PREFIX_LENGTH]
		});
		ret_val
	}
}

/// A helper trait for the [`Builder`] type to concatenate the given string at
/// the end of the other ones in the builder.
pub trait Concat<T> {
	type Output;

	/// Concatenate another string to the end of the builder.
	///
	/// # Errors
	///
	/// This function will return [`TkStrError::TooBig`] if the concatenated
	/// string is greater than [`MAX_LENGTH`].
	/// If more than `N` strings, the const generic value of the [`Builder`],
	/// are being concatenated, [`TkStrError::TooMany`] is returned.
	fn concat(self, s: T) -> Result<Self::Output, TkStrError>;
}

impl<'a, const N: usize> Concat<&'a TokenString>
	for Result<&'a mut Builder<'a, N>, TkStrError>
{
	type Output = &'a mut Builder<'a, N>;

	#[inline]
	fn concat(self, s: &'a TokenString) -> Result<Self::Output, TkStrError> {
		self?.concat_checked(s)
	}
}

impl<'a, const N: usize> Concat<&'a TokenString> for &'a mut Builder<'a, N> {
	type Output = &'a mut Builder<'a, N>;

	#[inline]
	fn concat(self, s: &'a TokenString) -> Result<Self::Output, TkStrError> {
		self.concat_checked(s)
	}
}

/// A helper trait for the [`Builder`] type to collect the strings to
/// concatenate into a single string.
pub trait Collect {
	/// Actually concatenate all strings given to the [`Builder`].
	///
	/// # Errors
	///
	/// Returns [`TkStrError::TooBig`] if the sum of all string lengths is
	/// greater than [`MAX_LENGTH`].
	fn collect(self) -> Result<TokenString, TkStrError>;
}

impl<'a, const N: usize> Collect
	for Result<&'a mut Builder<'a, N>, TkStrError>
{
	#[inline]
	fn collect(self) -> Result<TokenString, TkStrError> {
		self?.collect_checked()
	}
}

impl<'a, const N: usize> Collect for &'a mut Builder<'a, N> {
	#[inline]
	fn collect(self) -> Result<TokenString, TkStrError> {
		self.collect_checked()
	}
}

#[macro_export]
/// Return the number of arguments passed.
macro_rules! num_args {
    () => { 0_usize };
	($_x:tt $($xs:tt)*) => { 1_usize + $crate::num_args!($($xs)*) };
}

/// Concatenate all given strings and return a new [`TokenString`].
///
/// Warning: if passing a big number of strings to concatenate, this macro may
/// fail, as it uses recursion to compute the number of arguments.
///
/// Memory:
///
/// With `N` being the number of strings to concatenate, the [`Builder`] used
/// internally uses a static array of `N`  pointers to [`TokenString`]s and two
/// 8 byte fields, so (N + 2) * 8 bytes, (N + 2) * 64 bits.
///
/// # Errors
///
/// This function will return [`TkStrError::TooBig`] if the concatenated
/// string is greater than [`MAX_LENGTH`].
#[macro_export]
macro_rules! concat {
	( $x0:expr, $($xs:expr),+ ) => {{
		$crate::Builder::<'_, {$crate::num_args!($x0 $($xs)*)}>::new($x0)
		$( .concat($xs) )+.collect()
	}};
}

// =============================================================================
//                                  Tests
// =============================================================================


#[cfg(test)]
mod prefix {
	extern crate std;
	use assert2::check;

	use crate::{Builder, TokenString};

	#[test]
	fn concat_empty_unchecked() {
		let s1 = TokenString::default();
		let res = Builder::<6>::new(&s1).collect_unchecked();
		check!(res.prefix[0] == 0);
		check!(res.len == 0);
	}

	#[test]
	fn concat_2_empty_unchecked() {
		let s1 = TokenString::default();
		let s2 = TokenString::default();
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.collect_unchecked();
		check!(res.prefix[0] == 0);
		check!(res.len == 0);
	}

	#[test]
	fn concat_1_unchecked() {
		let s1 = TokenString::from_str_unchecked("12");
		let res = Builder::<6>::new(&s1).collect_unchecked();
		check!(&res.prefix[.. 2] == b"12");
		check!(res.len == 2);
	}

	#[test]
	fn concat_1_empty_unchecked() {
		let s1 = TokenString::from_str_unchecked("12");
		let s2 = TokenString::default();
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.collect_unchecked();
		check!(&res.prefix[.. 2] == b"12");
		check!(res.len == 2);
	}

	#[test]
	fn concat_empty_1_unchecked() {
		let s1 = TokenString::default();
		let s2 = TokenString::from_str_unchecked("12");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.collect_unchecked();
		check!(&res.prefix[.. 2] == b"12");
	}


	#[test]
	fn concat_2_unchecked() {
		let s1 = TokenString::from_str_unchecked("12");
		let s2 = TokenString::from_str_unchecked("34");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.collect_unchecked();
		check!(&res.prefix[.. 4] == b"1234");
		check!(res.len == 4);
	}

	#[test]
	fn concat_1_empty_1_unchecked() {
		let s1 = TokenString::from_str_unchecked("12");
		let s2 = TokenString::from_str_unchecked("");
		let s3 = TokenString::from_str_unchecked("34");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.concat_unchecked(&s3)
			.collect_unchecked();
		check!(&res.prefix[.. 4] == b"1234");
		check!(res.len == 4);
	}

	#[test]
	fn concat_3_unchecked() {
		let s1 = TokenString::from_str_unchecked("12");
		let s2 = TokenString::from_str_unchecked("34");
		let s3 = TokenString::from_str_unchecked("56");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.concat_unchecked(&s3)
			.collect_unchecked();
		check!(&res.prefix == b"123456");
		check!(res.len == 6);
	}
	#[test]
	fn concat_empty_checked() {
		let s1 = TokenString::default();
		let res = Builder::<6>::new(&s1).collect_checked().unwrap();
		check!(res.prefix[0] == 0);
		check!(res.len == 0);
	}

	#[test]
	fn concat_2_empty_checked() {
		let s1 = TokenString::default();
		let s2 = TokenString::default();
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(res.prefix[0] == 0);
		check!(res.len == 0);
	}

	#[test]
	fn concat_1_checked() {
		let s1 = TokenString::from_str_unchecked("12");
		let res = Builder::<6>::new(&s1).collect_checked().unwrap();
		check!(&res.prefix[.. 2] == b"12");
		check!(res.len == 2);
	}

	#[test]
	fn concat_1_empty_checked() {
		let s1 = TokenString::from_str_unchecked("12");
		let s2 = TokenString::default();
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix[.. 2] == b"12");
		check!(res.len == 2);
	}

	#[test]
	fn concat_empty_1_checked() {
		let s1 = TokenString::default();
		let s2 = TokenString::from_str_unchecked("12");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix[.. 2] == b"12");
	}


	#[test]
	fn concat_2_checked() {
		let s1 = TokenString::from_str_unchecked("12");
		let s2 = TokenString::from_str_unchecked("34");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix[.. 4] == b"1234");
		check!(res.len == 4);
	}

	#[test]
	fn concat_1_empty_1_checked() {
		let s1 = TokenString::from_str_unchecked("12");
		let s2 = TokenString::from_str_unchecked("");
		let s3 = TokenString::from_str_unchecked("34");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.concat_checked(&s3)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix[.. 4] == b"1234");
		check!(res.len == 4);
	}

	#[test]
	fn concat_3_checked() {
		let s1 = TokenString::from_str_unchecked("12");
		let s2 = TokenString::from_str_unchecked("34");
		let s3 = TokenString::from_str_unchecked("56");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.concat_checked(&s3)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix == b"123456");
		check!(res.len == 6);
	}
}

#[cfg(test)]
mod small {
	extern crate std;
	use assert2::check;

	use crate::{Builder, TokenString};


	#[test]
	fn concat_1_unchecked() {
		let s1 = TokenString::from_str_unchecked("1234567");
		let res = Builder::<6>::new(&s1).collect_unchecked();
		check!(&res.prefix == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { res.u.small[0] } == b'7');
		check!(res.len == 7);
	}

	#[test]
	fn concat_1_empty_unchecked() {
		let s1 = TokenString::from_str_unchecked("1234567");
		let s2 = TokenString::default();
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.collect_unchecked();
		check!(&res.prefix == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { res.u.small[0] } == b'7');
		check!(res.len == 7);
	}

	#[test]
	fn concat_empty_1_unchecked() {
		let s1 = TokenString::default();
		let s2 = TokenString::from_str_unchecked("1234567");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.collect_unchecked();
		check!(&res.prefix == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { res.u.small[0] } == b'7');
		check!(res.len == 7);
	}


	#[test]
	fn concat_2_unchecked() {
		let s1 = TokenString::from_str_unchecked("1234");
		let s2 = TokenString::from_str_unchecked("5678");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.collect_unchecked();
		check!(&res.prefix[.. 6] == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { &res.u.small[.. 2] } == b"78");
		check!(res.len == 8);
	}

	#[test]
	fn concat_2_pref_unchecked() {
		let s1 = TokenString::from_str_unchecked("1");
		let s2 = TokenString::from_str_unchecked("2345678");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.collect_unchecked();
		check!(&res.prefix[.. 6] == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { &res.u.small[.. 2] } == b"78");
		check!(res.len == 8);
	}

	#[test]
	fn concat_1_empty_1_unchecked() {
		let s1 = TokenString::from_str_unchecked("1234");
		let s2 = TokenString::from_str_unchecked("");
		let s3 = TokenString::from_str_unchecked("5678");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.concat_unchecked(&s3)
			.collect_unchecked();
		check!(&res.prefix[.. 6] == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { &res.u.small[.. 2] } == b"78");
		check!(res.len == 8);
	}

	#[test]
	fn concat_3_unchecked() {
		let s1 = TokenString::from_str_unchecked("1234");
		let s2 = TokenString::from_str_unchecked("5678");
		let s3 = TokenString::from_str_unchecked("90ABCD");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.concat_unchecked(&s3)
			.collect_unchecked();
		check!(&res.prefix[.. 6] == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { &res.u.small[.. 8] } == b"7890ABCD");
		check!(res.len == 14);
	}

	#[test]
	fn concat_1_checked() {
		let s1 = TokenString::from_str_unchecked("1234567");
		let res = Builder::<6>::new(&s1).collect_checked().unwrap();
		check!(&res.prefix == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { res.u.small[0] } == b'7');
		check!(res.len == 7);
	}

	#[test]
	fn concat_1_empty_checked() {
		let s1 = TokenString::from_str_unchecked("1234567");
		let s2 = TokenString::default();
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { res.u.small[0] } == b'7');
		check!(res.len == 7);
	}

	#[test]
	fn concat_empty_1_checked() {
		let s1 = TokenString::default();
		let s2 = TokenString::from_str_unchecked("1234567");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { res.u.small[0] } == b'7');
		check!(res.len == 7);
	}


	#[test]
	fn concat_2_checked() {
		let s1 = TokenString::from_str_unchecked("1234");
		let s2 = TokenString::from_str_unchecked("5678");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix[.. 6] == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { &res.u.small[.. 2] } == b"78");
		check!(res.len == 8);
	}

	#[test]
	fn concat_2_pref_checked() {
		let s1 = TokenString::from_str_unchecked("1");
		let s2 = TokenString::from_str_unchecked("2345678");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix[.. 6] == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { &res.u.small[.. 2] } == b"78");
		check!(res.len == 8);
	}

	#[test]
	fn concat_1_empty_1_checked() {
		let s1 = TokenString::from_str_unchecked("1234");
		let s2 = TokenString::from_str_unchecked("");
		let s3 = TokenString::from_str_unchecked("5678");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.concat_checked(&s3)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix[.. 6] == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { &res.u.small[.. 2] } == b"78");
		check!(res.len == 8);
	}

	#[test]
	fn concat_3_checked() {
		let s1 = TokenString::from_str_unchecked("1234");
		let s2 = TokenString::from_str_unchecked("5678");
		let s3 = TokenString::from_str_unchecked("90ABCD");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.concat_checked(&s3)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix[.. 6] == b"123456");
		// SAFETY:
		// The string should be in the `small` field of the union.
		check!(unsafe { &res.u.small[.. 8] } == b"7890ABCD");
		check!(res.len == 14);
	}
}

#[cfg(test)]
mod heap {
	extern crate std;
	use assert2::check;

	use crate::{Builder, TokenString};


	#[test]
	fn concat_1_unchecked() {
		let s1 = TokenString::from_str_unchecked("1234567890ABCDE");
		let res = Builder::<6>::new(&s1).collect_unchecked();
		check!(&res.prefix == b"123456");
		check!(
			// SAFETY:
			// The string should be in the `ptr` field of the union.
			unsafe { res.u.ptr.as_string_manually(res.len as usize) }
				== "1234567890ABCDE"
		);
		check!(res.len == 15);
	}

	#[test]
	fn concat_1_empty_unchecked() {
		let s1 = TokenString::from_str_unchecked("1234567890ABCDE");
		let s2 = TokenString::default();
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.collect_unchecked();
		check!(&res.prefix == b"123456");
		check!(
			// SAFETY:
			// The string should be in the `ptr` field of the union.
			unsafe { res.u.ptr.as_string_manually(res.len as usize) }
				== "1234567890ABCDE"
		);
		check!(res.len == 15);
	}

	#[test]
	fn concat_empty_1_unchecked() {
		let s1 = TokenString::default();
		let s2 = TokenString::from_str_unchecked("1234567890ABCDE");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.collect_unchecked();
		check!(&res.prefix == b"123456");
		check!(
			// SAFETY:
			// The string should be in the `ptr` field of the union.
			unsafe { res.u.ptr.as_string_manually(res.len as usize) }
				== "1234567890ABCDE"
		);
		check!(res.len == 15);
	}


	#[test]
	fn concat_2_unchecked() {
		let s1 = TokenString::from_str_unchecked("1234");
		let s2 = TokenString::from_str_unchecked("567890ABCDE");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.collect_unchecked();
		check!(&res.prefix[.. 6] == b"123456");
		check!(
			// SAFETY:
			// The string should be in the `ptr` field of the union.
			unsafe { res.u.ptr.as_string_manually(res.len as usize) }
				== "1234567890ABCDE"
		);
		check!(res.len == 15);
	}

	#[test]
	fn concat_1_empty_1_unchecked() {
		let s1 = TokenString::from_str_unchecked("1234");
		let s2 = TokenString::from_str_unchecked("");
		let s3 = TokenString::from_str_unchecked("567890ABCDE");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.concat_unchecked(&s3)
			.collect_unchecked();
		check!(&res.prefix[.. 6] == b"123456");
		check!(
			// SAFETY:
			// The string should be in the `ptr` field of the union.
			unsafe { res.u.ptr.as_string_manually(res.len as usize) }
				== "1234567890ABCDE"
		);
		check!(res.len == 15);
	}

	#[test]
	fn concat_3_unchecked() {
		let s1 = TokenString::from_str_unchecked("1234");
		let s2 = TokenString::from_str_unchecked("5678");
		let s3 = TokenString::from_str_unchecked("90ABCDE");
		let res = Builder::<6>::new(&s1)
			.concat_unchecked(&s2)
			.concat_unchecked(&s3)
			.collect_unchecked();
		check!(&res.prefix[.. 6] == b"123456");
		check!(
			// SAFETY:
			// The string should be in the `ptr` field of the union.
			unsafe { res.u.ptr.as_string_manually(res.len as usize) }
				== "1234567890ABCDE"
		);
		check!(res.len == 15);
	}

	#[test]
	fn concat_1_checked() {
		let s1 = TokenString::from_str_unchecked("1234567890ABCDE");
		let res = Builder::<6>::new(&s1).collect_checked().unwrap();
		check!(&res.prefix == b"123456");
		check!(
			// SAFETY:
			// The string should be in the `ptr` field of the union.
			unsafe { res.u.ptr.as_string_manually(res.len as usize) }
				== "1234567890ABCDE"
		);
		check!(res.len == 15);
	}

	#[test]
	fn concat_1_empty_checked() {
		let s1 = TokenString::from_str_unchecked("1234567890ABCDE");
		let s2 = TokenString::default();
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix == b"123456");
		check!(
			// SAFETY:
			// The string should be in the `ptr` field of the union.
			unsafe { res.u.ptr.as_string_manually(res.len as usize) }
				== "1234567890ABCDE"
		);
		check!(res.len == 15);
	}

	#[test]
	fn concat_empty_1_checked() {
		let s1 = TokenString::default();
		let s2 = TokenString::from_str_unchecked("1234567890ABCDE");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix == b"123456");
		check!(
			// SAFETY:
			// The string should be in the `ptr` field of the union.
			unsafe { res.u.ptr.as_string_manually(res.len as usize) }
				== "1234567890ABCDE"
		);
		check!(res.len == 15);
	}


	#[test]
	fn concat_2_checked() {
		let s1 = TokenString::from_str_unchecked("1234");
		let s2 = TokenString::from_str_unchecked("567890ABCDE");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix[.. 6] == b"123456");
		check!(
			// SAFETY:
			// The string should be in the `ptr` field of the union.
			unsafe { res.u.ptr.as_string_manually(res.len as usize) }
				== "1234567890ABCDE"
		);
		check!(res.len == 15);
	}

	#[test]
	fn concat_1_empty_1_checked() {
		let s1 = TokenString::from_str_unchecked("1234");
		let s2 = TokenString::from_str_unchecked("");
		let s3 = TokenString::from_str_unchecked("567890ABCDE");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.concat_checked(&s3)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix[.. 6] == b"123456");
		check!(
			// SAFETY:
			// The string should be in the `ptr` field of the union.
			unsafe { res.u.ptr.as_string_manually(res.len as usize) }
				== "1234567890ABCDE"
		);
		check!(res.len == 15);
	}

	#[test]
	fn concat_3_checked() {
		let s1 = TokenString::from_str_unchecked("1234");
		let s2 = TokenString::from_str_unchecked("5678");
		let s3 = TokenString::from_str_unchecked("90ABCDE");
		let res = Builder::<6>::new(&s1)
			.concat_checked(&s2)
			.unwrap()
			.concat_checked(&s3)
			.unwrap()
			.collect_checked()
			.unwrap();
		check!(&res.prefix[.. 6] == b"123456");
		check!(
			// SAFETY:
			// The string should be in the `ptr` field of the union.
			unsafe { res.u.ptr.as_string_manually(res.len as usize) }
				== "1234567890ABCDE"
		);
		check!(res.len == 15);
	}
}