From cc7fc66899087bb28177dfd6e2c8bd2cfac41190 Mon Sep 17 00:00:00 2001
From: Patrick Pelissier <patrick.pelissier@gmail.com>
Date: Mon, 29 Aug 2022 19:31:19 +0200
Subject: [PATCH] Add bitset_ctz function

---
 m-bitset.h           | 27 +++++++++++++++++++++++++++
 tests/test-mbitset.c | 41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 68 insertions(+)

diff --git a/m-bitset.h b/m-bitset.h
index 24c43ed7..3d9a1827 100644
--- a/m-bitset.h
+++ b/m-bitset.h
@@ -786,6 +786,32 @@ m_bitset_clz(const m_bitset_t set)
   return s;
 }
 
+/* Count the number of trailing zero */
+static inline size_t
+m_bitset_ctz(const m_bitset_t set)
+{
+  M_B1TSET_CONTRACT(set);
+  size_t s = set->size;
+  if (M_UNLIKELY (s == 0)) {
+    return 0;
+  }
+  size_t i = 0, n = (s -1) / M_B1TSET_LIMB_BIT;
+  size_t m = s % M_B1TSET_LIMB_BIT;
+  m_b1tset_limb_ct limb = set->ptr[0];
+  s = 0;
+  while (limb == 0 && i < n) {
+    s += M_B1TSET_LIMB_BIT;
+    limb = set->ptr[++i];
+  }
+  if (i == n && m != 0) {
+    m_b1tset_limb_ct mask = (((m_b1tset_limb_ct)1) << m) - 1;
+    limb &= mask;
+  }
+  unsigned ctz = m_core_ctz64(limb);
+  s += (ctz == 64) ? m : ctz;
+  return s;
+}
+
 // For GCC or CLANG or ICC
 #if defined(__GNUC__)
 static inline size_t m_b1tset_popcount64(m_b1tset_limb_ct limb)
@@ -910,6 +936,7 @@ m_bitset_popcount(const m_bitset_t set)
 #define bitset_not m_bitset_not
 #define bitset_hash m_bitset_hash
 #define bitset_clz m_bitset_clz
+#define bitset_ctz m_bitset_ctz
 #define bitset_popcount m_bitset_popcount
 #define bitset_get_str m_bitset_get_str
 
diff --git a/tests/test-mbitset.c b/tests/test-mbitset.c
index 2b902ab0..3570b575 100644
--- a/tests/test-mbitset.c
+++ b/tests/test-mbitset.c
@@ -377,6 +377,47 @@ static void test_clz(void)
     bitset_set_str(s1, "[10000011100000110010101010110000011111111111111110000001110001110001110001110000]");
     assert(bitset_popcount(s1) == 40);
   }
+
+  M_LET(s1, bitset_t) {
+    bitset_set_str(s1, "[00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000]");
+    assert(bitset_ctz(s1) == 116);
+    bitset_set_str(s1, "[10000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000]");
+    assert(bitset_ctz(s1) == 0);
+    bitset_set_str(s1, "[01000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000]");
+    assert(bitset_ctz(s1) == 1);
+    bitset_set_str(s1, "[00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001]");
+    assert(bitset_ctz(s1) == 115);
+    bitset_set_str(s1, "[00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000010]");
+    assert(bitset_ctz(s1) == 114);
+    bitset_set_str(s1, "[000001]");
+    assert(bitset_ctz(s1) == 5);
+    bitset_set_str(s1, "[00000000000000000000000000000000000000000000000000000000000000001]");
+    assert(bitset_ctz(s1) == 64);
+    bitset_set_str(s1, "[000000000000000000000000000000000000000000000000000000000000000010000]");
+    assert(bitset_ctz(s1) == 64);
+    bitset_set_str(s1, "[0000000000000000000000000000000000000000000000000000000000000001]");
+    assert(bitset_ctz(s1) == 63);
+    bitset_set_str(s1, "[0000000000000000000000000000000000000000000000000000000000000001000]");
+    assert(bitset_ctz(s1) == 63);
+    bitset_set_str(s1, "[000000000000000000000000000000000000000000000000000000000000000001]");
+    assert(bitset_ctz(s1) == 65);
+    bitset_set_str(s1, "[0000000000000000000000000000000000000000000000000000000000000000010000]");
+    assert(bitset_ctz(s1) == 65);
+    bitset_set_str(s1, "[00001000001110000011001010101011000001111111111111111000000111000111000111000111000]");
+    assert(bitset_ctz(s1) == 4);
+    bitset_set_str(s1, "[000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001]");
+    assert(bitset_ctz(s1) == 128);
+    bitset_set_str(s1, "[00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001]");
+    assert(bitset_ctz(s1) == 127);
+    bitset_set_str(s1, "[0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001]");
+    assert(bitset_ctz(s1) == 129);
+    bitset_set_str(s1, "[0000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000000100000000000000000001]");
+    assert(bitset_ctz(s1) == 128);
+    bitset_set_str(s1, "[000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000000010000000000000000001000]");
+    assert(bitset_ctz(s1) == 127);
+    bitset_set_str(s1, "[00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000010000000000011000000000000000000000000]");
+    assert(bitset_ctz(s1) == 129);
+  }
 }
 
 int main(void)