Refactoring the field encoder interface.

Replaced the confusing pb_enc_* functions with new pb_encode_* functions that
have a cleaner interface. Updated documentation.

Got rid of the endian_copy stuff in pb_encode.c, instead using C casts to do it automatically.
This makes the code safer and also reduces binary size by about 5%.

Fixes Issue 6.

6 files changed, 179 insertions, 173 deletions

diff --git a/docs/reference.rst b/docs/reference.rst
index 31f4e9fd..4021c764 100644
--- a/docs/reference.rst
+++ b/docs/reference.rst
@@ -149,17 +149,13 @@ Encodes the contents of a structure as a protocol buffers message and writes it
 
 Normally pb_encode simply walks through the fields description array and serializes each field in turn. However, submessages must be serialized twice: first to calculate their size and then to actually write them to output. This causes some constraints for callback fields, which must return the same data on every call.
 
-pb_encode_varint
-----------------
-Encodes an unsigned integer in the varint_ format. ::
+.. sidebar:: Encoding fields manually
 
-    bool pb_encode_varint(pb_ostream_t *stream, uint64_t value);
+    The functions with names *pb_encode_\** are used when dealing with callback fields. The typical reason for using callbacks is to have an array of unlimited size. In that case, `pb_encode`_ will call your callback function, which in turn will call *pb_encode_\** functions repeatedly to write out values.
 
-:stream:        Output stream to write to. 1-10 bytes will be written.
-:value:         Value to encode.
-:returns:       True on success, false on IO error.
+    The tag of a field must be encoded separately with `pb_encode_tag_for_field`_. After that, you can call exactly one of the content-writing functions to encode the payload of the field. For repeated fields, you can repeat this process multiple times.
 
-.. _varint: http://code.google.com/apis/protocolbuffers/docs/encoding.html#varints
+    Writing packed arrays is a little bit more involved: you need to use `pb_encode_tag` and specify `PB_WT_STRING` as the wire type. Then you need to know exactly how much data you are going to write, and use `pb_encode_varint`_ to write out the number of bytes before writing the actual data. Substreams can be used to determine the number of bytes beforehand; see `pb_encode_submessage`_ source code for an example.
 
 pb_encode_tag
 -------------
@@ -169,7 +165,7 @@ Starts a field in the Protocol Buffers binary format: encodes the field number a
 
 :stream:        Output stream to write to. 1-5 bytes will be written.
 :wiretype:      PB_WT_VARINT, PB_WT_64BIT, PB_WT_STRING or PB_WT_32BIT
-:field_number:  Identifier for the field, defined in the .proto file.
+:field_number:  Identifier for the field, defined in the .proto file. You can get it from field->tag.
 :returns:       True on success, false on IO error.
 
 pb_encode_tag_for_field
@@ -195,107 +191,71 @@ STRING, BYTES, SUBMESSAGE PB_WT_STRING
 FIXED32                   PB_WT_32BIT
 ========================= ============
 
-pb_encode_string
+pb_encode_varint
 ----------------
-Writes the length of a string as varint and then contents of the string. Used for writing fields with wire type PB_WT_STRING. ::
-
-    bool pb_encode_string(pb_ostream_t *stream, const uint8_t *buffer, size_t size);
+Encodes a signed or unsigned integer in the varint_ format. Works for fields of type `bool`, `enum`, `int32`, `int64`, `uint32` and `uint64`::
 
-:stream:        Output stream to write to.
-:buffer:        Pointer to string data.
-:size:          Number of bytes in the string.
-:returns:       True on success, false on IO error.
-
-.. sidebar:: Field encoders
-
-    The functions with names beginning with *pb_enc_* are called field encoders. Each PB_LTYPE has an own field encoder, which handles translating from C data into Protocol Buffers data.
-
-    By using the *data_size* in the field description and by taking advantage of C casting rules, it has been possible to combine many data types to a single LTYPE. For example, *int32*, *uint32*, *int64*, *uint64*, *bool* and *enum* are all handled by *pb_enc_varint*.
-
-    Each field encoder only encodes the contents of the field. The tag must be encoded separately with `pb_encode_tag_for_field`_.
-
-    You can use the field encoders from your callbacks. Just be aware that the pb_field_t passed to the callback is not directly compatible with most of the encoders. Instead, you must create a new pb_field_t structure and set the data_size according to the data type you pass to *src*.
-
-pb_enc_varint
--------------
-Field encoder for PB_LTYPE_VARINT. Takes the first *field->data_size* bytes from src, casts them as *uint64_t* and calls `pb_encode_varint`_. ::
-
-    bool pb_enc_varint(pb_ostream_t *stream, const pb_field_t *field, const void *src);
+    bool pb_encode_varint(pb_ostream_t *stream, uint64_t value);
 
-:stream:        Output stream to write to.
-:field:         Field description structure. Only *data_size* matters.
-:src:           Pointer to start of the field data.
+:stream:        Output stream to write to. 1-10 bytes will be written.
+:value:         Value to encode. Just cast e.g. int32_t directly to uint64_t.
 :returns:       True on success, false on IO error.
 
-pb_enc_svarint
---------------
-Field encoder for PB_LTYPE_SVARINT. Similar to `pb_enc_varint`_, except first zig-zag encodes the value for more efficient negative number encoding. ::
+.. _varint: http://code.google.com/apis/protocolbuffers/docs/encoding.html#varints
 
-    bool pb_enc_svarint(pb_ostream_t *stream, const pb_field_t *field, const void *src);
+pb_encode_svarint
+-----------------
+Encodes a signed integer in the 'zig-zagged' format. Works for fields of type `sint32` and `sint64`::
 
-(parameters are the same as for `pb_enc_varint`_)
+    bool pb_encode_svarint(pb_ostream_t *stream, int64_t value);
 
-The number is considered negative if the high-order bit of the value is set. On big endian computers, it is the highest bit of *\*src*. On little endian computers, it is the highest bit of *\*(src + field->data_size - 1)*.
+(parameters are the same as for `pb_encode_varint`_
 
-pb_enc_fixed32
---------------
-Field encoder for PB_LTYPE_FIXED32. Writes the data in little endian order. On big endian computers, reverses the order of bytes. ::
+pb_encode_string
+----------------
+Writes the length of a string as varint and then contents of the string. Works for fields of type `bytes` and `string`::
 
-    bool pb_enc_fixed32(pb_ostream_t *stream, const pb_field_t *field, const void *src);
+    bool pb_encode_string(pb_ostream_t *stream, const uint8_t *buffer, size_t size);
 
 :stream:        Output stream to write to.
-:field:         Not used.
-:src:           Pointer to start of the field data.
+:buffer:        Pointer to string data.
+:size:          Number of bytes in the string. Pass `strlen(s)` for strings.
 :returns:       True on success, false on IO error.
 
-pb_enc_fixed64
---------------
-Field encoder for PB_LTYPE_FIXED64. Writes the data in little endian order. On big endian computers, reverses the order of bytes. ::
-
-    bool pb_enc_fixed64(pb_ostream_t *stream, const pb_field_t *field, const void *src);
-
-(parameters are the same as for `pb_enc_fixed32`_)
-
-The same function is used for both integers and doubles. This breaks encoding of double values on architectures where they are mixed endian (primarily some arm processors with hardware FPU).
-
-pb_enc_bytes
-------------
-Field encoder for PB_LTYPE_BYTES. Just calls `pb_encode_string`_. ::
-
-    bool pb_enc_bytes(pb_ostream_t *stream, const pb_field_t *field, const void *src);
+pb_encode_fixed32
+-----------------
+Writes 4 bytes to stream and swaps bytes on big-endian architectures. Works for fields of type `fixed32`, `sfixed32` and `float`::
 
-:stream:        Output stream to write to.
-:field:         Not used.
-:src:           Pointer to a structure similar to pb_bytes_array_t.
-:returns:       True on success, false on IO error.
+    bool pb_encode_fixed32(pb_ostream_t *stream, const void *value);
 
-This function expects a pointer to a structure with a *size_t* field at start, and a variable sized byte array after it. The platform-specific field offset is inferred from *pb_bytes_array_t*, which has a byte array of size 1.
+:stream:    Output stream to write to.
+:value:     Pointer to a 4-bytes large C variable, for example `uint32_t foo;`.
+:returns:   True on success, false on IO error.
 
-pb_enc_string
--------------
-Field encoder for PB_LTYPE_STRING. Determines size of string with strlen() and then calls `pb_encode_string`_. ::
+pb_encode_fixed64
+-----------------
+Writes 8 bytes to stream and swaps bytes on big-endian architecture. Works for fields of type `fixed64`, `sfixed64` and `double`::
 
-    bool pb_enc_string(pb_ostream_t *stream, const pb_field_t *field, const void *src);
+    bool pb_encode_fixed64(pb_ostream_t *stream, const void *value);
 
-:stream:        Output stream to write to.
-:field:         Not used.
-:src:           Pointer to a null-terminated string.
-:returns:       True on success, false on IO error.
+:stream:    Output stream to write to.
+:value:     Pointer to a 8-bytes large C variable, for example `uint64_t foo;`.
+:returns:   True on success, false on IO error.
 
-pb_enc_submessage
------------------
-Field encoder for PB_LTYPE_SUBMESSAGE. Calls `pb_encode`_ to perform the actual encoding. ::
+pb_encode_submessage
+--------------------
+Encodes a submessage field, including the size header for it. Works for fields of any message type::
 
-    bool pb_enc_submessage(pb_ostream_t *stream, const pb_field_t *field, const void *src);
+    bool pb_encode_submessage(pb_ostream_t *stream, const pb_field_t fields[], const void *src_struct);
 
 :stream:        Output stream to write to.
-:field:         Field description structure. The *ptr* field must be a pointer to a field description array for the submessage.
+:fields:        Pointer to the autogenerated field description array for the submessage type, e.g. `MyMessage_fields`.
 :src:           Pointer to the structure where submessage data is.
 :returns:       True on success, false on IO errors, pb_encode errors or if submessage size changes between calls.
 
 In Protocol Buffers format, the submessage size must be written before the submessage contents. Therefore, this function has to encode the submessage twice in order to know the size beforehand.
 
-If the submessage contains callback fields, the callback function might misbehave and write out a different amount of data on the second call. This situation is recognized and *false* is returned, but it is up to the caller to ensure that the receiver of the message does not interpret it as valid data.
+If the submessage contains callback fields, the callback function might misbehave and write out a different amount of data on the second call. This situation is recognized and *false* is returned, but garbage will be written to the output before the problem is detected.
 
 pb_decode.h
 ===========
diff --git a/example/server.c b/example/server.c
index aba0667b..9f27906d 100644
--- a/example/server.c
+++ b/example/server.c
@@ -38,7 +38,7 @@ bool listdir_callback(pb_ostream_t *stream, const pb_field_t *field, const void
         if (!pb_encode_tag_for_field(stream, field))
             return false;
         
-        if (!pb_enc_submessage(stream, field, &fileinfo))
+        if (!pb_encode_submessage(stream, FileInfo_fields, &fileinfo))
             return false;
     }
     
diff --git a/pb_encode.c b/pb_encode.c
index 995eb3de..804c1a6a 100644
--- a/pb_encode.c
+++ b/pb_encode.c
@@ -3,6 +3,7 @@
  * 2011 Petteri Aimonen <jpa@kapsi.fi>
  */
 
+#define NANOPB_INTERNALS
 #include "pb.h"
 #include "pb_encode.h"
 #include <string.h>
@@ -220,6 +221,40 @@ bool checkreturn pb_encode_varint(pb_ostream_t *stream, uint64_t value)
     return pb_write(stream, buffer, i);
 }
 
+bool checkreturn pb_encode_svarint(pb_ostream_t *stream, int64_t value)
+{
+    uint64_t zigzagged;
+    if (value < 0)
+        zigzagged = ~(value << 1);
+    else
+        zigzagged = value << 1;
+    
+    return pb_encode_varint(stream, zigzagged);
+}
+
+bool checkreturn pb_encode_fixed32(pb_ostream_t *stream, const void *value)
+{
+    #ifdef __BIG_ENDIAN__
+    uint8_t *bytes = value;
+    uint8_t lebytes[4] = {bytes[3], bytes[2], bytes[1], bytes[0]};
+    return pb_write(stream, lebytes, 4);
+    #else
+    return pb_write(stream, (uint8_t*)value, 4);
+    #endif
+}
+
+bool checkreturn pb_encode_fixed64(pb_ostream_t *stream, const void *value)
+{
+    #ifdef __BIG_ENDIAN__
+    uint8_t *bytes[8] = value;
+    uint8_t lebytes[8] = {bytes[7], bytes[6], bytes[5], bytes[4], 
+                          bytes[3], bytes[2], bytes[1], bytes[0]};
+    return pb_write(stream, lebytes, 8);
+    #else
+    return pb_write(stream, (uint8_t*)value, 8);
+    #endif
+}
+
 bool checkreturn pb_encode_tag(pb_ostream_t *stream, pb_wire_type_t wiretype, int field_number)
 {
     int tag = wiretype | (field_number << 3);
@@ -265,71 +300,85 @@ bool checkreturn pb_encode_string(pb_ostream_t *stream, const uint8_t *buffer, s
     return pb_write(stream, buffer, size);
 }
 
-/* Field encoders */
-
-/* Copy srcsize bytes from src so that values are casted properly.
- * On little endian machine, copy to start of dest
- * On big endian machine, copy to end of dest
- * destsize must always be larger than srcsize
- * 
- * Note: This is the reverse of the endian_copy in pb_decode.c.
- */
-static void endian_copy(void *dest, const void *src, size_t destsize, size_t srcsize)
+bool checkreturn pb_encode_submessage(pb_ostream_t *stream, const pb_field_t fields[], const void *src_struct)
 {
-#ifdef __BIG_ENDIAN__
-    memcpy((char*)dest + (destsize - srcsize), src, srcsize);
-#else
-    memcpy(dest, src, srcsize);
-#endif
+    /* First calculate the message size using a non-writing substream. */
+    pb_ostream_t substream = {0};
+    size_t size;
+    bool status;
+    
+    if (!pb_encode(&substream, fields, src_struct))
+        return false;
+    
+    size = substream.bytes_written;
+    
+    if (!pb_encode_varint(stream, size))
+        return false;
+    
+    if (stream->callback == NULL)
+        return pb_write(stream, NULL, size); /* Just sizing */
+    
+    if (stream->bytes_written + size > stream->max_size)
+        return false;
+        
+    /* Use a substream to verify that a callback doesn't write more than
+     * what it did the first time. */
+    substream.callback = stream->callback;
+    substream.state = stream->state;
+    substream.max_size = size;
+    substream.bytes_written = 0;
+    
+    status = pb_encode(&substream, fields, src_struct);
+    
+    stream->bytes_written += substream.bytes_written;
+    stream->state = substream.state;
+    
+    if (substream.bytes_written != size)
+        return false;
+    
+    return status;
 }
 
+/* Field encoders */
+
 bool checkreturn pb_enc_varint(pb_ostream_t *stream, const pb_field_t *field, const void *src)
 {
     uint64_t value = 0;
-    endian_copy(&value, src, sizeof(value), field->data_size);
+    
+    switch (field->data_size)
+    {
+        case 1: value = *(uint8_t*)src; break;
+        case 2: value = *(uint16_t*)src; break;
+        case 4: value = *(uint32_t*)src; break;
+        case 8: value = *(uint64_t*)src; break;
+        default: return false;
+    }
+    
     return pb_encode_varint(stream, value);
 }
 
 bool checkreturn pb_enc_svarint(pb_ostream_t *stream, const pb_field_t *field, const void *src)
 {
     uint64_t value = 0;
-    uint64_t zigzagged;
-    uint64_t signbitmask, xormask;
-    endian_copy(&value, src, sizeof(value), field->data_size);
     
-    signbitmask = (uint64_t)0x80 << (field->data_size * 8 - 8);
-    xormask = ((uint64_t)-1) >> (64 - field->data_size * 8);
-    if (value & signbitmask)
-        zigzagged = ((value ^ xormask) << 1) | 1;
-    else
-        zigzagged = value << 1;
+    switch (field->data_size)
+    {
+        case 4: value = *(int32_t*)src; break;
+        case 8: value = *(int64_t*)src; break;
+        default: return false;
+    }
     
-    return pb_encode_varint(stream, zigzagged);
+    return pb_encode_svarint(stream, value);
 }
 
 bool checkreturn pb_enc_fixed64(pb_ostream_t *stream, const pb_field_t *field, const void *src)
 {
-    #ifdef __BIG_ENDIAN__
-    uint8_t bytes[8] = {0};
-    memcpy(bytes, src, 8);
-    uint8_t lebytes[8] = {bytes[7], bytes[6], bytes[5], bytes[4], 
-                          bytes[3], bytes[2], bytes[1], bytes[0]};
-    return pb_write(stream, lebytes, 8);
-    #else
-    return pb_write(stream, (uint8_t*)src, 8);
-    #endif
+    return pb_encode_fixed64(stream, src);
 }
 
 bool checkreturn pb_enc_fixed32(pb_ostream_t *stream, const pb_field_t *field, const void *src)
 {
-    #ifdef __BIG_ENDIAN__
-    uint8_t bytes[4] = {0};
-    memcpy(bytes, src, 4);
-    uint8_t lebytes[4] = {bytes[3], bytes[2], bytes[1], bytes[0]};
-    return pb_write(stream, lebytes, 4);
-    #else
-    return pb_write(stream, (uint8_t*)src, 4);
-    #endif
+    return pb_encode_fixed32(stream, src);
 }
 
 bool checkreturn pb_enc_bytes(pb_ostream_t *stream, const pb_field_t *field, const void *src)
@@ -345,42 +394,9 @@ bool checkreturn pb_enc_string(pb_ostream_t *stream, const pb_field_t *field, co
 
 bool checkreturn pb_enc_submessage(pb_ostream_t *stream, const pb_field_t *field, const void *src)
 {
-    pb_ostream_t substream = {0};
-    size_t size;
-    bool status;
-    
     if (field->ptr == NULL)
         return false;
     
-    if (!pb_encode(&substream, (pb_field_t*)field->ptr, src))
-        return false;
-    
-    size = substream.bytes_written;
-    
-    if (!pb_encode_varint(stream, size))
-        return false;
-    
-    if (stream->callback == NULL)
-        return pb_write(stream, NULL, size); /* Just sizing */
-    
-    if (stream->bytes_written + size > stream->max_size)
-        return false;
-        
-    /* Use a substream to verify that a callback doesn't write more than
-     * what it did the first time. */
-    substream.callback = stream->callback;
-    substream.state = stream->state;
-    substream.max_size = size;
-    substream.bytes_written = 0;
-    
-    status = pb_encode(&substream, (pb_field_t*)field->ptr, src);
-    
-    stream->bytes_written += substream.bytes_written;
-    stream->state = substream.state;
-    
-    if (substream.bytes_written != size)
-        return false;
-    
-    return status;
+    return pb_encode_submessage(stream, (pb_field_t*)field->ptr, src);
 }
 
diff --git a/pb_encode.h b/pb_encode.h
index cec39138..59ec554b 100644
--- a/pb_encode.h
+++ b/pb_encode.h
@@ -48,25 +48,56 @@ bool pb_encode(pb_ostream_t *stream, const pb_field_t fields[], const void *src_
  * You may want to use these from your caller or callbacks.
  */
 
-bool pb_encode_varint(pb_ostream_t *stream, uint64_t value);
-bool pb_encode_tag(pb_ostream_t *stream, pb_wire_type_t wiretype, int field_number);
-/* Encode tag based on LTYPE and field number defined in the field structure. */
+/* Encode field header based on LTYPE and field number defined in the field structure.
+ * Call this from the callback before writing out field contents. */
 bool pb_encode_tag_for_field(pb_ostream_t *stream, const pb_field_t *field);
-/* Write length as varint and then the contents of buffer. */
+
+/* Encode field header by manually specifing wire type. You need to use this if
+ * you want to write out packed arrays from a callback field. */
+bool pb_encode_tag(pb_ostream_t *stream, pb_wire_type_t wiretype, int field_number);
+
+/* Encode an integer in the varint format.
+ * This works for bool, enum, int32, int64, uint32 and uint64 field types. */
+bool pb_encode_varint(pb_ostream_t *stream, uint64_t value);
+
+/* Encode an integer in the zig-zagged svarint format.
+ * This works for sint32 and sint64. */
+bool pb_encode_svarint(pb_ostream_t *stream, int64_t value);
+
+/* Encode a string or bytes type field. For strings, pass strlen(s) as size. */
 bool pb_encode_string(pb_ostream_t *stream, const uint8_t *buffer, size_t size);
 
-/* --- Field encoders ---
- * Each encoder writes the content for the field.
- * The tag/wire type has been written already.
+/* Encode a fixed32, sfixed32 or float value.
+ * You need to pass a pointer to a 4-byte wide C variable. */
+bool pb_encode_fixed32(pb_ostream_t *stream, const void *value);
+
+/* Encode a fixed64, sfixed64 or double value.
+ * You need to pass a pointer to a 8-byte wide C variable. */
+bool pb_encode_fixed64(pb_ostream_t *stream, const void *value);
+
+/* Encode a submessage field.
+ * You need to pass the pb_field_t array and pointer to struct, just like with pb_encode().
+ * This internally encodes the submessage twice, first to calculate message size and then to actually write it out.
+ */
+bool pb_encode_submessage(pb_ostream_t *stream, const pb_field_t fields[], const void *src_struct);
+
+/* --- Internal functions ---
+ * These functions are not terribly useful for the average library user, but
+ * are exported to make the unit testing and extending nanopb easier.
  */
 
+#ifdef NANOPB_INTERNALS
 bool pb_enc_varint(pb_ostream_t *stream, const pb_field_t *field, const void *src);
 bool pb_enc_svarint(pb_ostream_t *stream, const pb_field_t *field, const void *src);
 bool pb_enc_fixed32(pb_ostream_t *stream, const pb_field_t *field, const void *src);
 bool pb_enc_fixed64(pb_ostream_t *stream, const pb_field_t *field, const void *src);
-
 bool pb_enc_bytes(pb_ostream_t *stream, const pb_field_t *field, const void *src);
 bool pb_enc_string(pb_ostream_t *stream, const pb_field_t *field, const void *src);
+#endif
+
+/* This function is not recommended for new programs. Use pb_encode_submessage()
+ * instead, it has the same functionality with a less confusing interface. */
 bool pb_enc_submessage(pb_ostream_t *stream, const pb_field_t *field, const void *src);
 
+
 #endif
diff --git a/tests/encode_unittests.c b/tests/encode_unittests.c
index 166e6e83..9cdbc66e 100644
--- a/tests/encode_unittests.c
+++ b/tests/encode_unittests.c
@@ -1,3 +1,5 @@
+#define NANOPB_INTERNALS
+
 #include <stdio.h>
 #include <string.h>
 #include "pb_encode.h"
@@ -123,7 +125,6 @@ int main()
         uint8_t buffer[30];
         pb_ostream_t s;
         uint8_t value = 1;
-        int8_t svalue = -1;
         int32_t max = INT32_MAX;
         int32_t min = INT32_MIN;
         int64_t lmax = INT64_MAX;
@@ -132,8 +133,6 @@ int main()
         
         COMMENT("Test pb_enc_varint and pb_enc_svarint")
         TEST(WRITES(pb_enc_varint(&s, &field, &value), "\x01"));
-        TEST(WRITES(pb_enc_svarint(&s, &field, &svalue), "\x01"));
-        TEST(WRITES(pb_enc_svarint(&s, &field, &value), "\x02"));
         
         field.data_size = sizeof(max);
         TEST(WRITES(pb_enc_svarint(&s, &field, &max), "\xfe\xff\xff\xff\x0f"));
diff --git a/tests/test_encode_callbacks.c b/tests/test_encode_callbacks.c
index f0a046de..7fa017f9 100644
--- a/tests/test_encode_callbacks.c
+++ b/tests/test_encode_callbacks.c
@@ -29,7 +29,7 @@ bool encode_fixed32(pb_ostream_t *stream, const pb_field_t *field, const void *a
         return false;
     
     uint32_t value = 42;
-    return pb_enc_fixed32(stream, field, &value);
+    return pb_encode_fixed32(stream, &value);
 }
 
 bool encode_fixed64(pb_ostream_t *stream, const pb_field_t *field, const void *arg)
@@ -38,7 +38,7 @@ bool encode_fixed64(pb_ostream_t *stream, const pb_field_t *field, const void *a
         return false;
     
     uint64_t value = 42;
-    return pb_enc_fixed64(stream, field, &value);
+    return pb_encode_fixed64(stream, &value);
 }
 
 int main()