foo.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct line {
    char *foo, *bar, *  baz;
    long derp, herp;
};

int main(int argc, char const *argv[])
{
	long LOWER = atol(argv[1]);
	long UPPER = atol(argv[2]);
	long MAX_LINES = 1000000;
	FILE* input = stdin;
	if (argc == 4 && strcmp(argv[3],"-") != 0) {
		input = fopen(argv[3], "r");
	}

    struct line *lines = malloc(MAX_LINES * sizeof(*lines));
    struct line *fillptr, *left, *right_upper, *right_lower;
    fillptr = left = right_upper = right_lower = lines;

    while ( 5 == fscanf( input, "%ms %li %ms %li %ms\n", &fillptr->foo, &fillptr->derp, &fillptr->bar, &fillptr->herp, &fillptr->baz)) {
        fillptr++;
    }

    // assumes lines are sorted by `herp`.
    while ( left < fillptr) {
        while (right_lower->herp < left->herp + LOWER && right_lower < fillptr) {
            right_lower++;
        }
        while (right_upper->herp < left->herp + UPPER && right_upper < fillptr) {
            right_upper++;
        }
        struct line *ptr = right_lower;
        while (ptr < right_upper) {
            printf("%s\t%li\t%s\t%li\t%s", left->foo, left->derp, left->bar, left->herp, left->baz);
            printf("%s\t%li\t%s\t%li\t%s\n", ptr->foo, ptr->derp, ptr->bar, ptr->herp, ptr->baz);
            ptr++;
        }
        free(left->foo);
        free(left->bar);
        free(left->baz);
        left++;
    }

    free(lines);
    fclose(input);
    return 0;
}