span.h

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/*  span.h                                                                */
/**************************************************************************/
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#pragma once

#include "core/error/error_macros.h"
#include "core/typedefs.h"

template <typename LHS, typename RHS>
bool are_spans_equal(const LHS *p_lhs, const RHS *p_rhs, size_t p_size) {
	if constexpr (std::is_same_v<LHS, RHS> && std::is_fundamental_v<LHS>) {
		// Optimize trivial type comparison.
		// is_trivially_equality_comparable would help, but it doesn't exist.
		return memcmp(p_lhs, p_rhs, p_size * sizeof(LHS)) == 0;
	} else {
		// Normal case: Need to iterate the array manually.
		for (size_t j = 0; j < p_size; j++) {
			if (p_lhs[j] != p_rhs[j]) {
				return false;
			}
		}

		return true;
	}
}

// Equivalent of std::span.
// Represents a view into a contiguous memory space.
// DISCLAIMER: This data type does not own the underlying buffer. DO NOT STORE IT.
//  Additionally, for the lifetime of the Span, do not resize the buffer, and do not insert or remove elements from it.
//  Failure to respect this may lead to crashes or undefined behavior.
template <typename T>
class Span {
	const T *_ptr = nullptr;
	uint64_t _len = 0;

public:
	static constexpr bool is_string = std::disjunction_v<
			std::is_same<T, char>,
			std::is_same<T, char16_t>,
			std::is_same<T, char32_t>,
			std::is_same<T, wchar_t>>;

	_FORCE_INLINE_ constexpr Span() = default;

	_FORCE_INLINE_ Span(const T *p_ptr, uint64_t p_len) :
			_ptr(p_ptr), _len(p_len) {
#ifdef DEBUG_ENABLED
		// TODO In c++20, make this check run only in non-consteval, and make this constructor constexpr.
		if (_ptr == nullptr && _len > 0) {
			ERR_PRINT("Internal bug, please report: Span was created from nullptr with size > 0. Recovering by using size = 0.");
			_len = 0;
		}
#endif
	}

	// Allows creating Span directly from C arrays and string literals.
	template <size_t N>
	_FORCE_INLINE_ constexpr Span(const T (&p_array)[N]) :
			_ptr(p_array), _len(N) {
		if constexpr (is_string) {
			// Cut off the \0 terminator implicitly added to string literals.
			if (N > 0 && p_array[N - 1] == '\0') {
				_len--;
			}
		}
	}

	_FORCE_INLINE_ constexpr uint64_t size() const { return _len; }
	_FORCE_INLINE_ constexpr bool is_empty() const { return _len == 0; }

	_FORCE_INLINE_ constexpr const T *ptr() const { return _ptr; }

	// NOTE: Span subscripts sanity check the bounds to avoid undefined behavior.
	//       This is slower than direct buffer access and can prevent autovectorization.
	//       If the bounds are known, use ptr() subscript instead.
	_FORCE_INLINE_ constexpr const T &operator[](uint64_t p_idx) const {
		CRASH_COND(p_idx >= _len);
		return _ptr[p_idx];
	}

	_FORCE_INLINE_ constexpr const T *begin() const { return _ptr; }
	_FORCE_INLINE_ constexpr const T *end() const { return _ptr + _len; }

	template <typename T1>
	_FORCE_INLINE_ constexpr Span<T1> reinterpret() const {
		return Span<T1>(reinterpret_cast<const T1 *>(_ptr), _len * sizeof(T) / sizeof(T1));
	}

	// Algorithms.
	constexpr int64_t find(const T &p_val, uint64_t p_from = 0) const;
	template <typename T1 = T>
	constexpr int64_t find_sequence(const Span<T1> &p_span, uint64_t p_from = 0) const;
	constexpr int64_t rfind(const T &p_val, uint64_t p_from) const;
	_FORCE_INLINE_ constexpr int64_t rfind(const T &p_val) const { return rfind(p_val, size() - 1); }
	template <typename T1 = T>
	constexpr int64_t rfind_sequence(const Span<T1> &p_span, uint64_t p_from) const;
	template <typename T1 = T>
	_FORCE_INLINE_ constexpr int64_t rfind_sequence(const Span<T1> &p_span) const { return rfind_sequence(p_span, size() - p_span.size()); }
	constexpr uint64_t count(const T &p_val) const;
	/// Find the index of the given value using binary search.
	/// Note: Assumes that elements in the span are sorted. Otherwise, use find() instead.
	template <typename Comparator = Comparator<T>>
	constexpr uint64_t bisect(const T &p_value, bool p_before, Comparator compare = Comparator()) const;

	/// The caller is responsible to ensure size() > 0.
	constexpr T max() const;
};

template <typename T>
constexpr int64_t Span<T>::find(const T &p_val, uint64_t p_from) const {
	for (uint64_t i = p_from; i < size(); i++) {
		if (ptr()[i] == p_val) {
			return i;
		}
	}
	return -1;
}

template <typename T>
template <typename T1>
constexpr int64_t Span<T>::find_sequence(const Span<T1> &p_span, uint64_t p_from) const {
	for (uint64_t i = p_from; i <= size() - p_span.size(); i++) {
		if (are_spans_equal(ptr() + i, p_span.ptr(), p_span.size())) {
			return i;
		}
	}

	return -1;
}

template <typename T>
constexpr int64_t Span<T>::rfind(const T &p_val, uint64_t p_from) const {
	DEV_ASSERT(p_from < size());
	for (int64_t i = p_from; i >= 0; i--) {
		if (ptr()[i] == p_val) {
			return i;
		}
	}
	return -1;
}

template <typename T>
template <typename T1>
constexpr int64_t Span<T>::rfind_sequence(const Span<T1> &p_span, uint64_t p_from) const {
	DEV_ASSERT(p_from + p_span.size() <= size());
	for (int64_t i = p_from; i >= 0; i--) {
		if (are_spans_equal(ptr() + i, p_span.ptr(), p_span.size())) {
			return i;
		}
	}

	return -1;
}

template <typename T>
constexpr uint64_t Span<T>::count(const T &p_val) const {
	uint64_t amount = 0;
	for (uint64_t i = 0; i < size(); i++) {
		if (ptr()[i] == p_val) {
			amount++;
		}
	}
	return amount;
}

template <typename T>
template <typename Comparator>
constexpr uint64_t Span<T>::bisect(const T &p_value, bool p_before, Comparator compare) const {
	uint64_t lo = 0;
	uint64_t hi = size();
	if (p_before) {
		while (lo < hi) {
			const uint64_t mid = (lo + hi) / 2;
			if (compare(ptr()[mid], p_value)) {
				lo = mid + 1;
			} else {
				hi = mid;
			}
		}
	} else {
		while (lo < hi) {
			const uint64_t mid = (lo + hi) / 2;
			if (compare(p_value, ptr()[mid])) {
				hi = mid;
			} else {
				lo = mid + 1;
			}
		}
	}
	return lo;
}

template <typename T>
constexpr T Span<T>::max() const {
	DEV_ASSERT(size() > 0);
	T max_val = _ptr[0];
	for (size_t i = 1; i < _len; ++i) {
		if (_ptr[i] > max_val) {
			max_val = _ptr[i];
		}
	}
	return max_val;
}

template <typename LHS, typename RHS>
bool operator==(const Span<LHS> &p_lhs, const Span<RHS> &p_rhs) {
	return p_lhs.size() == p_rhs.size() && are_spans_equal(p_lhs.ptr(), p_rhs.ptr(), p_lhs.size());
}

template <typename LHS, typename RHS>
_FORCE_INLINE_ bool operator!=(const Span<LHS> &p_lhs, const Span<RHS> &p_rhs) {
	return !(p_lhs == p_rhs);
}

// Zero-constructing Span initializes _ptr and _len to 0 (and thus empty).
template <typename T>
struct is_zero_constructible<Span<T>> : std::true_type {};