Add error tracking

1 files changed, 48 insertions, 5 deletions

diff --git a/dda.c b/dda.c
index 6915d8b..55e2e1c 100644
--- a/dda.c
+++ b/dda.c
@@ -36,7 +36,7 @@ next_sample_y (double y)
 }
 
 static double
-sample_step_x (double x)
+sample_step_x_forward (double x)
 {
     if ((x - floor (x)) == 0.625)
 	return 0.5;
@@ -45,9 +45,18 @@ sample_step_x (double x)
 }
 
 static double
+sample_step_x_backward (double x)
+{
+    if ((x - floor (x)) == 0.125)
+	return 0.5;
+    else
+	return 0.25;
+}
+
+static double
 sample_step_y (double y)
 {
-    return sample_step_x (y);
+    return sample_step_x_forward (y);
 }
 
 static void
@@ -63,18 +72,52 @@ dda (test_data_t *testdata)
     double y = next_sample_y (y0);
     double x = (y - y0) * dxdy + x0;
     double xi = next_sample_x (x);
+    double e = xi - x;
+
+#if 0
+    printf ("= = = = %f %f %f %f = = = = \n", x0, y0, x1, y1);
+#endif
 
     while (y < next_sample_y (y1))
     {
 	if (testdata->points[i++] != xi		||
 	    testdata->points[i++] != y)
 	{
-	    printf ("error\n");
+	    printf ("error %f %f %f\n", x, testdata->points[i - 1], xi);
 	    exit (-1);
 	}
 
-	x += sample_step_y (y) * dxdy;
-	xi = next_sample_x (x);
+	if (dxdy >= 0)
+	{
+	    e -= sample_step_y (y) * dxdy;
+	    x += sample_step_y (y) * dxdy;
+
+	    while (e <= 0)
+	    {
+		double s = sample_step_x_forward (xi);
+
+		e += s;
+		xi += s;
+	    }
+	}
+	else
+	{
+	    double s;
+
+	    e -= sample_step_y (y) * dxdy;
+	    x += sample_step_y (y) * dxdy;
+
+	    s = sample_step_x_backward (xi);
+
+	    while (e > s)
+	    {
+		e -= s;
+		xi -= s;
+
+		s = sample_step_x_backward (xi);
+	    }
+	}
+
 	y += sample_step_y (y);
     }
 }