From 89d08bec6d3ff3199b6096cbe8e85abba3578044 Mon Sep 17 00:00:00 2001
From: Laura Berkowitz <leb239@cornell.edu>
Date: Mon, 6 Apr 2026 13:24:14 -0400
Subject: [PATCH] Add downsample method to analogSignalArray

Introduce analogSignalArray.downsample(new_fs) to decimate signals using MATLAB's resample() with a rational ratio (rat). The method handles empty data, prevents upsampling by requiring new_fs < old sampling rate, resamples columns, recomputes timestamps, and updates sampling_rate. Add unit test testDownsample that verifies sampling rate update, timestamp spacing, channel preservation, approximate sample count, and timestamp start.
---
 core/analogSignalArray.m      | 33 +++++++++++++++++++++++++++++++++
 test/test_analogSignalArray.m | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 67 insertions(+)

diff --git a/core/analogSignalArray.m b/core/analogSignalArray.m
index 333e497..2af2383 100644
--- a/core/analogSignalArray.m
+++ b/core/analogSignalArray.m
@@ -218,5 +218,38 @@ function disp(self)
                 varargin{:});
         end
 
+        function self = downsample(self, new_fs)
+            % Downsample analogSignalArray using MATLAB's resample()
+            %
+            % Usage:
+            %   asa.downsample(10)
+
+            if isempty(self.data)
+                warning('analogSignalArray is empty');
+                return;
+            end
+
+            old_fs = self.sampling_rate;
+
+            if new_fs >= old_fs
+                warning('new_fs must be less than old_fs, downsampling not performed')
+                return;
+            end
+
+            % Compute rational resampling ratio
+            [p, q] = rat(new_fs / old_fs);
+
+            % --- Resample data ---
+            % resample operates column-wise (perfect for [samples x signals])
+            self.data = resample(self.data, p, q);
+
+            % --- Recompute timestamps ---
+            n_samples_new = size(self.data, 1);
+            self.timestamps = self.timestamps(1) + (0:n_samples_new-1)' / new_fs;
+
+            % Update sampling rate
+            self.sampling_rate = new_fs;
+        end
+
     end
 end
diff --git a/test/test_analogSignalArray.m b/test/test_analogSignalArray.m
index 8678daa..69b1d01 100644
--- a/test/test_analogSignalArray.m
+++ b/test/test_analogSignalArray.m
@@ -68,3 +68,37 @@ function testIssorted(testCase)
 
     verifyEqual(testCase, asa.issorted(), true);
 end
+
+function testDownsample(testCase)
+    % Create synthetic signal
+    fs = 100; % original sampling rate
+    t = (0:1/fs:5)'; % 5 seconds
+    data = [sin(2*pi*5*t), cos(2*pi*2*t)]; % 2-channel signal
+
+    asa = analogSignalArray( ...
+        'data', data, ...
+        'timestamps', t, ...
+        'sampling_rate', fs);
+
+    % Downsample
+    new_fs = 10;
+    asa.downsample(new_fs);
+
+    % Check sampling rate updated
+    verifyEqual(testCase, asa.sampling_rate, new_fs);
+
+    % Check timestamps spacing
+    dt = diff(asa.timestamps);
+    verifyEqual(testCase, mean(dt), 1/new_fs, 'AbsTol', 1e-10);
+
+    % Check number of channels preserved
+    verifyEqual(testCase, size(asa.data, 2), 2);
+
+    % Check number of samples roughly correct
+    expected_n = floor(length(t) * (new_fs / fs));
+    verifyLessThanOrEqual(testCase, abs(size(asa.data,1) - expected_n), 1);
+
+    % Check timestamps start correctly
+    verifyEqual(testCase, asa.timestamps(1), t(1));
+end
+