From 04adb2fa79d8aa124bcf8944ae01bb74d78ccdca Mon Sep 17 00:00:00 2001
From: reuk <reuk@users.noreply.github.com>
Date: Thu, 12 Mar 2020 14:04:27 +0000
Subject: [PATCH] DSP:  Tidy up ProcessorChain implementation

---
 .../juce_dsp/processors/juce_ProcessorChain.h | 168 ++++++++++--------
 1 file changed, 97 insertions(+), 71 deletions(-)

diff --git a/modules/juce_dsp/processors/juce_ProcessorChain.h b/modules/juce_dsp/processors/juce_ProcessorChain.h
index 71fbb23a8e..956e54a18f 100644
--- a/modules/juce_dsp/processors/juce_ProcessorChain.h
+++ b/modules/juce_dsp/processors/juce_ProcessorChain.h
@@ -29,109 +29,135 @@ namespace juce
 namespace dsp
 {
 
+//==============================================================================
 #ifndef DOXYGEN
-namespace ProcessorHelpers  // Internal helper classes used in building the ProcessorChain
+/** The contents of this namespace are used to implement ProcessorChain and should
+    not be used elsewhere. Their interfaces (and existence) are liable to change!
+*/
+namespace detail
 {
-    template <int arg>
-    struct AccessHelper
+    template <typename Fn, typename Tuple, size_t... Ix>
+    constexpr void forEachInTuple (Fn&& fn, Tuple&& tuple, std::index_sequence<Ix...>)
+        noexcept (noexcept (std::initializer_list<int> { (fn (std::get<Ix> (tuple), Ix), 0)... }))
     {
-        template <typename ProcessorType>
-        static auto& get (ProcessorType& a) noexcept                 { return AccessHelper<arg - 1>::get (a.processors); }
-
-        template <typename ProcessorType>
-        static const auto& get (const ProcessorType& a) noexcept     { return AccessHelper<arg - 1>::get (a.processors); }
+        (void) std::initializer_list<int> { ((void) fn (std::get<Ix> (tuple), Ix), 0)... };
+    }
 
-        template <typename ProcessorType>
-        static void setBypassed (ProcessorType& a, bool bypassed)    { AccessHelper<arg - 1>::setBypassed (a.processors, bypassed); }
-    };
+    template <typename T>
+    using TupleIndexSequence = std::index_sequence_for<std::remove_cv_t<std::remove_reference_t<T>>>;
 
-    template <>
-    struct AccessHelper<0>
+    template <typename Fn, typename Tuple>
+    constexpr void forEachInTuple (Fn&& fn, Tuple&& tuple)
+        noexcept (noexcept (forEachInTuple (std::forward<Fn> (fn), std::forward<Tuple> (tuple), TupleIndexSequence<Tuple>{})))
     {
-        template <typename ProcessorType>
-        static auto& get (ProcessorType& a) noexcept                 { return a.getProcessor(); }
+        forEachInTuple (std::forward<Fn> (fn), std::forward<Tuple> (tuple), TupleIndexSequence<Tuple>{});
+    }
+}
+#endif
 
-        template <typename ProcessorType>
-        static const auto& get (const ProcessorType& a) noexcept     { return a.getProcessor(); }
+/** This variadically-templated class lets you join together any number of processor
+    classes into a single processor which will call process() on them all in sequence.
+*/
+template <typename... Processors>
+class ProcessorChain
+{
+public:
+    /** Get a reference to the processor at index `Index`. */
+    template <int Index>       auto& get()       noexcept { return std::get<Index> (processors).processor; }
 
-        template <typename ProcessorType>
-        static void setBypassed (ProcessorType& a, bool bypassed)    { a.isBypassed = bypassed; }
-    };
+    /** Get a reference to the processor at index `Index`. */
+    template <int Index> const auto& get() const noexcept { return std::get<Index> (processors).processor; }
 
-    //==============================================================================
-    template <bool isFirst, typename Processor, typename Subclass>
-    struct ChainElement
+    /** Set the processor at index `Index` to be bypassed or enabled. */
+    template <int Index>
+    void setBypassed (bool b) noexcept  { std::get<Index> (processors).isBypassed = b; }
+
+    /** Query whether the processor at index `Index` is bypassed. */
+    template <int Index>
+    bool isBypassed() const noexcept    { return std::get<Index> (processors).isBypassed; }
+
+    /** Prepare all inner processors with the provided `ProcessSpec`. */
+    void prepare (const ProcessSpec& spec)
     {
-        void prepare (const ProcessSpec& spec)
-        {
-            processor.prepare (spec);
-        }
+        detail::forEachInTuple ([&] (auto& item, size_t) { item.processor.prepare (spec); }, processors);
+    }
+
+    /** Reset all inner processors. */
+    void reset()
+    {
+        detail::forEachInTuple ([] (auto& item, size_t) { item.processor.reset(); }, processors);
+    }
 
-        template <typename ProcessContext>
-        void process (const ProcessContext& context) noexcept
+    /** Process `context` through all inner processors in sequence. */
+    template <typename ProcessContext>
+    void process (const ProcessContext& context) noexcept
+    {
+        detail::forEachInTuple ([&] (auto& item, size_t index) noexcept
         {
-            if (context.usesSeparateInputAndOutputBlocks() && ! isFirst)
+            if (context.usesSeparateInputAndOutputBlocks() && index != 0)
             {
                 jassert (context.getOutputBlock().getNumChannels() == context.getInputBlock().getNumChannels());
                 ProcessContextReplacing<typename ProcessContext::SampleType> replacingContext (context.getOutputBlock());
-                replacingContext.isBypassed = (isBypassed || context.isBypassed);
+                replacingContext.isBypassed = (item.isBypassed || context.isBypassed);
 
-                processor.process (replacingContext);
+                item.processor.process (replacingContext);
             }
             else
             {
                 ProcessContext contextCopy (context);
-                contextCopy.isBypassed = (isBypassed || context.isBypassed);
+                contextCopy.isBypassed = (item.isBypassed || context.isBypassed);
 
-                processor.process (contextCopy);
+                item.processor.process (contextCopy);
             }
-        }
-
-        void reset()
-        {
-            processor.reset();
-        }
+        }, processors);
+    }
 
-        bool isBypassed = false;
+private:
+    template <typename Processor>
+    struct ProcessorWithBypass
+    {
         Processor processor;
-
-        Processor& getProcessor() noexcept             { return processor; }
-        const Processor& getProcessor() const noexcept { return processor; }
-        Subclass& getThis() noexcept                   { return *static_cast<Subclass*> (this); }
-        const Subclass& getThis() const noexcept       { return *static_cast<const Subclass*> (this); }
-
-        template <int arg> auto& get() noexcept                      { return AccessHelper<arg>::get (getThis()); }
-        template <int arg> const auto& get() const noexcept          { return AccessHelper<arg>::get (getThis()); }
-        template <int arg> void setBypassed (bool bypassed) noexcept { AccessHelper<arg>::setBypassed (getThis(), bypassed); }
+        bool isBypassed = false;
     };
 
-    //==============================================================================
-    template <bool isFirst, typename FirstProcessor, typename... SubsequentProcessors>
-    struct ChainBase  : public ChainElement<isFirst, FirstProcessor, ChainBase<isFirst, FirstProcessor, SubsequentProcessors...>>
-    {
-        using Base = ChainElement<isFirst, FirstProcessor, ChainBase<isFirst, FirstProcessor, SubsequentProcessors...>>;
-
-        template <typename ProcessContext>
-        void process (const ProcessContext& context) noexcept  { Base::process (context); processors.process (context); }
-        void prepare (const ProcessSpec& spec)                 { Base::prepare (spec); processors.prepare (spec); }
-        void reset()                                           { Base::reset(); processors.reset(); }
+    std::tuple<ProcessorWithBypass<Processors>...> processors;
+};
 
-        ChainBase<false, SubsequentProcessors...> processors;
-    };
+/** Non-member equivalent of ProcessorChain::get which avoids awkward
+    member template syntax.
+*/
+template <int Index, typename... Processors>
+inline auto& get (ProcessorChain<Processors...>& chain) noexcept
+{
+    return chain.template get<Index>();
+}
 
-    template <bool isFirst, typename ProcessorType>
-    struct ChainBase<isFirst, ProcessorType>  : public ChainElement<isFirst, ProcessorType, ChainBase<isFirst, ProcessorType>> {};
+/** Non-member equivalent of ProcessorChain::get which avoids awkward
+    member template syntax.
+*/
+template <int Index, typename... Processors>
+inline auto& get (const ProcessorChain<Processors...>& chain) noexcept
+{
+    return chain.template get<Index>();
 }
-#endif
 
+/** Non-member equivalent of ProcessorChain::setBypassed which avoids awkward
+    member template syntax.
+*/
+template <int Index, typename... Processors>
+inline void setBypassed (ProcessorChain<Processors...>& chain, bool bypassed) noexcept
+{
+    chain.template setBypassed<Index> (bypassed);
+}
 
-//==============================================================================
-/**
-    This variadically-templated class lets you join together any number of processor
-    classes into a single processor which will call process() on them all in sequence.
+/** Non-member equivalent of ProcessorChain::isBypassed which avoids awkward
+    member template syntax.
 */
-template <typename... Processors>
-using ProcessorChain = ProcessorHelpers::ChainBase<true, Processors...>;
+template <int Index, typename... Processors>
+inline bool isBypassed (const ProcessorChain<Processors...>& chain) noexcept
+{
+    return chain.template isBypassed<Index>();
+}
 
 } // namespace dsp
 } // namespace juce