feat(sqlite): add SQLite writer for converting PostgreSQL schemas

- Implement SQLite DDL writer to convert PostgreSQL schemas to SQLite-compatible SQL statements.
- Include automatic schema flattening, type mapping, auto-increment detection, and function translation.
- Add templates for creating tables, indexes, unique constraints, check constraints, and foreign keys.
- Implement tests for writer functionality and data type mapping.

pkg/writers/sqlite/template_functions.go

@@ -0,0 +1,146 @@
+package sqlite
+
+import (
+	"fmt"
+	"strings"
+	"text/template"
+
+	"git.warky.dev/wdevs/relspecgo/pkg/models"
+	"git.warky.dev/wdevs/relspecgo/pkg/writers"
+)
+
+// GetTemplateFuncs returns template functions for SQLite SQL generation
+func GetTemplateFuncs(opts *writers.WriterOptions) template.FuncMap {
+	return template.FuncMap{
+		"quote_ident":      QuoteIdentifier,
+		"map_type":         MapPostgreSQLType,
+		"is_autoincrement": IsAutoIncrementCandidate,
+		"qualified_table_name": func(schema, table string) string {
+			return writers.QualifiedTableName(schema, table, opts.FlattenSchema)
+		},
+		"format_default": FormatDefault,
+		"format_constraint_name": func(schema, table, constraint string) string {
+			return FormatConstraintName(schema, table, constraint, opts)
+		},
+		"join":  strings.Join,
+		"lower": strings.ToLower,
+		"upper": strings.ToUpper,
+	}
+}
+
+// QuoteIdentifier quotes an identifier for SQLite (double quotes)
+func QuoteIdentifier(name string) string {
+	// SQLite uses double quotes for identifiers
+	// Escape any existing double quotes by doubling them
+	escaped := strings.ReplaceAll(name, `"`, `""`)
+	return fmt.Sprintf(`"%s"`, escaped)
+}
+
+// IsAutoIncrementCandidate checks if a column should use AUTOINCREMENT
+func IsAutoIncrementCandidate(col *models.Column) bool {
+	// Must be a primary key
+	if !col.IsPrimaryKey {
+		return false
+	}
+
+	// Must be an integer type
+	if !IsIntegerType(col.Type) {
+		return false
+	}
+
+	// Check AutoIncrement field
+	if col.AutoIncrement {
+		return true
+	}
+
+	// Check if default suggests auto-increment
+	if col.Default != nil {
+		defaultStr, ok := col.Default.(string)
+		if ok {
+			defaultLower := strings.ToLower(defaultStr)
+			if strings.Contains(defaultLower, "nextval") ||
+				strings.Contains(defaultLower, "autoincrement") ||
+				strings.Contains(defaultLower, "auto_increment") {
+				return true
+			}
+		}
+	}
+
+	// Serial types are auto-increment
+	typeLower := strings.ToLower(col.Type)
+	return strings.Contains(typeLower, "serial")
+}
+
+// FormatDefault formats a default value for SQLite
+func FormatDefault(col *models.Column) string {
+	if col.Default == nil {
+		return ""
+	}
+
+	// Skip auto-increment defaults (handled by AUTOINCREMENT keyword)
+	if IsAutoIncrementCandidate(col) {
+		return ""
+	}
+
+	// Convert to string
+	defaultStr, ok := col.Default.(string)
+	if !ok {
+		// If not a string, convert to string representation
+		defaultStr = fmt.Sprintf("%v", col.Default)
+	}
+
+	if defaultStr == "" {
+		return ""
+	}
+
+	// Convert PostgreSQL-specific functions to SQLite equivalents
+	defaultLower := strings.ToLower(defaultStr)
+
+	// Current timestamp functions
+	if strings.Contains(defaultLower, "current_timestamp") ||
+		strings.Contains(defaultLower, "now()") {
+		return "CURRENT_TIMESTAMP"
+	}
+
+	// Current date
+	if strings.Contains(defaultLower, "current_date") {
+		return "CURRENT_DATE"
+	}
+
+	// Current time
+	if strings.Contains(defaultLower, "current_time") {
+		return "CURRENT_TIME"
+	}
+
+	// Boolean values
+	sqliteType := MapPostgreSQLType(col.Type)
+	if sqliteType == TypeInteger {
+		typeLower := strings.ToLower(col.Type)
+		if strings.Contains(typeLower, "bool") {
+			return MapBooleanValue(defaultStr)
+		}
+	}
+
+	// UUID generation - SQLite doesn't have built-in UUID, comment it out
+	if strings.Contains(defaultLower, "uuid") || strings.Contains(defaultLower, "gen_random_uuid") {
+		return "" // Remove UUID defaults, users must handle this
+	}
+
+	// Remove PostgreSQL-specific casting
+	defaultStr = strings.ReplaceAll(defaultStr, "::text", "")
+	defaultStr = strings.ReplaceAll(defaultStr, "::integer", "")
+	defaultStr = strings.ReplaceAll(defaultStr, "::bigint", "")
+	defaultStr = strings.ReplaceAll(defaultStr, "::boolean", "")
+
+	return defaultStr
+}
+
+// FormatConstraintName formats a constraint name with table prefix if flattening
+func FormatConstraintName(schema, table, constraint string, opts *writers.WriterOptions) string {
+	if opts.FlattenSchema && schema != "" {
+		// Prefix constraint with flattened table name
+		flatTable := writers.QualifiedTableName(schema, table, opts.FlattenSchema)
+		return fmt.Sprintf("%s_%s", flatTable, constraint)
+	}
+	return constraint
+}