Add extension normalization command

internal/extension/parser.go

@@ -0,0 +1,77 @@
+package extension
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+)
+
+// ParseResult captures normalized CLI arguments for the extension command.
+type ParseResult struct {
+	SourcesCanonical []string
+	SourcesDisplay   []string
+	Duplicates       []string
+	NoOps            []string
+	Target           string
+}
+
+// ParseArgs validates extension command arguments and returns normalized tokens.
+func ParseArgs(args []string) (ParseResult, error) {
+	if len(args) < 2 {
+		return ParseResult{}, errors.New("at least one source extension and a target extension are required")
+	}
+
+	target := NormalizeTargetExtension(args[len(args)-1])
+	if target == "" {
+		return ParseResult{}, errors.New("target extension cannot be empty")
+	}
+	if !strings.HasPrefix(target, ".") {
+		return ParseResult{}, fmt.Errorf("target extension %q must start with '.'", target)
+	}
+	if len(target) == 1 {
+		return ParseResult{}, fmt.Errorf("target extension %q must include characters after '.'", target)
+	}
+
+	rawSources := args[:len(args)-1]
+	trimmedSources := make([]string, len(rawSources))
+	for i, src := range rawSources {
+		trimmed := strings.TrimSpace(src)
+		if trimmed == "" {
+			return ParseResult{}, errors.New("source extensions cannot be empty")
+		}
+		if !strings.HasPrefix(trimmed, ".") {
+			return ParseResult{}, fmt.Errorf("source extension %q must start with '.'", trimmed)
+		}
+		if len(trimmed) == 1 {
+			return ParseResult{}, fmt.Errorf("source extension %q must include characters after '.'", trimmed)
+		}
+		trimmedSources[i] = trimmed
+	}
+
+	canonical, display, duplicates := NormalizeSourceExtensions(trimmedSources)
+	targetCanonical := CanonicalExtension(target)
+
+	filteredCanonical := make([]string, 0, len(canonical))
+	filteredDisplay := make([]string, 0, len(display))
+	noOps := make([]string, 0)
+	for i, canon := range canonical {
+		if canon == targetCanonical {
+			noOps = append(noOps, display[i])
+			continue
+		}
+		filteredCanonical = append(filteredCanonical, canon)
+		filteredDisplay = append(filteredDisplay, display[i])
+	}
+
+	if len(filteredCanonical) == 0 {
+		return ParseResult{}, errors.New("all source extensions match the target extension; provide at least one distinct source extension")
+	}
+
+	return ParseResult{
+		SourcesCanonical: filteredCanonical,
+		SourcesDisplay:   filteredDisplay,
+		Duplicates:       duplicates,
+		NoOps:            noOps,
+		Target:           target,
+	}, nil
+}