Convert yi to be a sample number

1 files changed, 45 insertions, 13 deletions

diff --git a/dda.c b/dda.c
index 354a59d..25dbd6c 100644
--- a/dda.c
+++ b/dda.c
@@ -7,6 +7,12 @@
 #define N_STEPS  (4)
 #define STEP  (1/((double)N_STEPS))
 
+#define N_GRID_X               3
+#define N_GRID_Y               3
+
+#define BIG_STEP_Y             0.5
+#define SMALL_STEP_Y           0.25
+#define FIRST_STEP_Y           0.125
 
 static double
 next_sample_x (double x)
@@ -30,9 +36,30 @@ next_sample_x (double x)
 }
 
 static double
+sample_to_pos (int y)
+{
+    return (((unsigned)y) >> 8) + (y & 0xff) * SMALL_STEP_Y + FIRST_STEP_Y;
+}
+
+static int
 next_sample_y (double y)
 {
-    return next_sample_x (y);
+    int i = floor (y);
+    int sno = floor ((y - i - FIRST_STEP_Y) / SMALL_STEP_Y) + 1;
+    int r;
+
+    if (sno == N_GRID_X)
+	sno++;
+
+    if (sno > N_GRID_X)
+    {
+	sno -= (N_GRID_X + 1);
+	i++;
+    }
+
+    r = (i << 8) | sno;
+
+    return r;
 }
 
 static double
@@ -53,12 +80,6 @@ sample_step_x_backward (double x)
 	return 0.25;
 }
 
-static double
-sample_step_y (double y)
-{
-    return sample_step_x_forward (y);
-}
-
 static void
 dda (test_data_t *testdata)
 {
@@ -70,9 +91,9 @@ dda (test_data_t *testdata)
 
     double dx = (x1 - x0);
     double dy = (y1 - y0);
-    double yi = next_sample_y (y0);
+    int yi = next_sample_y (y0);
     double xi = next_sample_x (x0);
-    double e = (xi - x0) * dy - (yi - y0) * dx;
+    double e = (xi - x0) * dy - (sample_to_pos (yi) - y0) * dx;
 
 #if 0
     printf ("= = = = %f %f %f %f = = = = \n", x0, y0, x1, y1);
@@ -104,14 +125,25 @@ dda (test_data_t *testdata)
 	}
 
 	if (testdata->points[i++] != xi		||
-	    testdata->points[i++] != yi)
+	    testdata->points[i++] != sample_to_pos (yi))
 	{
-	    printf ("error %f %f\n", testdata->points[i - 1], xi);
+	    printf ("%d error %f %f\n", i, testdata->points[i - 1], sample_to_pos (yi));
 	    exit (-1);
 	}
 
-	e -= sample_step_y (yi) * dx;
-	yi += sample_step_y (yi);
+	if ((yi & 0xff) == (N_GRID_Y - 1))
+	{
+	    e -= BIG_STEP_Y * dx;
+
+	    yi += (1 << 8);
+	    yi &= 0xffffff00;
+	}
+	else
+	{
+	    e -= SMALL_STEP_Y * dx;
+
+	    yi++;
+	}
     }
 }