filter_iterator.hpp

// EVMC: Ethereum Client-VM Connector API.
// Copyright 2022 The EVMC Authors.
// Licensed under the Apache License, Version 2.0.
#pragma once
 
#include <iterator>
 
namespace evmc
{
inline constexpr bool isspace(char ch) noexcept
{
 // Implementation taken from LLVM's libc.
 return ch == ' ' || (static_cast<unsigned>(ch) - '\t') < 5;
}
 
inline constexpr bool is_not_space(char ch) noexcept
{
 return !isspace(ch);
}
 
template <typename BaseIterator,
 bool predicate(typename std::iterator_traits<BaseIterator>::value_type) noexcept>
struct filter_iterator
{
 using difference_type = typename std::iterator_traits<BaseIterator>::difference_type;
 
 using value_type = typename std::iterator_traits<BaseIterator>::value_type;
 
 using pointer = typename std::iterator_traits<BaseIterator>::pointer;
 
 using reference = typename std::iterator_traits<BaseIterator>::reference;
 
 using iterator_category = std::input_iterator_tag;
 
private:
 BaseIterator base;
 BaseIterator base_end;
 value_type value;
 
 constexpr void forward_to_next_value() noexcept
 {
 for (; base != base_end; ++base)
 {
 value = *base;
 if (predicate(value))
 break;
 }
 }
 
public:
 constexpr filter_iterator(BaseIterator it, BaseIterator end) noexcept : base{it}, base_end{end}
 {
 forward_to_next_value();
 }
 
 constexpr auto operator*() noexcept
 {
 // We should not read from an input base iterator twice. So the only read is in
 // forward_to_next_value() and here we return the cached value.
 return value;
 }
 
 constexpr void operator++() noexcept
 {
 ++base;
 forward_to_next_value();
 }
 
 constexpr bool operator==(const filter_iterator& o) const noexcept { return base == o.base; }
 
 constexpr bool operator!=(const filter_iterator& o) const noexcept { return base != o.base; }
};
 
template <typename BaseIterator>
struct skip_space_iterator : filter_iterator<BaseIterator, is_not_space>
{
 using filter_iterator<BaseIterator, is_not_space>::filter_iterator;
};
 
template <typename BaseIterator>
skip_space_iterator(BaseIterator, BaseIterator) -> skip_space_iterator<BaseIterator>;
} // namespace evmc