var_int

varint.c

#include <stdio.h>
#include <stdint.h>

void check_value(const char* p_title, uint64_t expect, uint64_t challange)
{
    printf("%s: ", p_title);
    if (challange == expect) {
        printf("OK!\n");
    } else {
        printf("NG......\n");
    }
}

uint64_t varint(const uint8_t *p_varint)
{
    uint64_t val;
    if (p_varint[0] < 0xfd) {
        val = p_varint[0];
    } else if (p_varint[0] == 0xfd) {
        val = ((uint16_t)p_varint[2] << 8) | p_varint[1];
    } else if (p_varint[0] == 0xfe) {
        val = ((uint32_t)p_varint[4] << 24) |
              ((uint32_t)p_varint[3] << 16) |
              ((uint32_t)p_varint[2] <<  8) |
              (uint32_t)p_varint[1];
    } else {
        val = ((uint64_t)p_varint[8] << 56) |
              ((uint64_t)p_varint[7] << 48) |
              ((uint64_t)p_varint[6] << 40) |
              ((uint64_t)p_varint[5] << 32) |
              ((uint64_t)p_varint[4] << 24) |
              ((uint64_t)p_varint[3] << 16) |
              ((uint64_t)p_varint[2] <<  8) |
              p_varint[1];
    }
    return val;
}

int main(void)
{
    uint64_t val;

    {
        // 1byte min
        const uint8_t data[] = { 0x00 };
        val = varint(data);
        check_value("1byte min", 0x00, val);
    }
    {
        // 1byte max
        const uint8_t data[] = { 0xfc };
        val = varint(data);
        check_value("1byte max", 0xfc, val);
    }
    {
        // 2byte min
        const uint8_t data[] = { 0xfd, 0xfd, 0x00 };
        val = varint(data);
        check_value("2byte min", 0xfd, val);
    }
    {
        // 2byte max
        const uint8_t data[] = { 0xfd, 0xff, 0xff };
        val = varint(data);
        check_value("2byte max", 0xffff, val);
    }
    {
        // 4byte min
        const uint8_t data[] = { 0xfe, 0x00, 0x00, 0x01, 0x00 };
        val = varint(data);
        check_value("4byte min", 0x010000, val);
    }
    {
        // 4byte max
        const uint8_t data[] = { 0xfe, 0xff, 0xff, 0xff, 0xff };
        val = varint(data);
        check_value("4byte max", 0xffffffff, val);
    }
    {
        // 8byte min
        const uint8_t data[] = { 0xff, 0x00, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00 };
        val = varint(data);
        check_value("8byte min", 0x0100000000, val);
    }
    {
        // 4byte max
        const uint8_t data[] = { 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff };
        val = varint(data);
        check_value("4byte max", 0xffffffffffffffff, val);
    }


    return 0;
}