// filename 'euler-solution-102.c'
// By: Louis Casillas, oxaric@gmail.com
// Euler Problem #102
// Determine which triangles encompass the origin.
// The file is 'triangles.txt'
#include <stdio.h>
char doesLineCrossXAxis( x1, x2 )
{
if ( ((x1 < 0) && (x2 > 0)) || ((x1 > 0) && (x2 < 0)) )
{
return 1;
}
return 0;
}
char doesLineCrossYAxis( y1, y2 )
{
if ( ((y1 < 0) && (y2 > 0)) || ((y1 > 0) && (y2 < 0)) )
{
return 1;
}
return 0;
}
char doesTriangleContainOrigin( x1, y1, x2, y2, x3, y3 )
{
char does_line1_cross_x_axis = 0;
char does_line2_cross_x_axis = 0;
char does_line3_cross_x_axis = 0;
char num_lines_cross_x_axis = 0;
// line1
if ( doesLineCrossXAxis( x1, x2 ) )
{
does_line1_cross_x_axis = 1;
num_lines_cross_x_axis++;
}
// line2
if ( doesLineCrossXAxis( x2, x3 ) )
{
does_line2_cross_x_axis = 1;
num_lines_cross_x_axis++;
}
// line3
if ( doesLineCrossXAxis( x1, x3 ) )
{
does_line3_cross_x_axis = 1;
num_lines_cross_x_axis++;
}
// two of the lines must cross the x-axis to make it possible to contain the origin
if ( num_lines_cross_x_axis < 2 )
{
return 0;
}
float line1_slope = 0;
float line2_slope = 0;
float line3_slope = 0;
float line1_x_intercept = 0;
float line2_x_intercept = 0;
float line3_x_intercept = 0;
if ( does_line1_cross_x_axis )
{
line1_slope = (1.0 * y1 - y2) / (x1 - x2);
line1_x_intercept = y1 - ( 1.0 * line1_slope * x1 );
}
if ( does_line2_cross_x_axis )
{
line2_slope = (1.0 * y2 - y3) / (x2 - x3);
line2_x_intercept = y2 - ( 1.0 * line2_slope * x2 );
}
if ( does_line3_cross_x_axis )
{
line3_slope = (1.0 * y1 - y3) / (x1 - x3);
line3_x_intercept = y3 - ( 1.0 * line3_slope * x3 );
}
if ( ((line1_x_intercept > 0) && (line2_x_intercept < 0)) ||
((line2_x_intercept > 0) && (line3_x_intercept < 0)) ||
((line1_x_intercept > 0) && (line3_x_intercept < 0)) ||
((line1_x_intercept < 0) && (line2_x_intercept > 0)) ||
((line2_x_intercept < 0) && (line3_x_intercept > 0)) ||
((line1_x_intercept < 0) && (line3_x_intercept > 0))
)
{
return 1;
}
return 0;
}
int main()
{
int number_holder = 0;
char current_point = 1;
char current_char;
char is_negative_number = 0;
int num_that_contain_origin = 0;
int x1 = 0;
int y1 = 0;
int x2 = 0;
int y2 = 0;
int x3 = 0;
int y3 = 0;
//printf( "\n\n%d\n\n", (int)doesTriangleContainOrigin( -340, 495, -153, -910, 835, -947 ) );
FILE *in_file = fopen("triangles.txt", "rt");
while( (current_char = fgetc( in_file )) != EOF )
{
if ( current_char == '\r' )
{
}
else if ( current_char == '\n' )
{
if ( is_negative_number )
{
number_holder *= -1;
}
y3 = number_holder;
number_holder = 0;
is_negative_number = 0;
// printf( "\n\n%d\t%d\t%d\t%d\t%d\t%d", x1, y1, x2, y2, x3, y3 );
//getchar();
if ( doesTriangleContainOrigin( x1, y1, x2, y2, x3, y3 ) )
{
num_that_contain_origin++;
}
current_point = 1;
x1 = 0;
y1 = 0;
x2 = 0;
y2 = 0;
x3 = 0;
y3 = 0;
}
else if ( current_char == ',' )
{
if ( is_negative_number )
{
number_holder *= -1;
}
switch( current_point )
{
case 1:
x1 = number_holder;
break;
case 2:
y1 = number_holder;
break;
case 3:
x2 = number_holder;
break;
case 4:
y2 = number_holder;
break;
case 5:
x3 = number_holder;
break;
}
current_point++;
number_holder = 0;
is_negative_number = 0;
}
else
{
// ascii value of '-' is 45, base 10
if ( (int)current_char == 45 )
{
is_negative_number = 1;
}
else
{
number_holder *= 10;
number_holder += (current_char - '0');
}
}
}
printf( "\n\nThe number of triangles that contain the origin is: %lu\n\n", num_that_contain_origin );
fclose( in_file );
printf("\n\n");
return 0;
}
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