Reorganize the field decoder interface.

This makes the field decoding functions more intuitive to use.
The old interface is still present if you specify NANOPB_INTERNALS.

Update issue 2
Status: FixedInGit

6 files changed, 150 insertions, 152 deletions

diff --git a/docs/reference.rst b/docs/reference.rst
index 3331c6d..7dd08ed 100644
--- a/docs/reference.rst
+++ b/docs/reference.rst
@@ -320,16 +320,6 @@ In addition to EOF, the pb_decode implementation supports terminating a message
 
 For optional fields, this function applies the default value and sets *has_<field>* to false if the field is not present.
 
-pb_decode_varint
-----------------
-Read and decode a varint_ encoded integer. ::
-
-    bool pb_decode_varint(pb_istream_t *stream, uint64_t *dest);
-
-:stream:        Input stream to read from. 1-10 bytes will be read.
-:dest:          Storage for the decoded integer. Value is undefined on error.
-:returns:       True on success, false if value exceeds uint64_t range or an IO error happens.
-
 pb_skip_varint
 --------------
 Skip a varint_ encoded integer without decoding it. ::
@@ -373,48 +363,41 @@ Remove the data for a field from the stream, without actually decoding it::
 :wire_type:     Type of field to skip.
 :returns:       True on success, false on IO error.
 
-.. sidebar:: Field decoders
+.. sidebar:: Decoding fields manually
     
-    The functions with names beginning with *pb_dec_* are called field decoders. Each PB_LTYPE has an own field decoder, which handles translating from Protocol Buffers data to C data.
+    The functions with names beginning with *pb_decode_* 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_decode`_ will call your callback function repeatedly, which can then store the values into e.g. filesystem in the order received in.
 
-    Each field decoder reads and decodes a single value. For arrays, the decoder is called repeatedly.
+    For decoding numeric (including enumerated and boolean) values, use `pb_decode_varint`_, `pb_decode_svarint`_, `pb_decode_fixed32`_ and `pb_decode_fixed64`_. They take a pointer to a 32- or 64-bit C variable, which you may then cast to smaller datatype for storage.
 
-    You can use the decoders from your callbacks. Just be aware that the pb_field_t passed to the callback is not directly compatible 
-    with the *varint* field decoders. Instead, you must create a new pb_field_t structure and set the data_size according to the data type 
-    you pass to *dest*, e.g. *field.data_size = sizeof(int);*. Other fields in the *pb_field_t* don't matter.
+    For decoding strings and bytes fields, the length has already been decoded. You can therefore check the total length in *stream->state* and read the data using `pb_read`_.
 
-    The field decoder interface is a bit messy as a result of the interface required inside the nanopb library.
-    Eventually they may be replaced by separate wrapper functions with a more friendly interface.
+    Finally, for decoding submessages in a callback, simply use `pb_decode`_ and pass it the *SubMessage_fields* descriptor array.
 
-pb_dec_varint
--------------
-Field decoder for PB_LTYPE_VARINT. ::
+pb_decode_varint
+----------------
+Read and decode a varint_ encoded integer. ::
 
-    bool pb_dec_varint(pb_istream_t *stream, const pb_field_t *field, void *dest)
+    bool pb_decode_varint(pb_istream_t *stream, uint64_t *dest);
 
 :stream:        Input stream to read from. 1-10 bytes will be read.
-:field:         Field description structure. Only *field->data_size* matters.
-:dest:          Pointer to destination integer. Must have size of *field->data_size* bytes.
-:returns:       True on success, false on IO errors or if `pb_decode_varint`_ fails.
-
-This function first calls `pb_decode_varint`_. It then copies the first bytes of the 64-bit result value to *dest*, or on big endian architectures, the last bytes.
+:dest:          Storage for the decoded integer. Value is undefined on error.
+:returns:       True on success, false if value exceeds uint64_t range or an IO error happens.
 
-pb_dec_svarint
---------------
-Field decoder for PB_LTYPE_SVARINT. Similar to `pb_dec_varint`_, except that it performs zigzag-decoding on the value. ::
+pb_decode_svarint
+-----------------
+Similar to `pb_decode_varint`_, except that it performs zigzag-decoding on the value. This corresponds to the Protocol Buffers *sint32* and *sint64* datatypes. ::
 
-    bool pb_dec_svarint(pb_istream_t *stream, const pb_field_t *field, void *dest);
+    bool pb_decode_svarint(pb_istream_t *stream, int64_t *dest);
 
-(parameters are the same as `pb_dec_varint`_)
+(parameters are the same as `pb_decode_varint`_)
 
-pb_dec_fixed32
---------------
-Field decoder for PB_LTYPE_FIXED32. ::
+pb_decode_fixed32
+-----------------
+Decode a *fixed32*, *sfixed32* or *float* value. ::
 
-    bool pb_dec_fixed32(pb_istream_t *stream, const pb_field_t *field, void *dest);
+    bool pb_decode_fixed32(pb_istream_t *stream, void *dest);
 
 :stream:        Input stream to read from. 4 bytes will be read.
-:field:         Not used.
 :dest:          Pointer to destination *int32_t*, *uint32_t* or *float*.
 :returns:       True on success, false on IO errors.
 
@@ -422,9 +405,9 @@ This function reads 4 bytes from the input stream.
 On big endian architectures, it then reverses the order of the bytes.
 Finally, it writes the bytes to *dest*.
 
-pb_dec_fixed64
---------------
-Field decoder for PB_LTYPE_FIXED64. ::
+pb_decode_fixed64
+-----------------
+Decode a *fixed64*, *sfixed64* or *double* value. ::
 
     bool pb_dec_fixed(pb_istream_t *stream, const pb_field_t *field, void *dest);
 
@@ -433,53 +416,16 @@ Field decoder for PB_LTYPE_FIXED64. ::
 :dest:          Pointer to destination *int64_t*, *uint64_t* or *double*.
 :returns:       True on success, false on IO errors.
 
-Same as `pb_dec_fixed32`_, except this reads 8 bytes.
-
-pb_dec_bytes
-------------
-Field decoder for PB_LTYPE_BYTES. Reads a length-prefixed block of bytes. ::
-
-    bool pb_dec_bytes(pb_istream_t *stream, const pb_field_t *field, void *dest);
+Same as `pb_decode_fixed32`_, except this reads 8 bytes.
 
-**Note:** This is an internal function that is not useful in decoder callbacks. To read bytes fields in callbacks, use 
-*stream->bytes_left* and `pb_read`_.
-
-:stream:        Input stream to read from.
-:field:         Field description structure. Only *field->data_size* matters.
-:dest:          Pointer to a structure similar to pb_bytes_array_t.
-:returns:       True on success, false on IO error or if length exceeds the array size.
-
-This function expects a pointer to a structure with a *size_t* field at start, and a variable sized byte array after it. It will deduce the maximum size of the array from *field->data_size*.
-
-pb_dec_string
--------------
-Field decoder for PB_LTYPE_STRING. Reads a length-prefixed string. ::
+pb_make_string_substream
+------------------------
+Decode the length for a field with wire type *PB_WT_STRING* and create a substream for reading the data. ::
 
-    bool pb_dec_string(pb_istream_t *stream, const pb_field_t *field, void *dest);
-
-**Note:** This is an internal function that is not useful in decoder callbacks. To read string fields in callbacks, use 
-*stream->bytes_left* and `pb_read`_.
-
-:stream:        Input stream to read from.
-:field:         Field description structure. Only *field->data_size* matters.
-:dest:          Pointer to a character array of size *field->data_size*.
-:returns:       True on success, false on IO error or if length exceeds the array size.
-
-This function null-terminates the string when successful. On error, the contents of the destination array is undefined.
-
-pb_dec_submessage
------------------
-Field decoder for PB_LTYPE_SUBMESSAGE. Calls `pb_decode`_ to perform the actual decoding. ::
-
-    bool pb_dec_submessage(pb_istream_t *stream, const pb_field_t *field, void *dest)
-
-**Note:** This is an internal function that is not useful in decoder callbacks. To read submessage fields in callbacks, use 
-`pb_decode`_ directly.
-
-:stream:        Input stream to read from.
-:field:         Field description structure. Only *field->ptr* matters.
-:dest:          Pointer to the destination structure.
-:returns:       True on success, false on IO error or if `pb_decode`_ fails.
+    bool pb_make_string_substream(pb_istream_t *stream, pb_istream_t *substream);
 
-The *field->ptr* should be a pointer to *pb_field_t* array describing the submessage.
+:stream:        Original input stream to read the length and data from.
+:substream:     New substream that has limited length. Filled in by the function.
+:returns:       True on success, false if reading the length fails.
 
+This function uses `pb_decode_varint`_ to read an integer from the stream. This is interpreted as a number of bytes, and the substream is set up so that its `bytes_left` is initially the same as the length. The substream has a wrapper callback that in turn reads from the parent stream.
diff --git a/example_unions/decode.c b/example_unions/decode.c
index edd568c..a7cc781 100644
--- a/example_unions/decode.c
+++ b/example_unions/decode.c
@@ -44,10 +44,11 @@ const pb_field_t* decode_unionmessage_type(pb_istream_t *stream)
 
 bool decode_unionmessage_contents(pb_istream_t *stream, const pb_field_t fields[], void *dest_struct)
 {
-    pb_field_t field = {}; /* NB: Could get rid of this wrapper by fixing issue #2. */
-    field.ptr = fields;
+    pb_istream_t substream;
+    if (!pb_make_string_substream(stream, &substream))
+        return false;
     
-    return pb_dec_submessage(stream, &field, dest_struct);
+    return pb_decode(&substream, fields, dest_struct);
 }
 
 int main()
diff --git a/pb_decode.c b/pb_decode.c
index 9df4a89..59efb3d 100644
--- a/pb_decode.c
+++ b/pb_decode.c
@@ -11,6 +11,7 @@
     #define checkreturn __attribute__((warn_unused_result))
 #endif
 
+#define NANOPB_INTERNALS
 #include "pb.h"
 #include "pb_decode.h"
 #include <string.h>
@@ -192,7 +193,7 @@ static bool substream_callback(pb_istream_t *stream, uint8_t *buf, size_t count)
     return pb_read(parent, buf, count);
 }
 
-static bool checkreturn make_string_substream(pb_istream_t *stream, pb_istream_t *substream)
+bool checkreturn pb_make_string_substream(pb_istream_t *stream, pb_istream_t *substream)
 {
     uint32_t size;
     if (!pb_decode_varint32(stream, &size))
@@ -294,7 +295,7 @@ static bool checkreturn decode_field(pb_istream_t *stream, pb_wire_type_t wire_t
                 /* Packed array */
                 size_t *size = (size_t*)iter->pSize;
                 pb_istream_t substream;
-                if (!make_string_substream(stream, &substream))
+                if (!pb_make_string_substream(stream, &substream))
                     return false;
                 
                 while (substream.bytes_left && *size < iter->current->array_size)
@@ -329,7 +330,7 @@ static bool checkreturn decode_field(pb_istream_t *stream, pb_wire_type_t wire_t
             {
                 pb_istream_t substream;
                 
-                if (!make_string_substream(stream, &substream))
+                if (!pb_make_string_substream(stream, &substream))
                     return false;
                 
                 while (substream.bytes_left)
@@ -467,78 +468,104 @@ bool checkreturn pb_decode(pb_istream_t *stream, const pb_field_t fields[], void
 
 /* Field decoders */
 
-/* Copy destsize bytes from src so that values are casted properly.
- * On little endian machine, copy first n bytes of src
- * On big endian machine, copy last n bytes of src
- * srcsize must always be larger than destsize
- */
-static void endian_copy(void *dest, void *src, size_t destsize, size_t srcsize)
+bool pb_decode_svarint(pb_istream_t *stream, int64_t *dest)
 {
-#ifdef __BIG_ENDIAN__
-    memcpy(dest, (char*)src + (srcsize - destsize), destsize);
-#else
-    UNUSED(srcsize);
-    memcpy(dest, src, destsize);
-#endif
+    uint64_t value;
+    if (!pb_decode_varint(stream, &value))
+        return false;
+    
+    if (value & 1)
+        *dest = ~(value >> 1);
+    else
+        *dest = value >> 1;
+    
+    return true;
+}
+
+bool pb_decode_fixed32(pb_istream_t *stream, void *dest)
+{
+    #ifdef __BIG_ENDIAN__
+    uint8_t *bytes = (uint8_t*)dest;
+    uint8_t lebytes[4];
+    
+    if (!pb_read(stream, lebytes, 4))
+        return false;
+    
+    bytes[0] = lebytes[3];
+    bytes[1] = lebytes[2];
+    bytes[2] = lebytes[1];
+    bytes[3] = lebytes[0];
+    return true;
+    #else
+    return pb_read(stream, (uint8_t*)dest, 4);
+    #endif   
+}
+
+bool pb_decode_fixed64(pb_istream_t *stream, void *dest)
+{
+    #ifdef __BIG_ENDIAN__
+    uint8_t *bytes = (uint8_t*)dest;
+    uint8_t lebytes[8];
+    
+    if (!pb_read(stream, lebytes, 8))
+        return false;
+    
+    bytes[0] = lebytes[7];
+    bytes[1] = lebytes[6];
+    bytes[2] = lebytes[5];
+    bytes[3] = lebytes[4];
+    bytes[4] = lebytes[3];
+    bytes[5] = lebytes[2];
+    bytes[6] = lebytes[1];
+    bytes[7] = lebytes[0];
+    return true;
+    #else
+    return pb_read(stream, (uint8_t*)dest, 8);
+    #endif   
 }
 
 bool checkreturn pb_dec_varint(pb_istream_t *stream, const pb_field_t *field, void *dest)
 {
-    uint64_t temp;
-    bool status = pb_decode_varint(stream, &temp);
-    endian_copy(dest, &temp, field->data_size, sizeof(temp));
+    uint64_t value;
+    bool status = pb_decode_varint(stream, &value);
+    
+    switch (field->data_size)
+    {
+        case 1: *(uint8_t*)dest = value; break;
+        case 2: *(uint16_t*)dest = value; break;
+        case 4: *(uint32_t*)dest = value; break;
+        case 8: *(uint64_t*)dest = value; break;
+        default: return false;
+    }
+    
     return status;
 }
 
 bool checkreturn pb_dec_svarint(pb_istream_t *stream, const pb_field_t *field, void *dest)
 {
-    uint64_t temp;
-    bool status = pb_decode_varint(stream, &temp);
-    temp = (temp >> 1) ^ -(int64_t)(temp & 1);
-    endian_copy(dest, &temp, field->data_size, sizeof(temp));
+    int64_t value;
+    bool status = pb_decode_svarint(stream, &value);
+    
+    switch (field->data_size)
+    {
+        case 4: *(int32_t*)dest = value; break;
+        case 8: *(int64_t*)dest = value; break;
+        default: return false;
+    }
+    
     return status;
 }
 
 bool checkreturn pb_dec_fixed32(pb_istream_t *stream, const pb_field_t *field, void *dest)
 {
-#ifdef __BIG_ENDIAN__
-    uint8_t bytes[4] = {0};
-    bool status = pb_read(stream, bytes, 4);
-    if (status) {
-        uint8_t *d = (uint8_t*)dest;
-        d[0] = bytes[3];
-        d[1] = bytes[2];
-        d[2] = bytes[1];
-        d[3] = bytes[0];
-    }
-    return status;
-#else
     UNUSED(field);
-    return pb_read(stream, (uint8_t*)dest, 4);
-#endif
+    return pb_decode_fixed32(stream, dest);
 }
 
 bool checkreturn pb_dec_fixed64(pb_istream_t *stream, const pb_field_t *field, void *dest)
 {
-#ifdef __BIG_ENDIAN__
-    uint8_t bytes[8] = {0};
-    bool status = pb_read(stream, bytes, 8);
-    if (status) {
-        uint8_t *d = (uint8_t*)dest;
-        d[0] = bytes[7];
-        d[1] = bytes[6];
-        d[2] = bytes[5]; 
-        d[3] = bytes[4];
-        d[4] = bytes[3];
-        d[5] = bytes[2];
-        d[6] = bytes[1];
-        d[7] = bytes[0];
-    }
-    return status;
-#else
     UNUSED(field);
-    return pb_read(stream, (uint8_t*)dest, 8);
-#endif
+    return pb_decode_fixed64(stream, dest);
 }
 
 bool checkreturn pb_dec_bytes(pb_istream_t *stream, const pb_field_t *field, void *dest)
@@ -577,7 +604,7 @@ bool checkreturn pb_dec_submessage(pb_istream_t *stream, const pb_field_t *field
 {
     pb_istream_t substream;
     
-    if (!make_string_substream(stream, &substream))
+    if (!pb_make_string_substream(stream, &substream))
         return false;
     
     if (field->ptr == NULL)
diff --git a/pb_decode.h b/pb_decode.h
index 9cc67e8..27535c1 100644
--- a/pb_decode.h
+++ b/pb_decode.h
@@ -44,20 +44,38 @@ bool pb_decode(pb_istream_t *stream, const pb_field_t fields[], void *dest_struc
  * You may want to use these from your caller or callbacks.
  */
 
+/* Decode the tag for the next field in the stream. Gives the wire type and
+ * field tag. At end of the message, returns false and sets eof to true. */
 bool pb_decode_tag(pb_istream_t *stream, pb_wire_type_t *wire_type, uint32_t *tag, bool *eof);
+
+/* Skip the field payload data, given the wire type. */
 bool pb_skip_field(pb_istream_t *stream, pb_wire_type_t wire_type);
 
+/* Decode an integer in the varint format. This works for bool, enum, int32,
+ * int64, uint32 and uint64 field types. */
 bool pb_decode_varint(pb_istream_t *stream, uint64_t *dest);
 
-bool pb_skip_varint(pb_istream_t *stream);
-bool pb_skip_string(pb_istream_t *stream);
+/* Decode an integer in the zig-zagged svarint format. This works for sint32
+ * and sint64. */
+bool pb_decode_svarint(pb_istream_t *stream, int64_t *dest);
+
+/* Decode a fixed32, sfixed32 or float value. You need to pass a pointer to
+ * a 4-byte wide C variable. */
+bool pb_decode_fixed32(pb_istream_t *stream, void *dest);
+
+/* Decode a fixed64, sfixed64 or double value. You need to pass a pointer to
+ * a 8-byte wide C variable. */
+bool pb_decode_fixed64(pb_istream_t *stream, void *dest);
 
-/* --- Field decoders ---
- * Each decoder takes stream and field description, and a pointer to the field
- * in the destination struct (dest = struct_addr + field->data_offset).
- * For arrays, these functions are called repeatedly.
+/* Make a limited-length substream for reading a PB_WT_STRING field. */
+bool pb_make_string_substream(pb_istream_t *stream, pb_istream_t *substream);
+
+/* --- 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_dec_varint(pb_istream_t *stream, const pb_field_t *field, void *dest);
 bool pb_dec_svarint(pb_istream_t *stream, const pb_field_t *field, void *dest);
 bool pb_dec_fixed32(pb_istream_t *stream, const pb_field_t *field, void *dest);
@@ -67,4 +85,8 @@ bool pb_dec_bytes(pb_istream_t *stream, const pb_field_t *field, void *dest);
 bool pb_dec_string(pb_istream_t *stream, const pb_field_t *field, void *dest);
 bool pb_dec_submessage(pb_istream_t *stream, const pb_field_t *field, void *dest);
 
+bool pb_skip_varint(pb_istream_t *stream);
+bool pb_skip_string(pb_istream_t *stream);
+#endif
+
 #endif
diff --git a/tests/decode_unittests.c b/tests/decode_unittests.c
index 6ba6d4f..039c9fa 100644
--- a/tests/decode_unittests.c
+++ b/tests/decode_unittests.c
@@ -1,3 +1,5 @@
+#define NANOPB_INTERNALS
+
 #include <stdio.h>
 #include <string.h>
 #include "pb_decode.h"
diff --git a/tests/test_decode_callbacks.c b/tests/test_decode_callbacks.c
index 714b7bb..95824d1 100644
--- a/tests/test_decode_callbacks.c
+++ b/tests/test_decode_callbacks.c
@@ -37,7 +37,7 @@ bool print_int32(pb_istream_t *stream, const pb_field_t *field, void *arg)
 bool print_fixed32(pb_istream_t *stream, const pb_field_t *field, void *arg)
 {
     uint32_t value;
-    if (!pb_dec_fixed32(stream, NULL, &value))
+    if (!pb_decode_fixed32(stream, &value))
         return false;
     
     printf((char*)arg, (long)value);
@@ -47,7 +47,7 @@ bool print_fixed32(pb_istream_t *stream, const pb_field_t *field, void *arg)
 bool print_fixed64(pb_istream_t *stream, const pb_field_t *field, void *arg)
 {
     uint64_t value;
-    if (!pb_dec_fixed64(stream, NULL, &value))
+    if (!pb_decode_fixed64(stream, &value))
         return false;
     
     printf((char*)arg, (long long)value);