Fix incorrect DROP_SIZE usage

Previously, measure() only recorded timings for indices in the middle
range [DROP_SIZE, N_MEASURES - DROP_SIZE), while update_statistics()
assumed all entries were starting available from index 10. This
mismatch allowed zero-valued exec_times from unmeasured head and tail
indices to be included in the t-test, reducing sample means, inflating
variances, and suppressing the t-value, which may lead to
incorrect results or prevent detection thresholds from being reached.

After the fix, all samples are measured and
update_statistics() discards DROP_SIZE samples at both ends. This
ensures correct sample accounting, prevents overestimating the number
of measurements required, and avoids false negatives due to
uninitialized timing data.

Change-Id: Ibb1515043da5f56d72fe34fd5c78e2283df9a993

Author: Mike1117

dudect/constant.c

@@ -77,7 +77,7 @@ bool measure(int64_t *before_ticks,
 
     switch (mode) {
     case DUT(insert_head):
-        for (size_t i = DROP_SIZE; i < N_MEASURES - DROP_SIZE; i++) {
+        for (size_t i = 0; i < N_MEASURES; i++) {
             char *s = get_random_string();
             dut_new();
             dut_insert_head(
@@ -94,7 +94,7 @@ bool measure(int64_t *before_ticks,
         }
         break;
     case DUT(insert_tail):
-        for (size_t i = DROP_SIZE; i < N_MEASURES - DROP_SIZE; i++) {
+        for (size_t i = 0; i < N_MEASURES; i++) {
             char *s = get_random_string();
             dut_new();
             dut_insert_head(
@@ -111,7 +111,7 @@ bool measure(int64_t *before_ticks,
         }
         break;
     case DUT(remove_head):
-        for (size_t i = DROP_SIZE; i < N_MEASURES - DROP_SIZE; i++) {
+        for (size_t i = 0; i < N_MEASURES; i++) {
             dut_new();
             dut_insert_head(
                 get_random_string(),
@@ -129,7 +129,7 @@ bool measure(int64_t *before_ticks,
         }
         break;
     case DUT(remove_tail):
-        for (size_t i = DROP_SIZE; i < N_MEASURES - DROP_SIZE; i++) {
+        for (size_t i = 0; i < N_MEASURES; i++) {
             dut_new();
             dut_insert_head(
                 get_random_string(),
@@ -147,7 +147,7 @@ bool measure(int64_t *before_ticks,
         }
         break;
     default:
-        for (size_t i = DROP_SIZE; i < N_MEASURES - DROP_SIZE; i++) {
+        for (size_t i = 0; i < N_MEASURES; i++) {
             dut_new();
             dut_insert_head(
                 get_random_string(),

dudect/fixture.c

@@ -103,7 +103,7 @@ static void update_statistics(const int64_t *exec_times,
                               uint8_t *classes,
                               int64_t *percentiles)
 {
-    for (size_t i = 0; i < N_MEASURES; i++) {
+    for (size_t i = DROP_SIZE; i < N_MEASURES - DROP_SIZE; i++) {
         int64_t difference = exec_times[i];
         /* CPU cycle counter overflowed or dropped measurement */
         if (difference <= 0)