We are getting somewhere with aubio

src/aubio_cpp.cpp

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+#include "aubio_cpp.hpp"
+
+#include <algorithm>
+#include <iterator>
+#include <limits>
+
+#include <aubio/fvec.h>
+
+namespace aubio {
+    std::optional<std::size_t> onset_detector::detect(const std::vector<sf::Int16>& samples) {
+        std::unique_ptr<fvec_t, decltype(&del_fvec)> out{new_fvec(2), del_fvec};
+        std::vector<float> input;
+        std::ranges::transform(samples, std::back_inserter(input), [](const sf::Int16 i) -> float {
+            return static_cast<float>(i) / std::numeric_limits<sf::Int16>::max();
+        });
+        fvec_t _input = {
+            .length=static_cast<uint_t>(input.size()),
+            .data=input.data(),
+        };
+        aubio_onset_do(this->get(), &_input, out.get());
+        if (out->data[0] != 0) {
+            return aubio_onset_get_last(this->get());
+        } else {
+            return {};
+        }
+    }
+}

src/aubio_cpp.hpp

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+#pragma once
+
+#include <functional>
+#include <memory>
+
+#include <aubio/aubio.h>
+#include <aubio/onset/onset.h>
+#include <SFML/Config.hpp>
+
+// Thanks stack overflow !
+// https://stackoverflow.com/a/54121092/10768117
+template <auto F>
+struct Functor {
+    template <typename... Args>
+    auto operator()(Args&&... args) const { return std::invoke(F, std::forward<Args>(args)...); }
+};
+
+namespace aubio {
+    using _aubio_onset_t_unique_ptr = std::unique_ptr<aubio_onset_t, Functor<del_aubio_onset>>;
+    struct onset_detector : _aubio_onset_t_unique_ptr {
+        template <typename... Args>
+        onset_detector(Args&&... args) : _aubio_onset_t_unique_ptr(new_aubio_onset(std::forward<Args>(args)...)) {}
+        std::optional<std::size_t> detect(const std::vector<sf::Int16>& samples);
+    };
+}

src/guess_tempo.cpp

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+#include "guess_tempo.hpp"
+#include <SFML/Config.hpp>
+#include <SFML/System/Utf.hpp>
+#include <deque>
+
+#include "aubio_cpp.hpp"
+#include "custom_sfml_audio/precise_sound_stream.hpp"
+
+std::vector<TempoEstimate> guess_tempo(feis::InputSoundFile& music) {
+    const auto onsets = detect_onsets(music);
+}
+
+std::vector<sf::Time> detect_onsets(feis::InputSoundFile& music) {
+    auto samplerate = music.getSampleRate();
+    std::size_t win_s = 1024; // window size
+    std::size_t hop_size = win_s / 4;
+    std::size_t read = 0;
+    std::vector<sf::Int16> samples(hop_size); // input audio buffer
+    std::vector<sf::Time> onsets;
+    auto detector = aubio::onset_detector("default", win_s, hop_size, samplerate);
+    do {
+        read = music.read(samples.data(), hop_size);
+        auto onset = detector.detect(samples);
+        // do something with the onsets
+        if (onset) {
+            onsets.push_back(samples_to_time(*onset, music.getSampleRate(), music.getChannelCount()));
+        }
+    } while ( read == hop_size );
+    aubio_cleanup();
+    return onsets;
+}

src/guess_tempo.hpp

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+#pragma once
+
+#include <SFML/System/Time.hpp>
+#include <filesystem>
+#include <vector>
+
+#include "special_numeric_types.hpp"
+#include "utf8_sfml_redefinitions.hpp"
+
+struct TempoEstimate {
+    Decimal bpm;
+    sf::Time offset;
+    float fitness;
+};
+
+std::vector<TempoEstimate> guess_tempo(feis::InputSoundFile& music);
+std::vector<sf::Time> detect_onsets(feis::InputSoundFile& music);