iloan.c

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/** @file iloan.c
    @brief Installment Loan Calculator
    @author Bill Waller
    Copyright (c) 2025
    MIT License
    billxwaller@gmail.com
    @date 2026-02-09
 */
 
#include <math.h>
#include <signal.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
 
#define FALSE 0
#define TRUE 1
char in_str[BUFSIZ + 1];
 
void numbers(char *d, char *s);
double calculate_i(double, double, double);
double calculate_n(double, double, double);
double calculate_pmt(double, double, double);
double calculate_pv(double, double, double);
int is_numeric(char *);
void accept_str(char *s);
char *format_currency(float);
char *format_interest(float);
double accept_pv();
double accept_n();
double accept_i();
double accept_pmt();
void error_press_any_key(char *);
void ABEND(int);
int f_pv = 0;
int f_n = 0;
int f_i = 0;
int f_pmt = 0;
bool f_quiet = false;
 
/** @brief iloan is a trivial application to demonstrate how a command-line
  program can be integrated into C-Menu Form with simple file, argument, or pipe
  i-o.
  @details Positional arguments:
        present_value The present value of the loan (the amount borrowed).
        number_of_payments The total number of payments to be made.
        interest_rate The annual interest rate (as a percentage).
        payment_amount The amount of each payment.
  The program calculates the missing value based on the three provided
  values. For example, if the present value, number of payments, and interest
  rate are provided, it will calculate the payment amount. If the present value,
  interest rate, and payment amount are provided, it will calculate the number
  of payments, and so on.
  The program can be used in a non-interactive way by passing the four
  values as arguments, with the value to be calculated set to 0. For example, to
  calculate the payment amount for a $10,000 loan with a 5% annual interest rate
  and 60 monthly payments, you could run:
  @code
    iloan 10000 60 5 0
  @endcode
  This is proof-of-concept code, and is not intended for production use.
  It is not designed to be robust, and does not handle all edge cases or input
  errors. It is intended solely to demonstrate how a simple command-line program
  can be integrated into C-Menu Form.
  In the future, more sophisticated abstractions, such as async event
  handlers, serialization, rpc, database, and a standardized plugin interface
  will be integrated into C-Menu.
  Feel free to modify and enhance this code as needed, but please do not
  use it in production without proper testing and validation. It is provided
  as-is without any warranty or support. Use at your own risk.
*/
int main(int argc, char **argv) {
    double pv = 0, pmt = 0, i = 0, n = 0;
    char tmp_str[BUFSIZ];
 
    signal(SIGINT, ABEND);
    signal(SIGQUIT, ABEND);
    signal(SIGHUP, ABEND);
 
    if (argc > 1 &&
        ((strcmp(argv[1], "--help") == 0 || strcmp(argv[1], "-h") == 0) ||
         argc < 4)) {
        printf("Usage: iloan [present_value number_of_payments "
               "interest_rate payment_amount]\n\n");
        exit(EXIT_SUCCESS);
    }
    if (argc > 4) {
        numbers(tmp_str, argv[1]);
        sscanf(tmp_str, "%lf", &pv);
        sscanf(argv[2], "%lf", &n);
        sscanf(argv[3], "%lf", &i);
        numbers(tmp_str, argv[4]);
        sscanf(tmp_str, "%lf", &pmt);
        if (pv != 0)
            f_pv = 1;
        if (n != 0)
            f_n = 1;
        if (i != 0)
            f_i = 1;
        if (pmt != 0)
            f_pmt = 1;
        if (f_pv + f_n + f_i + f_pmt < 3) {
            error_press_any_key(
                "Error: At least three values must be greater than zero.");
            exit(EXIT_FAILURE);
        }
        f_quiet = true;
    } else {
        if (argc != 1) {
            printf("Usage: iloan [present_value number_of_payments "
                   "interest_rate payment_amount]\n\n");
            exit(EXIT_FAILURE);
        } else {
            if (argc == 1) {
                printf("\nInstallment Loan Calculator\n\n");
                printf("Three of these values must be greater than 0.  The "
                       "field\n");
                printf("with a value of 0 will be calculated.\n\n");
                while (f_pv + f_n + f_i + f_pmt < 3) {
                    if (pv == 0) {
                        pv = accept_pv();
                        if (pv != 0)
                            f_pv = 1;
                    }
                    if (n == 0) {
                        n = accept_n();
                        if (n != 0)
                            f_n = 1;
                    }
                    if (i == 0) {
                        i = accept_i();
                        if (i != 0)
                            f_i = 1;
                    }
                    if (pmt == 0) {
                        pmt = accept_pmt();
                        if (pmt != 0)
                            f_pmt = 1;
                    }
                    if (f_pv + f_n + f_i + f_pmt < 3) {
                        error_press_any_key("Error: At least three values must "
                                            "be greater than zero.");
                    }
                }
                printf("\nYou entered:\n\n");
                if (pv != 0)
                    printf("Present Value - - - - - -> %s\n",
                           format_currency(pv));
                if (n != 0)
                    printf("Number of Payments  - - -> %s\n",
                           format_currency(n));
                if (i != 0)
                    printf("Interest Rate - - - - - -> %s\n",
                           format_interest(i));
                if (pmt != 0)
                    printf("Payment Amount  - - - - -> %s\n",
                           format_currency(pmt));
                printf("\n\nCalculation result:\n\n");
            }
        }
    }
    if (pv == 0)
        pv = calculate_pv(n, i, pmt);
    else if (n == 0)
        n = calculate_n(pv, i, pmt);
    else if (i == 0)
        i = calculate_i(pv, n, pmt);
    else if (pmt == 0)
        pmt = calculate_pmt(pv, n, i);
 
    if (f_quiet) {
        printf("%s\n", format_currency(pv));
        printf("%s\n", format_currency(n));
        printf("%s\n", format_interest(i));
        printf("%s\n", format_currency(pmt));
    }
    signal(SIGINT, SIG_DFL);
    signal(SIGQUIT, SIG_DFL);
    signal(SIGHUP, SIG_DFL);
}
 
double accept_pv() {
    double pv;
    while (1) {
        accept_str("Present Value - - -> ");
        if (in_str[0] == '\0') {
            pv = 0;
            break;
        } else {
            if (is_numeric(in_str)) {
                pv = atof(in_str);
                if (pv < 0)
                    error_press_any_key("Present Value can't be less than 0");
                else
                    break;
            } else
                error_press_any_key("Present Value must be numeric");
        }
    }
    return (pv);
}
 
double accept_n() {
    double n;
    while (1) {
        accept_str("Number of Payments > ");
        if (is_numeric(in_str)) {
            n = atof(in_str);
            if (n < 0)
                error_press_any_key("Number of Payments can't be less than 0");
            else
                break;
        } else
            error_press_any_key("Number of Payments must be numeric");
    }
    return (n);
}
 
double accept_i() {
    double i;
    while (1) {
        accept_str("Rate (annual) - - -> ");
        if (in_str[0] == '\0') {
            i = 0;
            break;
        } else {
            if (is_numeric(in_str)) {
                i = atof(in_str);
                if (i < 0)
                    error_press_any_key("interest Rate can't be less than 0");
                else
                    break;
            } else
                error_press_any_key("Interest Rate must be numeric");
        }
    }
    return (i);
}
 
double accept_pmt() {
    double pmt;
    while (1) {
        accept_str("Payment Amount  - -> ");
        if (in_str[0] == '\0') {
            pmt = 0;
            break;
        } else {
            if (is_numeric(in_str)) {
                pmt = atof(in_str);
                break;
            } else
                error_press_any_key("Payment Amount must be numeric");
        }
    }
    return (pmt);
}
 
void error_press_any_key(char *s) {
    printf("%s", s);
    getc(stdin);
    printf("\n\n");
}
 
double calculate_pv(double n, double i, double pmt) {
    double i1, pv;
 
    if (n == 0 || i == 0 || pmt == 0)
        error_press_any_key(
            "3 non-zero values required to calculate Present Value");
    i1 = i / 1200;
    pv = pmt * (1 - pow(1 + i1, -n)) / i1;
    if (!f_quiet)
        printf("Present Value - - - - - -> %s\n", format_currency(pv));
    return (pv);
}
 
double calculate_n(double pv, double i, double pmt) {
    double i1, n;
    if (pv == 0 || i == 0 || pmt == 0)
        error_press_any_key("3 non-zero values required to calculate "
                            "Number of Payments");
    i1 = i / 1200;
    n = -log(1 - pv * i1 / pmt) / log(1 + i1);
    if (!f_quiet)
        printf("Number of Payments  - - -> %s\n", format_currency(n));
    return (n);
}
 
double calculate_i(double pv, double n, double pmt) {
    double i1 = 0, i;
    double delta = 0.0000001;
    double xdelta;
    double fdelta;
    double fprimi;
    double ffact;
    double fi;
    double fman;
    double fmann;
    if (pv == 0 || n == 0 || pmt == 0)
        error_press_any_key(
            "3 non-zero values required to calculate Interest Rate");
    ffact = pv / pmt;
    xdelta = 0;
    if (ffact < n) {
        i1 = 0.0125;
        xdelta = 0.9;
    }
    while (xdelta > delta) {
        fman = 1 / (1 + i1);
        fmann = pow(fman, n);
        fi = ffact * i1 + fmann - 1;
        fprimi = ffact - n * fmann * fman;
        fdelta = fi / fprimi;
        i1 = i1 - fdelta;
        if (fdelta < 0)
            xdelta = -fdelta;
        else
            xdelta = fdelta;
    }
    i = i1 * 1200;
    if (!f_quiet)
        printf("interest Rate - - - - - -> %s\n", format_interest(i));
    return (i);
}
 
double calculate_pmt(double pv, double n, double i) {
    double i1, pmt;
    if (pv == 0 || n == 0 || i == 0)
        error_press_any_key(
            "3 non-zero values required to calculate Payment Amount");
    i1 = i / 1200;
    pmt = pv * i1 / (1 - pow(1 + i1, -n));
    if (!f_quiet)
        printf("Payment Amount  - - - - -> %s\n", format_currency(pmt));
    return (pmt);
}
 
int is_numeric(char *s) {
    char c;
 
    while ((c = *s++) != '\0' && c != '\n')
        if ((c < '0' || c > '9') && c != '.' && c != ',')
            return (FALSE);
    return (TRUE);
}
 
void accept_str(char *s) {
    fprintf(stderr, "%s", s);
    read(0, in_str, BUFSIZ);
}
 
char *format_currency(float a) {
    int digit_count, left_of_point;
    static char sstr[80];
    static char fstr[80];
    char *src_ptr, *dst_ptr, *sptr, *FPtr;
 
    sprintf(sstr, "%-18.2f", a);
    sptr = sstr;
    src_ptr = sstr;
    FPtr = fstr;
    dst_ptr = fstr;
    while (*src_ptr++ != '\0')
        ;
    src_ptr -= 2;
    left_of_point = FALSE;
    digit_count = 0;
    while (src_ptr >= sptr) {
        if (left_of_point) {
            if (*src_ptr >= '0' && *src_ptr <= '9') {
                if (digit_count == 3) {
                    *dst_ptr++ = ',';
                    digit_count = 1;
                } else
                    digit_count++;
            }
        } else if (*src_ptr == '.')
            left_of_point = TRUE;
        *dst_ptr++ = *src_ptr--;
    }
    src_ptr = dst_ptr - 1;
    dst_ptr = sptr;
    while (src_ptr >= FPtr)
        *dst_ptr++ = *src_ptr--;
    while (*--dst_ptr == ' ')
        ;
    *++dst_ptr = '\0';
    return (sptr);
}
 
char *format_interest(float a) {
    static char sstr[80];
    char *s;
 
    sprintf(sstr, "%-3.5f", a);
    s = sstr;
    return (s);
}
 
void ABEND(int e) {
    printf("ABEND: Error %d:\n", e);
    exit(EXIT_FAILURE);
}
 
void numbers(char *d, char *s) {
    while (*s != '\0') {
        if (*s == '-' || *s == '.' || (*s >= '0' && *s <= '9'))
            *d++ = *s++;
        else
            s++;
    }
    *d = '\0';
}