From 3e8be112fe4f35c24d5b9eb81779f3f7216140fa Mon Sep 17 00:00:00 2001
From: Winni Neessen <winfried.neessen@cleverbridge.com>
Date: Sun, 24 Oct 2021 20:13:37 +0200
Subject: [PATCH] Updated GetChar() method to use a much faster and optimized
 algorithm

---
 .idea/workspace.xml                           | 10 ++--
 cmd/apg/apg.go                                |  2 +-
 cmd/apg/apg_test.go                           | 12 ++---
 .../simple-password-generator/main.go         |  2 +-
 random/random.go                              | 48 ++++++++++++++-----
 5 files changed, 50 insertions(+), 24 deletions(-)

diff --git a/.idea/workspace.xml b/.idea/workspace.xml
index 5ef3a48..7178bd0 100644
--- a/.idea/workspace.xml
+++ b/.idea/workspace.xml
@@ -4,9 +4,12 @@
     <option name="autoReloadType" value="ALL" />
   </component>
   <component name="ChangeListManager">
-    <list default="true" id="e32960c0-29e5-4669-9fc2-ef12314486ce" name="Changes" comment="Updated README.md and added code-example for programmatic use">
+    <list default="true" id="e32960c0-29e5-4669-9fc2-ef12314486ce" name="Changes" comment="Some parts of the Config struct don't need exporting">
       <change beforePath="$PROJECT_DIR$/.idea/workspace.xml" beforeDir="false" afterPath="$PROJECT_DIR$/.idea/workspace.xml" afterDir="false" />
-      <change beforePath="$PROJECT_DIR$/config/config.go" beforeDir="false" afterPath="$PROJECT_DIR$/config/config.go" afterDir="false" />
+      <change beforePath="$PROJECT_DIR$/cmd/apg/apg.go" beforeDir="false" afterPath="$PROJECT_DIR$/cmd/apg/apg.go" afterDir="false" />
+      <change beforePath="$PROJECT_DIR$/cmd/apg/apg_test.go" beforeDir="false" afterPath="$PROJECT_DIR$/cmd/apg/apg_test.go" afterDir="false" />
+      <change beforePath="$PROJECT_DIR$/example-code/simple-password-generator/main.go" beforeDir="false" afterPath="$PROJECT_DIR$/example-code/simple-password-generator/main.go" afterDir="false" />
+      <change beforePath="$PROJECT_DIR$/random/random.go" beforeDir="false" afterPath="$PROJECT_DIR$/random/random.go" afterDir="false" />
     </list>
     <option name="SHOW_DIALOG" value="false" />
     <option name="HIGHLIGHT_CONFLICTS" value="true" />
@@ -107,7 +110,8 @@
     <MESSAGE value="More README.md updates" />
     <MESSAGE value="Updated CLI usage text to reflect the changes of #27" />
     <MESSAGE value="Updated README.md and added code-example for programmatic use" />
-    <option name="LAST_COMMIT_MESSAGE" value="Updated README.md and added code-example for programmatic use" />
+    <MESSAGE value="Some parts of the Config struct don't need exporting" />
+    <option name="LAST_COMMIT_MESSAGE" value="Some parts of the Config struct don't need exporting" />
   </component>
   <component name="VgoProject">
     <integration-enabled>true</integration-enabled>
diff --git a/cmd/apg/apg.go b/cmd/apg/apg.go
index b72aa78..78561d9 100644
--- a/cmd/apg/apg.go
+++ b/cmd/apg/apg.go
@@ -102,7 +102,7 @@ func main() {
 			sylList[pwString] = pwSyls
 		default:
 			charRange := chars.GetRange(&cfgObj)
-			pwString, err := random.GetChar(&charRange, pwLength)
+			pwString, err := random.GetChar(charRange, pwLength)
 			if err != nil {
 				log.Fatalf("error generating random character range: %s\n", err)
 			}
diff --git a/cmd/apg/apg_test.go b/cmd/apg/apg_test.go
index 7cd41ff..a9febf0 100644
--- a/cmd/apg/apg_test.go
+++ b/cmd/apg/apg_test.go
@@ -77,7 +77,7 @@ func TestGenLength(t *testing.T) {
 			cfgObj.MaxPassLen = testCase.maxLength
 			pwLength := config.GetPwLengthFromParams(&cfgObj)
 			for i := 0; i < 1000; i++ {
-				pwString, err := random.GetChar(&charRange, pwLength)
+				pwString, err := random.GetChar(charRange, pwLength)
 				if err != nil {
 					t.Errorf("getRandChar returned an error: %q", err)
 				}
@@ -99,7 +99,7 @@ func TestGenLength(t *testing.T) {
 func TestGetRandChar(t *testing.T) {
 	t.Run("return_value_is_A_B_or_C", func(t *testing.T) {
 		charRange := "ABC"
-		randChar, err := random.GetChar(&charRange, 1)
+		randChar, err := random.GetChar(charRange, 1)
 		if err != nil {
 			t.Fatalf("Random character generation failed => %v", err.Error())
 		}
@@ -110,7 +110,7 @@ func TestGetRandChar(t *testing.T) {
 
 	t.Run("return_value_has_specific_length", func(t *testing.T) {
 		charRange := "ABC"
-		randChar, err := random.GetChar(&charRange, 1000)
+		randChar, err := random.GetChar(charRange, 1000)
 		if err != nil {
 			t.Fatalf("Random character generation failed => %v", err.Error())
 		}
@@ -122,7 +122,7 @@ func TestGetRandChar(t *testing.T) {
 
 	t.Run("fail", func(t *testing.T) {
 		charRange := "ABC"
-		randChar, err := random.GetChar(&charRange, -2000)
+		randChar, err := random.GetChar(charRange, -2000)
 		if err == nil {
 			t.Fatalf("Generated random characters expected to fail, but returned a value => %v",
 				randChar)
@@ -255,7 +255,7 @@ func BenchmarkGetRandNum(b *testing.B) {
 func BenchmarkGetRandChar(b *testing.B) {
 	charRange := "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890\"#/!\\$%&+-*.,?=()[]{}:;~^|"
 	for i := 0; i < b.N; i++ {
-		_, _ = random.GetChar(&charRange, 20)
+		_, _ = random.GetChar(charRange, 20)
 	}
 }
 
@@ -269,7 +269,7 @@ func BenchmarkConvertChar(b *testing.B) {
 	cfgObj.HumanReadable = false
 	charRange := chars.GetRange(&cfgObj)
 	for i := 0; i < b.N; i++ {
-		charToConv, _ := random.GetChar(&charRange, 1)
+		charToConv, _ := random.GetChar(charRange, 1)
 		charBytes := []byte(charToConv)
 		_, _ = spelling.ConvertCharToName(charBytes[0])
 	}
diff --git a/example-code/simple-password-generator/main.go b/example-code/simple-password-generator/main.go
index cf31235..7289abb 100644
--- a/example-code/simple-password-generator/main.go
+++ b/example-code/simple-password-generator/main.go
@@ -19,7 +19,7 @@ func main() {
 	}
 	pl := config.GetPwLengthFromParams(&c)
 	cs := chars.GetRange(&c)
-	pw, err := random.GetChar(&cs, pl)
+	pw, err := random.GetChar(cs, pl)
 	if err != nil {
 		panic(err)
 	}
diff --git a/random/random.go b/random/random.go
index 2918d94..ec0fdb9 100644
--- a/random/random.go
+++ b/random/random.go
@@ -2,28 +2,50 @@ package random
 
 import (
 	"crypto/rand"
+	"encoding/binary"
 	"fmt"
 	"math/big"
+	"strings"
+)
+
+// Bitmask sizes for the string generators (based on 93 chars total)
+const (
+	letterIdxBits = 7                    // 7 bits to represent a letter index
+	letterIdxMask = 1<<letterIdxBits - 1 // All 1-bits, as many as letterIdxBits
+	letterIdxMax  = 63 / letterIdxBits   // # of letter indices fitting in 63 bits
 )
 
 // GetChar generates random characters based on given character range
 // and password length
-func GetChar(charRange *string, pwLength int) (string, error) {
-	if pwLength <= 0 {
-		err := fmt.Errorf("provided pwLength value is <= 0: %v", pwLength)
-		return "", err
+func GetChar(cr string, l int) (string, error) {
+	if l < 1 {
+		return "", fmt.Errorf("length is negative")
+	}
+	rs := strings.Builder{}
+	rs.Grow(l)
+	crl := len(cr)
+
+	rp := make([]byte, 8)
+	_, err := rand.Read(rp)
+	if err != nil {
+		return rs.String(), err
 	}
-	availCharsLength := len(*charRange)
-	charSlice := []byte(*charRange)
-	returnString := make([]byte, pwLength)
-	for i := 0; i < pwLength; i++ {
-		randNum, err := GetNum(availCharsLength)
-		if err != nil {
-			return "", err
+	for i, c, r := l-1, binary.BigEndian.Uint64(rp), letterIdxMax; i >= 0; {
+		if r == 0 {
+			_, err := rand.Read(rp)
+			if err != nil {
+				return rs.String(), err
+			}
+			c, r = binary.BigEndian.Uint64(rp), letterIdxMax
+		}
+		if idx := int(c & letterIdxMask); idx < crl {
+			rs.WriteByte(cr[idx])
+			i--
 		}
-		returnString[i] = charSlice[randNum]
+		c >>= letterIdxBits
+		r--
 	}
-	return string(returnString), nil
+	return rs.String(), nil
 }
 
 // GetNum generates a random number with given maximum value