string_formatter.c

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

// Naive algorithm to "shift" a decimal number left `length` digits.
int stretch(int n, int length) {
    for (int i = 0; i < length; ++i) {
        n *= 10;
    }

    return n;
}

#define XOR_SWAP(a,b) do\
    {\
      a ^= b;\
      b ^= a;\
      a ^= b;\
    } while (0)


// String reversing function copied straight from stack overflow.
void reverse_string(char * str)
{
  if (str)
  {
    char * end = str + strlen(str) - 1;

    // walk inwards from both ends of the string,
    // swapping until we get to the middle
    while (str < end)
    {
      XOR_SWAP(*str, *end);
      str++;
      end--;
    }
  }
}

// mult takes a number and pads it out with the specified number of zeroes. It
// then writes that number to the provided buffer, making sure to group into
// threes for readability.
void mult(int number, int size, char* buffer, int buffer_length) {
    int i = 0, digits_in_group = 0;

    // Add the specified number of "0" digits to some temporary number
    int temp = stretch(number, size);

    while (temp > 0 && i < buffer_length) {
        // Get the rightmost digit and add it to our buffer
        int current_digit = temp % 10;
        buffer[i] = current_digit + '0';
        digits_in_group++;

        // Add the extra space if necessary (making sure to skip when the final
        // string length is a multiple of 3)
        if (digits_in_group > 2 && temp >= 10) {
            i++;
            buffer[i] = ' ';
            digits_in_group = 0;
        }

        temp /= 10;
        i++;
    }

    // Finally, we need to reverse the string because we populated it backwards
    reverse_string(buffer);
}

typedef struct Input {
    int number;
    int length;
    char* should_be;
} Input;


int main() {
    Input inputs[7] = {
        {51, 0, "51"},
        {51, 1, "510"},
        {51, 2, "5 100"},
        {51, 3, "51 000"},
        {51, 4, "510 000"},
        {51, 5, "5 100 000"},
        {51, 6, "51 000 000"},
    };

    for (int i = 0; i < 7; ++i) {
        Input input = inputs[i];

        printf("mult(%d, %d) => %10s   ", input.number, input.length, input.should_be);

        // Create a temporary buffer and clear it
        char buffer[15];
        memset(buffer, 0, sizeof(buffer));

        mult(input.number, input.length, buffer, 15);

        if (strcmp(buffer, input.should_be)) {
            printf("✘ (got \"%s\")\n", buffer);
        } else {
            printf("✓\n");
        }
    }
}