amort.c

Go to the documentation of this file.

/** @file amort.c
    @brief Installment Loan Amortization
    @author Bill Waller
    Copyright (c) 2026
    MIT License
    billxwaller@gmail.com
    @date 2026-05-05
 */
#define _GNU_SOURCE
#include <locale.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#ifdef __USE_GNU_GETTEXT
#include <libintl.h>
#define _(String) gettext(String)
#define gettext_noop(String) String
#define N_(String) gettext_noop(String)
#else
#define _(String) (String)
#define N_(String) String
#define textdomain(Domain)
#define bindtextdomain(Package, Directory)
#endif
char month[12][4] = {_("Jan"), _("Feb"), _("Mar"), _("Apr"), _("May"), _("Jun"),
                     _("Jul"), _("Aug"), _("Sep"), _("Oct"), _("Nov"), _("Dec")};
typedef struct {
    double pv;
    double pmt;
    double i;
    double n;
    double interest;
    double principal;
    double year_total_interest;
} Amort;
void print_totals(Amort *);
 
int main(int argc, char **argv) {
    setlocale(LC_NUMERIC, "");
    if (argc > 1 &&
        ((strcmp(argv[1], _("--help")) == 0 || strcmp(argv[1], "-h") == 0) ||
         argc < 5)) {
        printf(_("Usage: amort [present_value][number_of_payments] [interest_rate][payment_amount][yyyy-mm-dd]\n\n"));
        exit(EXIT_SUCCESS);
    }
    struct tm tm;
    Amort *amort = malloc(sizeof(Amort));
    sscanf(argv[1], _("%lf"), &amort->pv);
    sscanf(argv[2], _("%lf"), &amort->n);
    sscanf(argv[3], _("%lf"), &amort->i);
    sscanf(argv[4], _("%lf"), &amort->pmt);
    if (amort->pv == 0 || amort->n == 0 || amort->i == 0 || amort->pmt == 0 ||
        argv[5][0] == '\0') {
        fprintf(stderr, _("Error: All arguments must be non-zero.\n"));
        exit(EXIT_FAILURE);
    }
    amort->interest = amort->pv * amort->i / 1200;
    if (amort->interest > amort->pmt) {
        fprintf(stderr, _("Error: Payment amount less than interest\n"));
        exit(EXIT_FAILURE);
    }
    memset(&tm, 0, sizeof(tm));
    if (strptime(argv[5], _("%Y-%m-%d"), &tm) == NULL)
        if (strptime(argv[5], _("%Y%m%d"), &tm) == NULL)
            printf(_("Error: Invalid date format. Use yyyy-mm-dd or yyyymmdd.\n")),
                exit(EXIT_FAILURE);
    amort->year_total_interest = 0;
 
    printf("First Payment: %04d-%02d-%02d\n\n", tm.tm_year + 1900,
           tm.tm_mon + 1, tm.tm_mday);
    printf(_(" Per  Mth Year    Balance     Payment   Principal  Interest\n"));
    printf(_("----  --- ---- ------------ ---------- ---------- ----------\n"));
    int m = 0;
    for (int x = 0; x < amort->n; x++) {
        if (amort->pv <= 0)
            break;
        m = (x + tm.tm_mon) % 12;
        int y = ((x + tm.tm_mon) / 12) + tm.tm_year + 1900;
        amort->interest = amort->pv * amort->i / 1200;
        amort->year_total_interest += amort->interest;
        if (amort->pv + amort->interest < amort->pmt) {
            amort->pmt = amort->pv + amort->interest;
            amort->principal = amort->pv;
        } else
            amort->principal = amort->pmt - amort->interest;
        amort->pv -= amort->principal;
        printf("%4d %4s %4d %'12.2f %'10.2f %'10.2f %'10.2f\n", x + 1, month[m],
               y, amort->pv, amort->pmt, amort->principal, amort->interest);
        if (m == 11)
            print_totals(amort);
    }
    if (m != 11)
        print_totals(amort);
    free(amort);
    return EXIT_SUCCESS;
}
void print_totals(Amort *amort) {
    printf(_("                                                  ----------\n"));
    printf(_("                                                  %'10.2f\n"),
           amort->year_total_interest);
    printf(_("                                                             ==========\n\n"));
    amort->year_total_interest = 0;
}