Here is a follow-up to #18002, again with the goal of simplifying (potential) BIP341 code.
Instead of passing a begin and end iterator of the initial stack to ExecuteWitnessScript, they are turned into a Span<const valtype>, representing a span of valtypes in memory. This allows VerifyWitnessProgram to operate on that span directly, instead of juggling iterators around (which would be exacerbated by #17977 if trying to avoid copying the stack).
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utACK https://github.com/bitcoin/bitcoin/pull/18388/commits/c7b0131e3bc784263ce59c837be7d3ede59c45cd
wondering if there’s some short-hand we could do to “split” the Spans or something like pop_back which also returns the popped element since that seems to be a common use-case of it now and going forward.
pop_back couldn’t be added to Span, but since C++20’s std::span doesn’t have it, I’d rather not in order to maintain compatibility. Maybe a utility function T& PopSpan(Span<T>& span) is useful (which pops, and returns the popped element).
58@@ -57,4 +59,13 @@ constexpr Span<A> MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
59 template<typename V>
60 constexpr Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type> MakeSpan(V& v) { return Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type>(v.data(), v.size()); }
61
62+/** Pop the last element off a span, and return a reference to that element. */
63+template<typename T>
nit, missing space(git-clang-format)
0template <typename T>
58@@ -57,4 +59,13 @@ constexpr Span<A> MakeSpan(A (&a)[N]) { return Span<A>(a, N); }
59 template<typename V>
60 constexpr Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type> MakeSpan(V& v) { return Span<typename std::remove_pointer<decltype(std::declval<V>().data())>::type>(v.data(), v.size()); }
61
62+/** Pop the last element off a span, and return a reference to that element. */
63+template<typename T>
64+T& SpanPopBack(Span<T>& span)
65+{
66+ T& back = span.back();
26@@ -27,6 +27,8 @@ class Span
27 constexpr C* data() const noexcept { return m_data; }
28 constexpr C* begin() const noexcept { return m_data; }
29 constexpr C* end() const noexcept { return m_data + m_size; }
30+ constexpr C& front() const noexcept { return m_data[0]; }
I’d rather not add asserts here. std::span doesn’t have them, so if we’re expecting to someday replace Span with that, it should be subject to the same interface. And that interface requires the caller to make sure values are not out of range.
Spans may also be used in pretty tight loops some day, in which case asserts may be actually undesirable.
This allows for very cheap transformations on the range of elements that
are to be passed to ExecuteWitnessScript.
27@@ -27,6 +28,8 @@ class Span
28 constexpr C* data() const noexcept { return m_data; }
29 constexpr C* begin() const noexcept { return m_data; }
30 constexpr C* end() const noexcept { return m_data + m_size; }
31+ constexpr C& front() const noexcept { return m_data[0]; }
front() and back() are no longer used in this PR, but are used in the taproot implementation, so there’s no harm in including them here.
