eccodes/examples/C/grib_precision.c

/*
 * (C) Copyright 2005- ECMWF.
 *
 * This software is licensed under the terms of the Apache Licence Version 2.0
 * which can be obtained at http://www.apache.org/licenses/LICENSE-2.0.
 *
 * In applying this licence, ECMWF does not waive the privileges and immunities granted to it by
 * virtue of its status as an intergovernmental organisation nor does it submit to any jurisdiction.
 */

/*
 * C Implementation: grib_precision
 *
 * Description: how to control decimal precision when packing fields.
 *
 */
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <assert.h>

#include "eccodes.h"

#define EPSILON 1e-6
#define EXPECTED_MIN 270.466796875
#define EXPECTED_MAX 311.096796875

int main(int argc, char** argv)
{
    int err     = 0;
    size_t size = 0;

    FILE* in            = NULL;
    const char* infile  = "../../data/regular_latlon_surface.grib1";
    FILE* out           = NULL;
    const char* outfile = "out.precision.grib1";
    codes_handle* h     = NULL;
    const void* buffer  = NULL;
    double* values1     = NULL;
    double* values2     = NULL;
    double maxa         = 0;
    double maxv = 0, minv = 0;
    double maxr = 0, r = 0;
    long decimalPrecision;
    long bitsPerValue1 = 0, bitsPerValue2 = 0;
    int i = 0;

    in = fopen(infile, "rb");
    if (!in) {
        fprintf(stderr, "Error: unable to open input file %s\n", infile);
        return 1;
    }

    out = fopen(outfile, "wb");
    if (!out) {
        fprintf(stderr, "Error: unable to open output file %s\n", outfile);
        fclose(in);
        return 1;
    }

    /* create a new handle from a message in a file */
    h = codes_handle_new_from_file(0, in, PRODUCT_GRIB, &err);
    if (h == NULL) {
        fprintf(stderr, "Error: unable to create handle from file %s\n", infile);
        return 1;
    }

    /* bitsPerValue before changing the packing parameters */
    CODES_CHECK(codes_get_long(h, "bitsPerValue", &bitsPerValue1), 0);
    assert(bitsPerValue1 == 16);

    /* get the size of the values array*/
    CODES_CHECK(codes_get_size(h, "values", &size), 0);
    assert(size == 496);

    values1 = (double*)malloc(size * sizeof(double));
    /* get data values before changing the packing parameters*/
    CODES_CHECK(codes_get_double_array(h, "values", values1, &size), 0);

    /* changing decimal precision to 2 means that 2 decimal digits
     are preserved when packing.  */
    decimalPrecision = 2;
    CODES_CHECK(codes_set_long(h, "changeDecimalPrecision", decimalPrecision), 0);

    /* bitsPerValue after changing the packing parameters */
    CODES_CHECK(codes_get_long(h, "bitsPerValue", &bitsPerValue2), 0);
    assert(bitsPerValue2 == 12);

    values2 = (double*)malloc(size * sizeof(double));
    /* get data values after changing the packing parameters*/
    CODES_CHECK(codes_get_double_array(h, "values", values2, &size), 0);

    /* computing error */
    maxa = 0;
    maxr = 0;
    maxv = values2[0];
    minv = maxv;
    for (i = 0; i < size; i++) {
        double a = fabs(values2[i] - values1[i]);
        if (values2[i] > maxv) maxv = values2[i];
        if (values2[i] < minv) minv = values2[i];
        if (values2[i] != 0) r = fabs((values2[i] - values1[i]) / values2[i]);
        if (a > maxa) maxa = a;
        if (r > maxr) maxr = r;
    }
    printf("max absolute error = %g\n", maxa);
    printf("max relative error = %g\n", maxr);
    printf("min value = %g\n", minv);
    printf("max value = %g\n", maxv);
    assert(fabs(minv - EXPECTED_MIN) < EPSILON);
    assert(fabs(maxv - EXPECTED_MAX) < EPSILON);

    /* get the coded message in a buffer */
    CODES_CHECK(codes_get_message(h, &buffer, &size), 0);

    /* write the buffer in a file*/
    if (fwrite(buffer, 1, size, out) != size) {
        perror(argv[1]);
        exit(1);
    }

    codes_handle_delete(h);

    fclose(in);
    fclose(out);
    free(values1);
    free(values2);

    return 0;
}