diff --git a/core/src/com/google/zxing/oned/Code128Writer.java b/core/src/com/google/zxing/oned/Code128Writer.java
index 279722217..2809cea74 100644
--- a/core/src/com/google/zxing/oned/Code128Writer.java
+++ b/core/src/com/google/zxing/oned/Code128Writer.java
@@ -16,7 +16,10 @@
 
 package com.google.zxing.oned;
 
+import java.util.Enumeration;
 import java.util.Hashtable;
+import java.util.Vector;
+
 import com.google.zxing.BarcodeFormat;
 import com.google.zxing.WriterException;
 import com.google.zxing.common.BitMatrix;
@@ -28,85 +31,131 @@ import com.google.zxing.common.BitMatrix;
  */
 public final class Code128Writer extends UPCEANWriter {
 
-    public BitMatrix encode(String contents,
-                            BarcodeFormat format,
-                            int width,
-                            int height,
-                            Hashtable hints) throws WriterException {
-      if (format != BarcodeFormat.CODE_128) {
-        throw new IllegalArgumentException("Can only encode CODE_128, but got " + format);
-      }
-      return super.encode(contents, format, width, height, hints);
+  private static final int CODE_START_B = 104;
+  private static final int CODE_START_C = 105;
+  private static final int CODE_CODE_B = 100;
+  private static final int CODE_CODE_C = 99;
+  private static final int CODE_STOP = 106;
+
+  public BitMatrix encode(String contents,
+                          BarcodeFormat format,
+                          int width,
+                          int height,
+                          Hashtable hints) throws WriterException {
+    if (format != BarcodeFormat.CODE_128) {
+      throw new IllegalArgumentException("Can only encode CODE_128, but got " + format);
     }
+    return super.encode(contents, format, width, height, hints);
+  }
 
-    public byte[] encode(String contents) {
-      int length = contents.length();
-      if (length > 80) {
-        throw new IllegalArgumentException(
-            "Requested contents should be less than 80 digits long, but got " + length);
+  public byte[] encode(String contents) {
+    int length = contents.length();
+    // Check length
+    if (length < 1 || length > 80) {
+      throw new IllegalArgumentException(
+          "Contents length should be between 1 and 80 characters, but got " + length);
+    }
+    // Check content
+    for (int i = 0; i < length; i++) {
+      char c = contents.charAt(i);
+      if (c < ' ' || c > '~') {
+        throw new IllegalArgumentException("Contents should only contain characters between ' ' and '~'");
+      }
+    }
+    
+    Vector patterns = new Vector(); // temporary storage for patterns
+    int checkSum = 0;
+    int checkWeight = 1;
+    int codeSet = 0; // selected code (CODE_CODE_B or CODE_CODE_C)
+    int position = 0; // position in contents
+    
+    while (position < length) {
+      //Select code to use
+      int requiredDigitCount = codeSet == CODE_CODE_C ? 2 : 4;
+      int newCodeSet;
+      if (length - position >= requiredDigitCount && isDigits(contents, position, requiredDigitCount)) {
+        newCodeSet = CODE_CODE_C;
+      } else {
+        newCodeSet = CODE_CODE_B;
       }
       
-      //Determine which code we should use (C or B)
-      boolean useCodeC = true;
-      for (int i = 0; i < length; i++) {
-        char c = contents.charAt(i);
-        if (c < '0' || c > '9') {
-        	useCodeC = false;
-        	break;
-        }
-      }
-
-      int codeWidth = 11 + 11 + 13; //start plus check plus stop character
-      byte[] result;
-      int pos;
-      int check;
-      
-      if (useCodeC) {
-    	  //Optionnaly add "0" to have pairs
-        if (length % 2 != 0) {
-        	contents = '0' + contents;
-        	length++;
-        }
-        //get total code width for this barcode
-        for (int i = 0; i < length; i += 2){
-          int[] patterns = Code128Reader.CODE_PATTERNS[Integer.parseInt(contents.substring(i, i + 2))];
-          for (int j = 0; j < patterns.length; j++) {
-            codeWidth += patterns[j];
-          }
-        }
-        result = new byte[codeWidth];
-        pos = appendPattern(result, 0, Code128Reader.CODE_PATTERNS[105], 1);
-        check = 105;
-        //append next character to bytematrix
-        for (int i = 0; i < length; i += 2) {
-          int pair = Integer.parseInt(contents.substring(i, i + 2));
-          check += pair * (i / 2 + 1);
-          pos += appendPattern(result, pos, Code128Reader.CODE_PATTERNS[pair],1);
+      //Get the pattern index
+      int patternIndex;
+      if (newCodeSet == codeSet) {
+        // Encode the current character
+        if (codeSet == CODE_CODE_B) {
+          patternIndex = contents.charAt(position) - ' ';
+          position += 1;
+        } else { // CODE_CODE_C
+          patternIndex = Integer.parseInt(contents.substring(position, position + 2));
+          position += 2;
         }
       } else {
-        //get total code width for this barcode
-        for (int i = 0; i < length; i++) {
-          int[] patterns = Code128Reader.CODE_PATTERNS[contents.charAt(i) - ' '];
-          for (int j = 0; j < patterns.length; j++)  {
-            codeWidth += patterns[j];
+        // Should we change the current code?
+        // Do we have a code set?
+        if (codeSet == 0) {
+          // No, we don't have a code set
+          if (newCodeSet == CODE_CODE_B) {
+            patternIndex = CODE_START_B;
+          } else {
+            // CODE_CODE_C
+            patternIndex = CODE_START_C;
           }
+        } else {
+          // Yes, we have a code set
+          patternIndex = newCodeSet;
         }
-        result = new byte[codeWidth];
-        pos = appendPattern(result, 0, Code128Reader.CODE_PATTERNS[104], 1);
-        check = 104;
-        //append next character to bytematrix
-        for (int i = 0; i < length; i++) {
-          check += (contents.charAt(i) - ' ') * (i + 1);
-          pos += appendPattern(result, pos, Code128Reader.CODE_PATTERNS[contents.charAt(i) - ' '],1);
-        }
+        codeSet = newCodeSet;
       }
       
-      //compute checksum and append it along with end character and quiet space
-      check %= 103;
-      pos += appendPattern(result,pos,Code128Reader.CODE_PATTERNS[check],1);
-      pos += appendPattern(result,pos,Code128Reader.CODE_PATTERNS[106],1);
-
-      return result;
+      // Get the pattern
+      patterns.addElement(Code128Reader.CODE_PATTERNS[patternIndex]);
+      
+      // Compute checksum
+      checkSum += patternIndex * checkWeight;
+      if (position != 0) {
+        checkWeight++;
+      }
     }
+    
+    // Compute and append checksum
+    checkSum %= 103;
+    patterns.addElement(Code128Reader.CODE_PATTERNS[checkSum]);
+    
+    // Append stop code
+    patterns.addElement(Code128Reader.CODE_PATTERNS[CODE_STOP]);
+    
+    // Compute code width
+    int codeWidth = 0;
+    Enumeration patternEnumeration = patterns.elements();
+    while (patternEnumeration.hasMoreElements()) {
+      int[] pattern = (int[]) patternEnumeration.nextElement();
+      for (int i = 0; i < pattern.length; i++) {
+        codeWidth += pattern[i];
+      }
+    }
+    
+    // Compute result
+    byte[] result = new byte[codeWidth];
+    patternEnumeration = patterns.elements();
+    int pos = 0;
+    while (patternEnumeration.hasMoreElements()) {
+      int[] pattern = (int[]) patternEnumeration.nextElement();
+      pos += appendPattern(result, pos, pattern, 1);
+    }
+    
+    return result;
+  }
+
+  private static boolean isDigits(String value, int start, int length) {
+    int end = start + length;
+    for (int i = start; i < end; i++) {
+      char c = value.charAt(i);
+      if (c < '0' || c > '9') {
+        return false;
+      }
+    }
+    return true;
+  }
 
 }