1 files changed, 124 insertions, 0 deletions
diff --git a/internal/passgen/passgen.go b/internal/passgen/passgen.go
new file mode 100644
index 0000000..1c3241d
--- /dev/null
+++ b/internal/passgen/passgen.go
@@ -0,0 +1,124 @@
+package passgen
+
+import (
+	"crypto/rand"
+	"errors"
+	"fmt"
+	"io"
+	"math"
+)
+
+const (
+	lowerChars  = "abcdefghijklmnopqrstuvwxyz"
+	upperChars  = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
+	digitChars  = "0123456789"
+	symbolChars = "!@#$%^&*()-_=+[]{};:,.?/"
+)
+
+var ErrEmptyAlphabet = errors.New("alphabet is empty")
+
+type Options struct {
+	Length     int
+	UseLower   bool
+	UseUpper   bool
+	UseDigits  bool
+	UseSymbols bool
+	Random     io.Reader
+}
+
+func DefaultOptions() Options {
+	return Options{
+		Length:     24,
+		UseLower:   true,
+		UseUpper:   true,
+		UseDigits:  true,
+		UseSymbols: true,
+	}
+}
+
+func Generate(opts Options) (string, error) {
+	if opts.Length <= 0 {
+		return "", fmt.Errorf("length must be positive")
+	}
+
+	alphabet := buildAlphabet(opts)
+	if alphabet == "" {
+		return "", ErrEmptyAlphabet
+	}
+
+	random := opts.Random
+	if random == nil {
+		random = rand.Reader
+	}
+
+	password := make([]byte, opts.Length)
+	for i := range password {
+		n, err := randomIndex(len(alphabet), random)
+		if err != nil {
+			return "", fmt.Errorf("choose password character: %w", err)
+		}
+		password[i] = alphabet[n]
+	}
+
+	return string(password), nil
+}
+
+func EntropyBits(length int, alphabetSize int) (float64, error) {
+	if length <= 0 {
+		return 0, fmt.Errorf("length must be positive")
+	}
+	if alphabetSize <= 1 {
+		return 0, fmt.Errorf("alphabet size must be greater than 1")
+	}
+
+	return float64(length) * math.Log2(float64(alphabetSize)), nil
+}
+
+func AlphabetSize(opts Options) int {
+	return len(buildAlphabet(opts))
+}
+
+func buildAlphabet(opts Options) string {
+	alphabet := ""
+	if opts.UseLower {
+		alphabet += lowerChars
+	}
+	if opts.UseUpper {
+		alphabet += upperChars
+	}
+	if opts.UseDigits {
+		alphabet += digitChars
+	}
+	if opts.UseSymbols {
+		alphabet += symbolChars
+	}
+	return alphabet
+}
+
+func randomIndex(size int, random io.Reader) (int, error) {
+	if size <= 0 {
+		return 0, ErrEmptyAlphabet
+	}
+	if size > 256 {
+		return 0, fmt.Errorf("alphabet size %d exceeds supported maximum 256", size)
+	}
+
+	cutoff := 256 - (256 % size)
+	for {
+		value, err := randomByte(random)
+		if err != nil {
+			return 0, err
+		}
+		if int(value) < cutoff {
+			return int(value) % size, nil
+		}
+	}
+}
+
+func randomByte(random io.Reader) (byte, error) {
+	var b [1]byte
+	if _, err := io.ReadFull(random, b[:]); err != nil {
+		return 0, fmt.Errorf("read random byte: %w", err)
+	}
+	return b[0], nil
+}