Solution to DailyProgrammer Day 351 [Intermediate]

intermediate.c

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

#define ever (;;)

typedef struct {
    size_t size;
    char *names;

    size_t *positions;
} Programs;

static Programs *programs_new(char *names) {
    Programs *programs  = malloc(sizeof(Programs));
    programs->size      = strlen(names);
    programs->names     = names;
    programs->positions = calloc(programs->size, sizeof(size_t));
    for (size_t i = 0; i < programs->size; ++i) programs->positions[i] = i;
    return programs;
}

static void programs_free(Programs *programs) {
    free(programs->names);
    free(programs->positions);
    free(programs);
}

static void programs_print(const Programs *programs) {
    char *result = calloc(programs->size + 1, sizeof(char));

    for (size_t i = 0, l = programs->size; i < l; ++i) {
        result[programs->positions[i]] = programs->names[i];
    }

    printf("%s\n", result);
    free(result);
}

static void programs_spin(Programs *programs, size_t n) {
    for (size_t i = 0, l = programs->size; i < l; ++i) {
        programs->positions[i] = (programs->positions[i] + n) % l;
    }
}

static void programs_exchange(Programs *programs, size_t a, size_t b) {
    size_t a_index = programs->size,
           b_index = programs->size;

    for (size_t i = 0, l = programs->size; i < l; ++i) {
        size_t position = programs->positions[i];

        if (position == a) {
            a_index = i;
        }
        if (position == b) {
            b_index = i;
        }

        if (a_index < l && b_index < l) {
            size_t temp = programs->positions[a_index];
            programs->positions[a_index] = programs->positions[b_index];
            programs->positions[b_index] = temp;
            return;
        }
    }

    fputs("Wrongful exchange.", stderr);
    exit(EXIT_FAILURE);
}

static void programs_partner(Programs *programs, size_t a, size_t b) {
    size_t temp = programs->positions[a];
    programs->positions[a] = programs->positions[b];
    programs->positions[b] = temp;
}

int main(/*int argc, char *argv[]*/) {
    char *line = NULL;
    size_t _n = 0;
    if (getline(&line, &_n, stdin) == -1) {
        perror("Error reading program names.");
        exit(EXIT_FAILURE);
    }

    const size_t line_length = strlen(line);
    if (line[line_length - 1] == '\n') {
        line[line_length - 1] = 0;
    }

    Programs *programs = programs_new(line);

    for ever {
        int c = getchar();
        size_t a, b;

        switch(c) {
            case 's':
                scanf("%lu,", &a);
                programs_spin(programs, a);
                break;

            case 'x':
                scanf("%lu/%lu,", &a, &b);
                programs_exchange(programs, a, b);
                break;

            case 'p':
                scanf("%lu/%lu,", &a, &b);
                programs_partner(programs, a, b);
                break;

            case EOF:
                goto cleanup;

            case '\n':
                break;

            default:
                fprintf(stderr, "Unknown dance move '%c'\n", c);
                exit(EXIT_FAILURE);
        }

    }

cleanup:
    programs_print(programs);
    programs_free(programs);
}