Re: compound interest

/*
Here is your opportunity to show us how it can be improved.
This is an interest calculation simulation.
Please improve the algorithm so that it is significantly faster.
*/
#include <math.h>
#include <time.h>
#include <stdio.h>
#include <stdlib.h>

double yearly_interest(double principal, double rate, double
years)
{
double amount = principal * pow(1.0 + rate, years) -
principal;
return amount;
}

double quarterly_interest(double principal, double rate,
double years)
{
double amount = principal * pow(1.0 + rate * 0.25, 4.0 *
years) - principal;
return amount;
}

static const double years_per_month = 1.0 / 12.0;

double monthly_interest(double principal, double rate, double
years)
{
double amount = principal * pow(1.0 + rate *
years_per_month, 12.0 * years) - principal;
return amount;
}

static const double years_per_day = 1.0 / 365.2421875;

double daily_interest(double principal, double rate, double
years)
{
double amount = principal * pow(1.0 + rate *
years_per_day, 365.2421875 * years) - principal;
return amount;
}

double continuous_interest(double principal, double rate,
double years)
{
double amount = principal * exp(rate * years) -
principal;
return amount;
}

typedef enum frequency {
YEARLY = 0, QUARTERLY, MONTHLY, DAILY, CONTINUOUSLY
} frequency;

typedef struct customer_record {
int custid;
double last_balance;
double deposits;
double withdrawls;
double fees;
double rate;
time_t date_of_last_balance;
enum frequency freq;
} customer_record;

static const double seconds_per_day = 1.0 / 86400.0;

/* round number n to d decimal points */

static double round(double n, unsigned d)
{
return floor(n * pow(10., d) + .5) / pow(10., d);
}

void make_500k_customers(void)
{
FILE *foutput = fopen("customer_extract.inp", "wb");
customer_record cr = {0};
int i;
for (i = 1; i <= 500000; i++) {
int irate;
cr.custid = i;
cr.last_balance = rand() * rand() + rand() / (rand() + 1);
cr.last_balance = round(cr.last_balance, 2);
cr.deposits = rand() + rand() / (rand() + 1);
cr.withdrawls = rand() / (rand() + 1);
cr.fees = rand() / (rand() + 1);
irate = 3 + rand() / (rand() + 1);
cr.rate = irate % 200;
cr.rate *= .01;
cr.date_of_last_balance = time(NULL) - rand();
cr.freq = (frequency) rand() % 5;
if (fwrite(&cr, sizeof cr, 1, foutput) != 1) {
printf("Error processing customer %d\n", cr.custid);
}
}
fclose(foutput);
}

int main(int argc, char **argv)
{
double average_balance;
double interest;
customer_record cr;
FILE *finput;
FILE *foutput;
clock_t start, end;
if (argc > 1) {
start = clock();
puts("Creating customers...");
puts("Time includes customer creation.");
make_500k_customers();

}
else
{
puts("Time does not include customer creation.");
start = clock();
}
finput = fopen("customer_extract.inp", "rb");
foutput = fopen("customer_extract.out", "wb");
while (fread(&cr, sizeof cr, 1, finput) == 1) {
double ending_balance = cr.last_balance + cr.deposits
- (cr.withdrawls + cr.fees);
time_t now = time(NULL);
double duration_in_seconds = difftime(now,
cr.date_of_last_balance);
double days = duration_in_seconds * seconds_per_day;
double years = days * years_per_day;
average_balance = (cr.last_balance + ending_balance) * 0.5;
if (average_balance > 0) {
switch (cr.freq) {
case YEARLY:
interest = yearly_interest(average_balance, cr.rate,
years);
break;
case QUARTERLY:
interest = quarterly_interest(average_balance,
cr.rate, years);
break;
case MONTHLY:
interest = monthly_interest(average_balance, cr.rate,
years);
break;
case DAILY:
interest = daily_interest(average_balance, cr.rate,
years);
break;
case CONTINUOUSLY:
interest = continuous_interest(average_balance,
cr.rate, years);
break;
default:
printf("Unexpected interst rate type for customer %d
\n", cr.custid);
break;
}
cr.last_balance += interest;
cr.last_balance = round(cr.last_balance, 2);
cr.deposits = 0;
cr.withdrawls = 0;
cr.fees = 0;
cr.date_of_last_balance = now;
if (fwrite(&cr, sizeof cr, 1, foutput) != 1) {
printf("Error processing customer %d\n", cr.custid);
}
fflush(foutput);
}
}
if (finput)
fclose(finput);
if (foutput)
fclose(foutput);
end = clock();
printf("Total clock ticks to process the interest rates in the
customer file was %u\n", (unsigned) end - start);
return 0;
}
/*
Sample output:
C:\tmp\inp\Release>inp 1
Creating customers...
Time includes customer creation.
Total clock ticks to process the interest rates in the customer file
was 2468

C:\tmp\inp\Release>inp
Time does not include customer creation.
Total clock ticks to process the interest rates in the customer file
was 1875

C:\tmp\inp\Release>inp
Time does not include customer creation.
Total clock ticks to process the interest rates in the customer file
was 1765
*/


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