From 7408c48dff1e7679ec0adb73d370ca66e6407caf Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Nils=20O=2E=20Sel=C3=A5sdal?= <nos@utel.tech>
Date: Sun, 20 Jul 2025 21:44:11 +0200
Subject: [PATCH] Add uc_bv_find_first_zero

---
 include/ucore/bitvec.h |  7 ++++++-
 src/bitvec.c           | 18 ++++++++++++++++++
 test/test_bitvec.c     | 27 +++++++++++++++++++++++++++
 3 files changed, 51 insertions(+), 1 deletion(-)

diff --git a/include/ucore/bitvec.h b/include/ucore/bitvec.h
index bb5dfe1..a141bdc 100644
--- a/include/ucore/bitvec.h
+++ b/include/ucore/bitvec.h
@@ -68,11 +68,16 @@ void uc_bv_set_all(struct UCBitVec *v);
 /** Initialize bits from a array of ints, each array element maps to one bit.
  * The bits are initialized from the int array so zero maps to zero and non-zero maps to one.
  * e..g to set the 5 first bits, to 01110:
- * int a[] = {0,1,1,1,0};
+ * int a[] = {0,1,1,1,0};
  * set_bits_from_array(v,a,5);
  * */
 void uc_bv_set_bits_from_array(struct UCBitVec *v,const char *array,size_t array_len);
 
+/** Find the index of the first zero bit in the bitvec.
+ * @param v The bitvec to search
+ * @return The index of the first zero bit, or -1 if all bits are set
+ */
+int uc_bv_find_first_zero(const struct UCBitVec *v);
 
 #ifdef __cplusplus
 }
diff --git a/src/bitvec.c b/src/bitvec.c
index 6fb5892..d168f84 100644
--- a/src/bitvec.c
+++ b/src/bitvec.c
@@ -2,6 +2,7 @@
 #include <stdio.h>
 #include <string.h>
 #include <assert.h>
+#include "ucore/utils.h"
 #include "ucore/bitvec.h"
 
 #ifdef __GNUC__
@@ -12,6 +13,8 @@
 #define unlikely(expr)  (expr)
 #endif
 
+UC_STATIC_ASSERT(CHAR_BIT % 8 == 0);
+
 static inline int bit_index(int b)
 {
      return b / (sizeof(uc_bv_integer) * CHAR_BIT);
@@ -88,3 +91,18 @@ void uc_bv_set_all(struct UCBitVec *v)
     memset(v->vec,0xff,v->vec_len * sizeof(uc_bv_integer));
 }
 
+int uc_bv_find_first_zero(const struct UCBitVec *v)
+{
+    const size_t bits_per_word = sizeof(uc_bv_integer) * CHAR_BIT;
+
+    for (size_t i = 0; i < v->vec_len; i++) {
+        uc_bv_integer inverted = ~v->vec[i];
+
+        if (inverted != 0) {
+            int bit_pos = __builtin_ffsl(inverted) - 1; // ffsl returns 1-based index
+            return (int)(i * bits_per_word + bit_pos);
+        }
+    }
+
+    return -1;
+}
diff --git a/test/test_bitvec.c b/test/test_bitvec.c
index 4676114..a3ec81c 100644
--- a/test/test_bitvec.c
+++ b/test/test_bitvec.c
@@ -79,6 +79,31 @@ START_TEST (test_bitvec_setall_clearall)
 
     uc_bv_free(v);
 
+}
+END_TEST
+START_TEST (test_bitvec_find_first_zero)
+{
+    uc_bv_integer buf[8];
+    struct UCBitVec v = UC_BV_STATIC_INIT(buf);
+    uc_bv_set_all(&v);
+
+    int bit = uc_bv_find_first_zero(&v);
+    ck_assert_int_eq(bit, -1);
+
+    uc_bv_clr_bit(&v, 0);
+    bit = uc_bv_find_first_zero(&v);
+    ck_assert_int_eq(bit, 0);
+
+    uc_bv_set_bit(&v, 0);
+    uc_bv_clr_bit(&v, 63);
+    bit = uc_bv_find_first_zero(&v);
+    ck_assert_int_eq(bit, 63);
+
+    uc_bv_set_bit(&v, 63);
+    uc_bv_clr_bit(&v, sizeof buf * CHAR_BIT - 1);
+    bit = uc_bv_find_first_zero(&v);
+    ck_assert_int_eq(bit, sizeof buf * CHAR_BIT - 1);
+
 }
 END_TEST
 
@@ -91,6 +116,8 @@ Suite *bitvec_suite(void)
     tcase_add_test(tc, test_bitvec_set_bits_from_array);
     tcase_add_test(tc, test_bitvec_clearbit);
     tcase_add_test(tc, test_bitvec_setall_clearall);
+    tcase_add_test(tc, test_bitvec_find_first_zero);
+
     suite_add_tcase(s, tc);
 
     return s;