feat: output system — table/json/yaml/raw printers with struct tag driven tables

internal/output/table.go

@@ -0,0 +1,120 @@
+package output
+
+import (
+	"fmt"
+	"io"
+	"reflect"
+	"strings"
+)
+
+func printTable(w io.Writer, data any) error {
+	v := reflect.ValueOf(data)
+	if v.Kind() == reflect.Ptr {
+		v = v.Elem()
+	}
+	if v.Kind() == reflect.Slice {
+		return printTableSlice(w, v)
+	}
+	return printTableSingle(w, v)
+}
+
+func printTableSlice(w io.Writer, v reflect.Value) error {
+	if v.Len() == 0 {
+		fmt.Fprintln(w, "(empty)")
+		return nil
+	}
+	elemType := v.Index(0).Type()
+	if elemType.Kind() == reflect.Ptr {
+		elemType = elemType.Elem()
+	}
+	headers, cols := extractTableTags(elemType)
+	if len(headers) == 0 {
+		return fmt.Errorf("no table tags found on type %s", elemType.Name())
+	}
+
+	// Collect all rows
+	rows := make([][]string, v.Len())
+	for i := 0; i < v.Len(); i++ {
+		rows[i] = extractRow(v.Index(i), cols)
+	}
+
+	// Calculate column widths
+	widths := make([]int, len(headers))
+	for i, h := range headers {
+		widths[i] = len(h)
+	}
+	for _, row := range rows {
+		for i, cell := range row {
+			if len(cell) > widths[i] {
+				widths[i] = len(cell)
+			}
+		}
+	}
+
+	// Print headers
+	sep := "  "
+	renderRow(w, headers, widths, sep)
+
+	// Print separator line
+	separator := make([]string, len(headers))
+	for i, ww := range widths {
+		separator[i] = strings.Repeat("-", ww)
+	}
+	renderRow(w, separator, widths, sep)
+
+	// Print data rows
+	for _, row := range rows {
+		renderRow(w, row, widths, sep)
+	}
+
+	return nil
+}
+
+func renderRow(w io.Writer, cells []string, widths []int, sep string) {
+	parts := make([]string, len(cells))
+	for i, cell := range cells {
+		parts[i] = fmt.Sprintf("%-*s", widths[i], cell)
+	}
+	fmt.Fprintln(w, strings.Join(parts, sep))
+}
+
+func printTableSingle(w io.Writer, v reflect.Value) error {
+	if v.Kind() == reflect.Ptr {
+		v = v.Elem()
+	}
+	t := v.Type()
+	for i := 0; i < t.NumField(); i++ {
+		tag := t.Field(i).Tag.Get("table")
+		if tag == "" {
+			continue
+		}
+		val := fmt.Sprintf("%v", v.Field(i).Interface())
+		fmt.Fprintf(w, "%-10s %s\n", tag+":", val)
+	}
+	return nil
+}
+
+func extractTableTags(t reflect.Type) ([]string, []int) {
+	var headers []string
+	var cols []int
+	for i := 0; i < t.NumField(); i++ {
+		tag := t.Field(i).Tag.Get("table")
+		if tag == "" {
+			continue
+		}
+		headers = append(headers, strings.ToUpper(tag))
+		cols = append(cols, i)
+	}
+	return headers, cols
+}
+
+func extractRow(v reflect.Value, cols []int) []string {
+	if v.Kind() == reflect.Ptr {
+		v = v.Elem()
+	}
+	row := make([]string, len(cols))
+	for i, idx := range cols {
+		row[i] = fmt.Sprintf("%v", v.Field(idx).Interface())
+	}
+	return row
+}