fix: ColumnType.Conflicts for Decimal/Nullable/Array/LowCardinality

1. `a,b=b,a` would leave `b==c`, always returning false
2. fix Decimal256 conflicting with Decimal(76, 38)
3. propagate conflicts to inner types for Nullable/Array/LowCardinality

proto/column.go

@@ -2,6 +2,7 @@ package proto
 
 import (
 	"fmt"
+	"strconv"
 	"strings"
 
 	"github.com/go-faster/errors"
@@ -83,30 +84,58 @@ func (c ColumnType) Base() ColumnType {
 	return c[:start]
 }
 
+// reduces Decimal(P, ...) to Decimal32/Decimal64/Decimal128/Decimal256
+// returns c if any errors occur during conversion
+func (c ColumnType) decimalDowncast() ColumnType {
+	if c.Base() != ColumnTypeDecimal {
+		return c
+	}
+	elem := c.Elem()
+	precStr, _, _ := strings.Cut(string(elem), ",")
+	precStr = strings.TrimSpace(precStr)
+	prec, err := strconv.Atoi(precStr)
+	if err != nil {
+		return c
+	}
+	switch {
+	case prec < 10:
+		return ColumnTypeDecimal32
+	case prec < 19:
+		return ColumnTypeDecimal64
+	case prec < 39:
+		return ColumnTypeDecimal128
+	case prec < 77:
+		return ColumnTypeDecimal256
+	default:
+		return c
+	}
+}
+
 // Conflicts reports whether two types conflict.
 func (c ColumnType) Conflicts(b ColumnType) bool {
 	if c == b {
 		return false
 	}
-	{
-		a := c
-		if b.Base() == ColumnTypeEnum8 || b.Base() == ColumnTypeEnum16 {
-			a, b = b, a
-		}
-		switch {
-		case a.Base() == ColumnTypeEnum8 && b == ColumnTypeInt8:
-			return false
-		case a.Base() == ColumnTypeEnum16 && b == ColumnTypeInt16:
-			return false
-		}
+	cBase := c.Base()
+	bBase := b.Base()
+	if (cBase == ColumnTypeEnum8 && b == ColumnTypeInt8) ||
+		(cBase == ColumnTypeEnum16 && b == ColumnTypeInt16) ||
+		(bBase == ColumnTypeEnum8 && c == ColumnTypeInt8) ||
+		(bBase == ColumnTypeEnum16 && c == ColumnTypeInt16) {
+		return false
+	}
+	if cBase == ColumnTypeDecimal || bBase == ColumnTypeDecimal {
+		return c.decimalDowncast() != b.decimalDowncast()
 	}
-	if c.Base() != b.Base() {
+	if cBase != bBase {
 		return true
 	}
 	if c.normalizeCommas() == b.normalizeCommas() {
 		return false
 	}
-	switch c.Base() {
+	switch cBase {
+	case ColumnTypeArray, ColumnTypeNullable, ColumnTypeLowCardinality:
+		return c.Elem().Conflicts(b.Elem())
 	case ColumnTypeDateTime, ColumnTypeDateTime64:
 		// TODO(ernado): improve check
 		return false

proto/column_test.go

@@ -60,6 +60,8 @@ func TestColumnType_Elem(t *testing.T) {
 				{A: "Map(String,String)", B: "Map(String, String)"},
 				{A: "Enum8('increment' = 1, 'gauge' = 2)", B: "Int8"},
 				{A: "Int8", B: "Enum8('increment' = 1, 'gauge' = 2)"},
+				{A: "Decimal256", B: "Decimal(76, 38)"},
+				{A: "Nullable(Decimal256)", B: "Nullable(Decimal(76, 38))"},
 			} {
 				assert.False(t, tt.A.Conflicts(tt.B),
 					"%s ~ %s", tt.A, tt.B,
@@ -76,6 +78,7 @@ func TestColumnType_Elem(t *testing.T) {
 				{A: ColumnTypeArray.Sub(ColumnTypeInt32), B: ColumnTypeArray.Sub(ColumnTypeInt64)},
 				{A: "Map(String,String)", B: "Map(String,Int32)"},
 				{A: "Enum16('increment' = 1, 'gauge' = 2)", B: "Int8"},
+				{A: "Int8", B: "Enum16('increment' = 1, 'gauge' = 2)"},
 			} {
 				assert.True(t, tt.A.Conflicts(tt.B),
 					"%s !~ %s", tt.A, tt.B,