feat(sdcard): add SD card storage capacity query (closes #206)

src/Daqifi.Core.Tests/Communication/Producers/ScpiMessageProducerTests.cs

@@ -475,6 +475,14 @@ public void GetSdBenchmarkResults_ReturnsCorrectCommand()
         AssertMessageFormat(message);
     }
 
+    [Fact]
+    public void GetSdSpace_ReturnsCorrectCommand()
+    {
+        var message = ScpiMessageProducer.GetSdSpace;
+        Assert.Equal("SYSTem:STORage:SD:SPACe?", message.Data);
+        AssertMessageFormat(message);
+    }
+
     [Theory]
     [InlineData(StreamInterface.Usb, 0)]
     [InlineData(StreamInterface.WiFi, 1)]

src/Daqifi.Core.Tests/Device/SdCard/SdCardOperationsTests.cs

@@ -1024,6 +1024,139 @@ public async Task FormatSdCardAsync_WhenNotStreaming_StillSendsStopCommand()
 
         #endregion
 
+        #region GetSdCardStorageAsync Tests
+
+        [Fact]
+        public async Task GetSdCardStorageAsync_WhenDisconnected_Throws()
+        {
+            var device = new DaqifiStreamingDevice("TestDevice");
+
+            await Assert.ThrowsAsync<InvalidOperationException>(
+                () => device.GetSdCardStorageAsync());
+        }
+
+        [Fact]
+        public async Task GetSdCardStorageAsync_WhenConnected_SendsCorrectCommands()
+        {
+            var device = new TestableSdCardStreamingDevice("TestDevice");
+            device.CannedTextResponse = new List<string> { "1024,4096" };
+            device.Connect();
+
+            await device.GetSdCardStorageAsync();
+
+            var sentCommands = device.SentMessages.Select(m => m.Data).ToList();
+            Assert.Contains("SYSTem:COMMunicate:LAN:ENAbled 0", sentCommands); // PrepareSdInterface
+            Assert.Contains("SYSTem:STORage:SD:ENAble 1", sentCommands);       // PrepareSdInterface
+            Assert.Contains("SYSTem:STORage:SD:SPACe?", sentCommands);         // GetSdSpace
+        }
+
+        [Fact]
+        public async Task GetSdCardStorageAsync_ParsesResponseCorrectly()
+        {
+            var device = new TestableSdCardStreamingDevice("TestDevice");
+            device.CannedTextResponse = new List<string> { "1048576000,2097152000" };
+            device.Connect();
+
+            var storage = await device.GetSdCardStorageAsync();
+
+            Assert.Equal(1_048_576_000L, storage.FreeBytes);
+            Assert.Equal(2_097_152_000L, storage.TotalBytes);
+            Assert.Equal(1_048_576_000L, storage.UsedBytes);
+        }
+
+        [Fact]
+        public async Task GetSdCardStorageAsync_RestoresLanInterface()
+        {
+            var device = new TestableSdCardStreamingDevice("TestDevice");
+            device.CannedTextResponse = new List<string> { "1024,4096" };
+            device.Connect();
+
+            await device.GetSdCardStorageAsync();
+
+            var sentCommands = device.SentMessages.Select(m => m.Data).ToList();
+            Assert.Contains("SYSTem:STORage:SD:ENAble 0", sentCommands);       // PrepareLanInterface
+            Assert.Contains("SYSTem:COMMunicate:LAN:ENAbled 1", sentCommands); // PrepareLanInterface
+        }
+
+        [Fact]
+        public async Task GetSdCardStorageAsync_DefensivelySendsStopStreaming()
+        {
+            var device = new TestableSdCardStreamingDevice("TestDevice");
+            device.CannedTextResponse = new List<string> { "1024,4096" };
+            device.Connect();
+            Assert.False(device.IsStreaming);
+
+            await device.GetSdCardStorageAsync();
+
+            var sentCommands = device.SentMessages.Select(m => m.Data).ToList();
+            Assert.Contains("SYSTem:StopStreamData", sentCommands);
+        }
+
+        [Fact]
+        public async Task GetSdCardStorageAsync_WithScpiError_RetriesAndReturnsStorage()
+        {
+            var device = new RetryableSdCardStreamingDevice("TestDevice");
+            device.ResponseSequence.Enqueue(new List<string> { "**ERROR: -200, \"Execution error\"" });
+            device.ResponseSequence.Enqueue(new List<string> { "1024,4096" });
+            device.Connect();
+
+            var storage = await device.GetSdCardStorageAsync();
+
+            Assert.Equal(1024L, storage.FreeBytes);
+            Assert.Equal(4096L, storage.TotalBytes);
+            Assert.Equal(2, device.ExecuteTextCommandCallCount);
+        }
+
+        [Fact]
+        public async Task GetSdCardStorageAsync_WithPersistentScpiError_ThrowsSdCardOperationException()
+        {
+            var device = new RetryableSdCardStreamingDevice("TestDevice");
+            device.ResponseSequence.Enqueue(new List<string> { "**ERROR: -200, \"Execution error\"" });
+            device.ResponseSequence.Enqueue(new List<string> { "**ERROR: -200, \"Execution error\"" });
+            device.Connect();
+
+            var ex = await Assert.ThrowsAsync<SdCardOperationException>(
+                () => device.GetSdCardStorageAsync());
+            Assert.Equal(2, device.ExecuteTextCommandCallCount);
+            Assert.Contains("**ERROR", ex.LastScpiError);
+        }
+
+        [Fact]
+        public async Task GetSdCardStorageAsync_WithNoSdCardDetected_ThrowsSdCardNotPresentException()
+        {
+            var device = new RetryableSdCardStreamingDevice("TestDevice");
+            var response = new List<string>
+            {
+                "Error !! No SD Card Detected",
+                "**ERROR: -200, \"Execution error\""
+            };
+            device.ResponseSequence.Enqueue(response);
+            device.ResponseSequence.Enqueue(new List<string>(response));
+            device.Connect();
+
+            var ex = await Assert.ThrowsAsync<SdCardNotPresentException>(
+                () => device.GetSdCardStorageAsync());
+            Assert.Contains("**ERROR", ex.LastScpiError);
+        }
+
+        [Fact]
+        public async Task GetSdCardStorageAsync_OnError_StillRestoresLanInterface()
+        {
+            var device = new RetryableSdCardStreamingDevice("TestDevice");
+            device.ResponseSequence.Enqueue(new List<string> { "Error !! No SD Card Detected", "**ERROR: -200" });
+            device.ResponseSequence.Enqueue(new List<string> { "Error !! No SD Card Detected", "**ERROR: -200" });
+            device.Connect();
+
+            await Assert.ThrowsAsync<SdCardNotPresentException>(
+                () => device.GetSdCardStorageAsync());
+
+            var sentCommands = device.SentMessages.Select(m => m.Data).ToList();
+            Assert.Contains("SYSTem:STORage:SD:ENAble 0", sentCommands);
+            Assert.Contains("SYSTem:COMMunicate:LAN:ENAbled 1", sentCommands);
+        }
+
+        #endregion
+
         #region DownloadSdCardFileAsync Tests
 
         [Fact]

src/Daqifi.Core.Tests/Device/SdCard/SdCardSpaceParserTests.cs

@@ -0,0 +1,100 @@
+using Daqifi.Core.Device.SdCard;
+
+namespace Daqifi.Core.Tests.Device.SdCard;
+
+public class SdCardSpaceParserTests
+{
+    [Fact]
+    public void TryParse_ValidResponse_ReturnsStorageInfo()
+    {
+        Assert.True(SdCardSpaceParser.TryParse("1048576000,2097152000", out var result));
+        Assert.NotNull(result);
+        Assert.Equal(1_048_576_000L, result.FreeBytes);
+        Assert.Equal(2_097_152_000L, result.TotalBytes);
+        Assert.Equal(1_048_576_000L, result.UsedBytes);
+    }
+
+    [Fact]
+    public void TryParse_WithSurroundingWhitespace_ReturnsStorageInfo()
+    {
+        Assert.True(SdCardSpaceParser.TryParse("  100 , 500  ", out var result));
+        Assert.NotNull(result);
+        Assert.Equal(100L, result.FreeBytes);
+        Assert.Equal(500L, result.TotalBytes);
+    }
+
+    [Fact]
+    public void TryParse_FullCard_ReturnsZeroFree()
+    {
+        Assert.True(SdCardSpaceParser.TryParse("0,1000000", out var result));
+        Assert.NotNull(result);
+        Assert.Equal(0L, result.FreeBytes);
+        Assert.Equal(1_000_000L, result.TotalBytes);
+        Assert.Equal(1_000_000L, result.UsedBytes);
+    }
+
+    [Theory]
+    [InlineData(null)]
+    [InlineData("")]
+    [InlineData("   ")]
+    public void TryParse_NullOrWhitespace_ReturnsFalse(string? input)
+    {
+        Assert.False(SdCardSpaceParser.TryParse(input, out var result));
+        Assert.Null(result);
+    }
+
+    [Theory]
+    [InlineData("1048576000")]            // no comma
+    [InlineData(",2097152000")]           // missing free
+    [InlineData("1048576000,")]           // missing total
+    [InlineData("abc,2097152000")]        // non-numeric free
+    [InlineData("1048576000,xyz")]        // non-numeric total
+    [InlineData("-1,2097152000")]         // negative free
+    [InlineData("1048576000,-1")]         // negative total
+    [InlineData("**ERROR: -200, \"Execution error\"")] // SCPI error
+    public void TryParse_Malformed_ReturnsFalse(string input)
+    {
+        Assert.False(SdCardSpaceParser.TryParse(input, out var result));
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void TryParseLines_FindsFirstValidLine()
+    {
+        var lines = new[]
+        {
+            "",
+            "some preamble",
+            "1024,4096",
+            "trailing text"
+        };
+
+        Assert.True(SdCardSpaceParser.TryParseLines(lines, out var result));
+        Assert.NotNull(result);
+        Assert.Equal(1024L, result.FreeBytes);
+        Assert.Equal(4096L, result.TotalBytes);
+    }
+
+    [Fact]
+    public void TryParseLines_NoValidLines_ReturnsFalse()
+    {
+        var lines = new[] { "", "garbage", "**ERROR: -200" };
+
+        Assert.False(SdCardSpaceParser.TryParseLines(lines, out var result));
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void TryParseLines_EmptySequence_ReturnsFalse()
+    {
+        Assert.False(SdCardSpaceParser.TryParseLines([], out var result));
+        Assert.Null(result);
+    }
+
+    [Fact]
+    public void UsedBytes_ComputesDifference()
+    {
+        var info = new SdCardStorageInfo(FreeBytes: 300, TotalBytes: 1000);
+        Assert.Equal(700L, info.UsedBytes);
+    }
+}

src/Daqifi.Core/Communication/Producers/ScpiMessageProducer.cs

@@ -207,6 +207,17 @@ public static IOutboundMessage<string> SetSdMaxFileSize(long bytes)
     /// </remarks>
     public static IOutboundMessage<string> GetSdMaxFileSize => new ScpiMessage("SYSTem:STORage:SD:MAXSize?");
 
+    /// <summary>
+    /// Creates a query message to get the free and total byte counts of the SD card.
+    /// </summary>
+    /// <remarks>
+    /// Returns a single line of the form <c>"free,total"</c>, where both values are
+    /// unsigned byte counts.
+    /// Command: SYSTem:STORage:SD:SPACe?
+    /// Example: messageProducer.Send(ScpiMessageProducer.GetSdSpace);
+    /// </remarks>
+    public static IOutboundMessage<string> GetSdSpace => new ScpiMessage("SYSTem:STORage:SD:SPACe?");
+
     /// <summary>
     /// Creates a command message to run an SD card write speed benchmark.
     /// </summary>

src/Daqifi.Core/Device/DaqifiStreamingDevice.cs

@@ -392,6 +392,94 @@ public async Task<IReadOnlyList<SdCardFileInfo>> GetSdCardFilesAsync(Cancellatio
             return files;
         }
 
+        /// <summary>
+        /// Retrieves the free and total byte counts of the device's SD card.
+        /// </summary>
+        /// <param name="cancellationToken">A cancellation token to observe while waiting for the task to complete.</param>
+        /// <returns>A task that represents the asynchronous operation, containing the SD card storage info.</returns>
+        /// <exception cref="InvalidOperationException">Thrown when the device is not connected.</exception>
+        /// <exception cref="OperationCanceledException">Thrown when the operation is canceled.</exception>
+        /// <exception cref="SdCardNotPresentException">Thrown when no SD card is installed in the device.</exception>
+        /// <exception cref="SdCardOperationException">Thrown when the device returned a SCPI error or an unparseable response.</exception>
+        public async Task<SdCardStorageInfo> GetSdCardStorageAsync(CancellationToken cancellationToken = default)
+        {
+            if (!IsConnected)
+            {
+                throw new InvalidOperationException("Device is not connected.");
+            }
+
+            cancellationToken.ThrowIfCancellationRequested();
+
+            // Defensive: always send stop command even if IsStreaming is stale (see issue #118)
+            Send(ScpiMessageProducer.StopStreaming);
+            IsStreaming = false;
+
+            IReadOnlyList<string> lines;
+            try
+            {
+                lines = await ExecuteTextCommandAsync(() =>
+                {
+                    PrepareSdInterface();
+
+                    // Allow the device firmware to complete the SPI bus switch
+                    // before querying the SD card. Without this delay, the device
+                    // can return SCPI error -200 (Execution error).
+                    Thread.Sleep(SD_INTERFACE_SETTLE_DELAY_MS);
+
+                    Send(ScpiMessageProducer.GetSdSpace);
+                }, responseTimeoutMs: 3000, cancellationToken: cancellationToken);
+
+                if (ContainsScpiError(lines))
+                {
+                    for (var retry = 0; retry < SD_LIST_MAX_RETRIES; retry++)
+                    {
+                        cancellationToken.ThrowIfCancellationRequested();
+
+                        await Task.Delay(SD_INTERFACE_SETTLE_DELAY_MS, cancellationToken);
+
+                        lines = await ExecuteTextCommandAsync(() =>
+                        {
+                            PrepareSdInterface();
+                            Thread.Sleep(SD_INTERFACE_SETTLE_DELAY_MS);
+                            Send(ScpiMessageProducer.GetSdSpace);
+                        }, responseTimeoutMs: 3000, cancellationToken: cancellationToken);
+
+                        if (!ContainsScpiError(lines))
+                        {
+                            break;
+                        }
+                    }
+                }
+            }
+            finally
+            {
+                if (IsConnected)
+                {
+                    PrepareLanInterface();
+                }
+            }
+
+            if (SdCardSpaceParser.TryParseLines(lines, out var storage))
+            {
+                return storage;
+            }
+
+            // Parser failed — translate the firmware response into a typed exception.
+            var lastScpiError = lines.LastOrDefault(IsScpiErrorLine)?.Trim();
+
+            if (lines.Any(l => l.IndexOf("No SD Card Detected", StringComparison.OrdinalIgnoreCase) >= 0))
+            {
+                throw new SdCardNotPresentException(lines, lastScpiError);
+            }
+
+            throw new SdCardOperationException(
+                lastScpiError != null
+                    ? "The SD card storage query failed: " + lastScpiError
+                    : "The SD card storage query returned an unparseable response.",
+                lines,
+                lastScpiError);
+        }
+
         /// <summary>
         /// Starts logging data to the SD card.
         /// </summary>

src/Daqifi.Core/Device/SdCard/ISdCardOperations.cs

@@ -34,6 +34,16 @@ public interface ISdCardOperations
         /// <exception cref="SdCardOperationException">Thrown when the device returned an SCPI error that did not match a more specific condition. An empty directory returns an empty list rather than throwing.</exception>
         Task<IReadOnlyList<SdCardFileInfo>> GetSdCardFilesAsync(CancellationToken cancellationToken = default);
 
+        /// <summary>
+        /// Retrieves the free and total byte counts of the device's SD card.
+        /// </summary>
+        /// <param name="cancellationToken">A cancellation token to observe while waiting for the task to complete.</param>
+        /// <returns>A task that represents the asynchronous operation, containing the SD card storage info.</returns>
+        /// <exception cref="System.InvalidOperationException">Thrown when the device is not connected.</exception>
+        /// <exception cref="SdCardNotPresentException">Thrown when no SD card is installed in the device.</exception>
+        /// <exception cref="SdCardOperationException">Thrown when the device returned an SCPI error or an unparseable response.</exception>
+        Task<SdCardStorageInfo> GetSdCardStorageAsync(CancellationToken cancellationToken = default);
+
         /// <summary>
         /// Starts logging data to the SD card.
         /// </summary>