feat(nesutil): Add subcommand to decode Game Genie codes

cmd/nesutil/genie/cmd.go

@@ -0,0 +1,15 @@
+package genie
+
+import (
+	"gabe565.com/gones/cmd/nesutil/genie/decode"
+	"github.com/spf13/cobra"
+)
+
+func New() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "genie",
+		Short: "Game Genie code utilities",
+	}
+	cmd.AddCommand(decode.New())
+	return cmd
+}

cmd/nesutil/genie/decode/cmd.go

@@ -0,0 +1,111 @@
+package decode
+
+import (
+	"errors"
+	"fmt"
+	"strings"
+	"text/tabwriter"
+
+	"github.com/spf13/cobra"
+)
+
+func New() *cobra.Command {
+	cmd := &cobra.Command{
+		Use:   "decode code...",
+		Short: "Decode a Game Genie code",
+		Args:  cobra.MinimumNArgs(1),
+		RunE:  run,
+	}
+	return cmd
+}
+
+func run(cmd *cobra.Command, args []string) error {
+	cmd.SilenceUsage = true
+
+	w := tabwriter.NewWriter(cmd.OutOrStdout(), 0, 0, 3, ' ', 0)
+	if _, err := fmt.Fprintln(w, "CODE\tCPU ADDRESS\tREPLACE VALUE\tCOMPARE VALUE\t"); err != nil {
+		return err
+	}
+
+	var errs []error
+	for _, c := range args {
+		result, err := decode(c)
+		if err != nil {
+			errs = append(errs, err)
+			continue
+		}
+
+		if _, err := fmt.Fprintf(w,
+			"%s\t0x%04X\t0x%02X\t%s\t\n",
+			result.Code, result.Address, result.Replace, result.compareString(),
+		); err != nil {
+			return err
+		}
+	}
+
+	if err := w.Flush(); err != nil {
+		return err
+	}
+
+	return errors.Join(errs...)
+}
+
+type decodeResult struct {
+	Code    string
+	Address int
+	Replace int
+	Compare *int
+}
+
+var (
+	ErrInvalidCodeLen   = errors.New("invalid length")
+	ErrInvalidCharacter = errors.New("invalid character")
+)
+
+func decode(code string) (decodeResult, error) {
+	code = strings.ToUpper(code)
+	result := decodeResult{Code: code}
+
+	switch len(code) {
+	case 6, 8:
+	default:
+		return result, fmt.Errorf("%w %d in code %q; expected 6 or 8 characters", ErrInvalidCodeLen, len(code), code)
+	}
+
+	// Convert letters into integers (0x0-0xF)
+	const lookup = "APZLGITYEOXUKSVN"
+	codeValues := make([]int, 0, len(code))
+	for _, r := range code {
+		index := strings.IndexRune(lookup, r)
+		if index == -1 {
+			return result, fmt.Errorf("%w %q in code %q", ErrInvalidCharacter, r, code)
+		}
+		codeValues = append(codeValues, index)
+	}
+
+	// 24/32 bits (16 for address, 8 for replacement, 0 or 8 for compare)
+	var bigint int
+	loPosOrder := []int{3, 5, 2, 4, 1, 0, 7, 6}
+	for _, loPos := range loPosOrder[:len(code)] {
+		hiPos := (loPos - 1 + len(code)) % len(code)
+		bigint = (bigint << 4) | (codeValues[hiPos] & 8) | (codeValues[loPos] & 7)
+	}
+
+	// Split integer and set MSB of address
+	if len(code) == 8 {
+		compValue := bigint & 0xFF
+		result.Compare = &compValue
+		bigint >>= 8
+	}
+
+	result.Address = (bigint >> 8) | 0x8000
+	result.Replace = bigint & 0xFF
+	return result, nil
+}
+
+func (d decodeResult) compareString() string {
+	if d.Compare == nil {
+		return "<none>"
+	}
+	return fmt.Sprintf("0x%02X", *d.Compare)
+}

cmd/nesutil/root/cmd.go

@@ -2,6 +2,7 @@ package root
 
 import (
 	"gabe565.com/gones/cmd/nesutil/chr"
+	"gabe565.com/gones/cmd/nesutil/genie"
 	"gabe565.com/gones/cmd/nesutil/ines"
 	"gabe565.com/gones/cmd/nesutil/ls"
 	"gabe565.com/gones/cmd/options"
@@ -16,7 +17,7 @@ func New(opts ...options.Option) *cobra.Command {
 		SilenceErrors:     true,
 		DisableAutoGenTag: true,
 	}
-	cmd.AddCommand(ls.New(), ines.New(), chr.New())
+	cmd.AddCommand(ls.New(), ines.New(), chr.New(), genie.New())
 
 	for _, opt := range opts {
 		opt(cmd)

docs/nesutil.md

@@ -11,6 +11,7 @@ GoNES command-line utilities
 ### SEE ALSO
 
 * [nesutil chr](nesutil_chr.md)	 - CHR graphics data utilities
+* [nesutil genie](nesutil_genie.md)	 - Game Genie code utilities
 * [nesutil ines](nesutil_ines.md)	 - INES ROM utilities
 * [nesutil ls](nesutil_ls.md)	 - List ROM files and metadata
 

docs/nesutil_genie.md

@@ -0,0 +1,15 @@
+## nesutil genie
+
+Game Genie code utilities
+
+### Options
+
+```
+  -h, --help   help for genie
+```
+
+### SEE ALSO
+
+* [nesutil](nesutil.md)	 - GoNES command-line utilities
+* [nesutil genie decode](nesutil_genie_decode.md)	 - Decode a Game Genie code
+

docs/nesutil_genie_decode.md

@@ -0,0 +1,18 @@
+## nesutil genie decode
+
+Decode a Game Genie code
+
+```
+nesutil genie decode code... [flags]
+```
+
+### Options
+
+```
+  -h, --help   help for decode
+```
+
+### SEE ALSO
+
+* [nesutil genie](nesutil_genie.md)	 - Game Genie code utilities
+