compactgrid.c

/*
    File: compactgrid.c
Abstract: 
This is a standalone command line program to be run on Mac OS X that performs preprocessing to compact
a USGS tab separated earthquake hazard grid file into a smaller binary format that is faster to load
on a mobile device.
 
See http://earthquake.usgs.gov/hazards/products/conterminous/2008/data/.
 
 Version: 1.2
 
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*/
 
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
 
#include <libkern/OSByteOrder.h>
 
int main(int argc, char **argv) {
    if (argc != 3) {
        fprintf(stderr, "Usage %s gridfile.tab gridfile.bin\n", argv[0]);
        exit(1);
    }
    
    char *inpath = argv[1];
    char *outpath = argv[2];
    
    // read in the whole file first in order to establish the bounds of the grid.
    
    FILE *in = fopen(inpath, "r");
    
    size_t gridCount = 1000;
    int read = 0;
    float *grid = malloc(sizeof(float) * gridCount);
    
    double lastLat = -500;
    double lastLon = -500;
    float gridSize = -1;
    int32_t gridWidth = -1;
    double originLat = 0;
    double originLon = 0;
    
    char line[50];
    
    while (fgets(line, sizeof(line), in)) {
        double lon, lat;
        float value;
        if (3 != sscanf(line, "%lf %lf %f", &lon, &lat, &value)) {
            fprintf(stderr, "unparseable line: %s", line);
            exit(2);
        }
        
        if (lastLat < -200 && lastLon < -200) {
            // first value, set the origin
            originLat = lat;
            originLon = lon;
        } else if (gridSize == -1) {
            // second value, set the grid size
            gridSize = fabs(lon - lastLon);
        } else if (lat != lastLat && gridWidth < 0) {
            // end of first row, set the gridWidth
            gridWidth = read;
        }
        
        if (read == gridCount) {
            gridCount *= 2;
            grid = realloc(grid, sizeof(float) * gridCount);
        }
                
        grid[read] = value;
        
        lastLon = lon;
        lastLat = lat;
        
        read++;
    }
    
    int32_t gridHeight = read / gridWidth;
    
    fclose(in);
    
    // write out the binary version of the file
    // [4 bytes] - int32 little endian - grid origin latitudeE6
    // [4 bytes] - int32 little endian - grid origin longitudeE6
    // [4 bytes] - int32 little endian - grid step sizeE6
    // [4 bytes] - int32 little endian - grid width
    // [4 bytes] - int32 little endian - grid height
    // [N bytes] - list of width*height int16 little endian grid data as E4 fixed point
    
#define E6 (1000000.0)
#define E4 (10000.0)
    
    FILE *out = fopen(outpath, "wb");
        
    int32_t latE6 = OSSwapHostToLittleInt32(originLat * E6);
    int32_t lonE6 = OSSwapHostToLittleInt32(originLon * E6);
    int32_t gridSizeE6 = OSSwapHostToLittleInt32(gridSize * E6);
    int32_t widthSwapped = OSSwapHostToLittleInt32(gridWidth);
    int32_t heightSwapped = OSSwapHostToLittleInt32(gridHeight);
    
    fwrite(&latE6, sizeof(int32_t), 1, out);
    fwrite(&lonE6, sizeof(int32_t), 1, out);
    fwrite(&gridSizeE6, sizeof(int32_t), 1, out);
    fwrite(&widthSwapped, sizeof(int32_t), 1, out);
    fwrite(&heightSwapped, sizeof(int32_t), 1, out);
    
    int i;
    for (i = 0; i < read; i++) {
        int16_t valueSwapped = OSSwapHostToLittleInt16(grid[i] * E4);
        fwrite(&valueSwapped, sizeof(int16_t), 1, out);
    }
    
    fclose(out);
    
    return 0;
}