Support thousands separator in fmtnum

pkg/dsl/cst/builtin_function_manager.go

@@ -2000,10 +2000,11 @@ Note that NaN has the property that NaN != NaN, so you need 'is_nan(x)' rather t
 			name:  "fmtnum",
 			class: FUNC_CLASS_CONVERSION,
 			help: `Convert int/float/bool to string using printf-style format string (https://pkg.go.dev/fmt), e.g.
-'$s = fmtnum($n, "%08d")' or '$t = fmtnum($n, "%.6e")'. This function recurses on array and map values.`,
+'$s = fmtnum($n, "%08d")' or '$t = fmtnum($n, "%.6e")'. Miller-specific extension: "%_d" for comma-separated thousands. This function recurses on array and map values.`,
 			binaryFunc: bifs.BIF_fmtnum,
 			examples: []string{
-				`$x = fmtnum($x, "%.6f")`,
+				`$y = fmtnum($x, "%.6f")`,
+				`$o = fmtnum($n, "%_d")`,
 			},
 		},
 

pkg/mlrval/mlrval_format.go

@@ -4,6 +4,8 @@ import (
 	"fmt"
 	"strconv"
 	"strings"
+
+	"golang.org/x/text/message"
 )
 
 //----------------------------------------------------------------
@@ -103,9 +105,13 @@ func newFormatter(
 	goFormatString = strings.ReplaceAll(goFormatString, "le", "e")
 	goFormatString = strings.ReplaceAll(goFormatString, "lg", "g")
 
-	// MIller 5 and below required C format strings compatible with 64-bit ints
+	// Miller 5 and below required C format strings compatible with 64-bit ints
 	// and double-precision floats: e.g. "%08lld" and "%9.6lf". For Miller 6,
-	// We must still accept these for backward compatibility.
+	// we must still accept these for backward compatibility.
+	if strings.HasSuffix(goFormatString, "%_d") {
+		// Special sub-case of "d"; must be checked first
+		return newFormatterToSeparatedInt(goFormatString), nil
+	}
 	if strings.HasSuffix(goFormatString, "d") {
 		return newFormatterToInt(goFormatString), nil
 	}
@@ -164,6 +170,38 @@ func (formatter *formatterToFloat) FormatFloat(floatValue float64) string {
 
 // ----------------------------------------------------------------
 
+type formatterToSeparatedInt struct {
+	formatString string
+	printer      *message.Printer
+}
+
+func newFormatterToSeparatedInt(goFormatString string) IFormatter {
+	return &formatterToSeparatedInt{
+		formatString: "%d",
+		printer:      message.NewPrinter(message.MatchLanguage("en")),
+	}
+}
+
+func (formatter *formatterToSeparatedInt) Format(mv *Mlrval) *Mlrval {
+	intValue, isInt := mv.GetIntValue()
+	if isInt {
+		formatted := formatter.printer.Sprintf(formatter.formatString, intValue)
+		return TryFromIntString(formatted)
+	}
+	floatValue, isFloat := mv.GetFloatValue()
+	if isFloat {
+		formatted := formatter.printer.Sprintf(formatter.formatString, int(floatValue))
+		return TryFromIntString(formatted)
+	}
+	return mv
+}
+
+func (formatter *formatterToSeparatedInt) FormatFloat(floatValue float64) string {
+	return formatter.printer.Sprintf(formatter.formatString, int(floatValue))
+}
+
+// ----------------------------------------------------------------
+
 type formatterToInt struct {
 	goFormatString string
 }