NanoPB: Benutzerdefinierte Array-Konverter mit Offset und Größe in C

syntax = "proto3";

package example;

message CustomArrayMessage {
  repeated uint32 values = 1;
}

example.CustomArrayMessage.values max_count:20

protoc --nanopb_out=. custom_array.proto

#include <stdio.h>
#include <stdint.h>
#include <stddef.h>
#include <limits.h>
#include "custom_array.pb.h"
#include "pb_encode.h"
#include "pb_decode.h"

// Benutzerdefinierter Array-Konverter-Kontext
typedef struct {
    const uint32_t* data;
    size_t size;
    size_t offset;
    size_t count;
} array_context_t;

// Kodierungs-Callback für uint32-Array mit Offset und Größe
bool uint32_array_encode_callback(pb_ostream_t *stream, const pb_field_t *field, void * const *arg) {
    const array_context_t* ctx = (const array_context_t*)*arg;
    
    size_t start = ctx->offset;
    size_t end = (ctx->offset + ctx->count < ctx->size) ? 
                 ctx->offset + ctx->count : ctx->size;
    
    for (size_t i = start; i < end; i++) {
        if (!pb_encode_tag_for_field(stream, field))
            return false;
        
        if (!pb_encode_varint(stream, ctx->data[i]))
            return false;
    }
    
    return true;
}

int main() {
    // Puffer für kodierte Nachricht
    uint8_t buffer[256];
    size_t message_length;
    
    // Array mit 10 Elementen erstellen
    uint32_t values[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
    
    // --- KODIERUNG ---
    example_CustomArrayMessage message = example_CustomArrayMessage_init_zero;
    
    // Kontext einrichten: Elemente 3-7 kodieren (offset=2, count=5)
    array_context_t ctx = {
        .data = values,
        .size = 10,
        .offset = 2,
        .count = 5
    };
    
    // Callback einrichten
    message.values.funcs.encode = uint32_array_encode_callback;
    message.values.arg = &ctx;
    
    // Strom für Kodierung erstellen
    pb_ostream_t ostream = pb_ostream_from_buffer(buffer, sizeof(buffer));
    
    // Nachricht kodieren
    if (!pb_encode(&ostream, example_CustomArrayMessage_fields, &message)) {
        printf("Kodierung fehlgeschlagen: %s\n", PB_GET_ERROR(&ostream));
        return 1;
    }
    
    message_length = ostream.bytes_written;
    printf("%zu Bytes kodiert (Elemente 3-7)\n", message_length);
    
    // Hex-Dump ausgeben
    printf("Kodierte Daten: ");
    for (size_t i = 0; i < message_length; i++) {
        printf("%02x ", buffer[i]);
    }
    printf("\n");
    
    // Ausgeben was kodiert wurde
    printf("Kodierte Werte: ");
    for (size_t i = 2; i < 2 + 5 && i < 10; i++) {
        printf("%u ", values[i]);
    }
    printf("\n");
    
    return 0;
}

gcc -o custom_array_example custom_array_example.c custom_array.pb.c pb_common.c pb_encode.c pb_decode.c -I.

10 Bytes kodiert (Elemente 3-7)
Kodierte Daten: 08 03 08 04 08 05 08 06 08 07 
Kodierte Werte: 3 4 5 6 7 

#include <stddef.h>
#include <limits.h>

#define MAX_OFFSET_SIZE (SIZE_MAX / 2)

typedef struct {
    const void* data;
    size_t element_size;
    size_t size;
    size_t offset;
    size_t count;
    bool (*encode_element)(pb_ostream_t*, const pb_field_t*, const void*);
} generic_array_context_t;

bool generic_array_encode_callback(pb_ostream_t *stream, const pb_field_t *field, void * const *arg) {
    const generic_array_context_t* ctx = (const generic_array_context_t*)*arg;
    
    size_t start = ctx->offset;
    size_t end = (ctx->offset + ctx->count < ctx->size) ? 
                 ctx->offset + ctx->count : ctx->size;
    
    const uint8_t* data = (const uint8_t*)ctx->data;
    
    for (size_t i = start; i < end; i++) {
        const void* element = data + (i * ctx->element_size);
        if (!ctx->encode_element(stream, field, element)) {
            return false;
        }
    }
    
    return true;
}