fix: parquet negative DECIMAL loses sign (Issue #234) (#243)

Author: vmvarela

build.zig

@@ -2874,6 +2874,29 @@ pub fn build(b: *std.Build) void {
     const run_loader_unit_tests = b.addRunArtifact(loader_unit_tests);
     unit_test_step.dependOn(&run_loader_unit_tests.step);
 
+    // Unit tests for the Parquet loader (src/parquet.zig) — decimalToText sign handling
+    const parquet_unit_tests = b.addTest(.{
+        .root_module = b.createModule(.{
+            .root_source_file = b.path("src/parquet.zig"),
+            .target = target,
+            .optimize = optimize,
+            .link_libc = true,
+        }),
+    });
+    parquet_unit_tests.root_module.addImport("c", translate_c.createModule());
+    parquet_unit_tests.root_module.addImport("zig_parquet", zig_parquet.module("parquet"));
+    if (bundle_sqlite) {
+        parquet_unit_tests.root_module.addIncludePath(b.path("lib"));
+        parquet_unit_tests.root_module.addCSourceFile(.{
+            .file = b.path("lib/sqlite3.c"),
+            .flags = &.{ "-DSQLITE_OMIT_LOAD_EXTENSION=1", "-DSQLITE_ENABLE_MATH_FUNCTIONS=1" },
+        });
+    } else {
+        parquet_unit_tests.root_module.linkSystemLibrary("sqlite3", .{});
+    }
+    const run_parquet_unit_tests = b.addRunArtifact(parquet_unit_tests);
+    unit_test_step.dependOn(&run_parquet_unit_tests.step);
+
     // ─── --stats / --profile integration tests ──────────────────────────
 
     // Integration test: --stats on basic CSV with mixed types

src/parquet.zig

@@ -91,13 +91,19 @@ fn decimalToText(value: i64, scale: i32, buf: *[64]u8) []const u8 {
         // ponytail: manual int-to-text avoids bufPrint FixedWriter issues
         return intToBuf(value, buf);
     }
-    var pow10: i64 = 1;
+    var pow10: u64 = 1;
     for (0..@as(usize, @intCast(scale))) |_| pow10 *= 10;
-    const int_part = @divTrunc(value, pow10);
-    const prefix = intToBuf(int_part, buf);
+    // ponytail: sign tracked independently (Issue #234) — @divTrunc(-1, 100)
+    // is 0, so intToBuf(int_part) alone drops the sign of |value| < pow10
+    const neg = value < 0;
+    const mag: u64 = @intCast(@abs(value));
+    if (neg) buf[0] = '-';
+    const off: usize = @intFromBool(neg);
+    const digits = intToBuf(@as(i64, @intCast(mag / pow10)), buf[off..]);
+    const prefix = buf[0 .. off + digits.len];
     buf[prefix.len] = '.';
     const dot_pos = prefix.len + 1;
-    var f = @as(u64, @intCast(@abs(@rem(value, pow10))));
+    var f = mag % pow10;
     var pos: usize = @intCast(scale);
     while (pos > 0) {
         pos -= 1;
@@ -137,6 +143,15 @@ fn intToBuf(value: i64, buf: []u8) []const u8 {
     return buf[0..end];
 }
 
+test "decimalToText: negative sub-unit value keeps its sign (Issue #234)" {
+    var buf: [64]u8 = undefined;
+    try std.testing.expectEqualStrings("-0.01", decimalToText(-1, 2, &buf));
+    try std.testing.expectEqualStrings("0.01", decimalToText(1, 2, &buf));
+    try std.testing.expectEqualStrings("-123.45", decimalToText(-12345, 2, &buf));
+    try std.testing.expectEqualStrings("-5", decimalToText(-5, 0, &buf));
+    try std.testing.expectEqualStrings("0.00", decimalToText(0, 2, &buf));
+}
+
 /// Map a Parquet physical type to a SQLite ColumnType (no logical type mapping).
 fn physicalToAffinity(phys: parquet.format.PhysicalType) sqlite_mod.ColumnType {
     return switch (phys) {