From 495163ebb43fa8b8cea868c2be3df18aa64eb5df Mon Sep 17 00:00:00 2001
From: Chigozirim Igweamaka <cgzirim@gmail.com>
Date: Tue, 11 Mar 2025 09:18:29 +0100
Subject: [PATCH] refactor: update FindMatches function

---
 shazam/shazam.go      | 21 +++++++++++++++++++--
 shazam/spectrogram.go | 26 ++++++++++++++++++++++++--
 2 files changed, 43 insertions(+), 4 deletions(-)

diff --git a/shazam/shazam.go b/shazam/shazam.go
index 503b605..0683019 100644
--- a/shazam/shazam.go
+++ b/shazam/shazam.go
@@ -1,3 +1,6 @@
+//go:build !js && !wasm
+// +build !js,!wasm
+
 package shazam
 
 import (
@@ -21,7 +24,6 @@ type Match struct {
 // FindMatches analyzes the audio sample to find matching songs in the database.
 func FindMatches(audioSample []float64, audioDuration float64, sampleRate int) ([]Match, time.Duration, error) {
 	startTime := time.Now()
-	logger := utils.GetLogger()
 
 	spectrogram, err := Spectrogram(audioSample, sampleRate)
 	if err != nil {
@@ -31,6 +33,21 @@ func FindMatches(audioSample []float64, audioDuration float64, sampleRate int) (
 	peaks := ExtractPeaks(spectrogram, audioDuration)
 	sampleFingerprint := Fingerprint(peaks, utils.GenerateUniqueID())
 
+	sampleFingerprintMap := make(map[uint32]uint32)
+	for address, couple := range sampleFingerprint {
+		sampleFingerprintMap[address] = couple.AnchorTimeMs
+	}
+
+	matches, _, err := FindMatchesFGP(sampleFingerprintMap)
+
+	return matches, time.Since(startTime), nil
+}
+
+// FindMatchesFGP uses the sample fingerprint to find matching songs in the database.
+func FindMatchesFGP(sampleFingerprint map[uint32]uint32) ([]Match, time.Duration, error) {
+	startTime := time.Now()
+	logger := utils.GetLogger()
+
 	addresses := make([]uint32, 0, len(sampleFingerprint))
 	for address := range sampleFingerprint {
 		addresses = append(addresses, address)
@@ -55,7 +72,7 @@ func FindMatches(audioSample []float64, audioDuration float64, sampleRate int) (
 		for _, couple := range couples {
 			matches[couple.SongID] = append(
 				matches[couple.SongID],
-				[2]uint32{sampleFingerprint[address].AnchorTimeMs, couple.AnchorTimeMs},
+				[2]uint32{sampleFingerprint[address], couple.AnchorTimeMs},
 			)
 
 			if existingTime, ok := timestamps[couple.SongID]; !ok || couple.AnchorTimeMs < existingTime {
diff --git a/shazam/spectrogram.go b/shazam/spectrogram.go
index e18ae53..2488019 100644
--- a/shazam/spectrogram.go
+++ b/shazam/spectrogram.go
@@ -15,8 +15,7 @@ const (
 )
 
 func Spectrogram(sample []float64, sampleRate int) ([][]complex128, error) {
-	lpf := NewLowPassFilter(maxFreq, float64(sampleRate))
-	filteredSample := lpf.Filter(sample)
+	filteredSample := LowPassFilter(maxFreq, float64(sampleRate), sample)
 
 	downsampledSample, err := Downsample(filteredSample, sampleRate, sampleRate/dspRatio)
 	if err != nil {
@@ -53,6 +52,29 @@ func Spectrogram(sample []float64, sampleRate int) ([][]complex128, error) {
 	return spectrogram, nil
 }
 
+// LowPassFilter is a first-order low-pass filter that attenuates high
+// frequencies above the cutoffFrequency.
+// It uses the transfer function H(s) = 1 / (1 + sRC), where RC is the time constant.
+func LowPassFilter(cutoffFrequency, sampleRate float64, input []float64) []float64 {
+	rc := 1.0 / (2 * math.Pi * cutoffFrequency)
+	dt := 1.0 / sampleRate
+	alpha := dt / (rc + dt)
+
+	filteredSignal := make([]float64, len(input))
+	var prevOutput float64 = 0
+
+	for i, x := range input {
+		if i == 0 {
+			filteredSignal[i] = x * alpha
+		} else {
+
+			filteredSignal[i] = alpha*x + (1-alpha)*prevOutput
+		}
+		prevOutput = filteredSignal[i]
+	}
+	return filteredSignal
+}
+
 // Downsample downsamples the input audio from originalSampleRate to targetSampleRate
 func Downsample(input []float64, originalSampleRate, targetSampleRate int) ([]float64, error) {
 	if targetSampleRate <= 0 || originalSampleRate <= 0 {