feat(cli): add count and select subcommands

count reads row count from layout metadata (no data scan).
select projects columns via ScanOptions and streams CSV to stdout.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dfa1

cli/src/main/java/io/github/dfa1/vortex/cli/CountCommand.java

@@ -0,0 +1,32 @@
+package io.github.dfa1.vortex.cli;
+
+import io.github.dfa1.vortex.io.VortexReader;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+
+final class CountCommand {
+
+    private CountCommand() {
+    }
+
+    static int run(String[] args) {
+        if (args.length != 2) {
+            System.err.println("usage: count <file.vortex>");
+            return 1;
+        }
+        Path path = Path.of(args[1]);
+        if (!Files.exists(path)) {
+            System.err.println("file not found: " + path);
+            return 2;
+        }
+        try (VortexReader reader = VortexReader.open(path)) {
+            System.out.println(reader.layout().rowCount());
+            return 0;
+        } catch (IOException e) {
+            System.err.println("error: " + e.getMessage());
+            return 3;
+        }
+    }
+}

cli/src/main/java/io/github/dfa1/vortex/cli/SelectCommand.java

@@ -0,0 +1,42 @@
+package io.github.dfa1.vortex.cli;
+
+import io.github.dfa1.vortex.csv.CsvExporter;
+import io.github.dfa1.vortex.csv.ExportOptions;
+
+import java.io.IOException;
+import java.io.OutputStreamWriter;
+import java.io.Writer;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.Arrays;
+import java.util.List;
+
+final class SelectCommand {
+
+    private SelectCommand() {
+    }
+
+    static int run(String[] args) {
+        if (args.length < 3) {
+            System.err.println("usage: select <file.vortex> <col1> [col2 ...]");
+            return 1;
+        }
+        Path path = Path.of(args[1]);
+        if (!Files.exists(path)) {
+            System.err.println("file not found: " + path);
+            return 2;
+        }
+        List<String> columns = Arrays.asList(args).subList(2, args.length);
+        ExportOptions options = ExportOptions.defaults().withColumns(columns);
+        try {
+            Writer stdout = new OutputStreamWriter(System.out, StandardCharsets.UTF_8);
+            CsvExporter.exportCsv(path, stdout, options);
+            stdout.flush();
+            return 0;
+        } catch (IOException e) {
+            System.err.println("error: " + e.getMessage());
+            return 3;
+        }
+    }
+}

cli/src/main/java/io/github/dfa1/vortex/cli/VortexCli.java

@@ -20,6 +20,8 @@ public static void main(String[] args) {
             case "export"  -> ExportCommand.run(args);
             case "import"  -> ImportCommand.run(args);
             case "schema"  -> SchemaCommand.run(args);
+            case "count"   -> CountCommand.run(args);
+            case "select"  -> SelectCommand.run(args);
             default -> {
                 System.err.println("unknown subcommand: " + args[0]);
                 printUsage(System.err);
@@ -31,9 +33,11 @@ public static void main(String[] args) {
 
     static void printUsage(PrintStream out) {
         out.println("Usage: java -jar vortex.jar <subcommand> [args]");
-        out.println("  inspect <file.vortex>            print file structure");
-        out.println("  export  <file.vortex>            write CSV to stdout");
-        out.println("  import  <file.csv> [out.vortex]  convert CSV to Vortex");
-        out.println("  schema  <file.vortex>            print dtype (machine-readable)");
+        out.println("  inspect <file.vortex>               print file structure");
+        out.println("  export  <file.vortex>               write CSV to stdout");
+        out.println("  import  <file.csv> [out.vortex]     convert CSV to Vortex");
+        out.println("  schema  <file.vortex>               print dtype (machine-readable)");
+        out.println("  count   <file.vortex>               print row count");
+        out.println("  select  <file.vortex> <col> [...]   project columns to CSV on stdout");
     }
 }

csv/src/main/java/io/github/dfa1/vortex/csv/CsvExporter.java

@@ -66,15 +66,18 @@ private static void export(VortexReader reader, CsvWriter csvWriter, ExportOptio
         if (!(reader.dtype() instanceof DType.Struct schema)) {
             throw new VortexException("only struct root dtype supported for CSV export");
         }
-        List<String> colNames = schema.fieldNames();
+        List<String> colNames = options.hasProjection() ? options.columns() : schema.fieldNames();
         int colCount = colNames.size();
 
         if (options.writeHeader()) {
             csvWriter.writeRecord(colNames);
         }
 
+        ScanOptions scanOptions = options.hasProjection()
+                ? new ScanOptions(colNames, null, ScanOptions.NO_LIMIT)
+                : ScanOptions.all();
         String[] row = new String[colCount];
-        try (ScanIterator iter = reader.scan(ScanOptions.all())) {
+        try (ScanIterator iter = reader.scan(scanOptions)) {
             while (iter.hasNext()) {
                 ScanResult chunk = iter.next();
                 Array[] arrays = new Array[colCount];

csv/src/main/java/io/github/dfa1/vortex/csv/ExportOptions.java

@@ -1,11 +1,23 @@
 package io.github.dfa1.vortex.csv;
 
+import java.util.List;
+
 /// Options controlling Vortex → CSV export.
 public record ExportOptions(
         char delimiter,
-        boolean writeHeader
+        boolean writeHeader,
+        List<String> columns
 ) {
     public static ExportOptions defaults() {
-        return new ExportOptions(',', true);
+        return new ExportOptions(',', true, List.of());
+    }
+
+    /// Restrict output to specific columns (projection). Empty list = all columns.
+    public ExportOptions withColumns(List<String> cols) {
+        return new ExportOptions(delimiter, writeHeader, List.copyOf(cols));
+    }
+
+    public boolean hasProjection() {
+        return !columns.isEmpty();
     }
 }

csv/src/test/java/io/github/dfa1/vortex/csv/CsvExporterTest.java

@@ -84,7 +84,7 @@ void suppressesHeaderWhenConfigured(@TempDir Path tmp) throws Exception {
         Path csv = tmp.resolve("out.csv");
 
         // When
-        CsvExporter.exportCsv(vortex, csv, new ExportOptions(',', false));
+        CsvExporter.exportCsv(vortex, csv, new ExportOptions(',', false, java.util.List.of()));
 
         // Then
         List<String> lines = Files.readAllLines(csv);